SIGNAL DETECTOR

PURPOSE:To improve the detection rate and the detection throughput by a method wherein S/N of a photosignal is detected and then the beam scanning speed is controlled at optimum value to attain sufficient S/N even if the photosignal is faint. CONSTITUTION:When an output signal from an amplifier 8 is transmitted to an averaging circuit 7 with variable averaging rate initially set up at small value, the output value becomes almost equivalent to input value. Thus signal is divided into a signal component and a noise component by an S/N detector 6 to output a value indicating S/N. An optimum control circuit 5 sets up a higher averaging rate than the preceding rate to the averaging circuit 7 corresponding to the S/N output from the circuit 6 and external judgement criteria and control level data. Simultaneously, the circuit 5 transmits a command to a rotary controller 4 to decelerate the revolution of a motor 3. When the motor revolution is decelerated, the signal amount per unit time from a pattern can be increased so that the signal outputted from the circuit 7 may be provided with a waveform at higher S/N.