After the very fast isomerisation of the 11-cis-retinyl (11c-retinyl) group to the all-trans-retinyl (at-retinyl) group attached to opsins by light stimulation, slower events lead to exposure of at-retinyl group to the aqueous environment, resulting in the hydrolysis of the Schiff base linkage. Although other intermediate products are formed, the ultimate result is the release of all-trans-retinal (atRAL) from opsins, with apo-opsins being reformed (Baumann & Bender 1973). These series of slow decay reactions are called light bleaching of opsin and ends when atRAL, which can diffuse across membranes to the cytosol, is reduced to all-trans-retinol (atROL)