Trichromatic Colorimeter

a device for measuring color in one of the three-dimensional colorimetric systems—that is, a system in which it is assumed that any color may be represented as the result of optical addition of definite proportions of three colors, which are taken as the primary colors in the system.

Visual colorimeters In visual colorimeters, the amounts of color—the color coordinates—are selected by the viewer in such a way as to produce a color that is indistinguishable to the eye from the color being measured. The results of the selection are indicated on the measurement scales of the colorimeter. In the simplest visual colorimeter, a Maxwell disk, the optical mixing of the primary colors takes place with respect to time by rapid viewing of one color after another. The outer ring of the disk is divided into three sectors, one for each primary color. The size of each sector is regulated so that upon rapid rotation of the disk the observed color of the ring is indistinguishable from the color of the sample in the center of the disk.

Visual colorimeters, in which the optical mixing is achieved spatially by simultaneous illumination of a white surface by three light beams of different color, are more common. The contribution of each beam to the color produced is regulated by changing its intensity. The optical scheme of one of the better colorimeters of this type (developed by L. I. Demkina) is presented in Figure 1.

The results of measurement may be given in the form C = r’R + g’ G + b’B, where r’,g’, and b’ are the coordinates of C, read from the scale, in the system of primary colors R, G, and B of the device (usually red, green, and blue). If r’, g’, and b’ are known, the coordinates of C in any other three-dimensional colorimetric system (with other primary colors) may be calculated. For such a calculation it is necessary to know only the color coordinates of R, G, and B in the other system. Colorimeters are most often calibrated for recalculation of the results according to the international XYZ system.

Photoelectric colorimeters Photoelectric colorimeters (also called objective colorimeters) constitute another class. Relations that make possible the calculation of the color coordinates of the radiation being measured according to its spectral composition I (λ), the intensity of the illumination as a function of the wavelength, are used for measurements with such devices. These relations are integrals of the products of I (λ) and the “specific coordinates” of a color, which are known functions of the wavelength; in the international XYZ system, they are the functions x̄ (λ) ȳ (λ) and z̄(λ).

Photoelectric colorimeters are divided into spectral colorimeters and devices with selective detectors. In the first type, the illumination being measured is refracted by a dispersing prism or system of prisms into a spectrum, which is “read” by the photoelectric detector. The detector’s signals are multiplied by the functions x̄ (λ), ȳ (λ), and z̄ (λ) continuously or over small wavelength intervals and integrated over the entire visible spectrum; the results of the integration are the coordinates of the radiation being measured. Colorimeters with selective detectors use either three sensors with light filters or one detector in front of which three light filters are passed in turn. Each filter consists of a combination of colored glasses whose thickness is calculated to bring the spectral sensitivity of the photocells to the curves for x̄ (λ), ȳ (λ), and z̄ (λ) with maximum accuracy. If such sensitivity is achieved, the values for the three light beams are proportional to the color coordinates x, y, and z.

Figure 1. Optical diagram of the visual trichromatic colorimeter developed by L. I. Demkina. The field of vision observed in the eyepiece is divided by the photometer cube into two parts, one of which has the color of the sample and the other has the color of the screen, on which the primary colors of the device are mixed. The light from the source of illumination is incident on the screen through the stop, which contains three light filters (red, R; green, G; and blue, B) and three movable baffles. By varying the area of the filters using the baffles, the viewer changes the intensity of the red, green, and blue light beams in such a way that the color produced by mixing them is indistinguishable from the color of the sample. (/) illumination lamp; (L) lens; (M₁), (M₂), and (M₃) mirrors; (A) source illuminating the sample; (F₁) and (F₂) attenuating filters.

Various types of photoelectric colorimeters are used in industry for monitoring the color of light sources (UFK and UKL colorimeters), light filters and reflecting materials (KNO type), and screens of color and black-and-white television sets (TK type). The most accurate color data are given by spectral colorimeters. Photoelectric light comparators (EKTs and FKTsSh types), in which the light being measured is compared with the light of a standard sample having a similar spectral composition, also give high accuracy of measurement.