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Photometry is the science concerned with measuring the human visual response to light. Because the eye is a highly complex organ, this is by no means a simple task. It involves the meeting of many disciplines: psychology, physiology, and physics among them. Photometry can be said to have become a modern science in 1924, when the Commission Internationale de l'Eclairage (CIE) met to define the response of the average human eye. The Commission measured the light-adapted eyes of a sizable sample group, and compiled the data into the photopic curve. Simply stated, the curve reveals that people respond strongest to the color green, and are less sensitive to the spectral extremes, red and violet. The eye has an altogether different response in the dark-adapted state, wherein it also has difficulty determining color. This gave rise to a second set of measurements, and the scotopic curve. Having defined the eye's spectral response, CIE sought a standard light source to serve as a yardstick for luminous intensity. The first source was a specific type of candle, giving rise to the terms footcandle and candlepower. In an effort to improve repeatability, the standard was redefined in 1948 as the amount of light emitted from a given quantity of melting platinum. |
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