| 摘要 |
PURPOSE:To always obtain a constant detection output from a photodetector by controlling a driving current for a light emitting diode in terms of negative feedback so that a difference between the varied detection output from the photodetector and a reference value may be restored to zero when the detection output from the photodetector is varied because of temperature change or the lowering of the reflectance of a recording medium. CONSTITUTION:The device is provided with the light emitting diode LED1, the photodetector PD1, a mean value detection part 10, a light emission output setting part 11, a differential value amplification part 12 and a current output part 13. The difference signal between the detection signal from the photodetector PD1 and an output value from an output means is obtained and the driving current for the light emitting diode LED1 is controlled in terms of the negative feedback so that the difference signal may be restored to be at the zero level. Thus, the constant light power is always detected in the photodetector PD1 regardless of the temperature change or the lowering of the reflectance of the recording medium. |
