Beyond Lux: Methods for Species and Photoreceptor-Specific Quantification of Ambient Light for Mammals

Background Light is a key environmental regulator of physiology and behaviour. Mistimed or insufficient light disrupts circadian rhythms and is associated with impaired health and well-being across mammals. Appropriate lighting is therefore crucial for indoor housed mammals. The most commonly used measurement for lighting is lux. However, this employs a spectral weighting function based on human perceived brightness and is not suitable for ‘non-visual’ effects of light or use across species. In humans, a photoreceptor-specific (α-opic) metrology system has been proposed as a more appropriate way of measuring light. Results Here we establish technology to allow this α-opic measurement approach to be readily extended to any mammalian species, accounting for differences in photoreceptor types, photopigment spectral sensitivities, and eye anatomy. Since measuring photopigment spectral sensitivity can be hard to derive for novel animals and photoreceptors, we developed a high-throughput, easy-to-use, method to derive spectral sensitivities for recombinantly expressed melanopsins and use it to establish the spectral sensitivity of melanopsin from 12 non-human mammals. We further address the need for simple measurement strategies for species-specific α-opic measures by developing an accessible online toolbox for calculating these units and validating an open hardware, low-cost, multichannel light sensor for ‘point and click’ measurement. We finally demonstrate that species-specific α-opic measurements are superior to photopic lux as predictors of physiological responses to light in mice and allow ecologically relevant comparisons of photosensitivity between species. Conclusion Our study demonstrates that measuring light more accurately using species-specific α-opic units is superior to the existing unit of photopic lux and holds the promise of improvements to the health and welfare of animals, scientific research reproducibility, agricultural productivity, and energy usage. ### Competing Interest Statement RJL and TMB have received investigator-initiated grant funding from Signify/Philips Lighting and RJL has received honoraria from Samsung Electronics. RJL has received a financial support from NASA, UFAW (11-22/23) and the University of Manchester to organize a workshop about the measurement of species-specific light units for laboratory mammals (Manchester, 2023). * 9- cis : – 9- cis retinaldehyde 11- cis : – 11- cis retinaldehyde λmax : – The wavelength at which maximum activity/absorption occurs EDI : - equivalent daylight illumination MM light sensor : - multichannel miniaturised light sensor UV : - Ultraviolet