Light-Spectra-and-Human-Responses-11052021

Light Spectra and Human Responses A brief summary of human physiological responses to visible light with varying spectral content www.pinterest.com... For general lighting applications,

Light Spectra and Human Responses

A brief summary of human physiological responses to visible light with varying spectral content

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Human responses and interactions with light are complex

and reach beyond just the obvious vision systems to

hormonal and even basic cellular levels The science of

human photobiology is both well established and at the

same time rapidly evolving

For all the various human responses though, there is a

common and critical thread – that equal or possibly more

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?)

For general lighting applications, we are primarily interested in the spectrum of visible light, which broadly refers to electromagnetic radiation between about 350-380 nm to 750 -780 nm, bounded by ultraviolet radiation at the shorter wavelength end and infrared radiation at the longer wavelength end Figure 1 below provides an illustration of this

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

Image-forming

The primary vision response is produced in the eye by a

combination of ‘rod’ and ‘cone’ photoreceptors depending on

the intensity of light available

• Rod photoreceptors are active in low-light situations (<1

lux) and are therefore responsible for human night-time

vision (called scotopic vision), but do not transmit any

colour information Rods are highly sensitive, with a

peak response at roughly 500-510 nm, and they are

typically found around the outer sections of the retina

(the rear of the eye)

• Cone photoreceptors are less sensitive overall and are

therefore used for more general vision There are

actually three different cone cell types with distinctive

responses which broadly correspond to red, green, and

blue wavelengths, and collectively these generate an

overall combined photopic response for general vision

purposes (>30 lux) with a peak sensitivity at roughly

550-560 nm Cones are concentrated in the centre-of-vision

part of the retina, known as the fovea

When brightness falls within a relatively narrow band

overlapping scotopic and photopic vision, both the rods and

cones contribute to vision – referred to as mesopic vision

One practical example would be vision under bright

moonlight

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

field of view Night-time vision (rod-based, scotopic vision) is

most effective in the peripheral visual field rather than central

vision but has limited colour perception By contrast, colour

perception is most effective in the centre of vision but

deteriorates progressively in the peripheral field, and requires sufficient intensity to ensure photopic vision is active

Non-image-forming Responses While not contributing to vision (non-image-forming), a third photoreceptor type is present in the eye which has an equally important biological response Intrinsically photosensitive retinal ganglion cells (or ipRGCs) were only discovered relatively recently but have become increasingly important in modern lighting practice, as they have a direct impact on human health and wellbeing Through the interaction of light

on the pigment melanopsin, their influence is principally in synchronising circadian rhythms and the suppression or release of melatonin, the ‘sleep hormone’ In much the same way as rods and cones, the response of these cells to light again varies in sensitivity across the visible spectrum but is generally in the blue-green region with a peak sensitivity at

~480-490 nm

In addition to the circadian response, ganglion cells within the retina also transmit signals to the brain which control other non-image-forming responses, such as the pupillary

light reflex (controlling how much light the pupil allows into

the eye) and coordination of physical head and eye movements (such as tracking moving objects, shifting attention, and even reading)

Light Spectra and Human Responses

Figure 1: Visible light spectrum with nominal colour ranges by wavelength

Retina

Fovea

Optic Nerve

Pupil

Lens

Figure 2: Basic structure of the human eye

Non-retinal Interactions

Aside from the eye, however, there are other known

photosensitive biological responses In particular, the effects

of longer wavelength light on the activity of mitochondria (the

‘power stations’ within cells) have been studied in a number

of contexts and shown to improve cell performance and slow

cell aging processes This work has shown specific

biological benefits for eyes and skin from increased exposure

to light in the red to infrared parts of the spectrum, a process

referred to as photobiomodulation (PBM) Studies have

concluded that light in the spectral range from 600 to 1,300

nm can assist with wound healing, tissue repair and skin

rejuvenation In a sports performance context, results also

suggest that PBM can increase muscle mass gain and

reduce inflammation following exercise

Finally, it is equally important to consider potentially harmful

responses to the spectral content of visible light The

pupillary light reflex mentioned earlier has a direct influence

on how the eye manages glare, and the spectral sensitivity of

this response is similar to the melanopic response (i.e in the

shorter ‘blue’ wavelengths) Light in those wavelengths is

known to ‘scatter’, which compounds the impact on visual

comfort from what is already the ‘glariest’ portion of the

spectrum Beyond discomfort though, research indicates that

the risk of eye damage increases with increasing dosage of

particularly short wavelength ‘blue’ light (typically

characterised as wavelengths <450 nm) There are clearly

important health considerations associated with the spectral

content of light, especially in environments where users are

exposed to higher levels of intensity and/or longer durations

of exposure

Colour Vision

While the image-forming responses described previously

clearly include the detection of colour information (through

the short, medium, and long cone photoreceptors), colour

vision is an important field of study in itself, with important

implications for lighting practitioners It has been suggested

that the ability to discern colour has been a key part of the

evolutionary success of primates, presenting survival

advantages over other animals which lack that ability This

can be manifest in the selection of preferable food sources

(where colour is a reliable indicator of the freshness of

foods), the avoidance of conflict (where facial skin tones are

an indicator of emotion), and the detection of danger (where colour can help to identify objects that may otherwise be camouflaged by pattern alone)

In a modern context, the accurate perception of colour remains an important part of day-to-day life, and many of the same reasons still apply A few obvious examples include:

• Accurate rendering of skin tones is an important part of reading body language, and studies show consistent preferences for high colour rendering lighting in

interactions Skin tone rendering is also important for visual diagnosis in medical/clinical contexts, where changes in skin tone can be indicators of a variety of medical conditions (cyanosis, jaundice etc.) Accurate rendering of skin tones requires light with strong representation in the orange and red portions of the visible spectrum

• While food is typically purchased from a retailer now rather than foraged for, the accurate rendering of fresh produce (and other consumables for that matter) remains

an important part of the selection process Light colour quality in retail environments has long been accepted as

a critical specification and may focus on just a few select colours or span all portions of the visible spectrum depending on the specific application

• The majority of printed material is produced in full colour, both in a media context (newspapers, magazines etc.) and in a commercial context (business documents, plans etc.) The ability to accurately render printed colours is therefore important for some material to be correctly read and understood, especially where technical graphics rely

on colour scales to illustrate variations of a defined metric

A relatively high proportion of the population experience colour vision disorders which compromise their ability to perceive colours accurately It is estimated from collation of nation-level studies that some five to ten percent of men experience colour vision deficiencies, while significantly fewer women (usually less than one percent) are affected The problems typically relate to genetics rather than age or injury (unlike many general vision disorders), and the proportions are generally higher than general vision disorders (excluding those ailments related to age and injury and excluding refractive errors that can be corrected with glasses

or contact lenses)

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

Figure 3: Spectral responses (normalised) from retinal photoreceptors

Short cones (Blue response) ipRGCs (Melanopsin response)) Rods (low-light response) Medium cones (Green response) Long cones (Red response) Melatonin suppression

The matter of colour vision is therefore not a trivial

consideration in the broader context of overall visual acuity

It has been theorised that the human vision system is in fact

far more sensitive to colour accuracy than it is to lighting

intensity, and that negative reactions to a reduction in colour

rendering are more pronounced than the reactions to a

proportional reduction in illuminance This theory is yet to be

scientifically tested, but anecdotal evidence suggests it is

plausible and if it were able to be confirmed it could represent

a fundamental shift in how lighting requirements are defined

The introduction of the TM-30-18 Method for Evaluating Light

Source Colour Rendition by the Illuminating Engineering

Society (IES) is part of a growing acknowledgement in the

lighting profession for more detailed consideration and

specification of lighting performance for varying colour vision

purposes The previous Colour Rendering Index (CRI) framework ignored many important parts of the visible light spectrum, and so a more comprehensive framework was required to properly characterise those key aspects of overall colour performance The TM-30 method expands the colour evaluation from just eight sample colours as used in the CRI method to a far broader set of 99 sample colours Those evaluated results are summarised by an expanded set of metrics, characterising overall performance in terms of colour fidelity and colour gamut, and including other indices to describing shifts in chroma, hue and saturation A specific descriptor for the fidelity of skin tone rendering is also established

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

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

Figure 4: ‘Reconstituted’ 6000K daylight spectrum, with indicative spectral activity ranges for selected physiological responses

< Melanopic response

< Vision-forming (nominal peak sensitivity range)

Mitochondrial response >

< Glare, increased eye damage risk Skin tone rendering/diagnosis >

Development of Lighting Technologies

Historically, lighting technologies (at least those focused on

general lighting applications) have attempted to emit light

which stimulate the largest vision response possible from a

given input – or more simply, to create as much ‘useable’

light from the smallest possible energy input Each source of

light has its own ‘native’ emission spectrum, which may or

may not suit the desired application without modification For

discharge (i.e fluorescent) and more recently solid-state (i.e

LED) sources, it has been necessary for spectral modifiers to

be deployed to absorb and redistribute some of that ‘native’

emission into other sections of the spectrum to yield the

‘white light’ options desired for general lighting applications

Commercial LED sources are generally based on a ‘blue’

semiconductor design with a native emission in the

‘deep-blue’ part of the visible spectrum On its own, this would

have virtually no application for general lighting, but with the

addition of phosphor layers, that emission can be

manipulated to the point of achieving a ‘white light’ appearance The spectral response curves shown in Figure

3 demonstrate clearly why manufacturers using these technologies have tailored their white-light spectra to heavily favour the mid-spectrum green-yellow range Light emitted in that portion aligns well with the vision response, and therefore stimulates a maximum photopic response for the energy required to emit the light, which in turn yields higher light source efficacies

However, this pursuit of ever-increasing efficacy has largely been achieved through compromises in other important sections of the visible spectrum Notably, commercial LED sources typically have poor emission in the red and light-blue/light-green ranges, and retain high levels of emission in the deep-blue range This is demonstrated in Figure 5, where a typical commercial LED spectral power distribution (SPD) is shown in comparison to a reference daylight SPD of equivalent correlated colour temperature (CCT)

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

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’

colour specification) SPD shows the characteristic peak in

the deep-blue range (roughly corresponding to the ‘native’

emission wavelength), and then a broader hump of emission

spanning the mid-green through to light-red range

(corresponding to the peak sensitivity range of the

vision-forming response)

However, as established earlier in the preceding summary,

this SPD also demonstrates clearly that a number of other

important responses are somewhat neglected The

melanopic response (light blue to light green) aligns with the

distinct dip in emission, while emission is similarly low in the

strong red part of the spectrum which determines skin tone

rendering and mitochondrial responses Additionally, the

peak emission in the strong blue part of the spectrum aligns

with the negative outcomes of extra glare impact and

increased risk of eye damage

Development of ‘Full Spectrum’ LED lighting options have sought to address some of these spectral distribution deficiencies while retaining the preferred 4000K white light colour temperature preferred for general commercial interior environments

• A greater portion of the ‘native’ blue emission is absorbed by phosphors in the LED, reducing blue-light related outcomes

• Phosphors re-emit light in the light-blue/light-green range – boosting the melanopic response

• Phosphors also emit less in the yellow/orange range and instead emit in the deep red end of the spectrum – improving skin tone rendering and the mitochondrial response

Figure 6 demonstrates one such ‘Full Spectrum’ option, displayed alongside a typical commercial 840 LED option for comparison

Figure 6: SOLUS ‘Full Spectrum’ 4000K SPD compared with typical commercial ‘840’ LED

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

• Measuring and using light in the melanopsin age

• Action Spectrum for melatonin regulation in humans – Evidence for a novel circadian photoreceptor

• 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

• Experimental evidence that primate trichromacy is well suited for detecting primate social colour signals

Ecopoint Limited

2 Jarden Mile

Ngauranga, Wellington 6035 PO Box 12646 Thorndon, Wellington 6144

P: +64 4 499 3636

E: info@ecopoint.co.nz W: www.ecopoint.co.nz

Due to our commitment to ongoing technical development,

we reserve the right to change specifications without notice © Ecopoint Limited, 15/09/2020

LUMINAIRE DETAILS

300x1200 36W 4000K 600x600 36W 4000K

TEST DATA/DETAILS

12/06/2020

UNSW 19239.1 UNSW 20138.1.1

CIE 133 — 1995

R a 98

R10 97

R11 96

R12 81

R13 99

R14 97

R15 97

Melanopic Ratio (IWBI):

Describes the melanopic response (reaction to light for

regulating circadian rhythm) as a proportion of the visual

response (reaction to light for vision)

The Cyanosis Observation Index is established in AS/NZS 1680.2.5, and the same standard recommends a COI of no greater than 3.3 for clinical and critical patient care areas

Photon Efficacy is a measure of how efficiently circuit power is converted into photons of visible light which can drive photosynthesis — i.e photosynthetic active radiation (PAR)

Spectral power distribution data is available in tabular form; contact Ecopoint for the appropriate files

R a

Note that data presented here is considered representative of all specific models/configurations within the indicated product family Where multiple test results are available with a product family, the adopted dataset presented here is an indication of ‘average’ performance

Light Spectra and Human Responses

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