Abstract:
An optical system for strip scanning of an object in both directions of relative reciprocal motion between the object and the optical system. The object is first scanned in one direction, then the image orientation is rotated 180° about the axis of propagation for scanning in the reverse direction. Properly oriented images are thus projected onto an image surface and move in the same direction during both directions of scan.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to optical systems and in particular to a strip scanning optical system for scanning an object and progressively projecting a composite image of the same at an image surface, such as a xerographic or other photosensitive surface in a photocopying process. 
     There are three general types of strip scanning arrangements known to the prior art. In one, the optical system is fixed and the object and image surface move relative to the optics. In the second, the object is fixed and the optical system and image surface move at appropriate speeds relative to the object. In the third, the object is fixed and is scanned by a rotating mirror to relay an image of the object onto a moving image surface. 
     Typically, these prior art systems have in common the fact that they are capable of projecting a usable image only in one direction of scan motion. The return motion, whether it be return of the object, the optics, or the mirror, projects an image onto the moving image surface, but the image is moving in a direction opposite to that of the image surface and is therefore useless. Accordingly, the prior art has variously resorted to flyback arrangements to minimize &#34;lost time&#34; involved in the return motion of the scan mechanism. One system by which to accomplish scanning in both directions of object reciprocation is shown in U.S. Pat. No. 3,574,459 to Hartwig and Schnell, in which a single optical axis is rotated 180° between successive scans by a prism. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel form of strip scanning optical system which is capable of projecting a useful image of an object in both directions of reciprocal scan movement, thus greatly decreasing lost time in the operation of a photocopying apparatus. 
     Another object is to provide such a strip scanning optical system which does not require the cyclic reciprocation of the image surface in addition to either the object or the optics. 
     Another object is to provide an optical system with a novel means to selectively effect the 180° rotation of an image about its axis of propagation. 
     Briefly, this invention is practiced in one form by an optical system having alternative optical paths between object and image. 
     For a better understanding of this invention, reference is made to the following detailed description given in connection with the accompanying drawings. 
    
    
     DRAWINGS 
     FIG. 1 is a front elevation view of an optical system according to the present invention. 
     FIG. 2 is a side elevation view from the right side of FIG. 1. 
     FIG. 3 is a plan view taken along the line III--III of FIG. 1. 
    
    
     DESCRIPTION 
     Referring now to FIG. 1, the optical system of this invention is generally indicated at 2 and extends between an object plane or surface 4 and an image plane or surface 6 along an optical axis 8. The optical axis 8 of the system includes parallel branch paths 8a and 8b. 
     A first reflector 10 is located along the optical axis and is mounted for rotation between a first position shown in solid line in FIG. 1 and a second position shown in phantom line. In its first position, reflector 10 deflects optical axis 8 along branch path 8a to a second fixed reflector 12. In its second or alternate position, reflector 10 deflects optical axis 8 along branch path 8b to an alternate second reflector 14. Reflector 12 deflects the branch optical path 8a to a third reflector 16 which is mounted for rotation between alternate positions shown in FIG. 3 as solid and phantom lines respectively. 
     A projection lens 18 is disposed along the optical axis 8 to project an image I of an object O from the object surface 4 to the image surface 6. A fourth reflector 20 deflects the optical axis toward the image surface 6. The fourth reflector may or may not be necessary, depending on whether the optical system of this invention is used in an image transfer type system or in a direct imaging system. An image transfer system requires an even number of reflections (or zero reflections) between object and image and a direct imaging system requires an odd number of reflections. In any case, the particulars of that part of the optical system from the lens to the image surface are not material to this invention. 
     With the first reflector 10 in its first position (solid line) and the third reflector 16 in its first position (solid line) the optical path 8 extends from object surface 4 to first reflector 10, to second reflector 12, to third reflector 16, to the projection lens 18, and on to the image surface 6. Along this path 8, 8a, light from an object O on the object surface 4 propagates to form an Image I at the image surface 6. The image wavefront propagating along this axis of propagation is, after its reflection from the third reflector 16, in a first orientation relative to the axis of propagation 8. 
     With the first reflector 10 in its second position (phantom lines) indicated at 10&#39;, and with the third reflector in its second position (phantom lines) indicated at 16&#39;, the optical path 8 extends from object surface 4 to the first reflector 10, to alternate reflector 14, to third reflector 16, to the projection lens 18, and on to the image surface 6. Along this path 8, 8b, light from an object O on the object surface 4 propagates to form an image I at the image surface 6. The image wavefront propagating along this axis of propagation is, after its reflection from the third reflector 16, in a second orientation relative to the axis of propagation 8. This second image orientation is rotated 180° about the axis of propagation relative to the first image orientation. 
     In a photocopying environment, the image I of object O is projected onto a moving photoreceptor at the image surface 6. This continual uni-directional movement of the photoreceptor is indicated by the arrow 7. In a scanning mode of operation, the object O is reciprocated on the object plane 4 relative to the optical axis 8 along a path of travel indicated by the arrow 5. During this scanning, only a narrow band of the object is exposed to the photoreceptor at any time. This continual and progressive exposure results in a composite image of the scanned object. When the object O is moving in the scan direction 5a, its image is propagated along the optical axis 8 and its branch path 8a  and laid down on the moving photoreceptor, in synchronism with the photoreceptor, and in a first orientation. When the object O is moved in the scan direction 5b, its image is propagated along the optical axis 8 and its branch path 8b and laid down on the moving photoreceptor, still in synchronism with the photoreceptor, in a second orientation, rotated 180° about the axis of propagation from the first orientation. Mirrors 10 and 16 are in their solid line positions during scanning in direction 5a, and in their phantom line positions 10&#39; and 16&#39; respectively for scanning in direction 5b. 
     A drive means represented at 22 is operatively connected to the optical system to effect relative reciprocations between the object O and the optical axis 8. Drive means 22 is also operatively connected to the first reflector 10 and the third reflector 16 to rotate the two reflectors between their alternate positions at one or the other end of reciprocatory movement between object O and optical axis 8. The drive means 22 may take several forms, the details of which are not material to the present invention. The essential thing is that the reflectors 10 and 16 are in synchronism with the reciprocating motion of the object O. Thus, when the object O is moving in scan direction 5a, reflectors 10 and 16 are in position to relay an image of the object along optical axis 8 and its branch path 8a. At the end of the stroke in direction 5a, and before the return stroke in direction 5b, reflectors 10 and 16 are shifted to positions 10&#39; and 16&#39; so as to relay the image along optical axis 8 and its branch path 8b. 
     Reduced to its basics, this invention involves rotating a plane 180° about each of the coordinate axes lying in the plane. The resultant effect of this is to rotate the plane 180° about the third coordinate axis, the axis normal to the plane. In terms of an image projection system as is involved here, the propagating image wavefront is taken as a plane for the purpose of a frame of reference in describing and defining the geometry. The axis of propagation of the image is the axis normal to the wavefront plane and is the axis about which it is desired to rotate the image by 180°. Such concepts in the claims as image wavefront, planar wavefront, and coordinate axes lying in the wavefront, are not to be strictly construed as limited to an actual plane wavefront environment. The planar wavefront image propagation concept is simply a useful aid to understanding the invention. 
     In the present case, the image wavefront propagating along axis 8, 8a is rotated 90° about each of the axes lying in the wavefront plane. Similarly, the image wavefront propagating along axis 8, 8b is rotated 90° about each of the axes lying in the wavefront plane, but in directions opposite to the rotations along axis 8, 8a. The total relative rotational displacement of the image wavefront plane, between scans in directions 5a and 5b, is therefore 180° about each of its axes with a resultant 180° rotation of the image wavefront about its axis of propagation. 
     It will be appreciated that the invention described herein provides an optical system with a novel means to selectively effect the 180° rotation of an image about its axis of propagation. Such a system can find practical use in a scanning photocopying environment since it enables the scanning and projecting of ordered images during both directions of reciprocal scan movement. 
     The description given above has been with reference to a system in which the optics are stationary and the object is reciprocated. While this appears to be preferable, it will be appreciated that the concept of the invention is not limited to such an arrangement. It is, of course, possible to have the object remain stationary and to have the optical system reciprocate relative to it. 
     The foregoing description of an embodiment of this invention is given by way of illustration and not of limitation. The concept and scope of the invention are limited only by the following claims and equivalents thereof which may occur to others skilled in the art.