James Frysinger wrote in USMA 18778 >The candela *is* photometrically related. Its definition is absolute >only at one frequency (corresponding to ~555 nm in free space). For >other frequencies, the luminous efficiency function is used. The CIE >standard luminous efficiency function is photonic (daylight).
555 nm is the wavelength to which the human eye is most sensitve. The 16th CGPM (1979, Resolution 3): "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." Joseph B.Reid 17 Glebe Road West Toronto M5P 1C8 TEL. 416-486-6071
