Light is an essential component of the environment for all animals. It allows them to see and exerts widespread non-visual effects on circadian rhythms, sleep, hormone synthesis and affective and cognitive processes. However, current guidelines for light in laboratory animal housing are based upon the requirements of human vision rather than an appropriate consideration of how the animals themselves experience light. Indeed, the very units in which light in animal husbandry is measured - lux - presume that the recipient of the light is human. Lux adequately predicts brightness as perceived by a 'standard' human observer. However, the visual system of laboratory animals differs from that of humans in important ways. As a result, lights that are matched in lux (and appear very similar for us humans) can have quite divergent impact on animals. This can lead to animals being inadvertently exposed to inappropriate light. It also makes it impossible to standardise and regulate light exposure for lab animals. Together these can impact animal welfare and degrade the power and validity of scientific investigation.

This grant supported a workshop at which international experts in vision, photobiology and animal welfare addressed the questions of how we could better measure light for laboratory animals and what constitutes ‘healthy’ light for them. Over 3 days of discussion we agreed a measurement system that could be used to accurately measure light intensity as experienced by diverse species of laboratory mammals. We further defined evidence-based targets for light intensity at daytime and night expressed in these new units. The outcome of our discussion was collated in a consensus statement document that has been published (https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002535 .