In bright sunlight, we see the world in sparkling colours, but when night falls, colours fade away until, with less than a half moon, everything appears in 50 shades of grey. The reason for this lies in our duplex retina with three types of cones - noisy photoreceptors - and only one type of rods - receptors optimized for low noise. Colour discrimination is based on comparison of cone signals, and limited by receptor noise. Over a large intensity range, the level of receptor noise stays constant, but in dim light, photon shot noise statrts impairing colour discrimination, and the absolute threshold of cone-based vision is set by dark noise in cones. Like humans, other vertebrates with duplex retinae use colour-blind rod vision in dim light.
Nocturnal insects and vertebrates lacking rods have to use the same set of receptors day and night. They have evolved adaptations to increase the sensitivity of their eyes and photoreceptors, and to reduce receptor noise. This allows them to use chromatic vision in dimmer light intensities than humans. Frogs and toads that have two types of rod use opponent signals from rods to control phototaxis even at their visual threshold, at light intensities ten times darker than the absolute threshold of human vision. However, for tasks such as prey or mate choice, their colour discrimination abilities fail at brighter light intensities, only ten times darker than the humans colour vision threshold, limited by the dark noise in their cones.
In this presentation, I review what is known about dim-light colour vision and compare colour vision thresholds with the optical sensitivity of the photoreceptors in selected animal species with lens and compound eyes.