Iris anatomy: Encyclopedia II - Iris anatomy - Red eye

Iris anatomy - Red eye

When photographed with a flash, the iris only reacts to protect the retina, and not fast enough to avoid the red-eye effect. This represents reflection of light from the back of the eye, and is closely related to the term red reflex, used by ophthalmologists in describing appearances on fundal examination.
When used as a descriptive term in medicine, the meaning of "red eye" is quite different, and indicates that the bulbar conjunctiva is reddened due to dilatation of superficial blood vessels. Leaving aside rarities, it indicates surface infection (conjunctivitis), intraocular inflammation (e.g., iridocyclitis) or high intraocular pressure (acute glaucoma or occasionally severe, untreated chronic glaucoma). This use of "red eye" implies disease. The term is therefore not used in medicine for ocular albinism, in which the eye is otherwise healthy despite an obviously red pupil and a translucent pinkish iris due to reflected light from the fundus. "Red eye" is used more loosely in veterinary practice, where investigation of eye diseases can be difficult, but even so albinotic breeds are easily recognised and are usually described as having "pink eye" rather than "red eye".

Adapted from the Wikipedia article "Red eye", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki