FOCUSING DETECTOR

PURPOSE:To improve the precision of focusing detector by providing a switch circuit which switches two photoelectric outputs of an optical position detector corresponding to image formation positions of spot light alternately and makes it into an AC signal. CONSTITUTION:Output photocurrents I1 and I2 of the optical position detector which vary in level with the image formation position of reflected spot infrared light are passed through an analog switch 85 whose contacts (a) and (b) are changed over with switching pulse SP and processed by a current voltage converting and amplifying circuit 86a, band-pass filter 20, and logarithmic amplifying circuit 86c. The processed signal is peak-held 88 and 89 through an analog switch 87 which is synchronized with the switch 85 and their held values are amplified 83 differentially to obtain a distance signal. The filter 20 passes only a signal of the same frequency with pulses SP and continuous impulsive infrared- light emission is employed for distance measurement.