1. Field of the Invention
The present invention relates to a multi-beam scanning apparatus and an image forming apparatus using the same and, more particularly, to a light scanning optical system suitable for an image forming apparatus such as a laser beam printer, digital copying machine, or multifunctional printer having, e.g., an electrophotographic process in which a light beam emitted from a light source means is reflected and deflected by a polygon mirror serving as a deflection means to optically scan a target scanning surface through a scanning optical means, thereby recording image information. The present invention especially relates to a multi-beam scanning apparatus which simultaneously optically scans a plurality of light beams to increase the speed and resolution to always obtain a satisfactory image with low jitter.
2. Related Background Art
A conventional multi-beam scanning apparatus has a synchronization detecting means to accurately determine the write start position on a target scanning surface, i.e., a photosensitive image carrier. The synchronization detecting means has a photodetection element (BD sensor) for detecting incidence of a light beam (BD light beam) for synchronization detection and a slit (BD slit) for determining the synchronization position at which the light beam becomes incident on the BD sensor.
In this synchronization detecting means, some components (BD light beams) of a plurality of light beams reflected and deflected by a deflection means (optical deflector) optically scan the BD slit and reach the BD sensor when the light beams come near the opening portion of the BD slit. That is, the BD slit serves as a synchronization position determination means for determining the timing of synchronization detection. In some arrangements, an edge of a BD sensor is used as a synchronization position determination means in place of a BD slit.
Japanese Patent Laid-Open Application No. 2000-284194 discloses an arrangement example of a synchronization detecting means in which jitter that occurs when synchronization detecting light components (BD light beams) have a light quantity difference is corrected by tilting the slit of the synchronization detecting means by a predetermined angle.
Japanese Patent Laid-Open Application No. 10-151796 discloses another arrangement example of a synchronization detecting means in which the front edge of the light-receiving portion of a beam detection means is almost perpendicular to the main scanning direction, and the width of the light-receiving portion of the beam detection means is smaller than the gap between a plurality of beams LD1 and LD2 in the main scanning direction.
For these multi-beam scanning apparatuses, it is not mentioned that jitter generation amount changes depending on the smoothness of the synchronization position determination means.
To record high-resolution image information by a multi-beam scanning apparatus of this type, it is important that the plurality of light beams focus across the target scanning surface, and jitter (print position shift in the main scanning direction) is satisfactorily corrected.
However, if the edge of the BD slit or the edge of the BD sensor, which serves as the synchronization position determination means, is uneven because of any structural reason or a manufacturing error, the synchronization detection timing shifts relatively. This poses a problem because the timing shift generates jitter on the target scanning surface.
Note that jitter indicates a print position shift in the main scanning direction due to the difference between scanning line write start positions corresponding to the plurality of BD light beams.
It is an object of the present invention to provide a multi-beam scanning apparatus capable of always obtaining a satisfactory image without any jitter on a target scanning surface due to the smoothness of the edge of a BD slit or the edge of a BD sensor, which serves as a synchronization position determination means, and an image forming apparatus using the multi-beam scanning apparatus.