| 摘要 |
PURPOSE:To prevent destruction of a light emitting body under control and eliminate dispersion of a photo-sensor by putting a feedback loop in open state by a changeover means, adjusting the output of the photo-sensor by an adjusting means, closing the feedback loop, and controlling the light amount from the light emitting body. CONSTITUTION:A feedback loop is opened after initial setting, and an LED 25 is allowed to make light emission with the initially set LED current value, and the gain of an I-V converter 31 is adjusted so that the light reception mount signal of a light amount monitoring photo-transistor 61 at this time becomes the same level as the reference voltage, and thus destruction due to large current at the time of light amount control is prevented even though deterioration has arisen in the LED 25. Also dispersions in the light emitting efficiency of the LED 25 and the photo-electric conversion efficiency of the transistor 61 are absorbed by putting the light reception amount signal at the same level as the reference voltage, and light emission from the LED 25 is sensed by a photo-sensor 60 to make light amount control; so that eventual attachment of dust to an external scale 40, slit 50, etc., allows the scale signal to remain constant. |
