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Light Spectra and Human Responses
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A brief summary of human physiological responses to
visible light with varying spectral content
Light Spectra and Human Responses
Human responses and interactions with light are complex For general lighting applications, we are
and reach beyond just the obvious vision systems to primarily interested in the spectrum of
hormonal and even basic cellular levels. The science of visible light, which broadly refers to
human photobiology is both well established and at the electromagnetic radiation between about 350-380 nm to 750
same time rapidly evolving. -780 nm, bounded by ultraviolet radiation at the shorter
wavelength end and infrared radiation at the longer
For all the various human responses though, there is a wavelength end. Figure 1 below provides an illustration of
common and critical thread – that equal or possibly more this.
consideration must be given to spectral content as is given
to the more common focuses on light intensity (how ‘much’
light?) and duration (how long does the light operate?).
Ultraviolet
Infrared
Figure 1: Visible light spectrum with nominal colour ranges by wavelength
The following sections provide a very high-level overview of some aspects of human responses to light, both image-forming
and non-image-forming, and provides some context about how these responses can be considered when specifying and
design lighting for general applications. Finally, some key research references are listed for further study.
Retinal Interactions deteriorates progressively in the peripheral field, and requires
sufficient intensity to ensure photopic vision is active.
Image-forming
Non-image-forming Responses
The primary vision response is produced in the eye by a
combination of ‘rod’ and ‘cone’ photoreceptors depending on While not contributing to vision (non-image-forming), a third
the intensity of light available. photoreceptor type is present in the eye which has an equally
important biological response. Intrinsically photosensitive
• Rod photoreceptors are active in low-light situations (30 lux) with a peak sensitivity at roughly 550- light reflex (controlling how much light the pupil allows into
560 nm. Cones are concentrated in the centre-of-vision the eye) and coordination of physical head and eye
part of the retina, known as the fovea. movements (such as tracking moving objects, shifting
When brightness falls within a relatively narrow band attention, and even reading).
overlapping scotopic and photopic vision, both the rods and
Retina
cones contribute to vision – referred to as mesopic vision.
One practical example would be vision under bright
moonlight. Pupil
Fovea
Given the distribution of the rods and cones across the retina
and their specific sensitivities, both visibility and colour
perception vary by ambient lighting conditions and the
relative position of the subject within the viewer’s broader Lens
field of view. Night-time vision (rod-based, scotopic vision) is
Optic
most effective in the peripheral visual field rather than central Nerve
vision but has limited colour perception. By contrast, colour
perception is most effective in the centre of vision but Figure 2: Basic structure of the human eye
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Layering all of these retinal photoreceptor responses in the eye onto the visible spectrum illustrates
quite clearly that there are numerous (and overlapping) biological implications for light emissions
across the whole visible spectrum. Figure 3 presents the response curves for the three cone cell
types, the rod cells and the ipRGCs, which respectively trigger the trichromatic image-forming
response under ‘normal’ lighting conditions, the relatively-monochromatic image-forming response
under low-light conditions, and the melanopic non-image-forming hormonal response.
Short cones (Blue response)
ipRGCs (Melanopsin response))
Rods (low-light response)
Medium cones (Green response)
Long cones (Red response)
Melatonin suppression
Figure 3: Spectral responses (normalised) from retinal photoreceptors
Non-retinal Interactions foods), the avoidance of conflict (where facial skin tones are
an indicator of emotion), and the detection of danger (where
Aside from the eye, however, there are other known colour can help to identify objects that may otherwise be
photosensitive biological responses. In particular, the effects camouflaged by pattern alone).
of longer wavelength light on the activity of mitochondria (the In a modern context, the accurate perception of colour
‘power stations’ within cells) have been studied in a number remains an important part of day-to-day life, and many of the
of contexts and shown to improve cell performance and slow same reasons still apply. A few obvious examples include:
cell aging processes. This work has shown specific
biological benefits for eyes and skin from increased exposure • Accurate rendering of skin tones is an important part of
to light in the red to infrared parts of the spectrum, a process reading body language, and studies show consistent
referred to as photobiomodulation (PBM). Studies have preferences for high colour rendering lighting in
concluded that light in the spectral range from 600 to 1,300 environments which involve interpersonal/social
nm can assist with wound healing, tissue repair and skin interactions. Skin tone rendering is also important for
rejuvenation. In a sports performance context, results also visual diagnosis in medical/clinical contexts, where
suggest that PBM can increase muscle mass gain and changes in skin tone can be indicators of a variety of
reduce inflammation following exercise. medical conditions (cyanosis, jaundice etc.). Accurate
Finally, it is equally important to consider potentially harmful rendering of skin tones requires light with strong
responses to the spectral content of visible light. The representation in the orange and red portions of the
pupillary light reflex mentioned earlier has a direct influence visible spectrum.
on how the eye manages glare, and the spectral sensitivity of • While food is typically purchased from a retailer now
this response is similar to the melanopic response (i.e. in the rather than foraged for, the accurate rendering of fresh
shorter ‘blue’ wavelengths). Light in those wavelengths is produce (and other consumables for that matter) remains
known to ‘scatter’, which compounds the impact on visual an important part of the selection process. Light colour
comfort from what is already the ‘glariest’ portion of the quality in retail environments has long been accepted as
spectrum. Beyond discomfort though, research indicates that a critical specification and may focus on just a few select
the risk of eye damage increases with increasing dosage of colours or span all portions of the visible spectrum
particularly short wavelength ‘blue’ light (typically depending on the specific application.
characterised as wavelengths
The matter of colour vision is therefore not a trivial purposes. The previous Colour Rendering
consideration in the broader context of overall visual acuity. Index (CRI) framework ignored many
It has been theorised that the human vision system is in fact important parts of the visible light
far more sensitive to colour accuracy than it is to lighting spectrum, and so a more comprehensive
intensity, and that negative reactions to a reduction in colour framework was required to properly
rendering are more pronounced than the reactions to a characterise those key aspects of overall
proportional reduction in illuminance. This theory is yet to be colour performance. The TM-30 method
scientifically tested, but anecdotal evidence suggests it is expands the colour evaluation from just
plausible and if it were able to be confirmed it could represent eight sample colours as used in the CRI method to a far
a fundamental shift in how lighting requirements are defined. broader set of 99 sample colours. Those evaluated results
are summarised by an expanded set of metrics,
The introduction of the TM-30-18 Method for Evaluating Light characterising overall performance in terms of colour fidelity
Source Colour Rendition by the Illuminating Engineering and colour gamut, and including other indices to describing
Society (IES) is part of a growing acknowledgement in the shifts in chroma, hue and saturation. A specific descriptor for
lighting profession for more detailed consideration and the fidelity of skin tone rendering is also established.
specification of lighting performance for varying colour vision
Summary of Responses
When the primary sensitivity ranges of these various responses are overlaid on the nominal visible-light spectrum, it
becomes quite clear that the human body responds in a variety of ways across a broad range of radiation.
< Vision-forming (nominal peak sensitivity range)
< Melanopic response
< Glare, increased eye damage risk
Mitochondrial response >
Skin tone rendering/diagnosis >
Figure 4: ‘Reconstituted’ 6000K daylight spectrum, with indicative spectral activity ranges for selected physiological responses
This is obviously a very simplified illustration and lacks the nuance of specific sensitivity curves associated with each of the
responses, but it does demonstrate the concept of broad and often over-lapping physiological interactions with visible light
depending on spectral content.
Development of Lighting Technologies manipulated to the point of achieving a ‘white light’
appearance. The spectral response curves shown in Figure
Historically, lighting technologies (at least those focused on 3 demonstrate clearly why manufacturers using these
general lighting applications) have attempted to emit light technologies have tailored their white-light spectra to heavily
which stimulate the largest vision response possible from a favour the mid-spectrum green-yellow range. Light emitted in
given input – or more simply, to create as much ‘useable’ that portion aligns well with the vision response, and
light from the smallest possible energy input. Each source of therefore stimulates a maximum photopic response for the
light has its own ‘native’ emission spectrum, which may or energy required to emit the light, which in turn yields higher
may not suit the desired application without modification. For light source efficacies.
discharge (i.e. fluorescent) and more recently solid-state (i.e.
LED) sources, it has been necessary for spectral modifiers to However, this pursuit of ever-increasing efficacy has largely
be deployed to absorb and redistribute some of that ‘native’ been achieved through compromises in other important
emission into other sections of the spectrum to yield the sections of the visible spectrum. Notably, commercial LED
‘white light’ options desired for general lighting applications. sources typically have poor emission in the red and light-
blue/light-green ranges, and retain high levels of emission in
Commercial LED sources are generally based on a ‘blue’ the deep-blue range. This is demonstrated in Figure 5,
semiconductor design with a native emission in the ‘deep- where a typical commercial LED spectral power distribution
blue’ part of the visible spectrum. On its own, this would (SPD) is shown in comparison to a reference daylight SPD of
have virtually no application for general lighting, but with the equivalent correlated colour temperature (CCT).
addition of phosphor layers, that emission can be
Level 1, 2 Jarden Mile, Ngauranga, Wellington, New Zealand, 6035 www.ecopoint.co.nz 0800 695 949
Figure 5: Typical commercial LED SPD (Ra >80, 4000K) with reference daylight SPD at equivalent 4000K CCT
This typical Ra > 80/4000K (normally referred to as an ‘840’ Development of ‘Full Spectrum’ LED lighting options have
colour specification) SPD shows the characteristic peak in sought to address some of these spectral distribution
the deep-blue range (roughly corresponding to the ‘native’ deficiencies while retaining the preferred 4000K white light
emission wavelength), and then a broader hump of emission colour temperature preferred for general commercial interior
spanning the mid-green through to light-red range environments.
(corresponding to the peak sensitivity range of the vision-
forming response). • A greater portion of the ‘native’ blue emission is
absorbed by phosphors in the LED, reducing blue-light
However, as established earlier in the preceding summary, related outcomes.
this SPD also demonstrates clearly that a number of other • Phosphors re-emit light in the light-blue/light-green
important responses are somewhat neglected. The range – boosting the melanopic response
melanopic response (light blue to light green) aligns with the • Phosphors also emit less in the yellow/orange range
distinct dip in emission, while emission is similarly low in the and instead emit in the deep red end of the spectrum –
strong red part of the spectrum which determines skin tone improving skin tone rendering and the mitochondrial
rendering and mitochondrial responses. Additionally, the response.
peak emission in the strong blue part of the spectrum aligns
with the negative outcomes of extra glare impact and Figure 6 demonstrates one such ‘Full Spectrum’ option,
increased risk of eye damage. displayed alongside a typical commercial 840 LED option for
comparison.
Figure 6: SOLUS ‘Full Spectrum’ 4000K SPD compared with typical commercial ‘840’ LED
While a sacrifice in overall efficacy is needed to achieve these results, there are clear benefits to be weighed on the other
side of the balance, and that compromise may be considered easily justified in many instances depending on the priorities of
the application.
Level 1, 2 Jarden Mile, Ngauranga, Wellington, New Zealand, 6035 www.ecopoint.co.nz 0800 695 949
Resources for further information...
Lighting Research Centre, Rensselaer Polytechnic Institute. “Lighting for Healthy Living”
Accessed here: https://www.lrc.rpi.edu/healthyliving/
International Commission on Illumination. “CIE Position Statement on Non-Visual Effects of Light -
Recommending Proper Light at the Proper Time, 2nd Edition, October 2019”
Accessed here: https://cie.co.at/publications/position-statement-non-visual-effects-light-recommending-
proper-light-proper-time-2nd
Vetter et. al. (2021) “A Review of Human Physiological Responses to Light: Implications for the
Development of Integrative Lighting Solutions”, LEUKOS, DOI: 10.1080/15502724.2021.1872383
Accessed here: https://doi.org/10.1080/15502724.2021.1872383
Illuminating Engineering Society. “Forum for Illumination Research, Engineering, and Science (FIRES) -
Category: Light and Health”
Accessed here: https://www.ies.org/standards_cat/light-and-health/
Lighting Europe. “Joint position paper by LightingEurope and the International Association of Lighting
Designers (IALD) on Human Centric Lighting”
Accessed here: https://www.lightingeurope.org/images/publications/position-papers/
LightingEurope_and_IALD_Position_Paper_on_Human_Centric_Lighting_-_February_2017-
modified_version-v2.pdf
International Well Building Institute. “WELL Building Standard™ (WELL) Concept Overview – Light”
Accessed here: https://standard.wellcertified.com/light
Additionally, research and literature review journal articles can be provided on request. These cover a range of specific
topics, with titles covering:
• A Review of Human Physiological Responses to Light: Implications for the Development of Integrative Lighting
Solutions
• The Impact of LED Correlated Color Temperature on Visual Performance under Mesopic Conditions
• Measuring and using light in the melanopsin age
• Action Spectrum for melatonin regulation in humans – Evidence for a novel circadian photoreceptor
• Dim Light Adaptation Attenuates Acute Melatonin Suppression in Humans
• Eyeing up the Future of the Pupillary Light Reflex in Neurodiagnostics
• Research progress about the effect and prevention of blue light on eyes
• Aging retinal function is improved by near infrared light (670 nm) that is associated with corrected mitochondrial
decline
• A Controlled Trial to Determine the Efficacy of Red and Near-Infrared Light Treatment in Patient Satisfaction,
Reduction of Fine Lines, Wrinkles, Skin Roughness, and Intradermal Collagen Density Increase
• Photobiomodulation in human muscle tissue: an advantage in sports performance?
• Experimental evidence that primate trichromacy is well suited for detecting primate social colour signals
• The Reflectance Spectrum of Human Skin
• High Color Rendering Can Enable Better Vision without Requiring More Power
• Using TM-30 to Improve Your Lighting Design
Level 1, 2 Jarden Mile, Ngauranga, Wellington, New Zealand, 6035 www.ecopoint.co.nz 0800 695 949
LUMINAIRE DETAILS
PRODUCT Full-Spectrum Panel G2
MODELS 150x1200 20W 4000K
300x1200 36W 4000K
600x600 36W 4000K
TEST DATA/DETAILS
TEST DATE/S 05/09/2019
12/06/2020
REFERENCE/S UNSW 20138.2.1
UNSW 19239.1
UNSW 20138.1.1
Note that data presented here is considered representative of all specific
models/configurations within the indicated product family. Where multiple
Spectral power distribution data is available in tabular form; contact Ecopoint for the appropriate files. test results are available with a product family, the adopted dataset
presented here is an indication of ‘average’ performance.
CIE 133 — 1995
Ra 98
R1 98
R2 99
Ra Reference Colours
R3 95
R4 98
R5 99
R6 97
R7 99
R8 99
R9 98
R10 97
Special Colours
R11 96
R12 81
R13 99
R14 97
R15 97
Melanopic Ratio (IWBI): COI (AS/NZS 1680.2.5): PAR Photon Efficacy (400-700 nm):
0.738 0.3 1.57 µmol/J
Describes the melanopic response (reaction to light for The Cyanosis Observation Index is established in AS/NZS Photon Efficacy is a measure of how efficiently circuit power is
regulating circadian rhythm) as a proportion of the visual 1680.2.5, and the same standard recommends a COI of no converted into photons of visible light which can drive
response (reaction to light for vision). greater than 3.3 for clinical and critical patient care areas. photosynthesis — i.e. photosynthetic active radiation (PAR).
Ecopoint Limited P: +64 4 499 3636 Due to our commitment to
ongoing technical development,
2 Jarden Mile PO Box 12646 E: info@ecopoint.co.nz we reserve the right to change
Ngauranga, Wellington 6035 Thorndon, Wellington 6144 W: www.ecopoint.co.nz specifications without notice. © Ecopoint Limited, 15/09/2020Light Spectra and Human Responses www.pinterest.com
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