Light is necessary for our vision. It helps us see things and do our work. Beyond the visual requirements, light is also a potent stimulus for regulating hormonal, behavioural and the circadian systems which gets affected by the non-visual system of our eye. These biological & behavioural effects of light are influenced by a group of distinct photoreceptors in the eye, which acts as an effective entrainment. These are melanopsin containing intrinsically photosensitive retinal ganglion cells (ipRGCs), in addition to the conventional photo-receptors – the rods & cones.
Considering the neurophysiology of this sensory pathway which has an effect on non-visual responses, artificial lighting design of the indoor spaces must incorporate these factors for our well being.
The responses of photoreceptors / ipRGCs to light
All retinal photoreceptor classes are upstream of circadian, neuroendocrine, and neurobehavioral responses to light.
(A) Schematic of the relevant retinal circuitry in humans. Non-image (non-visual) forming responses originate in the retina and have been attributed to a particular class of retinal ganglion cell (ipRGC). ipRGCs are directly photosensitive owing to expression of melanopsin, which allows them to respond to light even when isolated from the rest of the retina. In situ they are connected to the outer retinal rod and cone photoreceptors via the conventional retinal circuitry. Shown above are major connections with on cone bipolar cells (on CBCs) connecting them to cone and, via amacrine cells (AII) and rod bipolar cells (RBC), rod photoreceptors. As a consequence, the firing pattern of ipRGCs can be influenced by both intrinsic melanopsin photoreception and extrinsic signals originating in rods and each of the spectrally distinct cone classes (shown in red, green, and blue).
(B) This feature is conceptualised in much simplified form as a number of photoreceptive mechanisms (depicted as R for rod opsin; M for melanopsin; SC for S cone opsin; MC for M cone opsin; and LC for L cone opsin), each of which absorbs light according to its own spectral sensitivity profile (shown in representative form as plots of log sensitivity against wavelength from 400 to 700 nm) to generate a distinct measure of illuminance. These five input signals are then combined by the retinal wiring, and within the ipRGC itself, to produce an integrated signal that is sent to non-image-forming centers in the brain. As each of the five representations of weighted irradiance is produced by a photopigment with its own spectral sensitivity profile, their relative significance for the integrated output defines the wavelength dependence of this signal, and hence of downstream responses.
Regulation of human physiology & behaviour by Light
It is a well demonstrated fact that retinal illumination (non-visual) influences many aspects of human physiology and behaviour directly. These types of light responses have been commonly referred to as Non Image Forming(NIF) or Non Visual (NV). A few of the examples which can be quoted – light constricts the pupil, suppresses pineal melatonin production, increases heart rate and core body temperature, stimulates cortisol production, and acts as a neurophysiological stimulant (increasing subjective and objective measures of alertness and psychomotor reaction time, and reducing lapses of attention).
Light has been shown to have anti-depressant properties, particularly in the treatment of seasonal affective disorder (SAD) and its subclinical variant sSAD, treatment for non-seasonal depression, menstrual-cycle-related problems, bulimia nervosa, and cognitive and fatigue problems associated with senile dementia, chemotherapy, and traumatic brain injury (TBI) and so on.
Human retinal photopigment complement (outputs): RPC
The primary method involved is to calculate equivalent “α-opic” illuminance values for each of the 5 photopigments in the human eye (λmax values based on Dartnall et al., with full absorption curves based on Govardovskii et al.,). This provides a weighted output, based on the 5 photopigments of the human eye, corrected for pre-receptoral filtering.
The incident light absorbed by the receptor depends on the peak axial optical density, effectively broadening the sensitivity curves. To account for this self- screening effect, the sensitivity template is also adjusted based on the optical density of photopigment. The mean values of photopigment optical density at peak absorptance are taken to be approximately 0.40 for the rods, 0.30 for the S-cones and 0.38 for the other cones [S4-S6]. It should be noted that these values are based on a 10 degree visual field, and the optical density may vary in the peripheral retina. Due to the low pigment concentration and lack of specialised membrane structures in ipRGCs, the presence of melanopsin is not thought to appreciably modify the sensitivity curve in the same manner.
This output is given in new photometric units equivalent “α-opic” lux, where α defines the photopigment. These values are provided to help place these measurements into the context most familiar with most biological researchers (i.e. lux values). Moreover, this enables the sensitivity of each photopigment channel to be addressed independently. This results in 5 measurement output values, corresponding to the human retinal photoreceptor complement.
Above are the examples of equivalent α-opic lux outputs. 100 photopic lux of three light sources (fluorescent, incandescent and 450 nm narrowband) are shown to compare their equivalent α-opic illuminance for the photopigment complement of the human retina. Upper panels show spectral power distribution (blue) with melanopic weighted power (red). Lower panels show lux equivalents for all 5 photoreceptors. 100 photopic lux of fluorescent light provides similar equivalent lux values for the chloropic and erythropic functions (mc and lc), as these photopigments have a similar peak sensitivity to the photopic sensitivity function V(λ). Lower levels, however, are apparent for rhodopic, melanopic and cyanopic functions (r, z and sc). For incandescent light, similar values are again obtained with much lower equivalent cyanopic lux (sc). For a narrowband 450 nm source (20 nm FWHM), 100 photopic lux produces a greatly enhanced equivalent cyanopic lux, with very low levels of rhodopic (r), chloropic (mc) and erythropic (lc) activation.
(References : CIE (2012). CIE S 017/E:2011 ILV: International Lighting Vocabulary. Vienna: Commission Internationale de l’Eclairage; 2012. eilv.cie.co.at  Dartnall HJ, Bowmaker JK, Mollon JD (1983). Proc R Soc Lond B. Nov 22;220(1218):115-30.  Govardovskii VI, Fyhrquist N, Reuter T, Kuzmin DG, Donner K (2000). In search of the visual pigment template. Vis Neurosci. Jul-Aug;17(4):509-528.)
The conventional objectives of architectural lighting include the provision of light that:
(a) is optimal for visual performance (optimal lux levels / CT / CRI / TM-30/15);
(b) is visually comfortable (UGR / flicker free);
(c) permits aesthetic appreciation of the space; and
(d) conserves energy (quality electronics / controls).
Light exposure in different environments has a broad range of effects on physiology and behavior of the occupants. These non-visual effects of light should be an additional consideration (both qualitative and quantitative) in the lighting application design. Further, light can be considered a drug, having the potential for both beneficial as well as deleterious effects if prescribed and used incorrectly.
These conflicting effects can occur concurrently, and in a single application and individual context. e.g., for night-shift employees, bright workspace lighting may improve immediate performance by enhancing visual perception and promoting alertness, but simultaneously suppress melatonin and shift the circadian clock to an undesirable phase which might lead to sleep related disorders. Conversely, tunable white and/or dim lighting may reduce ill effects on circadian timing, but may be detrimental to user comfort, adaptation to environment as well as performance.
The key lies in identifying the correct parameters of the lighting application and designing the lighting for spaces considering the non-visual aspects of light in addition to the visual factors.