What are scotopic and photopic vision?

Rods are sensitive to very low levels of illumination and are responsible for our ability to see in dim light (scotopic vision). They contain a pigment with a maximum sensitivity at about 510 nm, in the green part of the spectrum. The rod pigment is often called visual purple since when it is extracted by chemists in sufficient quantities the pigment has a purple appearance. Scotopic vision is completely lacking in colour; a single spectral sensitivity function is colour-blind and thus scotopic vision is monochromatic.

Colour vision is provided by the cones, of which there are three distinct classes each containing a different photosensitive pigment. The three pigments have maximum absorptions at about 430, 530, and 560 nm and the cones are often called blue, green, and red. The cones are not named after the appearance of the cone pigments but are named after the colour of light to which the cones are optimally sensitive. This terminology is unfortunate since monochromatic lights at 430, 530, and 560 nm are not blue, green, and red respectively but violet, blue-green, and yellow-green. The use of short-, medium-, and long-wavelength cones is a more logical nomenclature.

The existence of three spectral sensitivity functions provides a basis for colour vision since light of each wavelength will give rise to a unique ratio of short-, medium-, and long-wavelength cone responses. The cones therefore provide us with colour vision (photopic vision) that can distinguish remarkably fine wavelength changes.