Optical scanner

#The present invention relates to an optical scanner for use with a laser beam printer and other optical image forming apparatus. More specifically, the invention relates to a method of compensating or the deterioration in optical characteristics due to temperature variations.

#Prior Art

#A prior art optical scanner is shown schematically in FIG. 10. A laser beam, or rays of light issuing from a semiconductor laser 211, is collimated to parallel light by means of a collimator lens 221 and shaped to a convergent beam by means of a cylindrical lens 222. The convergent beam is convergent in one of the cross sections which are orthogonal to each other and include the optical axes of collimator lens 221. The shaped beam is deflected by a rotating polygonal mirror 230. The deflected beam is passed through scanning lenses 251 to be focused as a spot on the surface to be scanned 240. The image point is substantially equal to the beam waist of a Gaussian beam. To insure that the beam is focused to form a planar image on the surface 240, the scanning lenses 251 is designed such that it will not produce astigmatism or curvature of the field in greater amounts than specified values. In addition, the scanning lenses 251 is designed to create negative distortion so that the beam deflected from the rotating polygonal mirror 230 at uniform angular velocity will scan the surface 240 at uniform linear velocity.

#FIG. 1 is a perspective view showing the construct of an optical scanner according to the first or third example of the invention;

#FIG. 2 is a main scanning cross section of the optics used in the first and third examples of the invention;

#FIG. 3 is a sub-scanning cross section of the optics used in the first and third examples of the invention;

#Four examples of the invention will now be described with reference to accompanying drawings. FIG. 1 is a perspective view of an optical scanner according to the first example of the first as invention; FIG. 2 shows a main scanning cross section of the optics used in the optical scanner; and FIG. 3 shows a sub-scanning cross section of the optics which includes the optical axis and which is orthogonal to the main scanning cross section. As shown, a laser beam emitted from a semiconductor laser 11 is shaped by passage through a collimator lens 21 to have such a cross section that it provides generally parallel light. The beam thus shaped is passed through a cylindrical lens 22 and focused so as to form a line image on a deflecting surface 32 of a rotating polygonal mirror 31 in a direction parallel to the scanning direction. The beam deflected from the rotating polygonal mirror 31 is made convergent by passage through scanning lenses 51. The deflected convergent beam is reflected by a bending mirror 61 such that the direction of its travel is changed, thereby forming a focused spot of a specified size on the surface to be scanned 81. The scanning lens 51 has an f.theta. characteristic as already described in connection with the prior art.

#Prior to the scanning of the surface 81, the deflected beam is reflected by a synchronous detecting mirror 72 and launched into a synchronous detector 71 to generate the sync signal necessary to perform signal processing for each scan. The components described above are fixed on an optical base 91 which is resin molded in one piece. The scanning lenses 51 are composed of two lens elements 51a and 51b. Lens element 51a is formed of optical glass but lens element 51b is formed of a plastic material since the toric surface S9. (see FIGS. 2 and 3) from which the deflected beam emerges can be fabricated from plastics more easily than glass.

#A specific data sheet for the optics composed of the lens surfaces S1-S9 is given below in Table 1 in terms of the axial distance between lens surfaces (d), paraaxial radius (R) and refractive index (n). Symbol x in parentheses represents the paraaxial radius in a sub-scanning cross section and symbol y in parentheses represents the paraaxial radius in a main scanning cross section.