| 摘要 |
Exposure control apparatus adapted for use with a computer having an inexpensive, uncalibrated clock includes a pair of covariant, internal reference signal sources, an integrator and a comparator that produces a time interval signal when an electrical parameter of the integrator reaches a threshold level established by the comparator. With the covariant reference signal sources connected to the integrator, the computer calculates an initial clock correction factor that is stored in the computer's memory. A standard light source of known intensity, which is used only once, is then coupled to the integrator through a photoresponsive device and the initial clock correction factor is used by the computer to provide compensation for the computer clock during an exposure operation. In the field, the computer, prior to each exposure operation, connects the covariant reference signal sources to the integrator and calculates an updated clock correction factor. When the apparatus is measuring luminance of unknown intensity, for example ambient light, the updated clock correction factor is used by the computer to control a desired exposure parameter.
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