Abstract:
An optical scanning apparatus for optically scanning at least one scanning target surface, the optical scanning apparatus including: a light source; a light-flux dividing unit disposed on a main optical path of a main light flux emitted from the light source, and the light-flux dividing unit configured to spatially divide the main light flux; an optical deflector disposed on a divided optical path of the divided light flux, and the optical deflector configured to deflect the divided optical path; an optical path opening/closing switch unit disposed on the divided optical path between the light-flux dividing unit and the optical deflector, and the optical path opening/closing switch unit configured to interrupt or pass at least one of the divided optical path; and a controller configured to control operation of interrupting or passing the at least one of the divided optical path by the optical path opening/closing switch unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2012-257389 filed in Japan on Nov. 26, 2012 and Japanese Patent Application No. 2013-035063 filed in Japan on Feb. 25, 2013. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical scanning apparatus, a method for performing the same and an image forming apparatus. 
     The image forming apparatus according to the present invention performs image formation by an electrophotographic process. 
     That is, the image forming apparatus according to the present invention can be carried out as an optical printer such as a laser printer, an optical plotter, a digital electronic copier, a plain paper facsimile, and the like. 
     2. Description of the Related Art 
     Recently, for an image forming apparatus such as a laser printer, a digital electronic copier, and a plain paper facsimile, colorization of formed images, an increase in the speed, and downsizing of the apparatus have been demanded. 
     In response to such demands, various image forming apparatuses that use a plurality of photoconductive photoreceptors have been proposed and realized. 
     As such an image forming apparatus, an apparatus that “shares a light source for optical scanning” with respect to a plurality of photoreceptors has been proposed (Japanese Patent Application Laid-open No. 2012-145667). 
     As a light source for optical scanning in an image forming apparatus, a “semiconductor light-emitting element” such as a semiconductor laser or a surface-emitting semiconductor laser (so-called “vertical-cavity surface-emitting later (VCSEL)”) is generally used. 
     High-speed drive of these “semiconductor light-emitting elements” has been realized and a signal for modulating exposure energy based on image information becomes a “modulation signal of a higher frequency” from several to several tens of megahertz. 
     In a semiconductor light-emitting element used for such a “modulation signal of an extremely high frequency”, “offset light emission” is always performed for light-emission rise characteristics and stability of light-emitting power. 
     The offset light emission means to emit light with constant weak light emission intensity during optical scanning, even during a time when light emission does not contribute to image write based on image information. 
     When a semiconductor light-emitting element as a light source is shared by a plurality of photoreceptors and the “offset light emission” is performed by using the shared semiconductor light-emitting element, there are following problems. 
     For specific explanation, there is assumed a case where “one semiconductor light-emitting element is shared by two photoreceptors”, and an image A is formed on one of the photoreceptors and an image B is formed on the other. 
     The images A and B are, for example, a cyan image and a black image used for forming a color image. 
     In this case, when both the images A and B are formed, light emission from the semiconductor light-emitting element is continuously performed, and modulation is performed alternately by a modulation signal for writing the image A and a modulation signal for writing the image B. 
     When only the image A is formed, the photoreceptor for the image A is optically scanned by “modulated light”; however, the photoreceptor for the image B is optically scanned by “offset light emission”. 
     In this case, when the photoreceptor for the image B is stopped to save energy for driving the photoreceptor, the same spot of the photoreceptor is “optically scanned repeatedly by the offset light emission”. 
     Therefore, light-induced fatigue occurs in a “portion which is optically scanned repeatedly” of the photoreceptor, and the photosensitive property thereof tends to be deteriorated. Such deterioration of the photosensitive property occurs “in a line shape”. 
     When the image B is formed by using the “photoreceptor in which the photosensitive property is deteriorated in the line shape”, an abnormal image is formed such that streaky density unevenness appears due to the deterioration of the photosensitive property in the line shape. 
     Therefore, there is a need to provide an optical scanning apparatus that favorably prevents the abnormal image described above. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the invention, an optical scanning apparatus for optically scanning at least one scanning target surface is provided. The optical scanning apparatus includes: a light source; a light-flux dividing unit disposed on a main optical path of a main light flux emitted from the light source, and the light-flux dividing unit configured to spatially divide the main light flux; an optical deflector disposed on a divided optical path of the divided light flux, and the optical deflector configured to deflect the divided optical path; an optical path opening/closing switch unit disposed on the divided optical path between the light-flux dividing unit and the optical deflector, and the optical path opening/closing switch unit configured to interrupt or pass at least one of the divided optical path; and a controller configured to control operation of interrupting or passing the at least one of the divided optical path by the optical path opening/closing switch unit. 
     According to another aspect of the invention, an image forming apparatus is provided. The image forming apparatus includes: at least one photoreceptor; an optical scanning apparatus set forth in claim  1  for writing an electrostatic image onto the at least one photoreceptor; and a transferring unit configured to superimpose different color of toner images and transferring the superimposed image onto a common sheet-like recording medium, and fixing the transferred image thereon. 
     According to further aspect of the invention, a method for performing an optical scanning apparatus for optically scanning at least one scanning target surface is provided. The optical scanning apparatus includes: a light source; a light-flux dividing unit disposed on a main optical path of a main light flux emitted from the light source, and the light-flux dividing unit configured to spatially divide the main light flux; an optical deflector disposed on a divided optical path of the divided light flux, and the optical deflector configured to deflect the divided optical path; an optical path opening/closing switch unit disposed on the divided optical path between the light-flux dividing unit and the optical deflector, and the optical path opening/closing switch unit configured to interrupt or pass at least one of the divided optical path; and a controller configured to control operation of interrupting or passing the at least one of the divided optical path by the optical path opening/closing switch unit. The method includes: emitting the main light flux from the light source; dividing spatially the main light flux; interrupting the at least one of the divided optical path; and deflecting the divided optical path passing the optical path opening/closing switch unit. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram of an image forming apparatus according to one embodiment of the present invention; 
         FIG. 2  is an explanatory diagram of an optical scanning apparatus; 
         FIG. 3  is another explanatory diagram of the optical scanning apparatus; 
         FIG. 4  is still another explanatory diagram of the optical scanning apparatus; 
         FIG. 5  is an explanatory diagram of division of a light flux; 
         FIG. 6  is an explanatory diagram of optically scanning two scanning target surfaces with light fluxes from one light source; 
         FIGS. 7A and 7B  are explanatory diagrams of an example of optical path opening/closing by an optical-path opening/closing unit; 
         FIGS. 8A and 8B  are explanatory diagrams of another example of the optical path opening/closing by the optical-path opening/closing unit; 
         FIGS. 9A and 9B  are explanatory diagrams of another example of the optical path opening/closing by the optical-path opening/closing unit; 
         FIG. 10  is an explanatory diagram of still another example of the optical path opening/closing by the optical-path opening/closing unit; 
         FIGS. 11A and 11B  are explanatory diagram of still another example of the optical path opening/closing by the optical-path opening/closing unit; and 
         FIGS. 12A and 12B  are explanatory diagram of a VCSEL as an example of a semiconductor light-emitting element. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are explained below. 
       FIG. 1  is an explanatory diagram of an image forming apparatus according to one embodiment of the present invention. 
     The image forming apparatus is a “tandem color printer”. 
     A color printer denoted by reference sign  2000  is a multi-color printer that forms a full color image by superimposing four color images (black, cyan, magenta, and yellow). 
     The color printer  2000  includes two optical scanning apparatuses  2010   a  and  2010   b.    
     The color printer  2000  also includes photosensitive drums  2030   a ,  2030   b ,  2030   c , and  2030   d  as four photoreceptors. 
     A cleaning unit  2031   a , a charging device  2032   a , a developing roller  2033   a , and a toner cartridge  2034   a  are arranged around the photosensitive drum  2030   a.    
     The photosensitive drum  2030   a , the cleaning unit  2031   a , the charging device  2032   a , the developing roller  2033   a , and the toner cartridge  2034   a  form a “station K”. 
     The “station K” is an image forming station that forms a black image. 
     A cleaning unit  2031   b , a charging device  2032   b , a developing roller  2033   b , and a toner cartridge  2034   b  are arranged around the photosensitive drum  2030   b.    
     The photosensitive drum  2030   b , the cleaning unit  2031   b , the charging device  2032   b , the developing roller  2033   b , and the toner cartridge  2034   b  form a “station C”. 
     The “station C” is an image forming station that forms a cyan image. 
     A cleaning unit  2031   c , a charging device  2032   c , a developing roller  2033   c , and a toner cartridge  2034   c  are arranged around the photosensitive drum  2030   c.    
     The photosensitive drum  2030   c , the cleaning unit  2031   c , the charging device  2032   c , the developing roller  2033   c , and the toner cartridge  2034   c  form a “station M”. 
     The “station M” is an image forming station that forms a magenta image. 
     A cleaning unit  2031   d , a charging device  2032   d , a developing roller  2033   d , and a toner cartridge  2034   d  are arranged around the photosensitive drum  2030   d.    
     The photosensitive drum  2030   d , the cleaning unit  2031   d , the charging device  2032   d , the developing roller  2033   d , and the toner cartridge  2034   d  form a “station Y”. 
     The “station Y” is an image forming station that forms a yellow image. 
     A transfer belt  2040 , a transfer roller pair  2042 , and a fixing device  2050  are arranged below these stations K to Y. 
     A paper feed tray  2060 , a paper feed roller  2054 , and a timing roller pair  2056  are provided below these stations K to Y. 
     A paper delivery roller pair  2058  and a paper delivery tray  2070  are arranged in an upper part of the image forming apparatus body. 
     A communication control device  2080 , a printer control device  2090  that executes the overall control of the respective units, and the like are arranged in the upper part of the image forming apparatus body. 
     The communication control device  2080  controls bidirectional communication with a “higher-level device (for example, a computer)” via a network. 
     The printer control device  2090  includes a CPU, a ROM, a RAM, an AD converter circuit, and the like. 
     A program described in a code readable by the CPU, and various data to be used at the time of executing the program are stored in the ROM. 
     The RAM is a work memory, and the AD converter circuit converts analog data to digital data. 
     The printer control device  2090  transmits image information from the “higher-level device” to the optical scanning apparatuses  2010   a  and  2010   b.    
     The printer control device  2090  also executes various controls of optical scanning performed by the optical scanning apparatuses  2010   a  and  2010   b.    
     Circumferential surfaces of the respective photosensitive drums  2030   a  to  2030   d  are respectively formed as a photoconductive photosensitive layer. 
     Surfaces of the photosensitive layer of the photosensitive drums  2030   a  to  2030   d  are “scanning target surfaces” subjected to optical scanning. 
     When the full color image is formed, the photosensitive drums  2030   a  to  2030   d  are respectively rotated clockwise by a drive unit (not shown). 
     The respective photosensitive drums  2030   a  to  2030   d  are uniformly charged by the corresponding charging device  2032   a  to  2032   d.    
     In this exemplary embodiment, a corona discharge charging device is exemplified. However, the charging device is not limited thereto, and a contact/non-contact charging device such as a charging roller can be used. 
     Optical scanning is performed to the respective uniformly charged photosensitive drums by the optical scanning apparatus. 
     That is, the photosensitive drums  2030   a  and  2030   b  are optically scanned by the optical scanning apparatus  2010   a , and the photosensitive drums  2030   c  and  2030   d  are optically scanned by the optical scanning apparatus  2010   b.    
     Optical scanning is performed “between the charging device and the developing roller”. 
     The optical scanning apparatus  2010   a  performs optical scanning based on the respective pieces of image information of black and cyan supplied from the higher-level device via the printer control device  2090 . 
     The photosensitive drums  2030   a  and  2030   b  are optically scanned by the optical scanning. 
     A “K latent image” corresponding to the black image information is formed on the photosensitive drum  2030   a  by the optical scanning. 
     A “C latent image” corresponding to the cyan image information is formed on the photosensitive drum  2030   b.    
     Similarly, the optical scanning apparatus  2010   b  performs optical scanning based on magenta image information and yellow image information supplied from the higher-level device via the printer control device  2090 . 
     The photosensitive drums  2030   c  and  2030   d  are optically scanned by the optical scanning. 
     With the optical scanning, an “M latent image” corresponding to the magenta image information is formed on the photosensitive drum  2030   c , and a “Y latent image” corresponding to the yellow image information is formed on the photosensitive drum  2030   d.    
     The K to Y latent images formed on the corresponding photosensitive drums  2030   a  to  2030   d  are developed by the corresponding developing roller  2033   a  to  2033   d , respectively. 
     That is, the toner cartridge  2034   a  supplies black toner stored therein to the developing roller  2033   a.    
     The developing roller  2033   a  visualizes the K latent image formed on the photosensitive drum  2030   a  by the supplied black toner. 
     The toner cartridge  2034   b  supplies cyan toner stored therein to the developing roller  2033   b.    
     The developing roller  2033   b  visualizes the C latent image formed on the photosensitive drum  2030   b  by the supplied cyan toner. 
     The toner cartridge  2034   c  supplies magenta toner stored therein to the developing roller  2033   c.    
     The developing roller  2033   c  visualizes the M latent image formed on the photosensitive drum  2030   c  by the supplied magenta toner. 
     The toner cartridge  2034   d  supplies yellow toner stored therein to the developing roller  2033   d.    
     The developing roller  2033   d  visualizes the Y latent image formed on the photosensitive drum  2030   d  by the supplied yellow toner. 
     In this way, a black image, a cyan image, a magenta image, and a yellow image are formed on the photosensitive drums  2030   a  to  2030   d , respectively. 
     That is, different toner images are formed by an electrophotographic process on the photosensitive drums  2030   a  to  2030   d , which are a plurality of photoconductive photoreceptors, respectively. 
     The color images of black, cyan, magenta, and yellow formed as described above are sequentially transferred onto the transfer belt  2040  at a predetermined timing. 
     Transfer of the respective color images onto the transfer belt  2040  can be performed by a known appropriate transfer unit, and the transfer unit is not shown in FIG.  1 . 
     The respective color images to be transferred are superimposed on each other on the transfer belt  2040  to form a “color image”. 
     Transfer of the respective color images from the respective photosensitive drums onto the transfer belt  2040  is referred to as “primary transfer”. The color image is transferred to and fixed on a recording sheet, which is a sheet-like recording medium. 
     That is, a recording sheet S onto which the color image is transferred and fixed is stacked and stored in the paper feed tray  2060 , and is delivered and fed one by one by the paper feed roller  2054 . 
     A front end of the fed recording sheet S is nipped between the timing roller pair  2056 . 
     The timing roller pair  2056  delivers the nipped recording sheet S toward a “secondary transfer portion”, which is a portion at which the transfer belt  2040  and the transfer roller pair  2042  face each other, at a predetermined timing. 
     The color image on the transfer belt  2040  is secondarily transferred to the recording sheet S, when the recording sheet S passes through the secondary transfer portion. 
     The color image transferred to the recording sheet S is fixed thereon by the effects of heat and pressure by the fixing device  2050 , and the recording sheet S is delivered onto the paper delivery tray  2070  by the paper delivery roller pair  2058 . 
     The respective cleaning units  2031   a  to  2031   d  remove “transfer residual toner” remaining on the surfaces of the corresponding photosensitive drums  2030   a  to  2030   d.    
     The surfaces of the respective photosensitive drums, from which the transfer residual toner has been removed, return to a position facing the corresponding charging device again. 
     The optical scanning apparatuses  2010   a  and  2010   b  are explained next. Because the optical scanning apparatuses  2010   a  and  2010   b  have the same configuration, the optical scanning apparatus  2010   a  is explained below as an example. 
     An example of the optical scanning apparatus  2010   a  is explained with reference to  FIGS. 2 to 4 . 
     In  FIGS. 2 to 4 , reference sign  2200 A denotes a “single semiconductor light-emitting element” as a light source, reference signs QvA, Qa, and Qb denote a “quarter-wave plate” respectively, and reference sign  2201 A denotes a “coupling lens”. 
     Reference sign  2202 A denotes an “aperture plate”, reference sign  2203 A denotes a “light-flux dividing member”, which is a light-flux dividing unit, and reference signs  2204   a  and  2204   b  denote a “cylindrical lens” respectively. 
     Reference sign  2104 A denotes a “polygon mirror”. 
     Reference signs  2105   a  and  2105   b  denote “first scanning lenses”, and reference signs  2107   a  and  2107   b  denote “second scanning lenses”. 
     Reference signs  2106   a ,  2106   b ,  2108   a , and  2108   b  respectively denote an “optical-path bending mirror”. 
     These are arranged in an “optical housing” (not shown) in a predetermined position relation with each other. In  FIG. 2  and thereafter, a Z direction is a sub-scanning direction, and a Y direction is a main scanning direction. 
     The “main scanning direction” is a direction in which the optical scanning apparatuses  2010   a  and  2010   b  optically scan the corresponding photosensitive drums (scanning target surfaces). 
     Furthermore, the “sub-scanning direction” is a direction orthogonal to the main scanning direction on the scanning target surface. 
     In the following descriptions, directions corresponding to the main scanning direction and the sub-scanning direction are referred to as “main scanning direction” and “sub-scanning direction”, respectively, even on an optical axis and an optical path of the light flux extending from the semiconductor light-emitting element  2200 A to each of the scanning target surfaces. 
     In  FIG. 2 , the semiconductor light-emitting element  2200 A is a “semiconductor laser”, and emits a linearly polarized light flux (a single light flux) having a predetermined wavelength (in this example, a 780-nm band). 
     The emitted light flux enters into the quarter-wave plate QvA, and is provided with an “optical phase difference of a quarter-wavelength” and converted to circularly polarized light. 
     The quarter-wave plate QvA is inclined with respect to a surface orthogonal to a traveling direction of the light flux, and a light flux reflected by the quarter-wave plate QvA decreases an “amount returning to the semiconductor light-emitting element  2200 A”. 
     The light flux having passed through the quarter-wave plate QvA is changed to a substantially parallel light flux by the coupling lens  2201 A, and is so-called beam-shaped by an opening of the aperture plate  2202 A. 
     The light-flux dividing member  2203 A divides the beam-shaped light flux into two light fluxes. 
     Division of the light flux is explained with reference to  FIG. 5 . 
     In  FIG. 5 , reference sign L 0  denotes a light flux entering from the aperture plate  2202 A into the light-flux dividing member  2203 A. The light flux L 0  is “circularly polarized light”. 
     As shown in  FIG. 5 , the light-flux dividing member  2203 A is formed by combining a triangular prism P 1  having a sectional shape of right-angled triangle and a square prism P 2  having a sectional shape of parallelogram. 
     A bonded surface of these prisms P 1  and P 2  forms a “polarization separation surface”, which transmits a light flux L 1  having a P polarization component of the light flux L 0  incident thereto, and reflects a light flux L 2  having an S polarization component. 
     The light flux L 1  transmitted through the polarization separation surface “is emitted from the light-flux dividing member  2203 A, with a direction of the incident light flux L 0  being maintained”. 
     The light flux L 2  reflected by the polarization separation surface is reflected by a “reflecting mirror surface” on an upper surface of the square prism P 2 , “is separated in parallel” from the light flux L 1 , and is emitted from the light-flux dividing member  2203 A. 
     That is, the light flux L 0  from the semiconductor light-emitting element  2200 A is divided into two light fluxes L 1  and L 2  parallel to each other in the sub-scanning direction by the light-flux dividing member  2203 A. 
     In other words, the light flux emitted from the semiconductor light-emitting element  2200 A as the light source is spatially divided into plural (two) by the light-flux dividing member  2203 A, which is the light-flux dividing unit. 
     That is, the division number of the light flux by the light-flux dividing member  2203 A is two. 
     Furthermore, the light flux is divided by the light-flux dividing member  2203 A by using the polarization property. 
     As shown in  FIG. 3 , a first light flux (the light flux L 1 ), which is one of the two light fluxes emitted from the light-flux dividing member  2203 A, enters into the quarter-wave plate Qa and is converted to the circularly polarized light. 
     Similarly, a second light flux (the light flux L 2 ), which is the other of the two light fluxes emitted from the light-flux dividing member  2203 A, enters into the quarter-wave plate Qb and is converted to the circularly polarized light. 
     The light fluxes converted to the circularly polarized light in this manner enter into the cylindrical lens  2204   a ,  2204   b , respectively and are focused in the sub-scanning direction (the Z direction in  FIG. 3 ). 
     In the polygon mirror  2104 A as an “optical deflector”, a four-fold mirror having four deflective reflection surfaces is “arranged in two stages in the sub-scanning direction”. 
     As shown in  FIGS. 2 to 4 , in the four-fold mirror (first polygonal mirror) on the first stage (an upper stage), the light flux (the light flux L 2 ) from the cylindrical lens  2204   b  enters into the deflective reflection surface and is deflected. 
     In the four-fold mirror (second polygonal mirror) on the second stage (a lower stage), the light flux (the light flux L 1 ) from the cylindrical lens  2204   a  is deflected. 
     The respective light fluxes L 1  and L 2  are imaged as a “line image long in the main scanning direction” near the deflective reflection surface of the four-fold mirror, to which the light fluxes enter, by the operations of the cylindrical lenses  2204   a  and  2204   b.    
     In the “two-stage four-fold mirrors” forming the polygon mirror  2104 A, normal lines to the deflective reflection surfaces form 45 degrees with each other, and deflection for optical scanning is performed alternately on the first stage and the second stage. 
     In other words, the four-fold mirrors on the first stage and the second stage respectively “rotate with a phase being shifted by 45 degrees”. 
     The two first scanning lenses  2105   a  and  2105   b  shown in  FIGS. 2 and 4  respectively have an “fθ function”. 
     That is, the first scanning lenses  2105   a  and  2105   b  have a function of equalizing the main scanning speed on the corresponding photosensitive drum surface by the light flux deflected at an equiangular speed with rotation of the polygon mirror  2104 A. 
     As shown in  FIG. 4 , the first scanning lenses  2105   a  and  2105   b  are overlapped in the Z direction (the sub-scanning direction). 
     The first scanning lens  2105   a  faces the “lower four-fold mirror”, and the first scanning lens  2105   b  faces the “upper four-fold mirror”. 
     The light flux deflected by the “upper four-fold mirror” of the polygon mirror  2104 A is transmitted through the first scanning lens  2105   b , and the optical path thereof is bent by the optical-path bending mirror  2106   b.    
     The light flux is then emitted to the photosensitive drum  2030   b  via the second scanning lens  2107   b  and the optical-path bending mirror  2108   b  to form an optical spot. 
     The optical spot scans the photosensitive drum  2030   b  in the main scanning direction at the constant speed with the rotation of the polygon mirror  2104 A, thereby writing a cyan image. 
     Furthermore, the light flux deflected by the “lower four-fold mirror” of the polygon mirror  2104 A is transmitted through the first scanning lens  2105   a , and the optical path thereof is bent by the optical-path bending mirror  2106   a.    
     The light flux is then emitted to the photosensitive drum  2030   a  via the second scanning lens  2107   a  and the optical-path bending mirror  2108   a  to form an optical spot. 
     The optical spot scans the photosensitive drum  2030   a  in the main scanning direction at the constant speed with the rotation of the polygon mirror  2104 A, thereby writing a black image. 
     The respective optical-path bending mirrors are provided so that the respective optical path lengths from the polygon mirror  2104 A to the respective photosensitive drums match with each other. 
     The respective optical-path bending mirrors are also provided so that respective “incident positions and incident angles of the light flux to the photosensitive drum” are equivalent to each other. 
     The cylindrical lenses  2204   a  and  2204   b  and the second scanning lenses  2107   a  and  2107   b  corresponding thereto form a so-called “optical face tangle error correction system”. 
     That is, the “line image” described above formed by the cylindrical lens  2204   a  and the scanning direction of the photosensitive drum  2030   a  have a conjugate relation by the second scanning lens  2107   a  in the sub-scanning direction. 
     The “line image” described above formed by the cylindrical lens  2204   b  and the scanning direction of the photosensitive drum  2030   b  have also a conjugate relation by the second scanning lens  2107   b  in the sub-scanning direction. 
     The first scanning lenses  2105   a  and  2105   b , the second scanning lenses  2107   a  and  2107   b , and the optical-path bending mirrors  2106   a ,  2106   b ,  2108   a , and  2108   b  form a scanning optical system. 
     The configuration described above is a configuration of the optical scanning apparatus  2010   a  that optically scans the photosensitive drums  2030   a  and  2030   b.    
     Therefore, the first scanning lens  2105   a , the second scanning lens  2107   a , and the optical-path bending mirrors  2106   a  and  2108   a  form a “scanning optical system of the station K”. 
     Similarly, the first scanning lens  2105   b , the second scanning lens  2107   b , and the optical-path bending mirrors  2106   b  and  2108   b  form a “scanning optical system of the station C”. 
     As described above, the optical scanning apparatus  2010   b  that optically scans the photosensitive drums  2030   c  and  2030   d  have the same configuration as that of the optical scanning apparatus  2010   a.    
     An optical scanning area in the main scanning direction of each photosensitive drum in which image information is written is referred to as “effective scanning area”. 
     In  FIG. 2 , reference sign  2301 A denotes a “synchronization lens”, and reference sign  2302 A denotes a “synchronization detection sensor”. 
     The synchronization lens  2301 A is used for detecting a deflected light flux (the light flux L 1 ) deflected by the “lower-stage four-fold mirror” of the polygon mirror  2104 A. 
     That is, the synchronization lens  2301 A is arranged on an optical path of the deflected light flux transmitted through a “non-power portion having no power in the main scanning direction” at an end on a −Y side of the first scanning lens  2105   b.    
     The deflected light flux is focused onto a light-receiving surface of the synchronization detection sensor  2302 A. 
     The synchronization detection sensor  2302 A outputs a signal corresponding to an amount of light of the light-received deflected light flux to the printer control device  2090  that controls optical scanning. 
     The printer control device  2090  determines a “write start timing with respect to the photosensitive drums  2030   a  and  2030   b , based on the signal output from the synchronization detection sensor  2302 A. 
     The synchronization lens  2301 A and the synchronization detection sensor  2302 A constitute a “synchronization detection system”. 
     The deflected light flux light-received by the synchronization detection sensor  2302 A is referred to as “light flux for synchronization detection”. 
     The light flux for synchronization detection passes through the non-power portion of the first scanning lens  2105   a , and the optical path of the light flux for synchronization detection does not change regardless of deformation of the first scanning lens due to a change in the ambient temperature. 
     In this exemplary embodiment, when the two light fluxes L 1  and L 2  from the semiconductor light-emitting element  2200 A scan one of the photosensitive drums, the two light fluxes L 1  and L 2  do not reach the other photosensitive drum. 
     When the light flux L 1  optically scans the photosensitive drum  2030   a , a “light-source drive unit” (not shown) modulates and drives the semiconductor light-emitting element  2200 A based on the black image information. 
     Furthermore, when the light flux L 2  optically scans the photosensitive drum  2030   b , the light-source drive unit modulates and drives the semiconductor light-emitting element  2200 A based on the cyan image information. 
     A time chart for optical scanning with respect to the photosensitive drums  2030   a  and  2030   b  in this case is shown in  FIG. 6 . In  FIG. 6 , an “amount of light” of exposure is plotted on a vertical axis and “time” is plotted on a horizontal axis. 
     Exposure by the black image information and the cyan image information is performed by the light flux from the semiconductor light-emitting element  2200 A as a common light source. 
     That is, each of the spatially separated light fluxes L 1  and L 2  is “spatially separated” and deflected alternately by the polygon mirror  2104 A as the optical deflector. 
     Scanning of the different photoreceptor is performed by each of the deflected light fluxes. 
     The time chart shows a timing when the light is all turned on in the effective scanning areas of the photosensitive drums. 
     In  FIG. 6 , a solid line corresponds to a portion of the black image information, and a broken line corresponds to a portion of the cyan image information. 
     In  FIG. 6 , “scanning line  1 ” means a scanning line by the light flux L 1  (a trajectory of the optical spot that performs main scanning), and “scanning line  2 ” means a scanning line by the light flux L 2 . 
     When full-color image formation is performed, optical scanning of the respective photosensitive drums  2030   a  to  2030   d  is performed as described above by the optical scanning apparatuses  2010   a  and  2010   b.    
     The image forming apparatus in  FIG. 1  can perform “image formation using only a part” of the four image forming stations. 
     In this case, image formation is not performed by at least one of the four image forming stations. 
     As the simplest and most representative case, a case where “a black image is formed as a monochrome image by image formation by only the station K” is explained. 
     In this case, only the optical scanning apparatus  2010   a  that is required for forming a black image is operated, and the operation of the optical scanning apparatus  2010   b  is stopped. 
     Modulation drive of the semiconductor light-emitting element  2200 A that is required for forming a black image is performed after a determination of the write start timing of the black image and the cyan image by the synchronization detection system. 
     Therefore, until the write start timing is determined, the semiconductor light-emitting element  2200 A is forcibly lighted up and deflected, and the light flux for synchronization detection is received by the synchronization detection sensor  2302 A. 
     At this time, because the semiconductor light-emitting element  2200 A is forcibly lighted up, optical scanning is performed not only on the synchronization detection sensor  2302 A but also on the photosensitive drums  2030   a  and  2030   b.    
     As described above, offset light emission is performed by the semiconductor light-emitting element for improving rise characteristics and power stability of optical power of the semiconductor light-emitting element  2200 A. 
     Due to the offset light emission, “offset beams” are always emitted, although in a limited amount of light. 
     After a determination of the write start timing, when the photosensitive drum  2030   a  is optically scanned, modulation drive of the semiconductor light-emitting element  2200 A is performed based on the black image information. 
     In this case, the photosensitive drum  2030   b  for forming a cyan image is exposed to the offset beams. 
     The exposure of the photosensitive drum  2030   b  to the offset beams causes deterioration of the photosensitive drum  2030   b  due to light-induced fatigue. 
     Particularly, at the time of forming the black image, when the rotation of the photosensitive drum  2030   b  is stopped to reduce power consumption, it becomes a cause of the “abnormal image such as density unevenness” described above. 
     According to the present invention, this problem is solved by “at least one optical-path opening/closing unit and a control unit”. 
     The at least one optical-path opening/closing unit is arranged in at least one optical path of the light fluxes divided by the light-flux dividing unit to open or close the optical path independently. 
     The control unit controls opening/closing of the optical path by the at least one optical-path opening/closing unit. 
       FIGS. 7A and 7B  depict one embodiment of the optical-path opening/closing unit. 
     In the present embodiment, a shielding member  2109  (and a drive unit (not shown)) are provided as the optical-path opening/closing unit between the light-flux dividing member  2203 A and the scanning target surface. 
     The shielding member  2109  is provided to open or close the optical path of one of the divided light fluxes (the light flux L 2  that performs write of a cyan image). 
     That is, the shielding member  2109  shields the light flux L 2  guided in the optical path that is opened or closed. 
     The position where the shielding member  2109  is installed can be basically any position from a position after the light flux from the semiconductor light-emitting element is divided into two by the light-flux dividing unit up to a position of the photosensitive drum surface. 
     However, a position before the light flux is deflected by the optical deflector is advantageous in view of the installation space and cost, because the size of the shielding member  2109  can be decreased. 
     In this exemplary embodiment, the shielding member  2109  is arranged on an optical path of the light flux L 2  at a position immediately after the light-flux dividing member  2203 A. 
     The shielding member  2109  is driven to move parallel to a Y direction orthogonal to the drawing by a drive unit (not shown, being controlled by the printer control device  2090  in  FIG. 1 ). 
     Accordingly, the shielding member  2109  and the drive unit (not shown) form the “optical-path opening/closing unit”, and the printer control device  2090  forms the “control unit”. 
     The shielding member  2109  can switch the “opened or closed state of the optical path” depending on the input image information. 
     When the input image information requires all the divided light fluxes for image formation such as full color printing, the “optical paths of both light fluxes are opened” by the shielding member  2109 . 
     This state is shown in  FIG. 7A . The shielding member  2109  is retreated from the optical path of the light flux L 2 , as shown by a broken line, and does not close any optical path. 
     When the input image information is used to form a monochrome image (forming a black image), only one of the divided light fluxes (the light flux L 1 ) is required, and the other (the light flux L 2 ) is not required. 
     In this case, the shielding member  2109  is displaced in the Y direction by the drive unit (not shown), so that only the optical path of the light flux L 2  is shielded. This state is shown in  FIG. 7B . 
     With this method, the optical path of the light flux L 2  is closed by the shielding member  2109 . Therefore, the photosensitive drum  2030   b  is not optically scanned by the light flux L 2  (offset beams). 
     Accordingly, light-induced fatigue of the photosensitive drum  2030   b  due to unnecessary optical scanning by the offset beams can be suppressed, and an “abnormal image such as density unevenness” can be prevented, thereby enabling to form a high quality image for a long time. 
     In the present embodiment shown in  FIGS. 7A and 7B , the shielding member  2109  moves into and out from the optical path of the light flux L 2  with simple parallel displacement to open or close the optical path. 
     That is, the drive unit that displaces and drives the shielding member  2109  causes the shielding member  2109  to perform simple reciprocating parallel displacement. 
     As the drive unit that causes the shielding member  2109  to perform simple parallel displacement, a “known appropriate parallel displacement mechanism” can be used, and opening/closing control of the optical path can be executed according to the image to be formed. 
     Another embodiment of the present invention is explained with reference to  FIGS. 8A and 8B . 
     In the present embodiment, a shielding unit as the optical-path opening/closing unit is provided between the light-flux dividing member  2203 A and the scanning target surface. 
     The shielding unit is provided to open or close the optical path of one of the divided light fluxes (the light flux L 2  that performs write of a cyan image). 
     The shielding unit includes a swingable shielding member  2110  and a drive unit  2111  that drives the shielding member  2110 . The drive unit  2111  is a “stepping motor”. 
       FIG. 8A  depicts a state where the optical paths of both the light fluxes L 1  and L 2  are opened.  FIG. 8B  depicts a state where the optical path of the light flux L 2  is closed. 
     The light fluxes L 1  and L 2  are overlapped on each other in the sub-scanning direction orthogonal to the drawing. 
     The shielding member  2110  is rotated counterclockwise 90 degrees from the state in  FIG. 8A  by the stepping motor  2111  and is arranged in the optical path of the light flux L 2 . 
     The optical path of the light flux L 2  is closed by the arrangement of the shielding member  2110 . 
     When the shielding member  2110  is rotated clockwise 90 degrees from the state in  FIG. 8B , the shielding member  2110  is retreated from the optical path of the light flux L 2  to open the optical path of the light flux L 2 . 
     The working speed of the shielding member  2110  can be such that an opening/closing operation is complete within a time (within several hundreds of milliseconds) from an input of the image information until synchronization detection light is emitted. 
     Accordingly, the stepping motor  2111  as the drive unit only needs to be operated while being matched with the time. 
     The stepping motor  2111  can rotate at a certain angle according to an input signal, and can perform the opening/closing operation without executing any complicated control. 
     In the embodiment shown in  FIGS. 8A and 8B , an optical sensor  2112   b  and an actuator  2112   a  are provided. The actuator  2112   a  is integrally provided with the shielding member  2110 . 
     In the state shown in  FIG. 8A  where the optical path of the light flux L 2  is opened, the actuator  2112   a  puts the optical sensor  2112   b  in a shielded state. 
     As shown in  FIG. 8B , when the shielding member  2110  closes the optical path of the light flux L 2 , the actuator  2112   a  puts the optical sensor  2112   b  in an opened state, and the optical sensor  2112   b  is turned ON. 
     Accordingly, it is detected that the optical path of the light flux L 2  is closed. 
     By including the actuator  2112   a , when the opening/closing state cannot be controlled at the time of a failure or the like of the drive part, this state can be detected, and unnecessary optical scanning by the light flux L 2  can be prevented. 
     By using a black “member emitted with the light flux” such as the shielding members  2109  and  2110  in a piled form, optical absorptance can be increased, and reflection and scattering of the shielded light flux can be suppressed. 
     Accordingly, adverse effects on the photosensitive drum and the optical sensor and the like installed in the apparatus can be prevented, thereby enabling to form a high quality image. 
     Three examples of other embodiments are explained with reference to  FIGS. 9A and 9B . 
     In  FIGS. 9A and 9B , reference signs  2203 A,  2110 , and  2111  respectively denote the light-flux dividing member, the shielding member, and the stepping motor as in  FIGS. 8A and 8B . 
     The shielding member  2110  can rotate in forward and reverse directions, and rotates around a rotation shaft  2110 A in the forward and reverse directions to open and close the optical path of the light flux L 2 .  FIG. 9A  depicts an opened state, and  FIG. 9B  depicts a shielded state. 
     A rotary drive unit  2110 B is integrally formed with the shielding member  2110 . 
     The rotary drive unit  2110 B is in a “U shape” in this example, and has a gap parallel to a longitudinal direction thereof. 
     A pin  2120 A 1  fixed and provided near the end of an arm  2120 A of the drive member is inserted into the gap with a backlash. 
     The drive member includes another arm  2120 B integrally formed with the arm  2120 A. 
     The drive member is rotated in the forward and reverse directions around a shaft orthogonal to the drawing of  FIGS. 9A and 9B  by the stepping motor  2111  as the drive unit. 
     A rotation shaft of the drive member is coaxial with a drive shaft of the stepping motor  2111 . 
     The arm  2120 A of the drive member, the rotary drive unit  2110 B, and the pin  2120 A 1  constitute a link mechanism. 
     The drive member is rotated counterclockwise a predetermined angle by the stepping motor  2111 , from a state shown in  FIG. 9A  where the optical path of the light flux L 2  is opened. 
     The rotary drive unit  2110 B is then rotated clockwise, and as shown in  FIG. 9B , the shielding member  2110  closes the optical path of the light flux L 2 . 
     Although not shown in  FIGS. 9A and 9B , the optical path of the light flux L 1  explained with reference to  FIGS. 8A and 8B  is not opened or closed as in the embodiment shown in  FIGS. 8A and 8B . 
     In  FIGS. 9A and 9B , a part denoted by reference sign  2130  is a signal input unit to the stepping motor  2111 . 
     When the drive member is rotated clockwise by the stepping motor  2111 , the shielding member  2110  rotates counterclockwise substantially 90 degrees from the state in  FIG. 9B  to open the optical path. 
     The working speed of the shielding member  2110  can be such that the opening/closing operation is complete within a time (within several hundreds of milliseconds) from an input of the image information until synchronization detection light is emitted. 
     Accordingly, the stepping motor  2111  as the drive unit only needs to be operated while being matched with the time. The operation control is also executed by the printer control device  2090  in  FIG. 1 . 
     The stepping motor  2111  can rotate at a certain angle according to an input signal, and can perform the opening/closing operation without executing any complicated control. 
     In the embodiment shown in  FIGS. 9A and 9B , the optical-path opening/closing unit constitutes the “link mechanism” as described above, and a displacement amount of the arm  2120 A of the drive member and the shielding member  2110  can be set differently from each other. 
     Accordingly, design flexibility and layout flexibility with respect to the optical-path opening/closing unit can be considerably improved by the drive unit, as compared to a case where the shielding member is directly operated. 
     As a result, the optical-path opening/closing unit can be installed at a “position having little room for layout” such as in a pre-deflection optical system. 
     There is a “dead point at which an operation amount of a driven part becomes smaller than that of the drive part” in the link mechanism because of the configuration of the link mechanism. 
     In the optical path opening/closing operation, it is desired to realize the state in  FIG. 9A  where the optical path of the light flux L 2  is fully opened and the state in  FIG. 9B  where the optical path is fully closed near the dead point of the link mechanism. 
     The embodiment shown in  FIGS. 9A and 9B  realizes this state. 
     That is, in the state in  FIG. 9A  where the optical path is fully opened, the longitudinal direction of the gap in the rotary drive unit  2110 B is approximately parallel to a “shift direction of the pin  2120 A 1  due to the rotation of the drive member”. 
     Accordingly, when the drive member is rotated in the state in  FIG. 9A , a rotation angle of the shielding member  2110  is smaller than that of the drive member. 
     Similarly, in the state in  FIG. 9B  where the optical path is fully closed, the longitudinal direction of the gap in the rotary drive unit  2110 B is approximately parallel to the “shift direction of the pin  2120 A 1  due to the rotation of the drive member”. 
     Accordingly, when the drive member is rotated counterclockwise in the state in  FIG. 9B , the rotation angle of the shielding member  2110  is smaller than that of the drive member. 
     That is, an amount of displacement of the shielding member  2110  by the drive member of the optical path opening/closing member at the time of fully opening/closing the optical path is smaller than that at the time of halfway opening/closing the optical path. 
     The “fully opening/closing the optical path” means that opening/closing of the optical path is completely performed. 
     The “halfway opening/closing the optical path” means that opening/closing of the optical path is incomplete. 
     With this configuration, when the opening/closing state is switched by turning the drive member, even if a target amount of turn varies, fluctuations in the amount of rotation of the shielding member  2110  can be decreased. 
     Therefore, reliable opening/closing of the optical path of the light flux L 2  can be performed. 
       FIG. 10  is an explanatory diagram of another embodiment of the present invention. 
     In the embodiment explained with reference to  FIGS. 9A and 9B , a planar shape of the drive member is not axisymmetric to the rotation shaft of the stepping motor  2111 . 
     In this case, a center of gravity of the drive member is away from the rotation shaft of the stepping motor  2111 . 
     It is assumed here that a moment of inertia specific to the drive member is “I”, a distance between the rotation shaft of the stepping motor and the center of gravity of the drive member is “d”, and a mass of the drive member is “M”. 
     The moment of inertia associated with the rotation of the drive member by the stepping motor  2111  becomes “I+Md 2 ”. 
     That is, as the distance “d” increases, the moment of inertia increases, and a moment required for rotation of the drive member also increases. 
     A centrifugal force acting on the drive member at the time of rotation of the drive member also increases with an increase of the distance “d”, and counteraction thereof acts on the rotation shaft of the stepping motor  2111 . 
     The embodiment shown in  FIG. 10  is an exemplary embodiment considering this point. 
       FIG. 10  depicts a state as viewed from below the stepping motor  2111 . To avoid complexity, like reference signs to those of  FIGS. 9A and 9B  are added to like parts in  FIG. 10 , for which any confusion is unlikely to occur. 
     In  FIG. 10 , reference sign  2120  denotes a “drive member”. 
     The drive member  2120  includes arms  2120 A and  2120 D. 
     The arm  2120 D has a different shape from that of the arm  2120 B shown in  FIGS. 9A and 9B . 
     As shown in  FIG. 10 , the drive member  2120  has a “structural portion” denoted by reference sign  2120 C. 
     The structural portion  2120 C includes a “half-cut hollow cylindrical portion” and a radial “half-cut wheel shaft portion”, and is integrally formed with the drive member  2120 . 
     The structural portion  2120 C is a “counter-balanced portion” with respect to the arms  2120 A and  2120 D. 
     By providing the structural portion  2120 C, the center of gravity of the drive member  2120  can be set near the rotation shaft of the stepping motor  2111 , thereby enabling to decrease the distance “d”. 
     Ideally, it is desired to form the structural portion  2120 C so as to be “d=0”. However, when d is sufficiently small, “d” does not need to be 0. 
     The moment of inertia “I+Md 2 ” decreases with a decrease of “d”, a rotation driving force is reduced, and a centrifugal force acting on the stepping motor as the counteraction also decreases. 
     Accordingly, rotation of the drive member  2120  can be stabilized and the opening/closing operation of the shielding member  2110  can be also stabilized. 
     The embodiment shown in  FIGS. 11A and 11B  is an example in which a detecting unit  2140  that detects the opened/closed state of the optical path by the optical-path opening/closing unit is provided with respect to the embodiment explained with reference to  FIGS. 9A and 9B . 
     As shown in  FIG. 11A , the end of the arm  2120 B provided in the drive member is bent. 
     The end of the arm  2120 B is bent substantially parallel to the drive shaft of the stepping motor  2111 . 
     The bent portion is a shielding portion  2120 B 1  with respect to an optical sensor  2140 . 
     That is, the shielding portion  2120 B 1  shields “between a light emitting part and a light receiving part” of a sensor unit  2140 A of the optical sensor  2140  shown in  FIG. 11B . 
     In a state (a state in  FIG. 11A ) where the shielding member  2110  does not shield the optical path of the light flux L 2 , the shielding portion  2120 B 1  is positioned between the light emitting part and the light receiving part of the sensor unit  2140 A. 
     This state is a “Hi” state of the optical sensor  2140 . 
     When the drive member is rotated by the stepping motor  2111  and becomes a state of  FIG. 11B , the optical path of the light flux L 2  is shielded. 
     At this time, the shielding portion  2120 B 1  is retreated from between the light emitting part and the light receiving part of the sensor unit  2140 A, and the optical sensor  2140  is in a “Lo” state. 
     The “Hi” and “Lo” of the optical sensor  2140  are switched in this manner while being associated with the rotation of the drive member by the stepping motor  2111 . 
     According to this configuration, the opened/closed state of the optical path of the light flux L 2  can be reliably detected. 
     With this configuration, when the opened/closed state cannot be controlled at the time of a failure of the drive unit or the like, the state can be detected, thereby enabling to prevent unnecessary optical scanning by the light flux L 2 . 
     Even in the embodiments shown in  FIGS. 9 to 11 , the “member emitted with the light flux” of the shielding member  2110  can be formed in a black piled form to increase optical absorptance, and reflection and scattering of the shielded light flux can be suppressed. 
     Accordingly, adverse effects on the photosensitive drum and the optical sensor and the like installed in the apparatus can be prevented, thereby enabling to form a high quality image. 
     The shielding portion can be integrally formed with the shielding member  2110  instead of being provided in the drive member as shown in  FIG. 11 , so that rotation of the shielding member  2110  can be directly detected. 
     In this case, the opened/closed state of the optical path can be detected more reliably. 
     By detecting the opened/closed state of the optical path, occurrence of an abnormal image due to incomplete shielding of the optical path can be prevented, thereby enabling to form a high quality image for a long time. 
     In the embodiment in  FIG. 10 , the structural portion  2120 C is formed as a “counter balance” in the drive member. 
     Needless to mention, the structural portion  2120 C can be formed in the drive member in the embodiments shown in  FIGS. 9 and 11 . 
     The image forming apparatus shown in  FIG. 1  has four photoreceptors, and forms a four-color toner image thereon that is required for forming a color image. 
     The various types of optical scanning apparatuses explained above can be used as the optical scanning apparatuses  2010   a  and  2010   b  used in the image forming apparatus. 
     The present invention is not limited thereto, and any one of the optical scanning apparatuses  2010   a  and  2010   b  can be used to carry out an image forming apparatus that forms a two-color image such as red and black. 
     A case where the “semiconductor laser that emits a single laser beam” is used as a light source has been explained above as an example. 
     However, the light source is not limited thereto, and a light source “that includes a plurality of light emitting parts and can emit a plurality of light fluxes independently” can be also used. 
     That is, the light source is not limited to the one described above, and can be an edge-emitting semiconductor laser array or a surface-emitting semiconductor laser (VCSEL). 
     In such a semiconductor light-emitting element, “a plurality of light fluxes” are emitted from a single element. 
     In this case, a “light flux emitted from a light source” claimed in claim  1  means an “aggregate of light fluxes” to be emitted. 
     Accordingly, the light-flux dividing unit divides the light flux into a plurality of light fluxes, in a unit of “aggregate of light fluxes”, and the divided “one unit of light flux” includes a plurality of light fluxes. 
     Therefore, the divided one unit of light flux is focused as “two or more optical spots” on the surface to be optically scanned. “Multi-beam scanning” is performed by these plural spots. 
       FIGS. 12A and 12B  are explanatory diagrams of a “VCSEL” as an example of the semiconductor light-emitting element that emits the light fluxes independently. 
     As shown in  FIG. 12A , a VCSEL  100  includes “32 light emitting parts” two-dimensionally arranged on the same substrate, and electrode pads and wiring members arranged and connected to surround these light emitting parts. 
       FIG. 12B  depicts an arranged state of the light emitting parts. 
     The 32 light emitting parts are arranged such that adjacent light emitting parts are arranged in the main scanning direction (a direction corresponding to the main scanning in  FIG. 12A ) with a gap: X. 
     Furthermore, the light emitting parts are arranged such that adjacent light emitting parts are arranged in the sub-scanning direction (a direction corresponding to the sub-scanning in  FIG. 12B ) with a gap: d 2 . 
     In “one line of the light emitting parts” in the main scanning direction, the adjacent light emitting parts are “displaced” by a distance: d 1  in the sub-scanning direction. 
     This displacement is set so that a projected gap of the light emitting parts becomes an equal gap: d 1 , when all the light emitting parts are “orthographically projected on a virtual line extending in the sub-scanning direction”. 
     According to the optical scanning apparatus of the present invention, an optical path to a scanning target surface, which does not need to be optically scanned, among a plurality of scanning target surfaces can be closed. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.