Luminous efficacy of a light source may be defined in two ways. The radiant luminous efficacy (LER) is the ratio of the visible light flux emitted (the luminous flux) to the total power radiated over all wavelengths. The source luminous efficacy (LES) is the ratio of the visible light flux emitted (the luminous flux) to the total power input to the source, such as a lamp. Visible light is measured in lumens, a unit which is defined in part by the differing sensitivity of the human eye to different wavelengths of light. Not all wavelengths of visible electromagnetic energy are equally effective at stimulating the human eye; the luminous efficacy of radiant energy (LER) is a measure of how well the distribution of energy matches the perception of the eye. The units of luminous efficacy are "lumens per watt" (lpw). The maximum LER possible is 683 lm/W for monochromatic green light at 555 nanometers wavelength, the peak sensitivity of the human eye.