Photometry and Radiometry Standards
candela, cd: The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
Of all of the base units the candela is the one most obviously manmade. Because the human eye is more sensitive to some wavelengths of light than others we cannot rely purely on measurements of optical power to characterise the effectiveness of the lighting used in our houses, offices, and particularly on our roads. Pouring hundreds of kilowatts of ultraviolet or infrared light on our roads would not help us see at night. The candela provides a scale that measures the effectiveness of lighting systems made for the human eye.
To measure the effectiveness of lighting we must use a combination of a light detector and filter that responds to light in exactly the same way as the eye does. Such a meter is called a photometer. The SI definition of the candela specifies the sensitivity of the photometer at only one wavelength, the sensitivity at other wavelengths is specified by the CIE (Commission Internationale de l’Éclairage) according to a complex function based on many measurements of the sensitivity of the human eye.