Daylight: What makes the difference? - Centre for Chronobiology
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Lighting Res. Technol. 2019; 0: 1–20
Daylight: What makes the difference?
M Knoop PhDa , O Stefani PhDb, B Bueno PhDc, B Matusiak PhDd,
R Hobday PhDe, A Wirz-Justice PhDb, K Martiny DMScf,g , T Kantermann Dr.habilh,
MPJ Aarts MSci, N Zemmouri PhDj, S Appeltk and B Norton DScl
a
Department of Lighting Technology, Technische Universität Berlin, Berlin,
Germany
b
Centre for Chronobiology, Psychiatric Hospital of the University of Basel,
Basel, Switzerland
c
Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany
d
Department of Architecture and Technology, Norwegian University of
Science and Technology, Trondheim, Norway
e
Independent Researcher, Cwmbran, Torfaen, United Kingdom
f
Psychiatric Centre Copenhagen, University Hospital of Copenhagen,
Copenhagen, Denmark
g
Department of Clinical Medicine, University of Copenhagen, Copenhagen,
Denmark
h
FOM University of Applied Sciences, Essen, Germany
i
Department of the Built Environment, University of Technology Eindhoven,
Eindhoven, The Netherlands
j
Department of the Built Environment, University of Biskra, Biskra, Algeria
k
Langsames Licht/Slow Light, Vienna, Austria
l
Dublin Energy Laboratory, Technological University Dublin, Dublin, Ireland
Received 28 February 2019; Revised 26 June 2019; Accepted 23 July 2019
Light is necessary for vision; it enables us to sense and perceive our surroundings
and in many direct and indirect ways, via eye and skin, affects our physiological and
psychological health. The use of light in built environments has comfort, behav-
ioural, economic and environmental consequences. Daylight has many particular
benefits including excellent visual performance, permitting good eyesight, effective
entrainment of the circadian system as well as a number of acute non-image
forming effects and the important role of vitamin D production. Some human
responses to daylight seem to be well defined whilst others require more research
to be adequately understood. This paper presents an overview of current knowledge
on how the characteristics of daylight play a role in fulfilling these and other
functions often better than electric lighting as conventionally delivered.
1. Introduction lighting, daylight is highly dynamic,
changing within and across days, throughout
Daylight is the holistic combination of the the year, and with weather conditions in
luminous characteristics of sunlight from intensity, colour, diffuseness and direction.
direct solar radiation and skylight from dif- Daylighting refers to the illumination of
fuse solar radiation (Figure 1). Unlike electric indoor spaces by daylight delivered through
openings in the building skin.
This article arose from discussions between
Address for correspondence: M Knoop, Department of
Lighting Technology, Technische Universität Berlin, the authors at a seminar held in Berlin in June
Einsteinufer 19, Berlin 10587, Germany. 2018 and is not intended to be a
E-mail: martine.knoop@tu-berlin.de
ß The Chartered Institution of Building Services Engineers 2019 10.1177/1477153519869758
2 M Knoop et al. Figure 1 Daylight, a combination of sunlight and skylight (left and middle) or skylight only (right) (Copyright details: (left): Photo by Amy Chandra from Pexels https://www.pexels.com/photo/boat-on-ocean-789152/, (middle): Photo by Francesco Ungaro from Pexels https://www.pexels.com/photo/sky-blue-sun-ray-of-sunshine- 97558/, (right): Photo by Gabriela Palai from Pexels https://www.pexels.com/photo/blur-calm-waters-dawn-daylight- 395198/, https://www.pexels.com/photo-license/) comprehensive review paper. The purpose of preferences for natural light, as discussed by the seminar was to reflect an interdisciplinary Haans.15 discussion on the various scientific, technical Underlying the human preference for day- and creative aspects of the differences light are experiences that transcend immedi- between daylight and electric light. As a first ate physical stimuli, often orchestrated by step, this overview should provide a basis for their nature to be interwoven with context- further, more specific discussion and research. related knowledge. The sun has been wor- Numerous survey-based studies have shiped in many cultures, with sunlight and the shown that daylight is preferred to electric qualities of shadows and darkness being lighting in most settings.1–5 Boyce et al.6 state generally felt to be a source of spiritual and that ‘There is no doubt that people prefer aesthetic experience as well as of health and daylight over electric lighting as their primary well-being. Unlike daylight, electric lighting is source of illumination’ and provide an over- a controllable man-made light source asso- view of literature which shows that high ciated with advances in science and technol- percentages of survey respondents prefer to ogy that is easier both to study and to work by daylight. Most studies were per- engineer to achieve specific outcomes. In formed at latitudes around 508N2,3,7; one contrast, daylight as a natural source is study in the tropics indicates that the majority more difficult to control and the daily, of occupants prefer to work under daylight seasonal and annual dynamics of daylight as well.8 produce different outcomes in different loca- Many reviews document the importance of tions, additionally modified by weather con- daylight for health, well-being, and sustain- ditions. Due to these geographical differences, ability, and the consequences for architec- appropriate daylight utilisation can vary from ture.5–13 Veitch and Galasiu11 summarise: sun- and skylight exposure to complete exclu- ‘The reviews5,14 concluded that windows and sion of sunlight from buildings. In addition, daylighting are desired by most employees and the use of daylight openings in the building that they are contributors to health and well- envelope depends on the function of the being’. Here we show that the specific char- indoor space, as well as occupants’ require- acteristics and related benefits of daylight as ments for privacy, view, glare protection and summarised in Table 1 that produce this solar heat gain management. Individuals also human reaction go beyond subjective respond differently to daylight, as for Lighting Res. Technol. 2019; 0: 1–20
Daylight: What makes the difference? 3
Table 1 Characteristics of daylight and electric lighting.
Characteristics Daylight Electric lighting
Spectral Continuous spectral power distribu- Various spectral power distributions,
Figures 2 and 5 tion (containing all visible wave- some continuous, others discon-
lengths), with a strong short- tinuous. For typical general light-
wavelength component during ing: 380– 780 nm
daytime; includes infrared (IR) and
ultraviolet (UV) radiation
Outdoors: approx. 290– 2600 nm
Indoors: approx. 320– 2600 nm
Temporal and absolute photo- Temporal variations in intensity, Static or pre-programmed dynamic
metric and colorimetric spectral power distribution and intensities; static CCT (typically
characteristics. CCT 2700 K, 3000 K or 4000 K) or pre-
Illuminance and correlated Dusk and dawn: lower light intensity programmed dynamic colour
colour temperature (CCT) During daytime: high light intensities, change, available during day- and
Figures 3 and 4 variable CCT night-time
Spatial light distribution Daylighting from windows and sky- Typical, functional, electric lighting:
indoors lights: – lighting from above
Figures 6 and 7 – vertical surfaces can be illumi- – focus on horizontal surfaces
nated, with high light intensities – no parallel beams, distinct sha-
– under clear sky conditions: paral- dows or patches only possible with
lel beams, realise distinct shadows accent lighting
and sun patches
– under overcast sky conditions:
smooth transition from light to dark
Flicker and spectral fluctuation Stable on a short timescale (no flicker, Source can display flicker and/or have
no spectral fluctuations) spectral fluctuations
Polarisation Direct sunlight is not polarised. Partial polarisation is introduced in
Daylight from a particular region of lamp configurations involving
the sky (relative to the sun’s pos- specular reflections or direct trans-
ition) is partially polarised mission (e.g. through a flat glass
pane) where the light is incident on
the material close to its Brewster
angle
Energy requirements and costs Freely available during daytime Energy required for electric lighting
Figures 2 and 3 Costs for daylighting components to Costs for electric lighting components
deliver the daylight into the
building
example reviewed by Pierson et al.16 A 2. Visual performance
complex construct of individual, physio-
logical, cultural, geographical and seasonal Vision is the most developed sense in humans
preferences and characteristics underlies the and, therefore, our species significantly relies on
desire for daylight, and the subsequent the provision of light of adequate quality. Visual
human response, as well as the environmental performance, defined as the speed and accuracy
and monetary benefits. of processing visual information, is influenced
Lighting Res. Technol. 2019; 0: 1–20
4 M Knoop et al.
corresponding colour rendering qualities (according to IES TM-30-15 and CIE 013.3:1995); CRI: Colour Rendering Index, Rf: Fidelity Index, Rg: Gamut
Figure 2 Examples of the spectral power distribution of daylight indoors (left) and electric lighting (middle: fluorescent lamp, right: LED), with
by lighting conditions.17 Daylight is a very good
light source to support visual performance. It is
1000
a flicker-free light source with a continuous
CRI = 84
spectral power distribution covering the full
Rg = 97
Rf = 82
900
visible range (Figure 2). The high illuminances
(Figure 3) enable discrimination of fine details
800
supporting visual acuity. Glare must be con-
Wavelength [nm]
trolled both for daylight and electric light. The
700
spectral power distribution of daylight offers
600
optimal colour rendering and allows better
colour discrimination than most electric lighting,
500
whilst the directionality of both daylight and
electric light can produce shadows that enhance
400
details for three-dimensional tasks.
300
120
100
80
60
40
20
0
Normalised spectral power distribution
3. Good eyesight
1000
CRI = 80
Rg = 98
Rf = 77
Lack of daylight exposure seems to be linked to
900
developing myopia or short-sightedness.
800
Myopia is the most common visual disorder
affecting young people; it has reached epidemic
Wavelength [nm]
700
levels in East Asia and is increasing elsewhere.
Myopia is normally first diagnosed in school-
600
age children. Recent studies have revived the
500
idea that it is the environment in which children
learn that determines whether or not they
400
become short-sighted.18 It seems that children
who engage in outdoor activities have lower 300
levels of myopia.19 Thus, daylight exposure at
120
100
80
60
40
20
0
Normalised spectral power distribution
levels significantly higher than those typically
1000
found indoors (Figure 3) may be important in
CRI = 99
Rg = 99
Rf = 99
preventing myopia. The precise biological
900
mechanisms through which being outdoors
may protect children’s eyesight are not yet
800
fully understood. The hypotheses are that (i)
Wavelength [nm]
700
bright light stimulates the release of the retinal
neurotransmitter dopamine, which inhibits the
600
axial growth of the eye that causes short-
sightedness; (ii) since circadian rhythms in the
500
eye affect ocular growth, disruption of such
400
rhythms by low light levels has also been
proposed as a development factor20 and (iii)
300
120
100
80
60
40
20
0
there is a geographical, seasonal, component, as Normalised spectral power distribution
both eye elongation and myopia progression
Index.
increase as day-length shortens.21 The complex
protective effect of daylight may depend on
Lighting Res. Technol. 2019; 0: 1–20Daylight: What makes the difference? 5
300 lux 30000 lux 3000 lux
500 lux 9000 lux 7000 lux 1500 lux 2500 lux 1500 lux 700 lux
29000 lux 2750 lux
Figure 3 Range of approximate horizontal illuminance levels indoors (blue) and outdoors (black) in example
situations during winter time in Berlin, Germany (left: evening; middle: clear sky condition, afternoon; right: overcast
sky condition, afternoon).
many interlinked aspects including duration affecting an individual’s sleep quality,
and timing of daylight exposure, wavelength health, mood and cognitive abilities.25
and intensity. Excessive near-work may also Daylight, due to its temporal variations in
damage children’s eyesight; even though evi- spectral power distribution and intensity
dence for this is inconsistent, a recent review of (Figure 4), is the natural time cue (‘zeitgeber’)
myopia prevention by Lagrèze and Schaeffel22 for synchronisation of the circadian system
reported that ‘A person with little exposure to and the sleep–wake cycle. Dawn and dusk are
daylight has a fivefold risk of developing myopia, important cues for entrainment with high
which can rise as high as a 16-fold risk if that light levels during the day followed by dark-
person also performs close-up work’. ness at night being essential for optimal sleep.
The spectral component of daylight expos- Light input to the circadian system occurs
ure (Figure 2) may affect visual colour per- through intrinsically photosensitive retinal
formance. Reimchen23 showed that colour ganglion cells (ipRGCs) particularly sensitive
deficiencies are more common in northern to the short-wavelength ‘blue’ component of
latitudes, where twilight occupies a more light. Discovered in 2002,26,27 these cells are
significant part of the day than at the equator, connected to the circadian clock and other
where colour deficiencies are very uncommon. parts of the brain, affecting primarily non-
A study of visual perception in individuals visual functions.28 To support circadian func-
born below and above the Arctic Circle, in tionality, bright and short-wavelength light
different seasons, indicated that a reduction of exposure during daytime is important
daylight and an increase of exposure to together with avoidance of light during night-
twilight and electric lighting during infancy time. A study in the Antarctic region showed
changed colour sensitivity; participants born in better sleep quality of base personnel during
autumn above the Arctic Circle showed the the period of the year with daylight, with its
lowest overall colour performance.24 prevalent higher light levels, compared to
sleep quality during the polar winter with
only electric lighting. When comparing elec-
4. Circadian entrainment tric lighting conditions, blue-enriched
(17,000 K) light was more efficient than
Well-timed lighting can entrain the circadian bright white (5000 K) in supporting good
system, which is important for positively sleep–wake cycles.29 In interiors, reduced
Lighting Res. Technol. 2019; 0: 1–206 M Knoop et al.
Overcast sky, winter Clear sky, spring
25000 100000
20000 80000
Illuminance [lux]
Illuminance [lux]
15000 60000
10000 40000
5000 20000
0 0
4:00 8:00 12:00 16:00 20:00 4:00 8:00 12:00 16:00 20:00
Local time [hh:mm] Local time [hh:mm]
Hor. illuminance Vert. illuminance on a south facing façade Hor. illuminance Vert. illuminance on a south facing façade
Figure 4 Temporal characteristics: Light levels of daylight throughout example days for different weather conditions
in Berlin, Germany.
exposure to sunlight during the day together sunrise through a change of colour tempera-
with electric light exposure after sunset can ture (from 1090 K to 2750 K) and illuminance
delay timing of the circadian clock leading to at the eye (0–250 lx) resulted in better sub-
difficulties falling asleep at night and prob- jective mood and well-being,34 better cogni-
lems getting up on time in the morning. tive performance35 and could be a potential
Exclusive exposure to daylight synchronises protector for cardiac vulnerability in the
the circadian system to solar time.30,31 critical morning hours.36 Dynamic lighting
Roenneberg and Merrow32 proposed to treat that included lower lighting conditions and
and prevent circadian misalignment by colour temperatures in mornings and even-
‘strengthening light environments (more light ings resulted in significantly higher melatonin
during the day and less light during the night). production 1 hour prior to bedtime compared
This requires taking advantage of dynamic to static light.37
changes in spectral composition, and applying Daylight outdoors intrinsically provides
architectural solutions to get more daylight into temporal dynamics. Thus, the simplest solu-
buildings’. To artificially provide the high- tion to getting enough circadian stimulus is to
amplitude temporal dynamics of daylight by go outside. Nonetheless, people in the
means of electric lighting requires significant modern, industrialised, society spend up to
energy use. It is assumed that daylight is the 90% of their time indoors.38–41 In buildings,
best and appropriate light source for circa- the form and façade, as well as the choice of
dian entrainment, though conclusive research glazing material in the windows and shading
evidence for this is lacking. system modify intensity, colour and distribu-
Dawn and dusk signals are the most tion of daylight in the interior. Daylighting
powerful zeitgebers, not requiring high inten- conditions available to the occupant of a
sity light but a pattern of diurnal change with room also depend on their distance from the
sunrise and sunset. They depend on day of window, the geometry of the room and
year and latitude. Simulation studies have surface reflectances. Depending on the day-
shown a rapid phase advance with a single lighting design, indoor daylight can often
dawn pulse,33 and exposure to natural dawn provide an adequate stimulus and support to
and dusk immediately re-positions sleep to the circadian system, thus remaining as the
within the night.31 Compared to static light- usual light source for circadian support.
ing, dynamic lighting simulating a natural Office workers with access to windows have
Lighting Res. Technol. 2019; 0: 1–20Daylight: What makes the difference? 7
reported better sleep quality than those with- component indicate daylight has a high
out windows.42 Sleep quality increases with potential to support acute non-image forming
higher daylight availability in summer,43 with effects (Figure 5).
the duration over a threshold of 1000 lx or Investigations of these acute non-image
2500 lx at eye level being an indicator for forming effects of light have mostly been
better sleep quality.44 conducted with electric lighting. It has been
Comparing daylight to electric lighting shown, for example, that self-reported day-
conditions, Turner et al.45 state: time performance, alertness and ability to
concentrate, and reduction of daytime sleepi-
‘Typical residential illuminance [on aver-
ness, improve under static lighting with high
age 100 lux or less, due to electric light-
correlated colour temperature.48,49 Smolders
ing] is too low for circadian needs even in et al.50 found increased subjective alertness
young adults. Properly timed exposure to and vitality, as well as objective performance
sunlight or other bright light sources is and physiological arousal, when offering
vital for mental and physical well-being in 1000 lx instead of 200 lx at eye level in the
all age groups. [. . .] In general, several morning. Even though relevant studies with
hours of at least 2500 lux of blue weighted daylight have been limited, daylight would be
light exposure (ideally sunlight) starting expected to very effectively produce acute
early in the morning benefit most people. non-image forming effects during daytime
Bright light immediately and directly due to the availability of high light levels
enhances cognition, alertness, perform- together with the pronounced short-wave-
ance and mood, so bright environments length component in its spectrum. Though
throughout the day provide additional lamps have been specifically developed to
benefits, especially for middle-aged or support circadian and acute non-image form-
older adults.’ ing effects, daylight is the natural light source
to support these effects whilst incurring little,
or no, energy use.
5. Acute, non-image forming effects
Circadian responses, such as regulation of 6. Room, object and human appearance
sleep timing, are related to retinal-mediated
responses to light mediated by the ipRGCs. In The multiple characteristics of daylight (both
addition, some acute effects, such as mela- sunlight and skylight) affect room, object and
tonin suppression, increase of heart rate or human appearance, providing a specific per-
alertness, can also be realised by light through ceived room ambience that can influence the
the ipRGCs or a combination of photorecep- occupants’ emotional state. There is no con-
tors.46 Both intensity and spectral compos- clusive research on the impact of dynamic
ition of light play a role in inducing or changes of directionality and diffuseness due
avoiding these effects. Daylight can provide to variations of sunlight and skylight entering
high light levels. However, the spectral power built environments. However, users of a space
distribution of light from specific regions of are sensitive to the intensity, direction and
the sky can vary widely47; indoors, since the diffuseness of light in a space.51 Electric light
daylight received depends on the orientation systems usually deliver light from a number of
of a room, the colour of the light can be points distributed over a space leading to light
considerably cooler than the 6500 K cool rays of various intensities and directions
white often assumed. The related spectral creating overlapping shadows that can be
power distribution and short-wavelength perceived as visual noise. Conversely, daylight
Lighting Res. Technol. 2019; 0: 1–208Normalised spectral power distribution M Knoop et al.
Normalised spectral power distribution
180 1.2 120 1.2
Smel(λ) V(λ)
150 1 100 1
spectral sensitivity
spectral sensitivity
120 0.8 80 0.8
90 0.6 60 0.6
60 0.4 40 0.4
Smel(λ) V(λ)
30 0.2 20 0.2
0 0 0 0
380 480 580 680 780 380 480 580 680 780
Wavelength [nm] Wavelength [nm]
Daylight, 5500 K Daylight, 6500 K Daylight, 9000 K Fluorescent 4000 K LED 4000 K
Figure 5 Example spectral power distributions of daylight and electric lighting including spectral sensitivity of
ipRGCs (Smel()) and cones (V()).
Figure 6 Spatial light distribution due to daylight (left, right) and electric lighting (middle).
is delivered through a window or a skylight, perception of human faces and objects; day-
which has a main direction inward to the light through windows is effective in realising
room from the opening in the building skin. such spatial light distributions (Figure 6).54–56
This creates visual clarity that can provide an Research under electric lighting conditions
impression of serenity of the space. The showed that brightness of room surfaces,
spatial light distribution also affects room preferably greater than 30–40 cd/m2 in a
appearance, as well as the perceived repre- horizontal band of 208 above and below the
sentation of objects and human faces. The line of sight, give visual lightness and attract-
appearance of faces of people seated near the iveness to office rooms.57–60 Also important
window, side-lit by daylight, has been shown for perceived spaciousness is the amount of
to be labelled with positive attributes, and light,61,63 with wall-oriented lighting alone or
high luminance contrasts are not perceived as a combination with a low level of overhead
disturbing.52 Due to the size of a window, lighting seemingly beneficial for spacious-
shadows are typically ‘soft’, which is con- ness.61,63 A full-scale study (Figure 7) of a
sidered appropriate for good modelling.53 In series of room quality attributes showed that
addition, the light from the side, or a lateral high levels of daylight from large windows are
‘flow’ of light, seems to be preferred in the crucial in order to achieve a more pleasant,
Lighting Res. Technol. 2019; 0: 1–20Daylight: What makes the difference? 9
Figure 7 Spatial light levels due to daylight.
exciting, complex, legible, coherent and open on perceptions of room appearance and the
room.64 mood, stress and anxiety levels of partici-
Direct sunlight affects room appearance pants. Sunlight falling directly on the occu-
with a sun patch as well as clearly defined pant or reflected from a surface can cause
shadows produced by parallel beams of sun- visual and/or thermal discomfort. This dis-
light. Whilst the sun patch is seen as a visual comfort is linked to blind usage,70 which will
stimulus, research suggests that appropriate then block (part of the) direct sunlight and
sunlight penetration can induce relaxation.65 skylight from entering the building.
Sunlight penetration was found to have a Though most research on space and object
positive effect on job satisfaction and general appearance has been conducted using con-
well-being.66 A social survey in four different trolled electric lighting, the results are applic-
building types by Ne’eman et al.67 showed able to daylight conditions. The research
that sunshine has ‘a unique non-physical included in this section has mainly been
property which induces psychological well- performed in temperate climates and indicates
being’. One study used an artificial sky to that the spatial lighting realised by daylight
mimic daylight of a clear sky with defined supports good perception of room and object
(blue-toned) shadows, a sun patch, (produ- appearances. Direct sunlight seems to
cing a brightness ratio for the sunlight to enhance perceived room ambience and the
shaded areas as found outdoors), as well as a user’s emotional state, when visual and ther-
bright light source seen through the window mal comfort are maintained. Façade design
(having the appropriate perceived size of the considerations to maintain comfort in trop-
sun).68,69 The results indicated that these ical regions will affect indoor daylight condi-
lighting characteristics had a positive effect tions. Both the resulting room and object
Lighting Res. Technol. 2019; 0: 1–2010 M Knoop et al.
appearance, as well as the prevalence of sunny source. Glare can impact visual performance,
conditions might result in different subse- but even glare that does not necessarily impair
quent occupant responses. seeing objects can lead to fatigue. Research on
discomfort glare due to high luminances or
luminance contrast from daylight or electric
7. Comfort lighting indicates a greater tolerance when
mild discomfort glare arises from daylight76,77
The specific spectral power distribution and and/or a diversity of individual requirements
brightness of daylight can also affect human for visual comfort from daylight78 than met
physical comfort. Physical comfort is the from electric light sources with the same
feeling of well-being, when an environment’s luminance. Culture and climate are suggested
thermal and lighting conditions are experi- to influence perceived glare from daylight.79
enced as pleasant and associated with satis- Flicker can cause headaches, eye strain or
faction. The brightness and the strong seizures, and reduce visual performance.80
infrared component of daylight (Figure 2) Electric lighting can be a source of flicker,
may be appealing, but can cause visual and whilst daylight is flicker-free.
thermal discomfort. Nonetheless, interviews
in field studies showed that occupants can be
satisfied with daylight even though they 8. Well-being due to views through
sometimes experience visual discomfort.71 windows
Sunlight penetration heats up a room. In
addition, windows are a source of heat A window offers daylight, air exchange, a
transfer from and to the exterior. view, and information on the weather and
Differences between temperate zones and the activities outside. Window material proper-
extremes of polar or equatorial regions are ties, design and usage offer control over
typically reflected in architectural solutions, outdoor influences, such as smell, sound and
as the design approach should be different to heat. In addition, windows may provide an
give comfortable indoor environmental escape route. All these aspects play a role in
conditions.72 the feeling of control and safety in indoor
Thermal discomfort due to high or low environments. Enclosure, privacy, safety and
temperatures activates biological cooling (e.g. (subconscious) knowledge of escape routes
sweating) or heating (e.g. shivering), respect- relate to the functionality of a space. Stamps81
ively. Discomfort can also arise from the states that lightness of a scene is related to
thermal asymmetry between the temperatures judged safety (‘ability to move and the ability
of the cool internal surfaces of windows and to perceive’). No information about the wea-
those of warmer walls.73 A field study by ther and lack of a view were the reasons
Chinazzo et al.74 indicates that satisfaction female office workers dislike windowless
with the temperature in the room is affected offices, having feelings of isolation, depres-
by lighting conditions, with a lower satisfac- sion and tension (Ruys, reported in Collins1).
tion under lower lighting levels. This could The view from a window can affect several
suggest a greater tolerance for thermal dis- aspects of physical and mental well-being. It
comfort in situations with daylight, as previ- can, for example, support restorative pro-
ously proposed by Veitch and Galasiu.11 cesses, relieve stress or increase job satisfac-
Visual discomfort, referring to ‘discomfort tion. Research investigating the effects of view
or pain in or around the eyes’ (according to content suggests that busy and dense urban
Boyce and Wilkins75), can have several causes, areas with obstructions giving a short visual
including glare and flicker from the light range require constant accommodation and
Lighting Res. Technol. 2019; 0: 1–20Daylight: What makes the difference? 11
Figure 8 Examples of the view in four view quality categories (from left to right: insufficient, sufficient, good, excellent).
adaptation processes by the eye muscles, to that gave temporal information was amongst
keep an image fixed at the fovea. Conversely, the most frequently cited favourable factors
views into a deep space can relieve the eye and for residential spaces and a number of non-
the muscle tonus, and free the cerebral cortex residential spaces.88 According to Veitch and
from processing information, leading to cog- Galasiu11 ‘This information provision is an
nitive relaxation. Looking at a view speeds-up acknowledged function of a window’.
physiological recovery from a stressful Both content and perceived quality of a view
experience.82,83 can affect human responses to daylight. The
Less information is available comparing the number of view layers, the width and distance
relative restorative benefit of rooms with of the view, the perceived quality of the
window views, artificial windows and window- landscape elements and the composition of
less walls. Office occupants have a preference the view are important influential parameters
for real windows or an artificial window with a as shown in Figure 8.89 Tolerance of discomfort
dynamic view of nature,84 but the restorative glare from daylight through a window is partly
effect of artificial windows with dynamic determined by how interesting the scene outside
‘views’ seems to be lower.85 In windowless is,90,91 its attractiveness92 and its content.93
spaces, occupants seem to compensate for the A view outside adds to the desirable
lack of windows with nature elements, in the perception of daylight, especially for natural,
form of plants or pictures of natural scenery. attractive and interesting views, but the
Heerwagen and Orians86 found that small mechanisms for this are not yet fully under-
windowless offices are decorated with twice stood. Even though the contextual clues
the number of visual materials than windowed associated with daylight can be emulated,
rooms with views. Visual material (in window- research indicates that some benefits might
less offices) did not represent ‘surrogate’ views, not be reproduced by electric lighting.
but did include natural themes.
Windows also offer contextual clues about 9. Energy efficiency
time of day and about weather conditions,
that fix ourselves in time and space, both Daylight provision offers cost-free indoor
consciously and unconsciously. Patients in an lighting with a continuous spectral power
intensive therapy unit with a translucent distribution from 320 nm to 2600 nm which
window had a more accurate memory and has implications for the heating, cooling and
orientation and fewer hallucinations and lighting energy demand of a building.94
delusions then those in a windowless unit.87 Daylighting can directly reduce the electric
A questionnaire to understand the preference energy required to illuminate a room. The
for windows showed that the view outside extent to which daylight can displace times of
Lighting Res. Technol. 2019; 0: 1–2012 M Knoop et al.
use of electricity is obviously specific to the appropriately defined. The overall energy
design, location, purpose and use of a space demand depends on building type, form and
within a building. Care should be given to the construction, occupant activities and patterns
most suitable location of activities, for exam- together with geographical location, climate,
ple highly visual tasks should be done near a orientation and degree of obstruction.106–108
naturally lit building perimeter. A daylight Electric lighting requires energy. It may
design should be combined with electric light- also release heat to the building, depending on
ing controls that switch-off or reduce, but the light source that can increase the cooling
maintain the quality of, electric lighting to load but can also decrease heating energy
reduce electricity use. Lighting energy savings demands. A windowless building is often less
achieved through installing daylight responsive energy efficient than one with an appropriate
lighting controls range from 20% to 70%.95–99 selection and control of well located windows.
A meta-analysis by Williams et al.100 showed
average savings of approximately 30% in
10. Monetary value
various applications. Tsangrassoulis et al.101
indicate that a 40% reduction in lighting Daylight design can bring monetary benefits
energy consumption can reduce overall pri- by reducing the energy cost of electric lighting
mary energy consumption by 17%. Only when and by improving the productivity of building
the full potential of such designed-in occupants. Daylight can increase the latter by
approaches has been exhausted should consid- a combination of sharpened vision due to
eration be given to the introduction of techno- better colour rendering or higher light levels,
logical systems to convey daylight deeper into improved visual modelling of objects and
interior spaces by deflection at windows or the, faces, reduction of flicker and/or the provi-
often costly to install, transmission of daylight sion of contextual clues.109 Productivity has
from a roof through intervening floors by been shown to increase by 5–15% in compa-
mirrored pipes or fibre optic cables.102–104 nies that have moved into buildings with
Daylight openings affect thermal condi- more daylight.110,111 However, the exact role
tions in a building. Heat losses in wintertime of daylight on productivity in these kind of
can increase when the heat resistance of studies is still subject to future research, given
windows is less than walls. Heat gain arises the many other factors that change simultan-
from solar radiation through windows and eously with such a move. The impact of
depends on climate; this might be beneficial in daylight on productivity and related aspects,
winter but may require additional cooling in such as absenteeism, can only be investigated
summer. The energy saved as well as the cost- in field studies and epidemiological stu-
effectiveness of daylighting is thus less if dies,112,113 in which experimental control is
cooling energy is required. Modern glazing difficult and interpretation of results is
systems are capable of filtering-out a signifi- demanding.5 For now, insufficient results
cant fraction of the infrared component. are available to draw conclusions with respect
Solar heat gains can be modulated with to the impact of daylight availability on
shading devices or switchable glazing systems, productivity; further research is necessary.
which, ideally, should also balance provision An analysis of annual income and expense
of daylight and a view outside, and protection data for commercial buildings by Kim and
against glare.105 There are large differences in Wineman114 indicated that views have an
daylight composition and daylight availability economic value. In their study, higher build-
between temperate and equatorial regions for ings, likely to have a skyline and cityscape
which architectural solutions are usually views, had higher property values. In
Lighting Res. Technol. 2019; 0: 1–20Daylight: What makes the difference? 13
interviews, the majority of businesses stated Even though many characteristics of day-
that the view was a consideration in setting light can be mimicked by electric lighting, it
rents. A study by Heschong115 indicated that has not been demonstrated that all the
call centre workers with the best possible view diverse holistic positive outcomes associated
processed calls faster and scored better on with daylight can be reproduced artificially.
tests of mental function when compared with Indeed, the characteristics of the complex
those workers without a view. interaction of the dynamics of daylight with
An analysis of sales in stores with and individual human responses have not been
without skylights by Heschong et al.109 indi- readily quantifiable to-date. They remain
cated that stores with skylights had an key areas that require extensive further
increase in their sales index. Interviews research.
indicated that the skylight unconsciously led We suggest that future studies should
to the visual environment being perceived as address the impact of daylight on the follow-
cleaner and more spacious. ing aspects of human performance, health
As stated above, the detailed mechanisms and well-being that might lead to behaviours
behind these and other, secondary, monetary translating into monetary benefits:
benefits are largely unknown. In addition, an Differential impact of variations in the
increase in productivity can only be achieved spectral power distribution and light inten-
when unwanted effects from daylighting, such sity across the day and seasons at different
as glare, shadows, veiling reflections and geographical locations, for example
overheating, are avoided. through epidemiological studies further
exploring the effect of daylight provision
on good eyesight and circadian entrain-
11. Conclusion ment, restorative sleep and better health;
Differences in the impact of the source of
Intensity, spectral power distribution, and the light on room and object appearance,
spatial direction and diffuseness of daylight comparing electric lighting, and daylight
are characteristics that support room and through windows, skylights or light tubes,
object appearance as well as non-image which includes the differences between
forming effects. The dynamics of changes in static and dynamic lighting;
the intensity and colour of daylight naturally Statistical estimations of the variance in the
support circadian entrainment, mood and impact of daylight with concurrent exposure
alertness. Some human responses, such as to electric light, to elaborate their interactions
non-image forming effects, seem to be well including assessments of light history effects,
defined. Also the role of sunlight on the skin and to obtain a better insight into the acute,
to support vitamin D production is well non-image forming potential of daylight;
established. However, many benefits of day-
light and windows cannot yet be explained so In addition, some co-variables need atten-
straightforwardly. The higher onset of visual tion, for example:
discomfort glare in daylight conditions as well Qualitative assessments of the perception
as the positive effect of the contextual clues of an (e.g. work) environment to study
provided by a view are induced by mechan- the role of context and content under
isms that are not well understood. Some different lighting regimes including the
responses to light seem to be mediated absence of light and whether symptoms of
through both visual and non-image-forming such absence can be quantified/
pathways that require further research.116–118 operationalised;
Lighting Res. Technol. 2019; 0: 1–2014 M Knoop et al.
Quantification of the view and contextual research, authorship, and/or publication of
clues from windows. Metrics need to be this article.
developed for the quantity and quality of
the view out and a measure to evaluate the
importance of contextual clues, to balance Funding
different window functions, such as glare
protection, solar heat gain management The authors disclosed receipt of the following
and daylight provision; financial support for the research, authorship,
Prevalence of weather conditions and archi- and/or publication of this article: Attendance
tectural archetypes might influence occu- at the seminar was financed by the Daylight
pants’ expectations and responses, thus the Academy.
impact of climate and culture on light
source preference, room and object appear-
ance as well as comfort aspects should be ORCID iD
the subject of further investigation.
And finally, maybe what is most urgently M Knoop https://orcid.org/0000-0002-
needed and most difficult to devise would be a 5097-3623
(set of) metric(s) to measure the ‘naturalness’ K Martiny https://orcid.org/0000-0002-
of light. 7317-5958
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