For the quantitative measurement of light, a special set of concepts and units has been adopted that bear no direct relationship to those used in other domains of physical science. This is in contrast with measuring practice in other wavelength regions of the electromagnetic spectrum which is generally based on the familiar concepts of energy and power, and therefore use the SI units of joules and watts. The principal reason for this is that a lighting unit must not only take into account the energy content of the radiation but also the spectral distribution of the sensitivity of the human eye, which varies greatly with wavelength.
