What is "photometric exposure"?

Question

What is "photometric exposure," and how is it related to what is commonly called "exposure" and "exposure values"?

I have also seen the term "radiometric exposure" used to describe a similar property.

Answer

Radiometric exposure measures how much energy reaches a unit of surface in an unit of time, with no regard to wavelengths that carry the energy.

For photometric exposure, different weights are assigned to different wavelengths in order to mimic sensitivity of human eye to different wavelengths; for that purpose, a luminosity function has been agreed upon based on studies on visual photoreception. According to the function, energy of 555 nm (green) waves is assigned the highest weight and wavelengths outside visual spectrum are excluded by using weight of 0.

Unless based on the "human eye" attached to photographer, any exposure metering is based on calculations based on metered radiometric exposure. The metering device can then use a luminosity function to decide what the applicable exposure would be.

The luminosity function applied can be either based on the generic one (giving photometric exposure), or a custom one - either simplified (like measuring only green), or based on data about specific imaging media (an infrared or X-ray image would need to apply different weights to wavelengths for optimal metering). Since the function is more or less known in advance, radiometric exposure metering does not have to be sensitive to all wavelengths, just those that have some weight assigned in luminosity function.

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