Red, White or Blue? – a review of crepuscular and nocturnal light perception and reverse cycle
lighting

Frances Baines, M.A. Vet.M.B. M.R.C.V.S.

Introduction
At present, reverse cycle lighting in most Nocturnal Houses tends to consist of very low levels of coloured light to illuminate the enclosures during the day (the animals’ nocturnal period) and moderate levels of “white” light from fluorescent tubes to simulate daylight when it is actually night outside. The low-level “nocturnal” lighting needs to be sufficient to enable human visitors to see the animals in their enclosures, but ideally should have minimal effect upon the animals’ physiology or natural behaviours. In recent years, red or blue LED lamps have become increasingly popular owing to their bright colours and very low running costs, and “white” LEDs are also being considered by some zoos. The popular notion that “moonlight is blue” has led to widespread use of blue LEDs. In the past, red lighting was more often used, because it is widely believed that animals cannot see it. What is the truth behind these ideas? Is one colour really better than another? Would dim light from “white” LEDs be
better or worse than either of these?

Both the spectral quality of a light (the wavelengths it contains) and its intensity have profound effects upon animals, both during the day and at night.
Three major factors must be considered when providing the “nocturnal” part of reverse cycle lighting:
A. The effect of the light upon the animal’s vision – and that of the human observers
B. Non-visual perception of the light by the animal and its effect on circadian rhythms
C. The so-called “blue light hazard” – the damaging effects of shorter wavelengths on the retina.

The following points must also be considered when planning the “daytime lighting” (outside the scope of the current discussion):
D. The spectrum and intensity of the “daylight”
E. Cutaneous synthesis of vitamin D3 by a UVB component in the light
F. Light required for photosynthesis, if plants are to be grown in the enclosure.

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