Photoreceptor Enhanced Light Therapy (PELT): A Framework for Implementing BiologicallyDirected Integrative Lighting

Figures & data

Fig. 1. A portable photoreceptor enhanced light therapy (PELT) device with spectral components optimized to provide a gamut capable of generating circadian equivalent photoreceptor-directed light. The PELT stimulation protocol is specified with reference to the excitations of all five photoreceptor classes in the human eye and written to the microcontroller via a USB connection accessed through a port on the back of the device. Power is connected via the right-side panel. A protocol is started using the red button on the top right corner of the device. A notification speaker can provide alerting sounds for compliance.

Fig. 2. Circadian equivalent photoreceptor-directed light spectrums implemented in the PELT device. (a) Normalized component spectral outputs of the six primary lights (solid lines) and the replacement primary (dashed line). All lights are specified with reference to their excitation ratios of the three cones (L-, M, S-), the rhodopsin (R) and melanopsin (i) photopigments. (b) Spectra designed as daylight metamers across all five photoreceptors at correlated color temperatures (CCT) between 2500 and 7500 K (Table 1). The spectra can be programmed to change dynamically and match the photoreceptor excitation ratios with time of day or be set to a static spectrum with the five photoreceptor excitation ratios matching the specific time of day as designated by the CCT and preferred by the user. (c) Photoreceptor directed (PrD) spectra having the three cone excitations set to a reference CCT (e.g., 3500 K) and a circadian equivalent (CE) melanopsin and rhodopsin excitation set at a higher CCT (e.g., 4500–7500 K). The appearance of all higher circadian equivalent lights matches closely with the 3500 K reference white. The change in color appearance of the white light due to the higher melanopsin and rhodopsin excitations is circumvented by applying cone corrections (Δl, Δs at equiluminance), but which introduce minor deviations in the evaluated CCT (Table 2). (d) Personalized spectral outputs for a PELT protocol that selectively alters the melanopsin excitation between a depleted (dashed line) and enhanced state (solid line), with reference to a ~3500 K light. This circadian equivalent increase in melanopsin excitation approaches 5500 K (Δ24% melanopsin Weber contrast) (Table 2). (e) Personalized spectral outputs for a PELT protocol that selectively alters the rhodopsin excitation between a depleted (dashed line) and enhanced state (solid line), with reference to a ~3500 K light. This circadian equivalent increase in rhodopsin excitation approaches 5000 K (Δ15.5% rhodopsin Weber contrast). Although the maximum melanopsin or rhodopsin photoreceptor directed contrasts are achieved using nonwhite chromaticities with the implemented spectral components, the practical settings generate a white light with high PrD contrasts by allowing a small tolerance in the unmodulated photoreceptor contrast (Table 2).

Table 1. The five photoreceptor excitation ratios for typical daylight, circadian equivalent photoreceptor-directed lights (PrD).

	Daylight photoreceptor-directed lights (PrD)
	Photoreceptor excitations^a					Photopic-to-Total-Irradiance	CIE 1964 10° chromaticity coordinates
CCT	S	M	L	R	i^b		x	y
2000 K	0.1405	0.2625	0.7375	0.4388	0.3335	0.5118	0.5294	0.4127
2500 K	0.2604	0.2847	0.7153	0.5685	0.4721	0.5845	0.4804	0.4124
3000 K	0.3907	0.3002	0.6998	0.6762	0.5950	0.6371	0.4401	0.4029
3500 K	0.5202	0.3114	0.6886	0.7657	0.7011	0.6194	0.4083	0.3896
4000 K	0.6432	0.3194	0.6806	0.8410	0.7925	0.5984	0.3837	0.3754
4500 K	0.7346	0.3265	0.6735	0.9067	0.8724	0.5831	0.3650	0.3665
5000 K	0.816	0.3323	0.6677	0.9628	0.9419	0.5700	0.3497	0.3585
5500 K	0.9111	0.3367	0.6633	1.0092	1.0016	0.5558	0.3364	0.3477
6000 K	0.9975	0.3403	0.6597	1.0495	1.0541	0.5434	0.3254	0.3382
6500 K	1.0754	0.3435	0.6565	1.0847	1.1003	0.5327	0.3162	0.3299
7000 K	1.1453	0.3462	0.6538	1.1156	1.1412	0.5235	0.3085	0.3228
7500 K	1.2085	0.3485	0.6514	1.1429	1.1776	0.5154	0.3020	0.3165
8000 K	1.2653	0.3506	0.6494	1.1671	1.2099	0.5083	0.2964	0.3111

^aS = S-cone; M = M-cone; L = L-cone; R = rhodopsin; i = melanopsin.

^bIn our luminance normalized system, the metric Melanopic/Photopic = i and Scotopic/Photopic = R. The melanopic daylight (D65) efficacy is the i value at a specified CCT divided by the i value for the D65 illuminant (6500 K).

^cPhotopic-to-Total-Irradiance is a measure of the proportion of the total spectral irradiance that goes toward photopic luminance.

Table 2. Practical circadian equivalent stimulation settings for photoreceptor-enhanced light therapy (PELT).

(a) Photoreceptor excitations
			Photoreceptor excitations^a
Photoreceptor-directed light (PRD)				S	M	L	R	i
Melanopsin-directed (Practical)^b				0.5558	0.3089	0.6911	0.9016	0.7991
Rhodopsin-directed (Practical)^b				0.6002	0.3075	0.6925	0.8280	0.8515
Melanopsin-directed (Max PrD contrast)				1.4775	0.2903	0.7094	1.0624	1.1065
Rhodopsin-directed (Max PrD contrast)				0.8247	0.2575	0.7425	0.6344	0.7844
(b) Metrics.
							CIE 1964 10° chromaticity coordinates^d
Photoreceptor-directed light (PRD)	Contrast enhanced	Tolerance: Non-PrD contrast	Cone correction	CCT depleted^c	CCT enhanced	Photopic-to-Total-Irradiance^d	x	y
Melanopsin (Practical)^b	24.0% i	3% R	Yes	3477 K	3460 K	0.31 0.56	0.408 0.408	0.381 0.374
Rhodopsin (Practical)^b	15.5% R	3% i	Yes	3488 K	3537 K	0.58 0.26	0.406 0.404	0.371 0.369
Melanopsin (Max contrast)	25.6% i	0%	No	N/A	N/A	0.22 0.44	0.3591	0.2591
Rhodopsin (Max contrast)	27.7% R	0%	No	N/A	N/A	0.53 0.21	0.4446	0.3047

^aS = S-cone; M = M-cone; L = L-cone; R = rhodopsin; i = melanopsin.

^bThe SMLRi photoreceptor excitations are specified with reference to the depleted PrD condition. Contrast (enhanced) refers to the Weber contrast increase in the enhanced PrD condition. Note that depleted (low) and enhanced (high) are relative terms.

^cCCT: Minor deviations in CCT are evident due to the application of cone corrections that are required account for the color contributions of melanopsin and/or rhodopsin in the circadian equivalent PELT conditions.

^dPhotopic-to-Total-Irradiance and CIE 1964 10° x,y: Data order: depleted, enhanced

Data availability statement

Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.