Luminance Meter

a photometer that is used to measure luminance, or photometric brightness. Two optical diagrams of luminance meters with physical radiation detectors are shown in Figure 1,c and d in PHOTOMETER.

In a luminance meter built according to the optical design in Figure 1,c, the image of the luminous body in the light source (S) is formed in the plane of the diaphragm D, which limits the size of the part of the body being measured. When the objective is moved, the sensitivity of such a luminance meter is kept constant by means of an aperture stop D_a, which is fixed with respect to D.

In a simpler luminance meter, which is built according to the optical design in Figure 1,d, the bundle of rays being measured is limited by means of both a calibrated diaphragm D_c and the entrance pupil (see) of the radiation detector. The calibrated diaphragm D_c is placed either near the luminous body or, in the photometry of large objects, at some distance from the luminous body. The angle of the field of view of such a luminance meter may be varied discretely by superimposing a converging lens with a focal length of l₀ on the calibrated diaphragm. In this way, such a luminance meter is converted into a photometer corresponding to the optical design in Figure 1,c.

The human eye or an animal’s eye, either of which directly perceives brightness, is the simplest type of visual luminance meter. An equivalent optical diagram of such a luminance meter is shown in Figure 1,c.

Several types of luminance meters are manufactured by industry. They include photometers for measuring the luminance of continuous or pulsed light sources, visual photometers for measuring the equality of brightness, optical pyrometers, and exposure meters, which are used in photography. An exposure meter may be built into a camera or may be a separate instrument.