| 摘要 |
A scan start detecting device for a laser beam printer, comprising a wave length variation correcting unit arranged between a scanning unit including a scan disc hologram and a scan start detecting unit including a detector, the wave length variation correcting unit including a scan start hologram. The scan start hologram, which is a write hologram, serves to positional errors caused by a variation in wavelength, together with a pre-scan hologram arranged in front of the scan disc hologram. The correction of the scan start point is achieved, without using the cylindrical lens requiring a double surface finishing work. For the correction of the scan start point, the write hologram is used, which has the same function and yet inexpensive. Accordingly, the manufacture cost can be reduced. Where the write hologram is added with a read function, the performance thereof can be more improved. It is also possible to reduce the overall optical path of scan start beams. The number of constituting elements is also reduced, thereby enabling a compactness of products. There is also an advantage of a convenience in scan start beam alignment.
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