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July, 1944 SUBCOMMITTEE G ON EXPOSURE METERS
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grees. Definite test conditions have been set up to insure the conformance of meters to this requirement.
The meter and calculator combination on all makes of exposure meters will be designed to conform with the following law :
B X S
(1)
where /
//stop; 5 = brightness of a uniform brightness surface as described below in candles per sq ft; 5 = Exposure Index or ASA Speed Number; and T = Shutter time in sec.
The specific calibration methods used by the various manufacturers will not be specified; however, a uniform brightness screen for a
VIOLET BLUE GREEN YELLOW OR
RED
3600 4000 4400 4800 5200 5600 6000 6400 6800 7200 WAVE LENGTH OF LIGHT (ANGSTROM UNITS)
FIG. 1. Color sensitivity of various types of sensitive materials and cell compared with the energy in daylight.
transfer standard has been described in order to insure that all types of meters will conform to the above law for calibration. This uniform brightness screen has been described in general as a good quality pot opal glass illuminated approximately uniformly by a light of equivalent color temperature of between 2680 K to 2820 K at level of 100 to 250 candles per sq ft.
The importance of controlling spectral sensitivity of the lightsensitive cell has been recognized. The response of a typical light cell to light covers a range of wavelengths greater than that of the human eye and less than that of panchromatic film, as may be seen by Fig. 1. After comparing the spectral response of light-sensitive cells available today, an envelope curve was agreed upon which will control this characteristic within adequate working limits. The