Upon photon absorption, 11-cis-retinal (11cRAL) is isomerised to all-trans-retinal (atRAL). The structure of cone opsins which can ultimately activate G-proteins to initiate phototransduction is denoted as R* (Fan et al. 2002). R* is still bound to atRAL at this stage. The isomerisation is a very fast photochemical process (femtoseconds) (Schoenlein et al. 1991) followed by slower events described in the following reaction.