Which term describes the radiant intensity per projected area of a light source, used for extended sources?

The term that describes the radiant intensity per projected area of a light source, particularly in the context of extended sources, is radiance. Radiance is a measure of how much light is traveling in a particular direction from a surface area that is emitting light. It takes into account both the intensity of the light and how much of that light is spread over an area, which is particularly useful when dealing with extended light sources rather than point light sources.

Radiance is defined in terms of power emitted per unit solid angle per unit projected area. This definition captures the distribution of light emanating from a source and allows for the analysis of how light interacts with different surfaces and media. In applications such as visual perception, understanding radiance is crucial for predicting how a scene will appear to the observer, including factors such as brightness and color.

The other terms refer to different aspects of light. Radiant intensity refers specifically to the power emitted by a light source in a particular direction, while irradiance measures how much light power is received per unit area at a surface. Flux, on the other hand, refers to the total amount of light energy emitted by a source over time, without consideration for direction. Therefore, when discussing the radiant intensity per projected area specifically for extended