| 摘要 |
An apparatus is provided for detecting a light focus position of a scanning laser beam on a surface to be scanned. A laser beam splitting member splits part of the scanning laser beam, when the beam is scanning an unnecessary scanning area. A plurality of beamlets thus irradiate on a non-scanning direction line. A determination is made of an optical path differences between the split laser beamlets. Further, the spot size of each of the split beamlets is measured, thereby determining a split beamlet irradiating the surface to be scanned based on the spot size of each of the split beamlets.
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