| 摘要 |
PURPOSE:To detect the focal point of light beam over the entire area in the main scanning direction of the light beam and to bring the focal point position onto a scanning surface. CONSTITUTION:A sensor part 15 is installed along the main scanning direction in the end position of a movable table 2 existing on the same plane as the plane of the scanning surface. The sensor part 15 is provided with grating glass 16 having plural pieces of light transparent parts of approximately the same width as the beam diameter in the focal point position of the light beam and a photodiode existing behind this glass. The out-of-focus quantity at the respective points in the main scanning direction is led out in accordance with the detection of the quantity of the light beam over the entire area in the main scanning direction and an ftheta lens 13 is moved in a sub-scanning direction in accordance therewith and is turned around the axial center perpendicular to the sub-scanning direction, by which the light beam is focused. |
