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Photoreceptors are of two basic types, rods and cones. The rods have higher absolute sensitivity and function principally at low light levels (between dusk and dawn in natural environments) while the cones underpin vision during higher, daytime light levels. Cones are the photoreceptors that provide colour vision.

Every photoreceptor contains millions of photosensitive molecules (visual pigment), and each molecule is capable of trapping an individual photon of light. Trapping just a small number of photons is sufficient to kick off the process of phototransduction. This is a cascade of chemical changes which results in a neural signal being generated and transmitted to the brain via the optic nerve.

An important aspect of visual pigment molecules is that they do not absorb light of all wavelengths. Each molecule is selective within the spectrum, responding maximally to a relatively narrow range of wavelengths. Because only one type of photopigment occurs in each photoreceptor they sample the spectrum of the light falling upon them. The result is that for a particular eye the relative numbers of receptors with different photopigments will determine its sensitivity across the spectrum. This is explored in more detail in the ‘Types of cone photoreceptors’ section below and in Box 3.1.