The eye response is not a constant over the entire visible range. It peaks at Green 555 nm and falls off sharply toward the Blue at 400 nm, and in the Red at 700 nm. Detectors required for taking this type of measurement must be spectrally corrected so that their response matches the CIE curve selected. This correction is usually made by adding a specially designed photometric filter to the detector. Although many common terms are used to define photometric light output, the basic unit of measurement of light is the lumen.
All photometric quantities involve the lumen.
These terms are more scientific in their usage in that one watt of radiation in the ultraviolet is equal to one watt in the visible and one watt in the infrared range as well. Radiometric measurements are determined accurately when both the spectral power distribution of the light source measured is known along with the absolute responsivity of the detector are known. Intensity is useful since it is the flux per unit of solid angle directed toward the observer, and is most useful in a packaged LED lamp as the light in this lensed format is directed to the observer, usually along the axis of the lensing or optical system. This site is optimized for current versions of Internet Explorer or NetScape .