In our discussion of radiometry, light flux is measured in terms of power or energy. However, even under identical viewing conditions, equal power of light of different wavelengths does not produce equal brightness sensation in our visual perception. (In an extreme case, even a kilowatt infrared source will not help us to see.) Therefore, radiometric quantities are not always meaningful in our visualworld, especially in the field of illumination engineering. For example, in order to illuminate a library reading room, we need to know “how much” visible light our chosen light sources will provide for reading. For these types of applications, we need to measure light flux in quantities that are representative of its visual impact, such as brightness. Photometry deals with measurements of visible light in terms of its effectiveness to produce the “brightness” sensation in the human visual system. Given two stimuli of different spectral compositions, the basic goal of photometry is to set up a quantitative procedure for determining which stimulus will appear “brighter” or more luminous to an average observer.

Measuring visual quantities of light is complicated because light stimuli of different spectral compositions produce complex perceptions of light, such as bright red or dark green. It is not easy (if not impossible) to order these different color sensations along a single, intensive scale. In fact, years of research have not produced a completely satisfactory solution. However, the applications are so important that an agreed-upon, incomplete solution is better than no solution at all.