The measurement of the luminous intensities of visible light sources which is sometimes expanded to include near-infrared and near-ultraviolet light.

Photometric devices accurately measure the intensity of light from the stars and galaxies.

They are nearly all based on highly sensitive, light-detecting electron tubes, called photomultipliers.

In these devices, weak starlight causes electrons to be emitted from the surface of a photocathode, and each electron released can be made to produce many millions of secondary electrons.