| 摘要 |
PROBLEM TO BE SOLVED: To simply and surely inspect the scanning characteristic in the subscanning direction of the laser beam of a scanning optical unit. SOLUTION: A first optical grating 61 whose lattice direction forms a first angle with reference to the scanning direction of a laser beam 5 is provided. A second optical grating 62 whose lattice direction forms a second angle with reference to the scanning direction is provided. Then, an optical grating member 6 in which the arrangement width of the first and second optical gratings 61, 62 is small as compared with the scanning region of the laser beam is arranged in a position which is equivalent to a face to be scanned. The optical grating member is scanned by the laser beam of a scanning optical unit to be inspected. A change in the intensity of a laser beam which is intensity-modulated by the first and second optical gratings 61, 62 is converted into an electric signal. The passage time T1 which elapses until the laser beam 5 passes the prescribed grating position of the second optical grating 62 after the laser beam passes the first prescribed grating position of the first optical grating 61 is measured on the basis of the electric signal. On the basis of the difference between a predetermined reference time T2 and the passage time T1, the deviation with reference to the reference scanning position in the subscanning direction of the laser beam is detected.
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