Visible radiation is often described with units such as lumens, lux, phots, stibs, nits, footcandles, candelas, and lamberts. These all refer to a detector with a spectral response that mimics the visual response of the “Standard Observer.” When expressed in radiometric terms, this means: 1) a wavelength of maximum response at 550 nm; and 2) a sharply sloping spectral response (0.41% response at 700 nm relative to the peak, 0.12% at 400 nm). It is possible to convert from radiometric units to photometric units only if the complete spectral distribution is known.