| 摘要 |
PURPOSE:To simplify a circuit constitution and to improve the light measurement accuracy by judging whether or not light is back light from the result of the comparison of a radio showing the brightness of the peripheral part of a subject and a ratio indicating the brightness of the center part. CONSTITUTION:A 1st photoconductive element 1 senses the brightness of the peripheral part of the subject and a 2nd photoconductive element 2 senses the brightness of the subject center part. Switching circuits 8-10 connect the photoconductive elements 1 and 2 and a reference resistance element 3 to a time constant circuit 4 selectively as time constant determining elements and a gate circuit 11 constitutes a pulse generating circuit. An arithmetic circuit 13 compares the ratio of the counted value of a counting circuit 12A at the time of the connection of the reference resistance element 3 and the counted value of a counting circuit 12B at the time of the connection of the photoconductive element 1 with the ratio of the counted value at the time of the connection of the reference resistance element 3 and the counted value of a counting circuit 12C at the time of the connection of the photoconductive element 2 to detect a back light state. Consequently, the influence of time variations in the constants of circuit elements and source voltage and temperature variation is eliminated and back light detection accuracy is improved. |
