Abstract:
The present invention is directed to an optical system for a scanning device. The optical system employs a catadioptric lens which both refracts and reflects the light passing through it. In this manner, the majority of the image path portion of the light beam may be folded within the lens. This enables the required optical path length to be achieved while providing a smaller, more compact physical envelope for the imaging assembly. The catadioptric lens achieves focusing of the light beam through the use of mirrored surfaces on the lens. Several refractive surfaces are also provided to correct for various aberrations, such as, for example, spherical aberration.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to optical scanning devices and, more particularly, to a catadioptric lens system for an optical scanning device. 
     BACKGROUND OF THE INVENTION 
     Optical scanning devices are well-known in the art and produce machine-readable data which is representative of the image of an object, e.g., a page of printed text. Optical scanning devices generally employ line-focus systems which image an object by sequentially focusing narrow “scan line” portions of the object onto a linear photosensor array by sweeping a scanning head over the object. 
     In a line-focus system, a light beam from an illuminated line object is imaged by a lens on a linear photosensor array which is positioned remotely from the line object. The linear photosensor array is a single dimension array of photoelements which correspond to small area locations on the line object. These small area locations on the line object are commonly referred to as “picture elements” or “pixels.” In response to light from its corresponding pixel location on the line object, each photosensor pixel element in the linear photosensor array (sometimes referred to simply as a “pixel”) produces a data signal which is representative of the light intensity that it experiences during an immediately preceding interval of time known as a sampling interval. All of the photoelement data signals are received and processed by an appropriate data processing system. 
     In a color optical scanning device, a plurality of spectrally separated imaging beams (typically red, green and blue beams) must be projected onto photosensor arrays. Some color optical scanning devices employ beam splitter devices for spectrally separating an imaging light beam into color component beams. These separate color component beams are projected onto separate linear photosensor arrays. Other optical scanning devices project color component images on a single linear array in a series of separate scanning passes. 
     The construction and operation of color optical scanning devices employing beam splitter assemblies and photosensor arrays are disclosed in the following United States Patents: U.S. Pat. No. 5,410,347 of Steinle et al. for COLOR OPTICAL SCANNER WITH IMAGE REGISTRATION HOLDING ASSEMBLY; U.S. Pat. No. 4,870,268 of Vincent et al. for COLOR COMBINER AND SEPARATOR AND IMPLEMENTATIONS; U.S. Pat. No. 4,926,041 of Boyd for OPTICAL SCANNER (and corresponding EPO patent application no. 90306876.5 filed Jun. 22, 1990); U.S. Pat. No. 5,019,703 of Boyd et al. for OPTICAL SCANNER WITH MIRROR MOUNTED OCCLUDING APERTURE OR FILTER (and corresponding EPO patent application no. 90312893.2 filed Nov. 27, 1990); U.S. Pat. No. 5,032,004 of Steinle for BEAM SPLITTER APPARATUS WITH ADJUSTABLE IMAGE FOCUS AND REGISTRATION (and corresponding EPO patent application no. 91304185.1 filed May 9, 1991); U.S. Pat. No. 5,044,727 of Steinle for BEAM SPLITTER/COMBINER APPARATUS (and corresponding EPO patent application no. 91303860.3 filed Apr. 29, 1991); U.S. Pat. No. 5,040,872 of Steinle for BEAM SPLITTER/COMBINER WITH PATH LENGTH COMPENSATOR (and corresponding EPO patent application no. 90124279.2 filed Dec. 14, 1990 which has been abandoned); and U.S. Pat. No. 5,227,620 of Elder, Jr. et al. for APPARATUS FOR ASSEMBLING COMPONENTS OF COLOR OPTICAL SCANNERS (and corresponding EPO patent application no. 91304403.8 filed May 16, 1991), which are all hereby specifically incorporated by reference for all that is disclosed therein. 
     A hand-held optical scanning device is an optical scanner which is moved across a scanned object, e.g. a page of text, by hand. Rollers may be provided on a hand-held scanning device to guide the device across the object to be scanned and also to provide data to the scanning device microprocessor regarding the speed at which the scanning device is being moved over the scanned object. These rollers may also serve to control the speed at which an operator moves the scanning device across the scanned object. 
     The construction and operation of hand-held optical scanning devices employing such rollers is disclosed in United States patents: U.S. Pat. No. 5,381,020 of Kochis et al. for HAND-HELD OPTICAL SCANNER WITH ONBOARD BATTERY RECHARGING ASSEMBLY and U.S. Pat. No. 5,306,908 of McConica et al. for MANUALLY OPERATED HAND-HELD OPTICAL SCANNER WITH TACTILE SPEED CONTROL ASSEMBLY (and corresponding EPO patent application no. 94301507.3 filed Mar. 2, 1994), and in U.S. patent application Ser. No. 08/601,276 filed Jan. 29, 1996 of Kerschner et al. for HAND-HELD SCANNING DEVICE; U.S. patent application Ser. No. 08/592,904 filed Jan. 29, 1996 of Kerschner et al. for SCANNING DEVICE WITH NON-CONTACT OPTICAL COMPONENTS; U.S. patent application Ser. No. 08/878,110 filed Jun. 18, 1997, of Kerschner et. al. for SCANNING DEVICE WITH FLOATING WINDOW MEMBER; and U.S. patent application Ser. No. 08/878,429 filed Jun. 18, 1997, of Kerschner et al. for ILLUMINATION SYSTEM WITH WHITE LEVEL CALIBRATION FOR HAND-HELD SCANNER which are all hereby specifically incorporated by reference for all that is disclosed therein. 
     In a typical scanning device, a lens is generally provided which separates the light beam into an object path portion and an image path portion. The object path portion generally extends between the object being scanned and the lens while the image path portion generally extends between the lens and the photosensor array. In order to scan conventional size documents, most scanning devices have a length of at least about 8.5 inches. A typical linear photosensor array, however, may have a length of only about 1.21 inches. The imaging assembly of a scanning device, thus must be configured to reduce the scan line image to the size of the photosensor array, e.g., from about 8.5 inches to about 1.21 inches. 
     As is well known, the amount of image reduction caused by an imaging assembly is dictated by the relationship between the length of the object path and the length of the image path. Further, for a lens having a given focal length, the length of the object path and of the image path will be determined by the required image reduction. Accordingly, to achieve a given image reduction using a lens having a given focal length, the overall length of the imaging path must be a certain length. For example, if a lens having a focal length of 0.984 inches is used and an image reduction ratio of 7:1 is desired (as needed, e.g., to reduce a 8.5 inch long scan line to a 1.21 inch long photosensor array), then the length of the object path  50  must be about 7.87 inches and the length of the image path  52  must be about 1.125 inches. Thus, the overall length of the imaging path must be the sum of the object path and the image path lengths, or 8.995 inches. 
     The relationships set forth above dictate the geometry and physical size of the optical assembly of a conventional optical scanning device. Specifically, the necessity to maintain a light path having a particular length serves to limit the minimum size of the optical assembly and reduces the degree of compactness achievable for the imaging assembly and, thus, for the overall optical scanning device. 
     It is noted that it is possible to shorten the light path of an optical scanning device by using a shorter focal length lens. A shorter focal length lens, however, requires a greater field of view than a longer focal length lens. This greater field of view, in turn, worsens the optical aberrations, e.g., spherical aberration, which are inherent in lenses. Accordingly, it is not generally desirable to shorten the light path of an optical scanning device by merely reducing the focal length of the lens. 
     Optical systems for hand-held scanning devices must generally be very compact due to the relatively small size of hand-held scanning devices. Generally, such optical systems include various mirrors, and prisms to fold the light path in order to achieve the necessary optical path length in the smallest physical package feasible. Even with the use of such mirrors and prisms, however, the compactness of optical scanning devices is limited by the optical requirements set forth above. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an optical system for a scanning device. The optical system employs a catadioptric lens which both refracts and reflects the light passing through it. In this manner, the majority of the image path portion of the light beam may be folded within the lens. This enables the required optical path length to be achieved while providing a smaller, more compact physical envelope for the imaging assembly. 
     The catadioptric lens achieves focusing of the light beam through the use of mirrored surfaces on the lens. Several refractive surfaces are also provided to correct for various aberrations, such as, for example, spherical aberration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a conventional optical scanning device imaging assembly. 
     FIG. 2 is a side elevation view of a catadioptric lens assembly for use in an optical scanning device imaging assembly. 
     FIG. 3 is a front elevation view of the catadioptric tens assembly of FIG.  2 . 
     FIG. 4 is a rear elevation view of the catadioptric lens assembly of FIG.  2 . 
     FIG. 5 is a cross-sectional view of the catadioptric lens assembly of FIG. 2 taken along the line  5 — 5  of FIG.  2 . 
     FIG. 6 is a side elevation view of the catadioptric lens assembly of FIG. 2 schematically illustrating the passage of light therethrough. 
     FIG. 7 is a schematic illustration of an optical scanning device imaging assembly including the catadioptric lens assembly of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-7, in general, illustrate an optical assembly  10  for a photoelectric imaging apparatus in which a light path  26  extends between an object  24  which is to be imaged and a photosensor array  20 . The optical assembly  10  includes the photosensor array  20  and a plurality of optical components ( 12 ,  14 ,  100 ) arranged along the light path  26 . The plurality of optical components include a catadioptric lens  100 . 
     FIGS. 1-7 also illustrate, in general a method of imaging a portion  22  of an object  24  which is to be imaged onto a photosensor array  20  with a photoelectric imaging apparatus in which a light path  26  extends between the portion  22  of the object  24  and the photosensor array  20 . The method includes the steps of providing an optical assembly  10  including the photosensor array  20  and a plurality of optical components  12 ,  14 ,  100  arranged along the light path  26 , the optical components  12 ,  14 ,  100  including a lens  100 ; transmitting an imaging light beam  26  from the portion  22  of the object  24  to the photosensor array  20  along the light path  26  via the optical components  12 ,  14 ,  100 ; and reflecting the imaging light beam  26  within the lens  100 . 
     Having thus described the photoelectric imaging apparatus optical assembly in general, the device will now be described in further detail. 
     FIG. 1 schematically illustrates an imaging assembly  10  of the type which might be used, for example, in a conventional hand-held optical scanning device. Imaging assembly  10  may contain first and second mirrors  12  and  14 , respectively, a prism  16 , and an imaging lens  18 . These optical components fold and resize the imaging beam  26  and serve to focus an image of a line portion  22  of a scanned object  24  onto a photosensor array  20  in a manner that is well-known in the art. 
     Referring again to FIG. 1, the object  24  may generally be considered to lie within an object plane x-y wherein the y axis lies within the plane of FIG.  1  and the x axis extends in a perpendicular fashion with respect to the y axis and with respect to the plane of FIG. 1. A third axis z, as indicated in FIG. 1, extends in a direction perpendicular to both the axes x and y and lies within the plane of FIG.  1 . In an example where the object  24  is a standard 8.5 by 11 inch page of text or graphics, the y axis would generally correspond to the 11 inch dimension of the page and the x axis would generally correspond to the 8.5 inch dimension of the page. 
     In a similar manner, the image focused on the photosensor array  20  may also be considered to lie within an image plane x′-y′ wherein the y′ axis lies within the plane of FIG.  1  and the x′ axis extends in a perpendicular fashion with respect to the y′ axis and with respect to the plane of FIG. 1. A third axis z′, as indicated in FIG. 1, extends in a direction perpendicular to both the axes x′ and y′ and lies within the plane of FIG.  1 . 
     Referring again to FIG. 1, it can be seen that, due to the configuration of the mirrors  12  and  14  and the prism  16 , the image plane x′-y′ will not necessarily be parallel to the object plane x-y. It is noted, for example, that the image axis y′ is not parallel to the object axis y. From an optical perspective, however, the image plane x′-y′ does correspond to the object plane x-y. In other words, the axes x, y and z in the object plane directly correspond to the axes x′, y′ and z′, respectively, in the image plane. 
     The configuration of the mirrors  12  and  14 , the prism  16  and the photosensor array  20  cause the imaging light beam  26  to be folded into a plurality of imaging beam portions. Specifically, a first imaging beam portion  30  may extend between the line portion  22  and the first mirror  12  and may have a length, for example, of about 3.2 inches. A second imaging beam portion  32  may extend between the first mirror  12  and the second mirror  14  and may have a length, for example, of about 2.3 inches. A third imaging beam portion  34  may extend between the second mirror  14  and the prism  16  and may have a length, for example, of about 2.14 inches. A fourth imaging beam portion  36  may extend within the prism  16  and may have a length, for example, of about 0.7 inches. A fifth imaging beam portion  38  may extend between the prism  16  and the lens  18  and may have a length, for example, of about 0.12 inches. A sixth imaging beam portion  40  may extend within the lens  18  and may have a length, for example, of about 0.38 inches. A seventh imaging beam portion  42  may extend between the lens  18  and the photosensor array  20  and may have a length, for example, of about 0.78 inches. 
     As is generally true in all image focusing systems, the imaging beam  26  comprises an object path portion  50  and an image path portion  52 . Object path portion  50  generally extends between the object (i.e., the line portion  22 ) and the lens  18  and, thus, includes imaging beam portions  30 ,  32 ,  34 ,  36  and  38 . Image path portion  52  generally extends between the lens  18  and the photosensor array  20  and, thus, is comprised of imaging beam portion  42 . 
     During operation of a typical scanning device, the scanning device optical assembly  10  is moved relative to the object  24  in order to sequentially focus consecutive scan line portions of the object  24  onto the photosensor array  20  and thus acquire data representative of an image of the entire object  24 . In order to have the ability to scan conventional size documents, most scanning devices have a length of at least about 8.5 inches. Accordingly, referring again to FIG. 1, the scan line portion  22  may have a length, measured in the x axis direction, of at least about 8.5 inches. A typical linear photosensor array  20 , however, may have a length (measured in the x′ axis direction) of only about 1.21 inches. The imaging assembly  10 , thus must be configured to reduce the scan line image to the size of the photosensor array, e.g., from at least about 8.5 inches to about 1.21 inches. 
     As is well known, the amount of image reduction achieved by an imaging assembly, such as the imaging assembly  10 , is dictated by the relationship between the length of the object path  50  and the length of the image path  52 . Further, for a lens having a given focal length, the length of the object path  50  and of the image path  52  will be determined by the required image reduction. Accordingly, to achieve a given image reduction using a lens having a given focal length, the overall length of the imaging path  26  must be a fixed length. For example, if a lens having a focal length of 0.984 inches is used and an image reduction ratio of 7:1 is desired (as needed, e.g., to reduce a 8.5 inch long scan line to a 1.21 inch long photosensor array), then the length of the object path  50  must be about 7.87 inches and the length of the image path  52  must be about 1.125 inches. Thus, the overall length of the imaging path  26  must be the sum of the object path and the image path lengths, or about 8.995 inches. 
     The relationships set forth above dictate the geometry and physical size of the optical assembly  10  of a conventional optical scanning device. Specifically, the necessity to maintain the light path  26  having a particular length serves to limit the minimum size of the optical assembly  10  and reduces the degree of compactness achievable for the imaging assembly  10  and, thus, for the overall optical scanning device. 
     FIGS. 2-4 illustrate a catadioptric lens assembly  100  which allows a reduction of the physical envelope occupied by the image path portion  52  while maintaining its optical length as required for a given size reduction and lens focal length. To accomplish this reduction in the physical envelope, the catadioptric lens assembly  100  folds the image path portion  52  within the lens as shown, for example, in FIG. 5 and, thus, enables a more compact imaging assembly  10  and, accordingly, a more compact optical scanning device, as will be explained in further detail herein. 
     Referring to FIG. 2, the lens assembly  100  generally, may have a front surface  110 , a rear surface  120 , a first side surface  130  and a second side surface  140 . As can best be seen in FIGS. 3 and 4, the lens assembly  100  may also include a first end surface  150  and a second end surface  160 . The lens assembly  100  may have a height “a” of about 50.0 mm extending between the first side surface  130  and the second side surface  140 , FIG. 2, and a width “b” of about 12.0 mm extending between the first end surface  150  and the second end surface  160 , FIGS. 3 and 4. 
     Lens assembly  100  has a central longitudinal axis AA which bisects its length “a”, FIGS. 2-4, and is, thus, equidistant from the first and second side surfaces  130 ,  140 . Central longitudinal axis AA also bisects the width “b” of the lens assembly  100 , FIGS. 3 and 4, and is, thus, also equidistant from the first and second end surfaces  150 ,  160 . Referring to FIG. 2, lens assembly  100  has a depth “c” of about 16.3 mm extending between the front and rear surfaces  110 ,  120  measured at the central longitudinal axis AA. 
     Lens assembly  100  includes first and second lens members  170 ,  180 . First lens member  170  includes the lens assembly front surface  110 , previously described, and a first lens member second surface  172 , FIG.  2 . Lens assembly front surface  110  may be a spherical surface having a radius “r 1 ” of about 48.0 mm and a center of curvature “c 1 ” located on the lens assembly central longitudinal axis AA. First lens member second surface  172  may be a spherical surface having a radius “r 2 ” of about 70.6 mm and a center of curvature “c 2 ” located on the lens assembly central longitudinal axis AA at a distance “d” of about 11.3 mm from the center of curvature “c 1 ” of the radius “r 1 ” as shown. 
     Second lens member  180  includes the lens assembly rear surface  120 , previously described, and a second lens member second surface  182 , FIG.  2 . Lens assembly rear surface  120  may be a spherical surface having a radius “r 3 ” of about 44.8 mm and a center of curvature “c 3 ” located on the lens assembly central longitudinal axis AA at a distance “e” of about 39.0 mm from the center of curvature “c 1 ” of the radius r 1  as shown. Second lens member second surface  182  may be a spherical surface identical to the first lens member first surface  172  and, thus, may have a radius “r 2 ” of about 70.6 mm and a center of curvature “c 2 ” located on the lens assembly central longitudinal axis AA at a distance “d” of about 11.3 mm from the center of curvature “c 1 ” of the radius “r 1 ”. 
     First and second lens members  170 ,  180  may be attached to one another as shown in FIG. 2, with the first lens member second surface  172  being located directly adjacent the second lens member second surface  182  and an interface  102  being formed therebetween. First and second lens members  170 ,  180  may be attached in any conventional manner, such as by cementing. 
     First lens member  170  may be constructed of crown glass of the type commercially available from Schott Optical Glass, Inc. of Duryea, Pa. and sold as product specification Type BK  7 . Second lens member  180  may be constructed of flint glass of the type commercially available from Schott Optical Glass, Inc. of Duryea, Pa. and sold as product specification Type F 4. 
     Referring to FIG. 3, the lens assembly front surface  110  is provided with a centrally located strip  112  of reflective material as shown. The strip  112  may have a width “f” of about 4.6 mm and may extend for the entire length “a” of the lens assembly  100 . A pair of substantially transparent strips  114 ,  116  are located immediately adjacent and on either side of the strip  112 . Transparent strips  114 ,  116  may each have a width “g” of about 3.7 mm and extend for the entire length “a” of the lens assembly  100 . Reflective strip  112  may be formed by coating the lens assembly front surface  110 , in the area described above, with a reflective material. The applied reflective material may have a minimum reflectivity of 90 percent at 580 nm, measured from the glass side, i.e., from the inside of the lens assembly  100 . The reflective material may, for example, be a material such as silver. 
     After the reflective material is applied to the strip  112 , as described above, the entire lens assembly front surface  110 , including the strip  112 , may be overcoated with an anti-reflective coating which may be a broad band anti-reflective coating. Alternatively, the anti-reflective coating may be chosen to more specifically reflect the wavelength of light produced by the scanning device light source. In a preferred embodiment, the scanning device light source may provide light having a wavelength of about 580 nm and the anti-reflective coating may be a ¼ wavelength thick (at 580 nm) layer of magnesium fluoride. 
     Referring to FIG. 4, the rear surface  120  of the lens assembly  100  is provided with a first  122  and a second  124  strip of reflective material as shown. First reflective strip  122  may extend from the lens assembly second end surface  160  for a distance “h” of about 4.0 mm and may have a length equal to the entire length “a” of the lens assembly  100 . In a similar fashion, second reflective strip  124  may extend from the lens assembly first end surface  150  for a distance “i” of about 4.0 mm and may have a length equal to the entire length “a” of the lens assembly  100 . A centrally located transparent strip  126  is located immediately adjacent and between the reflective strips  122  and  124 . Transparent strip  126  may have a width “j” of about 4.0 mm and may extend for the entire length “a” of the lens assembly  100 . The reflective strips  122 ,  124  may be formed by coating the lens assembly rear surface  120 , in the areas described above, with a reflective material. The applied reflective material should have a minimum reflectivity of 90 percent at 580 nm, measured from the glass side, i.e., from the inside of the lens assembly  100 . The reflective material may, for example, be a material such as silver. 
     In a similar manner to the lens assembly front surface  110  as described above, the entire lens assembly rear surface  120 , including the strips  122  and  124 , may be overcoated with an anti-reflective coating which may be a broad band anti-reflective coating. Alternatively, the anti-reflective coating may be chosen to more specifically reflect the wavelength of light produced by the scanning device light source. In a preferred embodiment, the scanning device light source may provide light having a wavelength of about 580 nm and the anti-reflective coating may be a ¼ wavelength thick (at 580 nm) layer of magnesium fluoride. 
     FIG. 5 is a cross-sectional view of the lens assembly  100  viewed from the direction of the x′ axis, i.e., in a direction normal to the y′-z′ plane. FIG. 5 schematically illustrates how the lens assembly  100  images light onto a photosensor array  20  when the lens assembly is mounted within the imaging assembly  10  of a scanning device in a manner as generally illustrated in FIG.  7 . Photosensor array  20  may, in a conventional manner, be mounted on a printed circuit board substrate  28  as shown. Referring again to FIG. 5, light entering the lens  100  is schematically illustrated by three separate light beams  190 ,  200 ,  210 . As can be seen, centrally located light beam  200  impinges upon the rear surface of the reflective strip  112  located on the front surface  110  of the lens  100  and is thereby blocked from entering the lens assembly  100 . Light beam  190 , however, passes above, as viewed in FIG. 5, the reflective strip  112  and, thus passes through the transparent strip  116  of the lens assembly front surface  110  and into the interior of the lens assembly  100 . In a similar manner, light beam  210  passes below the reflective strip  112  and, thus passes through the transparent strip  114  of the lens assembly front surface  110  and into the interior of the lens assembly  100 . 
     The operation of the lens assembly  100  will now be described in detail with respect to the light beam  210 . As previously described, light beam  210  enters the lens assembly  100  through the lens assembly front surface transparent strip  114 . After entering the lens assembly  100 , the beam  210  passes through the interface  102  and is thereafter reflected a first time by the reflective strip  124  located on the lens assembly rear surface  120 . The beam  210  then passes through the interface  102  a second time and is thereafter reflected a second time by the reflective strip  112  located on the lens assembly front surface  110 . After this reflection, the beam  210  passes through the interface  102  a third time and subsequently passes through the transparent strip  126  located on the rear surface  120  of the lens assembly  100 , thus exiting the lens assembly. After exiting the lens assembly, the light beam  210  impinges upon the photosensor array  20  in order to form an image of the object thereon in a manner as previously described. 
     Light beam  210  generally consists of an object path portion  50 , as previously described with respect to the conventional lens arrangement of FIG. 1, and an image path portion. In contrast to the conventional lens arrangement of FIG. 1, however, the reflective strips  112 ,  122 ,  124  of the lens assembly  100 , as illustrated in FIG. 5, cause the image path portion of the light beam  210  to be folded into three light path portions  212 ,  214 ,  216  within the lens assembly  100 . Specifically, first light path portion  212  is located between the lens assembly front surface transparent strip  114  and the reflective strip  124 . Second light path portion  214  is located between the reflective strip  124  and the reflective strip  112 . The third light path portion  216  is located between the reflective strip  112  and the lens assembly rear surface transparent strip  126 . After exiting the lens assembly  100  through the transparent strip  126 , the image path portion of the light beam  210  continues through a short light path portion  218  until it impinges upon the photosensor array  20 . 
     The lens assembly  100 , thus, causes the majority of the imaging light beam image path portion to be folded internally within the lens assembly. Accordingly, the lens assembly  100  may be located at a close distance “k” from the photosensor array, FIG. 5, relative to the distance “m” that a conventional lens must be located from the photosensor array, as illustrated in FIG.  1 . The distance “m”, FIG. 1, which is substantially equal to the length of the light path portion  42 , may be about 0.78 inches. In contrast, the distance “k” in FIG. 7 may, for example, only be about 0.12 inches in a typical configuration. The lens assembly  100 , thus, provides for a more compact optical scanning device than is possible with a conventional lens assembly. 
     With reference again to FIG. 5, the image beam  26 , in addition to being reflected, is also refracted within the lens assembly  100 . Specifically, with reference to the light beam  210 , the light beam is refracted a first time as it passes through the front surface transparent strip  114  of the lens assembly  100 . It is refracted a second time as light path portion  212  passes through the interface  102 , a third time as the light path portion  214  passes through the interface  102 , a fourth time as the light path portion  216  passes through the interface  102  and a fifth time as the light beam  210  exits the lens assembly  100  through the rear surface  120 . 
     This refraction aids in the reduction of various well-known optical aberrations such as spherical and chromatic aberration and, thus, enhances the quality of the image impinging upon the photosensor array  20 . The index of refraction of the first and second lens members  170 ,  180 , as previously discussed, as well as the shape of the interface  102  may be chosen specifically to correct for these aberrations in a conventional manner. 
     FIG. 6 illustrates the lens assembly  100  viewed from the direction of the y′ axis, i.e., from a direction normal to the x′-y′ plane. FIG. 6 schematically illustrates the light beam  210 , as previously described with respect to FIG. 5, and two other light beams  230 ,  240  which are spaced from the light beam  210  in the x′ direction. With reference, for example, to the light beam  240 , the beam enters the lens assembly  100  through the lens assembly front surface transparent strip  114 . The beam  240  then passes through the interface  102  and is thereafter reflected a first time by the reflective strip  124  located on the lens assembly rear surface  120 . The beam  240  then passes through the interface  102  a second time and is thereafter reflected a second time by the reflective strip  112  located on the lens assembly front surface  110 . After this reflection, the beam  240  passes through the interface  102  a third time and subsequently passes through the transparent strip  126  located on the rear surface  120  of the lens assembly  100 , thus exiting the lens assembly. After exiting the lens assembly, the light beam  240  impinges upon the photosensor array  20  in order to form an image of the object thereon in a manner as previously described. 
     In a similar manner to the light beam  210  previously described, the light beam  240  is folded into three light path portions  242 ,  244 ,  246  within the lens assembly  100 . Specifically, first light path portion  214  is located between the lens assembly front surface transparent strip  114  and the reflective strip  124 . Second light path portion  244  is located between the reflective strip  124  and the reflective strip  112 . The third light path portion  246  is located between the reflective strip  112  and the lens assembly rear surface transparent strip  126 . After exiting the lens assembly  100  through the transparent strip  126 , the image path portion of the light beam  240  continues through a short light path portion  248  until it impinges upon the photosensor array  20 . 
     In a manner as described previously with respect to the light beam  210 , the light beam  240 , in addition to being reflected, is also refracted within the lens assembly  100 . Specifically, the light beam  240  is refracted a first time as it passes through the front surface transparent strip  114  of the lens assembly  100 . It is refracted a second time as light path portion  242  passes through the interface  102 , a third time as the light path portion  244  passes through the interface  102 , a fourth time as the light path portion  246  passes through the interface  102  and a fifth time as the light beam  240  exits the lens assembly  100  through the rear surface transparent strip  126 . 
     It is noted that, although reflection and refraction of the image beam  26  have been described in detail only with respect to the y′-z′ (FIG. 5) and x′-z′ (FIG. 6) planes, it will be understood that similar reflection and refraction occur in all planes within the lens assembly  100  as will be readily apparent to one skilled in the art. 
     It is further noted that, in FIG. 5, the thickness of the reflective strips  112 ,  122  and  124  has been greatly exaggerated for purposes of illustration. Similarly, in FIG. 6, the thickness of the surfaces  112  and  124  have likewise been greatly exaggerated. The actual thickness of the strips  112 ,  122  and  124  may be only about 0.002 inches. 
     Referring to FIG. 7, it can be seen that the imaging assembly  10  incorporating the catadioptric lens assembly  100  is significantly more compact than the conventional imaging assembly illustrated in FIG.  1 . Specifically, as previously described, the distance “k” between the catadioptric lens  100  and the photosensor array  20  is significantly less than the distance “m” between the conventional lens  18  and the photosensor array  20  in FIG.  1 . Because of this reduced distance, the prism  16 , FIG. 1, may be eliminated in the imaging assembly of FIG.  7 . 
     An aperture stop  300  may be located as shown schematically in FIG. 7 in order to reduce or eliminate off-axis aberration in a conventional manner. Ideally, the aperture stop should be located as close as possible to the center of curvature of the lens surfaces  110  and  120 . Accordingly, referring to FIG. 2, the aperture stop may preferably be located between the centers of curvature “C 1 ” and “C 3 ”. 
     It is noted that, while the lens assembly  100  has been primarily described in conjunction with the optical system of a hand-held scanning device, the lens assembly may be used in any type of scanning device optical system where compactness is desirable. 
     While an illustrative and presently preferred embodiment of the invention has been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.