| 摘要 |
PROBLEM TO BE SOLVED: To stabilize a synchronizing signal of an optical scanning system in a laser printer or the like with respect to variation of a laser power and to dispense with a capacity for holding a threshold value. SOLUTION: Two photo-diodes 1, 2 having different lengths are provided along a main scanning direction of a laser beam 6. Each of the outputs PD1, PD2 of the photodiodes is inputted to respective inverting input terminals of amplifiers 3, 4 to be subjected to current-voltage conversion. Each of feedback resisters Rf,αRf is connected between the output terminal and inverting input terminal of the respective amplifiers 3, 4. 'α' is determined to be in a range of 0<α<1, e.g. 0.5. The output signals Amp1, Amp2 of the amplifiers 3, 4 are compared by a comparator 5 to form a synchronizing signal. A center of the laser beam 6 passes through an intermediate portion between photo-diodes 1, 2.
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