1. Field of the Invention
The present invention relates to an optical scanning apparatus and, more particularly, to an optical scanning apparatus suitable for an apparatus such as a laser beam printer (LBP) or a laser beam copying machine, in which, when a light beam (also called as a beam) optically modulated on the basis of recording image information and emitted from a light source means is guided onto the scanning surface of a photosensitive drum or the like through a deflecting means (optical deflector) such as a rotary polygon mirror and a focusing means to be optically scanned, each element of a synchronous detecting system for receiving part of the light beam from the focusing means to set a scanning start position on the scanning surface is appropriately set, thereby performing high-precision optical scanning.
2. Related Background Art
In an optical scanning apparatus such as a laser beam printer (LBP), conventionally, image recording is performed by using a method proposed in, e.g., Japanese Patent Publication No. 62-36210. That is, a light beam optically modulated on the basis of an image signal and emitted from a light source means is periodically deflected by an optical deflector comprising, e.g., a polygon mirror. The light beam is focused on a photosensitive recording medium by a focusing optical system having an f-.theta. characteristic to form a spot and optically scanned, thereby performing image recording.
FIGS. 1 and 2 are a perspective view showing the main part of a conventional optical scanning apparatus and a sectional view showing the main part in a sub-scanning direction, respectively.
Referring to FIG. 1, a light beam (divergent beam) emitted from a light source means 51 is collimated by a collimator lens 52 into a parallel beam. The beam (light amount) is regulated by a stop 53 provided near the collimator lens 52 and incident on a cylindrical lens 54 having a predetermined refracting power in only the sub-scanning direction.
On a main scanning surface where the beam is deflected and scanned, part of the parallel beam incident on the cylindrical lens 54 emerges in the parallel beam state. On the sub-scanning surface perpendicular to the main scanning surface, the beam is focused to form a substantially linear image on a reflection surface (polygon surface) 55a of an optical deflector 55 comprising a polygon mirror.
The light beam reflected and deflected on the reflection surface 55a of the optical deflector 55 is guided onto a photosensitive drum surface (scanning surface) 57 through an f-.theta. lens system (focusing optical system) 56 comprising a spherical lens 56a and a toric lens 56b and having an f-.theta. characteristic.
The f-.theta. lens system 56 converts the light beam passing through the optical deflector 55 and deflected and scanned at a constant angular velocity such that the light spot is scanned at a constant speed on the photosensitive drum surface 57.
When the optical deflector 55 is rotated by a motor 61 serving as a driving means in a direction indicated by an arrow 55b, the photosensitive drum surface 57 is optically scanned in a direction indicated by an arrow 57a, thereby performing image information recording.
At this time, the light spot on the photosensitive drum surface 57 is repeatedly optically scanned in the direction indicated by the arrow 57a. However, if a dividing error is present on the reflection surface of the optical deflector 55, the timing of repeatedly optically scanning the light beam to write the image information is shifted.
In the conventional optical scanning apparatus, in order to adjust the timing of the scanning start position on the photosensitive drum surface 57 before optical scanning on the photosensitive drum surface 57, the light beam at the distal end portion (image noneffective portion), which is deflected and scanned by the optical deflector 55, is reflected by a fixed reflecting mirror 58 and guided by a focusing lens 59 to a detecting means 60 for detecting the scanning start position. By using a signal obtained from the detecting means 60, the timing of the scanning start position for image recording onto the photosensitive drum surface 57 is adjusted.
Referring to FIG. 2, the light beam on the sub-scanning section is focused on a deflection point (reflection position) X on the reflection surface 55a of the optical deflector 55 through the cylindrical lens 54 as described above.
The deflection point X and the photosensitive drum surface Y are optically almost conjugate to each other on the sub-scanning section by the f-.theta. lens system 56. Therefore, even if the reflection surface 55a is inclined with respect to a rotating shaft 62 on the sub-scanning section (that is, plane inclination occurs), the light beam is focused on the same scanning line on the photosensitive drum surface Y. An optical system for correcting so-called plane inclination of the reflection surface of the optical deflector 55 is thus constituted.
In order to obtain desired optical performance in the conventional optical scanning apparatus, for example, strict optical precision for the position or a change-with-time of each optical element constituting the optical scanning apparatus shown in FIG. 1 is required.
For this reason, in the conventional optical scanning apparatus, as a means for ensuring high precision, for example, a member called as an optical box which integrates all elements other than the photosensitive drum is attached to the apparatus to meet the above requirement.
The optical box is preferably as small as possible to realize a compact optical scanning apparatus.
In the conventional optical scanning apparatus, the fixed reflecting mirror 58 is provided as part of the synchronous detecting system for detecting the scanning start position on the scanning surface to appropriately bend the optical path, thereby realizing a compact apparatus.
However, as shown in FIG. 1, when the fixed reflecting mirror 58 is disposed near, e.g., the toric lens 56b, the light beam for detecting the scanning start position directed from the reflecting mirror 58 to the focusing lens 59 (referred to as a detecting light beam thereafter) passes near the toric lens 56b. At this time, part of the detecting light beam is reflected on a lens surface (exit surface) 56b1 of the toric lens 56b to generate stray light. This stray light is incident on the photosensitive drum surface 57 to generate so-called ghost or flare light. The stray light is also incident on the detecting means to generate noise to degrade optical scanning precision and the image quality.
If the distance (optical path length) from the toric lens 56b to the reflecting mirror 58 is shortened, the optical path from the reflecting mirror 58 to the focusing lens 59 is elongated, so that the size of the optical box in a direction perpendicular to the optical axis on the main scanning surface is elongated.
This makes it very difficult to guide the optical path. For example, when the synchronous detecting system is constituted by a plurality of reflecting mirrors, the peripheral portion of the optical deflector is excessively complicated, so that a desired compact apparatus can hardly be realized.