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Important Photometric Terms

1. Luminous flux . Radiant flux evaluted in terms of a standardized visual response. Measured in lumens, . where

   

   The above definition of luminous flux is for photopic vision and has the value . For scotopic vision is replaced by and is replaced by .

2. Luminous flux density, . This quantity is usually referred to as illuminance, , if luminous flux density is incident on a surface element, and luminous exitance, , if luminous flux density is leaving a surface element. Measured in .
3. Luminous intensity, . The luminous flux per unit solid angle in a certain direction. Measured in .
4. Luminance, . The definition is the same as radiance. The units are .