Photometric Standard

Photometric Standard 

a measure used to reproduce photometric units. The following have been used at various times as standards: 1 cm² of a platinum surface at the solidification temperature; electric incandescent lamps; and the flame of a candle or lamp with specified characteristics, such as the fuel and the dimensions of the flame. A distinction is made between a primary and a secondary photometric standard.

Figure 1. Apparatus of primary standard of light: (1) sight tube made of fused thorium oxide (ThO₂), whose temperature is maintained at 2042°K, the solidification temperature of platinum; (2) crucible of fused ThO₂ containing chemically pure paltinum (3); (4) quartz vessel containing a charge (5) of ThO₂; (6) sight tube aperture; (7) reflecting prism; (8) lens, which creates an image of the luminous aperture of the radiator on the white diffuse screen (10); (9) diaphragm; (11) comparison lamp, which illuminates the other side of the screen (10). The platinum in the crucible is heated by means of a high-frequency induction furnace (the melting point of ThO₂ is greater than 2042°K). By adjusting the distances between the photometer head, the complete radiator, and the comparison lamp, the illuminances on both sides of the screen (10) can be equalized. A photocell is often used instead of the screen; the photocell is illuminated alternately by the primary and secondary standards.

The primary standard for the unit of luminous intensity—the candela (cd)—is a complete, or blackbody, radiator at the solidification temperature of platinum. This standard is known as the primary standard of light. It is constant and is reproducible on the basis of the laws of thermal radiation. The apparatus consists of a small refractory sight tube immersed in metal that is heated by high-frequency currents (see Figure 1). This standard was developed in the USA and was adopted by an international agreement on Jan. 1, 1948; it is realized in eight national laboratories. The luminance of the standard is 6 X 10⁵ cd/m²; the magnitudes of the luminance obtained in the various national laboratories differ by no more than ± 0.6 percent, and the error within an individual laboratory is ± 0.2 percent.

The secondary standards for the units of luminous intensity, illuminance, and luminous flux consist of groups of incandescent lamps of different designs and different color temperatures.

V. E. KARTASHEVSKAIA