Poultry have four types of cone in the retina of the eye, and this means that they probably see colour differently from trichromatic humans. Notwith- standing the fact that humans and birds have maximum sensitivity in a similar part of the spectrum (545–575nm), poultry are likely to perceive light from various types of lamp at a different intensity from humans because they are more sensitive to the blue and red parts of the spectrum. Although colour has been confounded with illuminance in many trials, wavelength has an unquestionable effect on poultry production and behaviour. Growth and behaviour responses depend principally on retinal photoreception, whereas photosexual responses are mainly influenced by hypothalamic light reception. In turkeys and chickens growth under red illumination is inferior to that under blue or green light, and this may be a result of birds exposed to red light being more active and showing more aggression than birds exposed to shorter wavelength radiation. In contrast, the easier penetration of longer wavelength radiation to the hypothalamus makes red light more sexually stimulatory than blue or green, although the hypothalamic photo- receptors are more sensitive to blue/green light when illuminated directly. Egg production traits, however, appear to be minimally affected by wavelength.