A photo pigment is a light-sensitive molecule that converts electromagnetic energy of photons into neural activity. The retina receives light from the world. The brain cannot use this light information because it is an improper format. Rather, the brain's basic unit of computation is a nerve cell or neuron. The photo pigments convert light information into neural information.

A photo pigment is a molecule in a photoreceptor cell (a rod or cone) that can trap the energy in visible light and convert it into a signal that can trigger a nerve cell.

When the nerve cell is triggered, your nervous system knows that light is hitting that particular photoreceptor. The way pigments usually work is by having a 3-dimensional structure with a special arrangement of chemical bonds.

When light of the correct wavelength hits the pigment, the electrons in the bonds are moved around, and this changes the 3-D structure of the pigment. When the pigment changes shape, it causes the photoreceptor cell to send a nerve impulse to the brain. The pigment molecule then has to return to its original 3-D shape to be able to be stimulated again.