Spectrophotometer: A device which measures the amount of ultraviolet light absorbed by a substance. For example, vitamin A absorbs ultraviolet light so that a spectrophotometer can measure within minutes with almost perfect accuracy the amount of vitamin A in a given food. The spectrophotometer, a key instrument today in biomedical laboratories, was invented in 1939 by the American chemist Arnold O. Beckman (1900-2004).