Optical Detection of Dangerous Road Conditions - MDPI

Page created by Maurice Bennett
 
CONTINUE READING
Optical Detection of Dangerous Road Conditions - MDPI
sensors
Article
Optical Detection of Dangerous Road Conditions
Armando Piccardi * and Lorenzo Colace
 NooEL—Nonlinear Optics and OptoElectronics Laboratory and CNIT—Photonic Networks and Technologies
 National Lab, Department of Engineering, University ROMA TRE, Via Vito Volterra 62, 00146 Rome, Italy;
 lorenzo.colace@uniroma3.it
 * Correspondence: armando.piccardi@uniroma3.it
                                                                                                     
 Received: 13 February 2019; Accepted: 12 March 2019; Published: 19 March 2019                       

 Abstract: We demonstrated an optical method to evaluate the state of asphalt due to the presence of
 atmospheric agents using the measurement of the polarization/depolarization state of near infrared
 radiation. Different sensing geometries were studied to determine the most efficient ones in terms of
 performance, reliability and compactness. Our results showed that we could distinguish between a
 safe surface and three different dangerous surfaces, demonstrating the reliability and selectivity of
 the proposed approach and its suitability for implementing a sensor.

 Keywords: optical sensor; scattering from rough surfaces; polarized scattering; polarization
 contrast ratio

1. Introduction
     The assessment of surface conditions that are exposed to the effects of atmospheric agents is
crucial when dealing with safety, requiring the development of technologies to exploit sensing elements
to assess and reduce the associated risks.
     For instance, a number of methods have been developed for ice detection, each focused on a
specific application from aircraft safety to cooling systems or environmental monitoring: these exploit
combined measurements of temperature and humidity [1–3], capacitive [4–6] or piezoelectric [7]
sensors and fiber optic devices [8,9], and some of them are available as commercial systems [10].
Contact sensors for ice detection on roads or other surfaces are moderately reliable, but they have
problems in terms of installation and maintenance, since they need to be mounted directly on the
iced surface. For these reasons, contactless systems based on electrical resistance measurements [11],
reflected radiation at microwaves [12] or optical frequencies [13], and infrared imaging [14,15] have
been proposed.
     With reference to road safety, smart systems equipped on both vehicles and the roadside are
desirable when there is the need for a reliable way to distinguish between safe and potentially
dangerous driving situations [16,17].
     A number of methods have been developed based on the refractive index difference of asphalt,
water and ice, by measuring light reflection at different wavelengths. Unfortunately, they show limited
reliability due to their low sensitivity, thus preventing selective measurement [18–20]. More recently
it has been demonstrated that more information can be derived from the analysis of the reflected
polarization, which is very sensitive to different environmental conditions [21–24].
     Here we propose an optical method to detect different surface conditions based on the
measurement of two orthogonally polarized components of a light beam scattered by the surface. Since
the polarization of light is sensitive to the surface under investigation, its analysis gives information
on the surface itself and it allows to distinguish different conditions.

Sensors 2019, 19, 1360; doi:10.3390/s19061360                                    www.mdpi.com/journal/sensors
Optical Detection of Dangerous Road Conditions - MDPI
Sensors 2019, 19, 1360                                                                                2 of 8

     This method was proposed in recent paper where the working principle and some preliminary
results were reported [25]. It was based on the combined measurement of the polarization contrast ratio
and the amplitude of the scattered light, and it focused on the identification of ice on an asphalt surface.
     In this work, we started with the original idea and its preliminary results and developed an
optical method to distinguish different surface conditions. Even though, in principle, this method is
not dependent on the type of surface, our work focused on detecting the state of an asphalt surface
and assessing potentially dangerous road conditions.
     Using polarimetry measurement, we investigated different sensing configurations in order
to optimize both reliability and selectivity in the detection of asphalt conditions. Furthermore,
we implemented a sensing device and developed an electronic read-out suitable to handle the
optical signals.

2. Materials and Methods
       To investigate the different surfaces we employed a diode that emitted at λ = 980 nm. A near
infrared source has several advantages: first of all, it is invisible to human eye and therefore it avoids
the risk of annoying light scattering; it is in a wavelength range where the most common atmospheric
agents (e.g., water and ice) feature a high degree of selectivity [26]; and, additionally, low cost near
infrared optoelectronic devices are widely available. The radiation is modulated with a square wave at
a frequency of about 1 kHz, which optimizes the detection sensitivity and immunity to environmental
(i.e., continuous) light when a narrowband receiver is used.
       A lens was used to collimate the radiation and obtain a spot size of about 5 mm on the surface
under investigation. This solution compensates for beam diffraction, allowing maximization of the
light impinging on the detectors. A polarizer can be added to the source to study the sensor response
as a function of incident light polarization. Two further polarizers were used to select the polarization
components of the scattered light parallel and perpendicular to the incident plane and a couple
of photodiodes detected polarized radiation and converted the optical signals into current signals.
The front-end electronics performed a current to voltage conversion by means of trans-impedance
amplifiers (TIA). The signals corresponding to the orthogonal polarizations were detected by a
heterodyne receiver (i.e., a lock-in amplifier) in order to extract and process the information about the
surface condition even in the presence of large noise. The block diagram of the sensor is sketched in
Figure 1a.
       The figure of merit that was exploited to distinguish between different surfaces was the
polarization contrast ratio (PC). This parameter is a normalized quantity sensitive to the differences
between the polarization components, which was defined as PC = (ITE − ITM )/(ITE + ITM ), where
ITE/TM is the light intensity corresponding to the two polarization components. In addition, the setup
was equipped with a third photodetector in order to measure the total unpolarized light scattered from
the surface, thus providing a second useful parameter.
       Since Fresnel’s coefficients depend on the incidence angle [27], preliminary measurements were
performed to identify the optimal geometric configuration. Thus, we first found the angle for which
the distance among the polarization contrast values corresponding to different surface states was
maximized. Once this angle was found, the polarization contrast was calculated from the converted
voltage values (representing the light intensities) and it was plotted against the total (unpolarized)
scattered light. In this way, the position of the single point on the parameter space (PC vs unpolarized
scattering) is a function of the surface condition [25].
       We focused our experiments on asphalt surfaces with the aim of detecting different atmospheric
agents affecting road safety. The investigated samples refer to different asphalt conditions, as indicated
in Figures 2–5 (later in the text). Four kinds of surfaces were tested. The reference condition corresponds
to dry asphalt, which can be identified as a safe condition. Several kinds of asphalts were tested,
differing in both roughness and composition. Though no formal classification of the kinds of asphalt
Optical Detection of Dangerous Road Conditions - MDPI
Sensors 2019, 19, 1360                                                                                                            3 of 8

Sensors 2019, 19, x FOR PEER REVIEW                                                                                           3 of 8
was performed, we verified (using data in Figures 3 and 5, see Section 3) that all kinds of asphalt
sharedA first  potentially
          similar              dangerouswith
                   optical properties       condition
                                                 respectistothe
                                                              thepresence
                                                                   scatteredofpolarized
                                                                               water on radiation.
                                                                                            the asphalt surface. This has
been Astudied     in two different
         first potentially     dangerous states:  wet asphalt
                                            condition             and asphalt
                                                        is the presence           where
                                                                            of water             surface surface.
                                                                                       on the asphalt       was completely
                                                                                                                     This has
covered
been studiedby ainthick     layer (i.e.,
                    two different         a few
                                     states: wet millimetres)       of water.
                                                   asphalt and asphalt     whereA the
                                                                                   second
                                                                                        surfacehazardous     condition
                                                                                                  was completely          was
                                                                                                                      covered
determined     by the
by a thick layer         presence
                     (i.e.,         of ice: we used
                            a few millimetres)         an asphalt
                                                   of water.          surface
                                                               A second        that wascondition
                                                                            hazardous       covered by    a thin
                                                                                                        was       ice layer by
                                                                                                             determined     (1
mm).
the presence of ice: we used an asphalt surface that was covered by a thin ice layer (1 mm).
      Thus, we investigated
                   investigated the the scattered
                                         scattered radiation
                                                     radiation from
                                                                  from four
                                                                         four different
                                                                               different states,
                                                                                            states, identified
                                                                                                     identified as
                                                                                                                 as dry,
                                                                                                                     dry, wet,
                                                                                                                          wet,
water and
        and ice
              iceininthe
                       thefollowing.
                             following.We We  would    like   to emphasize     that  it  is not  within    the scope
                                            would like to emphasize that it is not within the scope of this paper      of this
paper    to investigate
to investigate    the optical the properties
                                  optical properties      of these
                                               of these kinds         kinds ofbut
                                                                  of surfaces,    surfaces,
                                                                                      only tobut      only the
                                                                                                validate    to validate
                                                                                                                approachthe to
approach
efficiently to  efficiently
             detect            detectcondtions.
                      the surface     the surface condtions.
      Two major geometries were investigated, as shown in Figure 1. The first was based on Fresnel
reflection with the source and detectors mounted in a θ–2θ              θ–2θ configuration
                                                                               configuration (Figure
                                                                                                  (Figure 1b).
                                                                                                             1b). Using this
geometry, the horizontal (vertical) polarized reflected radiation was maximized (minimized) at the
Brewster angle (like for a smooth surface) and thus featured a wide range of polarization contrast
ratio with benefits to the system sensitivity. Nevertheless, the system required the source (S) and the
detector (D) to be placed in different positions, increasing increasing the
                                                                          the space
                                                                              space needed
                                                                                      needed for for the
                                                                                                      the sensor.
                                                                                                          sensor.
      The alternative geometry measured the polarization components of the backscattered light that
was always present during scattering from a rough surface (Figure 1c). In this case, higher sensitivity
was needed
      neededfrom  from the the   receiver,
                            receiver, but thebut   the resulting
                                               resulting  sensor was  sensor
                                                                         muchwasmoremuchcompact.more     compact.
                                                                                                     In both         In both
                                                                                                              configurations,
configurations,
the presence of an  theadditional
                          presence of    an additional
                                      detector,  used to detector,
                                                          sense the used     to sensescattered
                                                                       unpolarized      the unpolarized
                                                                                                   light from scattered  light
                                                                                                                the direction
from   the  direction    of  the surface  normal,
of the surface normal, was considered as well.      was   considered     as well.

      Figure 1. Block
                  Blockdiagram
                         diagramandandset-up.
                                         set-up.(a)(a)Block
                                                        Blockdiagram
                                                               diagram  of of
                                                                           thethesensor.  TheThe
                                                                                    sensor.    radiation
                                                                                                  radiationfrom  thethe
                                                                                                              from    LED   (S)
                                                                                                                          LED
      was   collimated
      (S) was   collimatedby by
                              a lens
                                 a lens(L)(L)and
                                              andimpinged
                                                    impingedon    on the
                                                                      the surface             investigation. After
                                                                           surface under investigation.          After being
      reflected/scattered,
      reflected/scattered,ititwas
                                wasdetected
                                      detectedbybytwo twophotodiodes
                                                            photodiodes  (PD1
                                                                            (PD1  and  PD2).
                                                                                     and      TwoTwo
                                                                                          PD2).     polarizers   (P1 (P1
                                                                                                        polarizers    and and
                                                                                                                            P2)
      selected  the horizontal   and  vertical polarization    components.       Two   trans-impedance
      P2) selected the horizontal and vertical polarization components. Two trans-impedance amplifiers      amplifiers   (TIA)
      and
      (TIA)the
             andlock-in  amplifier
                  the lock-in       converted
                              amplifier          the the
                                         converted     optical signals
                                                           optical       into
                                                                    signals     two
                                                                             into  twovoltage
                                                                                        voltagelevels
                                                                                                 levelsready
                                                                                                        readyforforthe
                                                                                                                    the signal
                                                                                                                        signal
      processing.   (b) θ–2θ
      processing. (b)   θ–2θconfiguration.
                               configuration.TheTheincidence
                                                      incidence    angle
                                                               angle      θ was
                                                                       θ was        referred
                                                                               referred      to surface
                                                                                         to the  the surface   normal
                                                                                                          normal  and aand    a
                                                                                                                          third
      third   photodiode
      photodiode            (PD3)
                    (PD3) was    usedwas   used tothe
                                       to measure       measure     the unpolarized
                                                          unpolarized     scattering. (c)scattering.     (c) Backscattering
                                                                                            Backscattering    configuration.
      configuration.
      The set-up wasThe      set-up was
                         analogous         analogous
                                      to (b), but bothtophotodetectors
                                                             (b), but both collecting
                                                                               photodetectors     collectingscattering
                                                                                           the polarized      the polarized
                                                                                                                          were
      scattering
      mounted on  were
                     themounted
                         same sideon ofthe
                                        thesame   side of the source.
                                             source.

     The experimental set-up was realized with commercial components: the source was a LED940E
(Thorlabs) with a maximum
                   maximum drivedrive current
                                      current ofof 100 mA
                                                       mA for a power of 18 mW. The polarizers were
 LPV050 (Thorlabs) with an  extinction  ratio  higher than
         (Thorlabs) with an extinction ratio higher than    100000:1
                                                              100000:1at the employed
                                                                         at the   employed  wavelength  andand
                                                                                               wavelength   the
 detectors
the        were
    detectors   BPW34F
              were BPW34F(OSRAM)
                              (OSRAM) thatthat
                                            worked
                                               workedin the 780–1100
                                                        in the 780–1100nmnm spectral
                                                                                spectralrange  and
                                                                                           range andexhibited
                                                                                                      exhibiteda
                                                                                                  2
amaximum
  maximumresponsivity   ofof
             responsivity  0.70.7
                                A/W
                                  A/Watatλλ==950
                                               950nm
                                                   nmand
                                                       andaasensitive
                                                              sensitivearea        2.65×× 2.65
                                                                         areaofof2.65     2.65 mm
                                                                                               mm2.

3. Results
3. Results
    In the
    In the first
           first geometry,
                 geometry, Figure
                           Figure 1b,
                                  1b, the
                                      the source
                                          source and
                                                  and the
                                                       the detectors
                                                           detectors were
                                                                     were mounted    on two
                                                                            mounted on   two arms
                                                                                              arms in
                                                                                                   in aa
goniometric  system to preserve the symmetry   of  the incident and  reflection angles, with the source
goniometric system to preserve the symmetry of the incident and reflection angles, with the source
mounted approximately
mounted  approximately 3030 cm
                            cm from
                                from the
                                      the surface
                                          surface under
                                                   under investigation.
                                                          investigation. AA preliminary
                                                                             preliminary measurement
                                                                                          measurement
involved the evaluation of the polarization contrast as a function of the incident angle [28]. The
results in the case of unpolarized light from the source are shown in Figure 2a, after being averaged
over several acquisitions. Most of the curves corresponding to the different kind of surfaces were
Optical Detection of Dangerous Road Conditions - MDPI
Sensors 2019, 19, 1360                                                                                                       4 of 8

involved
Sensors 2019,the
              19, xevaluation of the polarization
                    FOR PEER REVIEW                    contrast as a function of the incident angle [28]. The results       4 of 8
 in the case of unpolarized light from the source are shown in Figure 2a, after being averaged
superposed,
 over severalpreventing
                  acquisitions. theMost
                                    identification   of thecorresponding
                                           of the curves     asphalt conditions. to theOnly     for θ kind
                                                                                          different   = 50° of were   the curves
                                                                                                                 surfaces    were
separated,   but    the polarization    contrast  range   was    small.   We   repeated     the  same
 superposed, preventing the identification of the asphalt conditions. Only for θ = 50 were the curves   measurement
                                                                                                             ◦              using
polarized
 separated,light,
               but theexploiting    the contrast
                         polarization     polarization
                                                   rangedependence
                                                           was small. We     of repeated
                                                                                 the Fresnel      coefficients.
                                                                                             the same    measurementFigureusing
                                                                                                                             2b,c
reports   on  the   angular   dependence      of the  scattered   light   from    a source    with   horizontal
 polarized light, exploiting the polarization dependence of the Fresnel coefficients. Figure 2b,c reports          and   vertical
polarization,
 on the angular  respectively.
                    dependenceWhen of the using  horizontal
                                           scattered           polarization
                                                      light from   a source with from   the source
                                                                                     horizontal    and(i.e., perpendicular
                                                                                                         vertical               to
                                                                                                                    polarization,
the   plane  of  incidence)     the  curves   exhibited   some     angles    (θ   > 50°)  where     the
 respectively. When using horizontal polarization from the source (i.e., perpendicular to the plane of   safe   condition    was
separated
 incidence)from      all the exhibited
               the curves     others, but    none
                                          some     of the(θ“dangerous”
                                                 angles      > 50◦ ) wherestates       were
                                                                                the safe       clearly distinguishable.
                                                                                           condition     was separated from   On
the   other
 all the     hand,
          others,     the
                    but    vertical
                         none   of thepolarized
                                        “dangerous”lightstates
                                                          (i.e., were
                                                                 polarization       parallel to the plane
                                                                         clearly distinguishable.         On the  of other
                                                                                                                      incidence)
                                                                                                                            hand,
guaranteed      a  larger  separation     between    the   different   conditions,      at  least   around
 the vertical polarized light (i.e., polarization parallel to the plane of incidence) guaranteed a larger      50°,   where   the
polarization    contrast   value   associated    with  water  (for   both   the ◦water   layer   and  wet
 separation between the different conditions, at least around 50 , where the polarization contrast value    cases)   exhibited   a
minimum
 associated.with
               This water
                      was due(fortoboth
                                     the the
                                          Brewster   angleand
                                              water layer    associated
                                                                  wet cases)to the   water surface
                                                                                 exhibited             (approximately
                                                                                              a minimum.       This was due  53°).
                                                                                                                                 to
The    transmitted     radiation    was   minimized,     increasing     the   intensity◦   associated
 the Brewster angle associated to the water surface (approximately 53 ). The transmitted radiation was    with    the   reflected
radiation,
 minimized,  and     thus increasing
                increasing                 the associated
                               the intensity    absolute value
                                                             with theof the    polarization
                                                                         reflected    radiation, contrast
                                                                                                    and thus  ratio.   Thus, we
                                                                                                                 increasing    the
concluded     that   vertical   polarized    radiation   offered    the  best   performances        for
 absolute value of the polarization contrast ratio. Thus, we concluded that vertical polarized radiation the   realization   of  a
sensor   in this  geometry.
 offered the best performances for the realization of a sensor in this geometry.

      Figure 2.2. θ–2θ
      Figure             configuration.Polarization
                   θ–2θconfiguration.      Polarization contrast
                                                     contrast     versus
                                                              versus      incidence
                                                                      incidence     angle
                                                                                angle     for unpolarized,
                                                                                      for (a)  (a) unpolarized,
                                                                                                           (b)
      (b) horizontally  and (c) vertically polarized incident light.
      horizontally and (c) vertically polarized incident light.

       After setting the vertically polarized source to 50◦ with respect to the surface normal, we measured
      After setting the vertically polarized source to 50° with respect to the surface normal, we
 the intensities associated with the two polarization components and calculated the polarization contrast
measured the intensities associated with the two polarization components and calculated the
 for all the surfaces under investigation. A large number of acquisitions were taken in order to evaluate
polarization contrast for all the surfaces under investigation. A large number of acquisitions were
 statistical errors and to simulate the “on field” conditions, including motion and different kinds of
taken in order to evaluate statistical errors and to simulate the “on field” conditions, including
 asphalt surfaces.
motion and different kinds of asphalt surfaces.
       To evaluate the surface state, we built two parameter planes. The first included the polarization
      To evaluate the surface state, we built two parameter planes. The first included the polarization
 contrast values versus the total unpolarized scattering, evaluated from the measurement of the
contrast values versus the total unpolarized scattering, evaluated from the measurement of the light
 light scattered along the surface normal and collected by the third photodetector. The second was
scattered along the surface normal and collected by the third photodetector. The second was
 composed of the polarization contrast versus the sum of the two polarization components. In this
composed of the polarization contrast versus the sum of the two polarization components. In this
 way, each surface condition could be defined by a sector of the parameter plane, as the values of
way, each surface condition could be defined by a sector of the parameter plane, as the values of the
 the experimental measurements corresponding to the different surfaces generated a group of points
experimental      measurements corresponding to the different surfaces generated a group of points
 (with
(with thethepoint
             pointdispersion
                     dispersionbeing
                                   beingdue
                                         duetoto measurement
                                               measurement          uncertainty,
                                                                uncertainty,    thethe different
                                                                                    different    kinds
                                                                                              kinds      of asphalt,
                                                                                                     of asphalt,   or
 or slightly  different surface   conditions, like water   or ice layer  thickness), which  should  be
slightly different surface conditions, like water or ice layer thickness), which should be included in  included    in
 the corresponding     part  of the
the corresponding part of the plane.plane.
       Figure 33 shows
      Figure      shows the
                          the graphs
                              graphs obtained
                                       obtained for
                                                 for aa vertical
                                                        vertical polarized
                                                                  polarizedsource
                                                                               sourceand
                                                                                       andincidence
                                                                                           incidenceangle
                                                                                                      angleof     50◦ .
                                                                                                              of50°.
 The former
The   former case,
              case, PC
                     PCversus
                         versusunpolarized
                                 unpolarizedscattering
                                               scattering(Figure
                                                            (Figure3a), exhibited
                                                                     3a),         a problem
                                                                          exhibited          withwith
                                                                                     a problem    the dry
                                                                                                      the (i.e., safe)
                                                                                                           dry (i.e.,
 and   iced (i.e., dangerous)    conditions,  where    the  two  groups    of points  were
safe) and iced (i.e., dangerous) conditions, where the two groups of points were partially  partially overlapping.
 Thus, the choice
overlapping.         of geometry
                 Thus,   the choice andofmeasured
                                          geometryparameters
                                                       and measureddid notparameters
                                                                             allow selective
                                                                                         did measurement     for the
                                                                                              not allow selective
 considered surface
measurement             states.
                   for the       Conversely,
                            considered        using
                                          surface     the sum
                                                   states.       of the orthogonal
                                                             Conversely,     using thepolarizations
                                                                                         sum of theallowed      us to
                                                                                                       orthogonal
polarizations allowed us to discriminate among all four different states even if the voltage signal
levels were slightly lower (thus, the required sensitivity was higher), as shown in Figure 3b. Even if
the water and wet conditions were very close, they almost correspond to the same kind of dangerous
Optical Detection of Dangerous Road Conditions - MDPI
Sensors 2019, 19, 1360                                                                                                       5 of 8

discriminate among all four different states even if the voltage signal levels were slightly lower (thus,
Sensors 2019, 19,
the required
Sensors 2019, 19, xx FOR PEER
                         PEER REVIEW
                   sensitivity
                     FOR      REVIEW
                               was higher),
                                          as shown in Figure 3b. Even if the water and wet conditions   55 of
                                                                                                           of 88

were very close, they almost correspond to the same kind of dangerous conditions, thus the risk of
conditions, thus
conditions, thus the
                  the risk
                       risk of
                            of confusing
                                confusing the twotwo surfaces
                                                       surfaces is
                                                                is not
                                                                   not critical
                                                                        critical and
                                                                                 and it
                                                                                     it would
                                                                                        would not
                                                                                               not affect
                                                                                                   affect the
                                                                                                           the
confusing the two   surfaces  is not criticalthe
                                               and it would  not affect the performance   of the sensor  in an
performance
performance   of the sensor  in an
             of theinvestigation.  application-based     investigation.
                     sensor in an application-based investigation.
application-based

      Figure
      Figure 3.
      Figure   3. Mirror-like
                  Mirror-likeconfiguration.
                  Mirror-like  configuration.Polarization
                               configuration.  Polarizationcontrast
                                               Polarization  contrast
                                                             contrast versus
                                                                    versus
                                                                      versus  (a)
                                                                           (a)(a) the
                                                                               thethe  total
                                                                                   total     scattering
                                                                                         scattering
                                                                                      total             and
                                                                                                    andand
                                                                                             scattering       (b)sum
                                                                                                         (b) the
                                                                                                              (b) the
                                                                                                                  the
      sum
      of theof
             twothe  two   orthogonal
                   orthogonal           polarized
                                polarized          components
                                           components
      sum of the two orthogonal polarized components    (V    and(V
                                                                  V TE and VTM, the voltage corresponding to
                                                                 (VTE  , the voltage  corresponding    to horizontal
                                                           TE       TMand VTM, the voltage corresponding to
      horizontal
      and vertical
      horizontal   and vertical polarization,
                    polarization,
                  and  vertical  polarization, respectively).
                                   respectively).
                                               respectively).

      The polarization
      The    polarization    contrast
             polarization contrast      was
                              contrast was     close
                                         was close    to
                                               close to   zero for
                                                       to zero  for both
                                                                     both the
                                                                           the dry
                                                                               dry asphalt
                                                                                     asphalt and
                                                                                               and the
                                                                                                    the ice
                                                                                                        ice cases,
                                                                                                            cases, which
                                                                                                                   which
indicates that
indicates     that the
                    the scattered
                        scattered light
                                    light was
                                           was almost
                                                 almost unpolarized,
                                                          unpolarized, probably
                                                                          probably duedue to
                                                                                           to the
                                                                                              the depolarization
                                                                                                  depolarization effects
                                                                                                                   effects
from   the
       the   high
             high    surface
                   surface    roughness
                            roughness     inin  both
                                             both      cases.
                                                    cases. TheThe   presence
                                                                 presence  of  of water
                                                                              water       enhanced
                                                                                     enhanced
from the high surface roughness in both cases. The presence of water enhanced light polarization,lightlight polarization,
                                                                                                       polarization, even
even    if
if the if
even        the
        groups    groups
                   of points
            the groups      of points    corresponding
                           ofcorresponding                   to
                                                  to the wettoand
                               points corresponding              the  wet
                                                                 thewater   and
                                                                      wet layer   water  layer
                                                                                  were quite
                                                                            and water            were
                                                                                         layerdispersed.quite
                                                                                                 were quite   dispersed.
                                                                                                            Nevertheless,
                                                                                                              dispersed.
Nevertheless,
this  geometry       this
                    allows geometry
                            us  to       allows
                                   discriminate     us
                                                    amongto  discriminate
                                                             some             among
                                                                     potentially        some
                                                                                  dangerous
Nevertheless, this geometry allows us to discriminate among some potentially dangerous          potentially
                                                                                               conditions.    dangerous
                                                                                                            Furthermore,
conditions.      Furthermore,
the third photodiode
conditions.     Furthermore,     the
                            is not
                                 the  third photodiode
                                             photodiode is
                                    necessary.
                                      third                 is not
                                                                not necessary.
                                                                    necessary.
      We performed
      We     performed the the same
                                same kind
                                       kind ofof measurement
                                                   measurement      procedure
                                                   measurement procedure
                                                                     procedure inin the
                                                                                    the backscattering
                                                                                         backscattering    configuration.
                                                                                         backscattering configuration.
                                                                                                           configuration.
As   for
    for    the
         the     former
               former     geometry,
                        geometry,    we we
                                        first first  verified
                                               verified  which  which
                                                                  angle  angle  maximized
                                                                         maximized    the
As for the former geometry, we first verified which angle maximized the separation between     the
                                                                                          separationseparation
                                                                                                       between   between
                                                                                                                 different
different
conditions,
different      conditions,
                 as sketched
              conditions,    as   sketched
                             asinsketched
                                   Figure 4, inin   Figure
                                               for Figure    4,  for
                                                    the unpolarized   the  unpolarized
                                                            4, for the(Figure              (Figure
                                                                                 4a) and (Figure
                                                                           unpolarized     polarized 4a)  and  polarized
                                                                                                       (horizontally
                                                                                                     4a)              and
                                                                                                          and polarized
(horizontally
vertically,        and
               Figure   vertically,
                       4b,c,         Figures
                             respectively)      4b,c,
                                              incidentrespectively)
                                                          radiation.   incident
(horizontally and vertically, Figures 4b,c, respectively) incident radiation.   radiation.

      Figure 4.4.Backscattering
      Figure       Backscatteringconfiguration. Polarization
                                    configuration.            contrast
                                                      Polarization     versus incidence
                                                                     contrast           angle for (a)
                                                                                versus incidence
                                                                                       incidence      unpolarized,
                                                                                                   angle  for (a)
                                                                                                               (a)
      Figure  4. Backscattering     configuration.    Polarization   contrast  versus              angle  for
      (b) horizontally
      unpolarized,  (b) and (c) vertically
                        horizontally  and  polarized
                                           (c)        incident
                                               vertically      light.incident light.
                                                          polarized
      unpolarized, (b) horizontally and (c) vertically polarized incident light.
      It can
     It  can bebe noted
                  notedthat
                         thatthe
                               thepolarization
                                     polarizationcontrast
                                                      contrast was
                                                                 waslower
                                                                        lowerwith  respect
                                                                                with        to the
                                                                                      respect       previous
                                                                                                to the
                                                                                                    the          case.case.
                                                                                                          previous      ThisThis
                                                                                                                              was
     It  can   be noted  that  the   polarization     contrast   was    lower   with  respect   to       previous     case. This
expected
was expectedconsidering
     expected considering the  lower
                   considering the      signal
                                  the lower     level
                                        lower signal    due
                                                signal level to different
                                                          level due
                                                                due to      geometry.
                                                                       to different      Nevertheless,
                                                                           different geometry.              here
                                                                                      geometry. Nevertheless,     the
                                                                                                     Nevertheless, herefour  road
                                                                                                                        here the
                                                                                                                               the
was
conditions
four  road     often exhibited
              conditions  often  quite   different
                                  exhibited     quitevalues.   In the
                                                        different       unpolarized
                                                                   values.    In  the  case the whole
                                                                                      unpolarized      case angle
                                                                                                              the  range below
                                                                                                                   whole    angle
four road    conditions often exhibited quite different values. In the unpolarized case the whole angle
θ = 40◦below
range     couldθbe= used,  evenbe
                     40° could
                         could     if the dry
                                      used,    condition
                                             even   if the   wascondition
                                                       the dry
                                                            dry   always close wasto  the iceclose
                                                                                    always    curve. toWhen   iceexamining
                                                                                                         the ice  curve. When
                                                                                                                           Whenthe
range below θ = 40°             be used,     even   if           condition was      always   close to   the       curve.
horizontal
examining the  polarized  case,
                the horizontal  we     can
                    horizontal polarized   easily
                                 polarized case,   see
                                                case, we that
                                                        we canthe  curves
                                                             can easily
                                                                  easily see were
                                                                            see thatsuperposed
                                                                                 that the
                                                                                       the curves  for
                                                                                            curves were almost    all
                                                                                                      were superposed the  angles
                                                                                                               superposed for  for
examining
almost all
almost    all the
              the angles
                   angles (except
                          (except for for θ
                                          θ == 65°,
                                               65°, where
                                                     where drydry and
                                                                   and iceice conditions
                                                                              conditions were
                                                                                            were still
                                                                                                    still too
                                                                                                          too close
                                                                                                                close to
                                                                                                                       to ensure
                                                                                                                          ensure
reliable   detection),  thus  this   case  was   also   not  taken   into  consideration.     The   vertical
reliable detection), thus this case was also not taken into consideration. The vertical polarized light         polarized    light
case   provided     much    better     results,  exhibiting      several    angles    where
case provided much better results, exhibiting several angles where all the curves were well    all  the    curves    were     well
separated. In
separated.     In particular,
                  particular, at
                               at θ θ == 20°
                                         20° the
                                              the graph
                                                  graph shows
                                                            shows almost
                                                                      almost equal
                                                                               equal distances
                                                                                       distances among
                                                                                                    among the  the curves
                                                                                                                    curves in in aa
Sensors 2019, 19, 1360                                                                                                 6 of 8

(except for θ = 65◦ , where dry and ice conditions were still too close to ensure reliable detection), thus
this case
Sensors     was
         2019, 19, xalso
                     FORnot
                          PEERtaken  into consideration. The vertical polarized light case provided much better
                                REVIEW                                                                                 6 of 8
results, exhibiting several angles where all the curves were well separated. In particular, at θ = 20◦
quite   largeshows
the graph       interval   of about
                         almost  equal0.12  for polarization
                                          distances  among the contrast.
                                                                   curvesAs  inaafurther  advantage
                                                                                  quite large  intervalwith  respect
                                                                                                         of about     to the
                                                                                                                   0.12  for
unpolarized       case, the As
polarization contrast.         dryacondition    had the highest
                                    further advantage               contrast
                                                           with respect        value,
                                                                           to the      and it was
                                                                                   unpolarized      well
                                                                                                 case,  theseparated   from
                                                                                                             dry condition
all
hadthethe other
            highest  potentially   dangerous
                        contrast value,    and it situations.   In addition,
                                                   was well separated       frominallthis
                                                                                      the case,
                                                                                           other the   verticaldangerous
                                                                                                  potentially    polarized
radiation
situations. was        considered
               In addition,    in thistocase,
                                          be the
                                              thevertical
                                                    best choice    to radiation
                                                           polarized    discriminate     among theto different
                                                                                   was considered        be the bestasphalt
                                                                                                                     choice
conditions.
to discriminate among the different asphalt conditions.
      Thus,
      Thus, onceonce thethe incident
                            incident angle
                                        angle was   set to
                                               was set  to about
                                                           about 20   ◦ , we
                                                                   20°,   we measured
                                                                               measured thethe two
                                                                                               two polarized
                                                                                                     polarized scattering
                                                                                                                 scattering
components.
components. The     The results
                         resultsare
                                  areshown
                                       shownininFigure
                                                   Figure5,5,with
                                                              withthethe  polarization
                                                                       polarization       contrast
                                                                                       contrast as aasfunction
                                                                                                       a function   of both
                                                                                                                of both  the
the  total   scattering     (Figure   5a)  and   the sum    of  the  signals   corresponding     to  the
total scattering (Figure 5a) and the sum of the signals corresponding to the two orthogonal polarizations two  orthogonal
polarizations
(Figure    5b). (Figure 5b).

                Backscattering configuration. Polarization contrast versus (a) the total scattering and (b) the
      Figure 5. Backscattering
      sum of the two orthogonal polarized components.

      Remarkably, the
     Remarkably,     the two
                          two graphs
                                graphs were
                                        were very
                                               very similar,
                                                     similar, which
                                                                 which made
                                                                         made the
                                                                                the third
                                                                                     third photodiode
                                                                                            photodiodeunnecessary.
                                                                                                           unnecessary.
In other words, the backscattered light contained all the information needed, allowing usreduce
In other words,   the  backscattered   light contained    all  the information    needed,   allowing   us to           the
                                                                                                               to reduce
sensor  complexity.
the sensor complexity.
      The dry
     The   dry asphalt
                asphalthad hadthe
                                thehighest value
                                     highest      of both
                                              value         contrast
                                                      of both          and total
                                                                  contrast         scattering,
                                                                            and total           thus defining
                                                                                         scattering,             a region
                                                                                                      thus defining      a
of safe condition   in  the parameter    space,  while  the   polarization   contrast   easily  allowed
region of safe condition in the parameter space, while the polarization contrast easily allowed the       the  detection
of the presence
detection  of theof  ice. Theof
                   presence     water  layerwater
                                  ice. The   showedlayertheshowed
                                                             lower value    of (back)scattering
                                                                      the lower                     due to the smooth
                                                                                   value of (back)scattering       due to
surface,  while  the  group    of points  corresponding       to the  wet  condition    was   in
the smooth surface, while the group of points corresponding to the wet condition was in the middlethe middle     between
water
betweenandwater
            dry, as
                  andexpected.    The results
                        dry, as expected.     forresults
                                            The   each group
                                                           for eachof points
                                                                       groupwere     well were
                                                                              of points   separated
                                                                                                  well from   the others
                                                                                                       separated     from
with  reasonable   dispersion    occurring,  which   allowed     us to divide  the  parameter
the others with reasonable dispersion occurring, which allowed us to divide the parameter space   space into  four   parts
that four
into wereparts
           associated    to the
                 that were      different to
                              associated   asphalt  conditions.
                                             the different          Forconditions.
                                                              asphalt   example, PC     = example,
                                                                                      For  0.05 would PCdivide
                                                                                                          = 0.05the    dry
                                                                                                                   would
asphalt  and  the ice  states.
divide the dry asphalt and the ice states.
      Hence, the
     Hence,    thegeometry
                      geometry thatthat
                                    employed    the backscattered
                                          employed                     radiation radiation
                                                        the backscattered         guaranteedguaranteed
                                                                                                 the best performance.
                                                                                                                the best
Moreover,   employing      only  two  photodiodes    mounted       on the same
performance. Moreover, employing only two photodiodes mounted on the same side   side  of the  source  allowed     for the
                                                                                                          of the source
most compact
allowed   for theconfiguration
                   most compact   for configuration
                                      the practical realization     of sensors
                                                       for the practical         based onof
                                                                             realization    this   method.
                                                                                               sensors   based on this
method.
4. Discussion
4. Discussion
     The presented results demonstrate the possibility of implementing an optical sensor to distinguish
several asphalt conditions. The information on the state is given by the position of the measured
     The presented results demonstrate the possibility of implementing an optical sensor to
values on the parameter plane: for instance, Figure 4 suggests that the plane can be divided into four
distinguish several asphalt conditions. The information on the state is given by the position of the
parts corresponding to the four different states. The edges of the four parts define threshold values
measured values on the parameter plane: for instance, Figure 4 suggests that the plane can be
of the polarization contrast and the sum of the components, which could be represented by straight
divided into four parts corresponding to the four different states. The edges of the four parts define
lines. In other words, our results can be used as a calibration of the system. A sensor employing this
threshold values of the polarization contrast and the sum of the components, which could be
method will measure the suitable quantities by placing the corresponding point on the parameter
represented by straight lines. In other words, our results can be used as a calibration of the system. A
sensor employing this method will measure the suitable quantities by placing the corresponding
point on the parameter plane. The surface state will be identified by comparing the position of the
measured point with the calculated thresholds.
     Future developments of our work will encompass the increased number of detected conditions,
including a study of the perturbations coming from disturbances such as the presence of dust or
Sensors 2019, 19, 1360                                                                                     7 of 8

plane. The surface state will be identified by comparing the position of the measured point with the
calculated thresholds.
     Future developments of our work will encompass the increased number of detected conditions,
including a study of the perturbations coming from disturbances such as the presence of dust or water
particles in the air, mixed surface conditions, or fast motion. Moreover, the implementation of an
algorithm to define the threshold values has to be performed. This should include the definition of
the boundaries for each zone, for instance by the statistical analysis of the groups of points over a
huge number of acquisitions. Finally, the development of electronics able to automatically process the
measured data and provide an alert signal linked to the state of the surface will be required.

5. Conclusions
     We proposed an optical method for the detection of dangerous road conditions based on the
measurement of the polarization state of the scattered radiation from an asphalt surface by two
photodiodes that detected two orthogonal polarized light components. Two main geometries were
investigated. The first measured the reflected light, while the second measured the backscattered
radiation. In both configurations we also evaluated the need for a third photodiode that measured
the total unpolarized scattering. The backscattering geometry was the most reliable, allowing for
discrimination among dry, wet, water and ice surfaces, and thus distinguishing the safe condition
from the other three different danger levels. In addition, in this case the information from the third
photodiode were redundant, allowing a compact sensor geometry.
     Thus, we demonstrated the feasibility of a contactless sensor that featured reliability, compactness
and was low cost due to the usage of commonly-used commercial components. We stress that the
method used here is not dependent on the surface under investigation, and thus it is not limited to the
automotive sector. In fact, the reference surface can change from case to case, e.g., a metallic surface
(rather than asphalt) in the case of ice detection on wind turbines or airplane wings. Thus, the same
approach can potentially be applied to any case where different surface conditions must be identified.

Author Contributions: Conceptualization, L.C.; methodology, investigation, A.P. and L.C.; validation, formal
analysis, data curation, A.P.; resources, L.C.; writing—draft preparation, review and editing, A.P. and L.C.;
visualization, A.P. and L.C.; supervision, project administration, funding acquisition, L.C.
Funding: This work was supported by ACTPHAST (Access CenTer for PHotonics InnovAtion Solutions and
Technology Support), grant number P2016-29 (SODARC—Sensor for the Optical detection of DAngerous Road
Conditions).
Conflicts of Interest: The authors declare no conflict of interest.

References
1.    Ikiades, A.; Howard, G.; Armstrong, D.J.; Konstantaki, M.; Crossley, S. Measurement of optical diffusion
      properties of ice for direct detection ice accretion sensors. Sens. Actuator A 2007, 140, 24–31. [CrossRef]
2.    Wollenweber, G.C. Icing Condition Detection Using Instantaneous Humidity Sensing. U.S. Patent App.
      15,275,013, 29 March 2018.
3.    Hydrologicalusa. Available online: https://www.hydrologicalusa.com/products/snow-ice-measurement/
      ice-detection-sensor/ (accessed on 6 February 2019).
4.    Zhi, X.; Cho, H.C.; Wang, B.; Ahn, C.H.; Moon, H.S.; Go, J.S. Development of a Capacitive Ice Sensor to
      Measure Ice Growth in Real Time. Sensors 2015, 15, 6688–6698. [CrossRef] [PubMed]
5.    Troiano, A.; Pasero, E.; Mesin, L. Remote Monitoring of Ice Formation over a Runway Surface. IEEE Trans.
      Instr. Meas. 2010, 60, 1091–1101. [CrossRef]
6.    Flatscher, M.; Neumayer, M.; Bretterklieber, T. Maintaining critical infrastructure under cold climate
      conditions: A versatile sensing and heating concept. Sens. Actuators A Phys. 2017, 267, 538–546. [CrossRef]
7.    Roy, S.; De Anna, R.G.; Mehregani, M.; Zakar, E. A capacitive ice detection microsensor. Sens. Mater. 2000,
      12, 1–14.
Sensors 2019, 19, 1360                                                                                          8 of 8

8.    Ikiades, A. Direct ice detection based on fiber optic sensor architecture. Appl. Phys. Lett. 2007, 91, 104104.
      [CrossRef]
9.    Amiropoulos, K.; Spasopoulos, D.; Ikiades, A. Fiber optic sensor for ice detection on aerodynamic surfaces
      using plastic optic fiber tapers. In Optical Sensors; OSA Technical Digest; Optical Society of America:
      Washington, DC, USA, 2018.
10.   Vaisala DRS511. Road & Runway Sensor. Available online: http://www.vaisala.com (accessed on 6
      February 2019).
11.   Tabatabai, H.; Aljuboori, M. A Novel Concrete-Based Sensor for Detection of Ice and Water on Roads and
      Bridges. Sensors 2017, 17, 2912. [CrossRef] [PubMed]
12.   Finkele, R. Optical road-ice detector operating in the near infrared. Electron. Lett. 1997, 33, 1153–1154.
      [CrossRef]
13.   Ogura, T.; Kageyama, I.; Nasukawa, K.; Miyashita, Y.; Kitagawa, H.; Imada, Y. Study on a road surface
      sensing system for snow and ice road. JSAE Rev. 2002, 23, 333–339. [CrossRef]
14.   Muñoz, C.; Márquez, F.; Tomás, J. Ice detection using thermal infrared radiometry on wind turbine blades.
      Measurement 2016, 93, 157–163. [CrossRef]
15.   Abdel-Moati, H.; Morris, J.; Zeng, Y.; Wesley Corie, M.; Garas Yanni, V. Near field ice detection using infrared
      based optical imaging technology. Opt. Laser Technol. 2018, 99, 402–410. [CrossRef]
16.   EPoSS Roadmap Automated Driving 2014. Available online: https://www.smart-systems-integration.org/
      publication/eposs-roadmap-automated-driving-2014 (accessed on 14 March 2019).
17.   Guerrero-Ibáñez, J.; Zeadally, S.; Contreras-Castillo, J. Sensor Technologies for Intelligent Transportation
      Systems. Sensors 2018, 18, 1212. [CrossRef] [PubMed]
18.   Giles, D.B.; Kemp, D.R. Method and Apparatus for Detecting Ice and Packed Snow. U.S. Patent No. 5,818,339,
      6 October 1998.
19.   Schmitt, K.; Schaube, W. Method for Determining the Condition of a Roadway Surface. U.S. Patent No.
      5,218,206, 8 June 1993.
20.   Casselgren, J.; Sjödahl, M.; LeBlanc, J. Angular spectral response from covered asphalt. Appl. Opt. 2007, 46,
      4277–4288. [CrossRef] [PubMed]
21.   Lu, Y.; Higgins-Luthman, M.J. Black Ice Detection and Warning System. U.S. Patent Appl. No. 2008/0129541
      A1, 5 June 2008.
22.   Stern, H.; Schaefer, J.; Maali, F. System for Detecting Ice or Snow on Surface Which Specularly Reflects Light.
      U.S. Patent No. 6,069,565, 30 May 2000.
23.   Zhao, D. Inflight Ice Detection System. U.S. Patent 7,370,525, 13 May 2008.
24.   Casselgren, J.; Sjödahl, M. Polarization resolved classification of winter road condition in the near-infrared
      region. Appl. Opt. 2012, 51, 3036–3045. [CrossRef] [PubMed]
25.   Colace, L.; Santoni, F.; Assanto, G. A near-infrared optoelectronic approach to detection of road conditions.
      Opt. Laser Eng. 2013, 51, 633–667. [CrossRef]
26.   Kou, L.; Labrie, D.; Chylek, P. Refractive indices of water and ice in the 0.65- to 2.5-µm spectral range.
      Appl. Opt. 1993, 32, 3531–3540. [CrossRef] [PubMed]
27.   O’Donnell, K.A.; Knotts, M.E. Polarization dependence of scattering from one-dimensional rough surfaces.
      J. Opt. Soc. Am. A 1991, 8, 1126–1131. [CrossRef]
28.   Videen, G.; Hsu, J.; Bickel, W.S.; Wolfe, W.L. Polarized light scattered from rough surfaces. J. Opt. Soc. Am. A
      1992, 9, 1111–1118. [CrossRef]

                         © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
                         article distributed under the terms and conditions of the Creative Commons Attribution
                         (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
You can also read