The present invention relates to a light scanning device.
Light scanning devices are known as a device for writing and reading information by scanning a light beam. In one type of such a light scanning device, a light beam emitted from a light source is focused as a linear image, and a rotating polygon mirror has a reflecting surface positioned near the linearly focused image for deflecting the light beam at a constant angular velocity. The deflected light beam is focused as a beam spot on a surface by a focusing lens system for scanning the surface.
FIG. 2 of the accompanying drawings illustrates a conventional light scanning device of the type described. A light beam emitted from a light source 1 is focused as a linear image near a reflecting surface 4 of a rotating polygon mirror 3 by a first focusing optical system 2. The light beam reflected by the rotating polygon mirror 3 is deflected at a constant angular velocity upon rotation of the rotating polygon mirror 3. The deflected light beam is then focused as a beam spot on a surface 7 by a second focusing optical system comprising lenses 5, 6 for scanning the surface 7.
The light scanning device employing a rotating multi-faceted polygon, however, suffers from the problem of a facet error. That is, the mirror facets of the polygon may not lie parallel to the axis of rotation of the polygon mirror. One known method of solving this problem is to use an anamorphic optical system as the second focusing optical system disposed between the rotating polygon and the surface to be scanned, and to position the reflecting position on the rotating polygon and the scanning surface in conjugate relationship with respect to an auxiliary scanning direction (vertical direction in FIG. 3). In FIG. 3, the second focusing optical system couples the reflecting position on the rotating polygon 3 and the scanned surface 7 in substantilly conjugate relationship as viewed in the auxiliary scanning direction. Therefore, even if a mirror facet 4 of the rotating polygon suffers from a deviant orientation as represented by 4', the focused position on the scanned surface 7 is not virtually moved in the auxiliary scanning direction by the second focusing optical system. The facet error is corrected in this manner.
When the polygon mirror 3 rotates, the reflecting surface or facet 4 rotates about an axis 3A, and there is developed an optical path length change (sag) between the first focusing optical system 2 and the reflecting surface 4. Therefore, a positional deviation .DELTA.X is produced between the position P of the focused linear image and the reflecting surface 4, and hence the position P' of a conjugate image of the linear image generated by the second focusing optical system or f.theta. lens system is deviated from the scanned surface 7 by .DELTA.'X.
The amount of deviation .DELTA.'X is given as .DELTA.'X=.beta..sup.2 .multidot..DELTA.X where .beta. is the lateral magnification of the lens system, as is well known.
Where the angle formed in the light deflecting plane between the optical axis of the lenses and the principal ray of the deflected light beam is expressed by .theta., the relationship between .theta. and .DELTA.X is shown in FIGS. 5 and 6.
The curves in FIG. 5 are plotted with an angle .alpha. (which is the angle between the principal ray of the light beam applied to the rotating polygon and the optical axis of the second focusing optical system) being 90.degree. and the radius R of a circle inscribed in the rotating polygon 3 being used as a parameter. In FIG. 6, the curves are plotted with the radius R of the inscribed circle being 40 mm and the angle .alpha. being used as a parameter.
As can be seen from FIGS. 5 and 6, .DELTA.X is greater as the radius R of the inscribed circle is greater and the angle .alpha. is smaller.
The relative positional deviation between the linear image and the reflecting surface upon rotation of the reflecting surface is developed two-dimensionally in the light deflecting plane and is asymmetrically moved with respect to the lens optical axis.
Therefore, with the light scanning device as shown in FIG. 2, it is necessary that the curvature of field in each of the main and auxiliary scanning directions of the second focusing optical system be well corrected. As described above, the positional deviation .DELTA.X is produced by the sag. Since the configuration of a rotating polygon, or optimum conditions thereof, i.e., the number of reflecting surfaces or facets and the position of the axis of rotation thereof, are determined by the radius of a light beam applied and the angle of incidence to the second focusing optical system, the sag is also determined as one of the characteristics of the rotating polygon. Japanese Laid-Open Patent Publication No. 59-147316 discloses a known light scanning device of the kind described above. However, the problem of curvature of field developed by sag has not sufficiently been studied in the disclosed light scanning device.