Retinal is a chromophore that forms photosensitive Retinylidene proteins  when covalently bound to proteins called opsins. Retinal can be photoisomerized by itself, but requires to be bound to an opsin protein to both trigger the phototransduction cascade and tune the spectral sensitivity to longer wavelengths, which enable color vision.
Retinal is a species of retinoid and the aldehyde form of Vitamin A. Retinal is interconvertible with retinol, the transport and storage form of vitamin A. During the visual cycle, retinal moves between several different isomers and is also converted to retinol and retinyl ester. Retinoids can be derived from the oxidation of carotenoids like beta carotene or can be consumed directly. To reach the retina, it is bound to Retinol Binding Protein (RBP) and Transthyretin, which prevents its filtration in the glomeruli.
As in transport via the RBP-Transthyretin pathway, retinoids must always be bound to Chaperone molecules, for several reasons. Retinoids are toxic, insoluble in aqueous solutions, and prone to oxidation, and as such they must be bound and protected when within the body. The body uses a variety of chaperones, particularly in the retina, to transport retinoids.