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26 J. M. WHITTENTON Vol 43, No. l
sired for such a general-purpose instrument was defined as "an instrument whose pointer would deflect at least 0.010 in. for an exposure of f/11 with a shutter speed of 1 sec and a film speed number of 100, when used by the reflected light method." This represented less sensitivity than had been previously supplied by most manufacturers, but this change was felt fully justified, since it would enable them to make more sturdy devices which would be necessary to meet the performance specifications which were to follow.
The over-all performance of the device was next considered. An exposure meter is essentially a sensitive microammeter used in combination with a light-sensitive cell of the barrier-layer type, equipped with a suitable light-restricting mechanism over the cell, and a calculator by which exposure can be determined. In view of this, it was felt that the basic mechanism should meet performance specifications similar to those which were set up recently for small paneltype indicating instruments. The American War Standard for Small Panel Instruments, C39.2-1944, was therefore used as a basis for making up this proposed American War Standard for Exposure Meters.
In this proposed standard, careful consideration has been given to the mechanical construction and electrical performance. From the standpoint of mechanical construction, the case design must be dusttight and moisture-proof, all component parts will be required to be adequately protected against rust and corrosion when subjected to all types of world climatic conditions, moving systems will be balanced within closer limits, and ready means of instrument adjustment will be required. The calculators will be designed to use standardized //stops, shutter times, and ASA exposure index speed numbers.
One of the prime purposes of this proposed standard was to set up requirements which would ultimately result in exposure meters made by various manufacturers indicating more nearly the same exposure. To attain this, rigid limits of angle of acceptance, method of calibration, cell spectral sensitivity, and performance were set up.
The angle of acceptance of light energy striking the light-sensitive cell surface was defined, in order to more definitely control the directional characteristics of the device. Essentially this means that some form of hood, barrier, or grid must be placed in front of the cell in such a manner that not less than 60 per cent of the luminous energy actuating the meter from a screen of uniform brightness shall come from an area included within a cone whose half-angle is 25 de