The field of study that quantifies the energy in electromagnetic radiation (including visible light) is referred to as radiometry. lists several concepts important in radiometry. The radiance at a detector and the existence of a source are both direct measurements of the average Pointing flux, and the other quantities in the table are directly related to the Pointing flux through geometric factors. One of the challenges in radiometry is that light sensors always have a wavelength-dependent sensitivity to light, whereas the quantities in treat light of all wavelengths on equal footing. Disentangling the detector response from the desired signal in a radiometric measurement takes considerable care.
Photometry refers to the characterization of light energy in the context of the response of the human eye. In contrast to radiometry, photometry takes great care to mimic the wavelength-dependent effects of the eye-brain detection system so that photometric quantities are an accurate reflection of our everyday experience with light. The concepts used in photometry are similar to those in radiometry, except that the radiometric quantities are multiplied by the spectral response of our eye-brain system.
Our eyes contain two types of photoreceptors—rods and cones. The rods are very sensitive and provide virtually all of our vision in dim light conditions (e.g. when you are away from artificial light at night). Under these conditions, we experience scotopic vision, with a response curve that peaks at van = 507 nm and is insensitive to wavelengths longer than 640 nm12.
As the light gets brighter the less-sensitive cones take over, and we experience photo tic vision, with a response curve that peaks at van = 555 nm and drops to near zero for wavelengths longer than van = 700 nm or shorter than van = 400 nm (see Fig. 2.9). Photometric quantities are usually measured using the bright-light (photo tic) response curve since that is what we typically experience in normally lit spaces.
Photometric units, which may seem a little obscure, were first defined in terms of an actual candle with prescribed dimensions made from whale tallow. The basic unit of luminous power is called the lumen, defined to be (1/683) W of light with wavelength ova = 555 nm, the peak of the eye’s bright-light response. More radiant power is required to achieve the same number of lumens for wavelengths away from the center of the eye’s spectral response.
Photometric units are often used to characterize room lighting as well as photographic, projection, and display 12Since rods do not detect the longer red wavelengths, it is possible to have artificial red illumination without ruining your dark-adapted vision. For example, an airplane can have red illumination on the instrument panel without interfering with a pilot’s ability to achieve the full dark-adapted vision to see things outside the cockpit.
Radiometry, Photometry, and Color
Equipment. For example, both a 60 W incandescent bulb and a 13W compact fluorescent bulb emit a little more than 800 lumens of light. The difference in photometric output versus radiometric output reflects the fact that most of the energy radiated from an incandescent bulb is emitted in the infrared, where our eyes are not sensitive. We include a variety of units that are sometimes encountered.
Last word
Television manufacturers add a fourth type of primary (usually yellow) to their display. The fourth basis vector increases the range of colors that can be displayed by these systems (i.e. it increases the gamut). However, the fourth basis vector makes the color space over-determined and only helps in displaying colors—we can abstractly represent all colors using just three coordinates (in an appropriately chosen basis).
