Mammalian eye
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Mammals normally have a pair of eyes. Although mammalian vision is not so excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families possessing a trichromatic color perception.
Human eye seen from the front in its orbit surrounded by the eyelid and eyelashes, showing the anterior segment with the iris (light brown in this individual), pupil, and sclera visible
Eyelids and eyelashes are a unique feature of mammalian eyes, and primarily serve to protect the eyes. However, the lids also play a role in nonverbal communication, and the lashes in human culture and sexuality.
Retinal
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Retinal is a polyene chromophore. Retinal, bound to proteins called opsins, is the chemical basis of visual phototransduction, the light-detection stage of visual perception (vision).
An opsin protein surrounds a molecule of 11-cis retinal, awaiting the arrival of a photon. Once the retinal molecule captures a photon, its configuration change causes it to push against the surrounding opsin protein which may cause the opsin to send a chemical signal to the brain indicating that light has been detected. Retinal is then converted back to its 11-cis configuration by ATP phosphorylation, and the cycle begins again.