Which term refers to luminous intensity per projected area of a light source?

The term that refers to luminous intensity per projected area of a light source is luminance. Luminance measures how much light is emitted, reflected, or transmitted from a particular area in a given direction. It is defined as the luminous flux per unit solid angle per unit projected area. This concept is integral in understanding how bright an object appears in a given context, as it takes into account both the amount of light emitted by the source and the area over which that light is distributed.

Luminous intensity, on the other hand, pertains to the power emitted by a light source in a particular direction, but does not consider the area over which this power is spread. Illuminance measures the total light incident on a surface, focusing on how much light hits a specified area rather than the characteristics of the emission itself. Brightness is a subjective perception related to how light is experienced by the observer, and it can vary based on numerous factors, including the surrounding environment and the observer's perception. Therefore, luminance is the correct choice as it specifically quantifies how the emitted light corresponds to an area in a defined direction.