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

Abstract

BackgroundLight 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.ResultsHere 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.ConclusionOur 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.

The transcription factor VAX1 in VIP neurons of the suprachiasmatic nucleus impacts circadian rhythm generation, depressive-like behavior, and the reproductive axis in a sex-specific manner in mice

“…Mice were maintained on a light/dark cycle of LD [12 h light, 12 h dark, average light intensity ~150-350 lux within the cage]. Based on the newly proposed light reporting method by ( 49 , 50 ), we determined the relative perception of light by mice using the mouse α-opics equivalent daylight illuminance (EDI, Supplementary Figure 1 ). We calculated that the α-opics for our experimental mice on LD to be melanopsin = 43.2 ± 16.9 lux, rod = 51.6 ± 19.8 lux, s-cone = 0.03 ± 0.01 lux, and m-cone = 57.1 ± 21.7 lux.…”

In the Eye of the Beholder: Measuring and Standardising Light for Laboratory Mammals

“…The most conceptually straightforward, and accurate, approach is to use an optical spectrometer to measure the spectral power distribution of light, ideally measured at animal eye level, and apply mathematical conversions based upon equation 1 to calculate α-opic irradiances (Figure 3). To facilitate such a process, we direct the reader to an online tool that will calculate species specific α-opic irradiances/EDIs from input spectral power distributions (https://alphaopics.shinyapps.io/animal_light_toolbox/) [39] based upon the 𝑠 functions in Supplementary Table 1. Sufficiently accurate spectrophotometers are available at moderate cost (>£500), but although relatively easy to use, may be intimidating for those unfamiliar with light.…”

“…A simple design could integrate a spectrophotometer with suitable data processing. Alternatively, cheap multichannel light sensors, that are increasingly applied to measure human α-opic irradiances [40,41] could be recalibrated to measure species specific metrics [39]. Examples of commercially available light meters and spectrophotometers are provided in Supplementary Table 2.…”