an electronic sensor; in the case of the eye, it is an array of visual receptors. With this simple geometrical similarity, based on the laws of optics, the eye functions as a transducer, as does a CCD camera.

In the visual system, retinal, technically called retinene₁ or "retinaldehyde", is a light-sensitive molecule found in the rods and cones of the retina. Retinal is