| 摘要 |
Disclosed is a scanner system which includes a light source and a photodetector. Circuitry is included for automatically adjusting the intensity of the light source so as to prevent saturation of the photodetector. Whenever the current through the photodetector exceeds a predetermined amount, a transistor switch is opened to connect a resistor in series with the light source and cause the intensity of the light source to decrease. When the output of the photodetector reduces due to decreased intensity of the light source, the transistor switch will close and thus cause the resistor to be bypassed and the intensity of the light source to return to its original level. The scanner is utilized to detect reference marks which are located on a contrasting background. Additional circuitry included to adjust the sensitivity of the system so as to be able to detect marks of varying contrast with respect to the background. A mark is determined to be present by detecting a predetermined amount of change in the output of the photodetector over a certain time period, which represents the change in reflectivity between a mark and the background. If the change in the photodetector exceeds the second predetermined level, a portion of the photodetector output is automatically shunted to ground, with a result that a greater change in the output in the photodetector is required to trigger the detection portion of the circuit. The portion of the output of the photodetector which is shunted is passed through a variable resistance. The resistance is determined by connecting one or more of a plurality of resistors into the shunt path by means of a multiplexer, the addressing of which is controlled by a counter.
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