Because the eye's sensitivity changes with the wavelength, the output of lamps is commonly measured in lumens, a measure of the power of light as perceived by the human eye. The luminous efficacy of lamps is the number of lumens produced for each watt of electrical power used. The luminous efficacy of a typical CFL is 50–70 lumens per watt (lm/W) and that of a typical incandescent lamp is . Compared to a theoretical 100%-efficient lamp (), CFL lamps have lighting efficiency ranges of 7–10%, versus 1.5–2.5% for incandescents.