Patent Publication Number: US-2015071658-A1

Title: Sensor drive and image forming apparatus incorporating same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119(a) to Japanese Patent Application Nos. 2013-188965 and 2014-130734, filed on Sep. 12, 2013, and Jun. 25, 2014, respectively, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     Example embodiments of the present invention generally relate to a sensor device and an image forming apparatus incorporating the sensor device. 
     2. Background Art 
     Image forming apparatuses such as digital copying machines and laser printers form an image by transferring a toner image on a recording medium, typified by a printing paper, and heating and pressurizing the recording medium under a specified condition for fixation. When an image is formed, conditions for image formation such as a development condition, a transfer condition, and a fixation condition should be taken into consideration. When a high-quality image is to be formed, in particular, the conditions for image formation need to be set for the type of a recording medium. 
     This is because the quality of the image is heavily dependent on the recording medium material, thickness, moisture content, smoothness, and coating status. In regard to the smoothness, for example, the stability of a toner becomes low at concave portions of the surface of a printing paper if a proper fixation condition is not chosen. For example, irregularities in color may develop if fixing conditions are not proper for the type of recording medium. 
     Further, due to the development of image forming apparatuses and the diversification of expression in recent years, hundreds or more types of printing paper have become available as recording media, and a number of brands exist for each of these types for different basis weights and thickness. In order to achieve high-quality image formation, detailed fixation conditions should be set for every one of these brands. 
     In recent years, the number of brands of paper is increasing for plain paper, coated paper typified by gloss coated paper, matte coaled paper, and art paper, plastic sheet, and special paper whose surface is embossed. In recent years, the number of brands of paper is increasing for plain paper, coated paper typified by gloss coated paper, matte coated paper, and art paper, plastic sheet, and special paper whose surface is embossed. 
     In currently-available image forming apparatuses, a user has to change settings for each tray depending on the brand of paper. For this reason, setting operation could be troublesome. Further, a user is required to know how to identify the type of paper in order to employ the proper settings, and an error in the setting operation leads to an undesired image developed on paper. When the brand of paper is unknown, it is difficult to choose appropriate settings. 
     SUMMARY 
     Embodiments of the present invention described herein provide an improved sensor device and an image forming apparatus incorporating the sensor device. The sensor device includes a light source, a photo-detection system configured to receive light emitted from the light source and reflected at a sheet-like object, and a housing accommodating the light source and the photo-detection system, where the housing has a slit into which the object is inserted. The sensor device includes a holding member configured to hold the object such that relative positions of a surface of the object inserted into the slit and the photo-detection system are fixed. The image forming apparatus includes an image forming unit configured to form an image on a recording medium, the sensor device, and an adjustment unit configured to adjust a condition for image formation, based on an output from the sensor device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of exemplary embodiments and the many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
         FIG. 1  is a schematic diagram the configuration of a color printer according to an example embodiment of the present invention. 
         FIG. 2  is an external view of a sensor device according to an example embodiment of the present invention. 
         FIG. 3  illustrates the configuration of a sensor device according to an example embodiment of the present invention. 
         FIGS. 4A to 4C  illustrate a housing according to an example embodiment of the present invention. 
         FIG. 5  illustrates the inner structure of a housing according to an example embodiment of the present invention. 
         FIGS. 6A to 6C  illustrate a supporting member according to an example embodiment of the present invention. 
         FIG. 7  illustrates spring members according to an example embodiment of the present invention. 
         FIG. 8  illustrates the relative positions of an optical system, a supporting member, and recording paper when the recording paper is inserted into a slit, according to an example embodiment of the present invention. 
         FIG. 9  illustrates the configuration of an optical system according to an example embodiment of the present invention. 
         FIG. 10  illustrates the relative positions of an optical system and a slit according to an example embodiment of the present invention. 
         FIG. 11  illustrates a surface emitting laser array according to an example embodiment of the present invention. 
         FIG. 12  illustrates the angle of incidence of irradiation tight on recording paper, according to an example embodiment of the present invention. 
         FIG. 13  illustrates the relative positions of a first photosensor and a second photosensor according to an example embodiment of the present invention. 
         FIG. 14  illustrates the arrangement of a third photosensor according to an example embodiment of the present invention. 
         FIG. 15A  illustrates surface regular reflection light. 
         FIG. 15B  illustrates surface diffuse reflection light. 
         FIG. 15C  illustrates internal reflection light. 
         FIG. 16  illustrates the light that enters a polarizing filter, according to an example embodiment of the present invention. 
         FIG. 17  illustrates the light received by the second photosensor, according to an example embodiment of the present invention. 
         FIG. 18  illustrates the light received by the second and third photosensors, according to an example embodiment of the present invention. 
         FIG. 19  illustrates the configuration of a processing device according to an example embodiment of the present invention. 
         FIGS. 20A to 20C  illustrate how a supporting member moves when recording paper is inserted into a slit, according to an example embodiment of the present invention. 
         FIG. 21  illustrates the pressing force exerted by a plurality of spring members on a supporting member, according to an example embodiment of the present invention. 
         FIG. 22  illustrates the pulling force applied to recording paper when the recording paper is inserted into a slit, according to an example embodiment of the present invention. 
         FIG. 23  illustrates brand-specific output level data according to an example embodiment of the present invention. 
         FIG. 24  is a flowchart of the processes executed by a CPU of a processing device when brand identification processes are performed, according to an example embodiment of the present invention. 
         FIGS. 25A and 25B  illustrate the relative movement of recording paper according to an example embodiment of the present invention. 
         FIG. 26  illustrates a plurality of detection positions according to an example embodiment of the present invention. 
         FIG. 27  illustrates a first modification (1) of a sensor device according to an example embodiment of the present invention. 
         FIG. 28  illustrates a first modification (2) of a sensor device according to an example embodiment of the present invention. 
         FIG. 29  illustrates the characteristics of transmission light according to an example embodiment of the present invention. 
         FIG. 30  illustrates brand-specific output level data that corresponds to the first modification of a sensor device, according to an example embodiment of the present invention. 
         FIG. 31  illustrates a second modification of a sensor device according to an example embodiment of the present invention. 
         FIG. 32  illustrates a third modification (1) of a sensor device according to an example embodiment of the present invention. 
         FIG. 33  illustrates a third modification (2) of a sensor device according to an example embodiment of the present invention. 
         FIG. 34  illustrates brand-specific output level data that corresponds to the third modification of a sensor device, according to an example embodiment of the present invention. 
         FIG. 35  illustrates a sensor for detecting the insertion of recording paper, according to an example embodiment of the present invention. 
         FIGS. 36A and 36B  illustrate the operation of a sensor for detecting the insertion of recording paper, according to an example embodiment of the present invention. 
         FIG. 37  illustrates a modification of a surface emitting laser array according to an example embodiment of the present invention. 
         FIG. 38  illustrates a fourth modification of a sensor device according to an example embodiment of the present invention. 
         FIG. 39  illustrates a fifth modification (1) of a sensor device according to an example embodiment of the present invention. 
         FIG. 40  illustrates a fifth modification (2) of a sensor device according to an example embodiment of the present invention. 
         FIG. 41  illustrates a sixth modification (1) of a sensor device according to an example embodiment of the present invention. 
         FIG. 42  illustrates a sixth modification (2) of a sensor device according to an example embodiment of the present invention. 
         FIG. 43  illustrates a seventh modification (1) of a sensor device according to an example embodiment of the present invention. 
         FIG. 44  illustrates a seventh modification (2) of a sensor device according to an example embodiment of the present invention. 
         FIG. 45  illustrates an irradiation area according to an example embodiment of the present invention. 
         FIG. 46  illustrates a desired shape of an opening according to an example embodiment of the present invention. 
         FIG. 47  illustrates a first modification of an opening according to an example embodiment of the present invention. 
         FIG. 48  illustrates the arrangement of a light source and a plurality of photosensors with reference to an opening of a modification, according to an example embodiment of the present invention. 
         FIG. 49A  illustrates a second modification of an opening according to an example embodiment of the present invention. 
         FIG. 49B  illustrates a third modification of an opening according to an example embodiment of the present invention. 
         FIG. 50A  illustrates a fourth modification of an opening according to an example embodiment of the present invention. 
         FIG. 50B  illustrates a fifth modification of an opening according to an example embodiment of the present invention. 
         FIGS. 51A and 51B  illustrate other methods of preventing recording paper from being caught at an edge of an opening, according to an example embodiment of the present invention. 
     
    
    
     The accompanying drawings are intended to depict exemplary embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “includes” and/or “including”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     In describing example embodiments shown in the drawings, specific terminology is employed for the sake of clarity. However, the present disclosure is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have the same structure, operate in a similar manner, and achieve a similar result. 
     Some embodiments of the present invention are described with reference to  FIGS. 1 to 26 . 
       FIG. 1  is a schematic diagram illustrating the configuration of a color printer  2000  according to an example embodiment of the present invention. 
     The color printer  2000  according to the present example embodiment is a tandem-type multicolor printer that forms a full-color image by superimposing multiple images of four colors (black, cyan, magenta, and yellow) on top of one another. The color printer  2000  includes a sensor device  100 , an optical scanner  2010 , four photoreceptor drums ( 2030   a,    2030   b,    2030   c,    2030   d ), four cleaning units ( 2031   a,    2031   b,    2031   c,    2031   d ), four charging devices ( 2032   a,    2032   b,    2032   c,    2032   d ), four development rollers ( 2033   a,    2033   b,    2033   c,    2033   d ), a transfer belt  2040 , a transfer roller  2042 , a fixing device  2050 , a paper feed roller  2054 , art ejection roller  2058 , a paper feed tray  2060 , a paper output tray  2070 , a communication controller  2080 , an optical sensor  2245 , a printer controller  2090 , an operation panel, and printer housing  2200 . 
     The communication controller  2080  controls bidirectional communication with a host device (for example, a personal computer (PC)) through a network or the like. 
     The printer controller  2090  includes, for example, a central processing unit (CPU), a read-only memory (ROM) in which a program described by CPU-readable codes and various kinds of data used for executing the program are stored, a random access memory (RAM) that serves as a working memory, an amplifier circuit, and an analog-to-digital (A/D) converter that converts an analog signal to a digital signal. Further, the printer controller  2090  controls elements under instructions from a host device, and transfers the image data sent from the host device to the optical scanner  2010 . Note that optimal developing conditions and transferring conditions for several brands of recording paper, which can be properly handled as a recording medium by the color printer  2000 , are stored in the ROM on a brand-by-brand basis as a “development and transfer table”. 
     The operation panel has a plurality of keys used by an operator to perform several kinds of settings or processes, and a display unit that displays several kinds of information. 
     The photoreceptor drum  2030   a,  the charging device  2032   a,  the development roller  2033   a,  and the cleaning unit  2031   a  are used as a unit, and together configure an image forming station that forms a black image. Hereinafter, this image forming station may be referred to as a K-station. 
     The photoreceptor drum  2030   b,  the charging device  2032   b,  the development roller  2033   b,  and the cleaning unit  2031   b  are used as a unit, and together configure an image forming station that forms a cyan image. Hereinafter, this image forming station may be referred to as a C-station. 
     The photoreceptor drum  2030   c,  the charging device  2032   c,  the development roller  2033   c,  and the cleaning unit  2031   c  are used as a unit, and together configure an image forming station that forms a magenta image. Hereinafter, this image forming station may be referred to as an M-station. 
     The photoreceptor drum  2030   d,  the charging device  2032   d,  the development roller  2033   d,  and the cleaning unit  2031   d  are used as a unit, and together configure an image forming station that forms a yellow image. Hereinafter, this image forming station may be referred to as a Y-station. 
     A photosensitive layer is formed on the surface of each of the photoreceptor drums. The photoreceptor drums rotate in the direction of the arrows as illustrated in  FIG. 1 . 
     Each of the charging devices evenly charges the surface of the associated photoreceptor drum. 
     The optical scanner  2010  scans each of the surfaces of the electrically-charged photoreceptor drums, with the light that is modulated for each color based on the color image data (i.e., black image data, cyan image data, magenta image data, and yellow image data) received from the printer controller  2090 . Note that the surface of each of the photoreceptor drums is scanned. Accordingly, a latent image that corresponds to the image data of each color is formed on the surface of each of the photoreceptor drums. Here, each of the photoreceptor drums is an image bearer that carries an image. Each of the latent images moves towards the corresponding development roller as the photoreceptor drum rotates. 
     A toner form the corresponding toner cartridge is thinly and evenly applied to the surface of each of the development rollers as it rotates. Then, the toner applied to the surface of each of the development rollers moves and adheres to the portions of the surface of the corresponding photoreceptor drum that are irradiated with light by the optical scanner  2010 . In other words, the development roller renders a latent image manifest by making the toner adhere to the latent image formed on the surface of the corresponding photoreceptor drum. Each of the toner images moves towards the transfer belt  2040  as the photoreceptor drum rotates. 
     Each of the toner images of yellow, magenta, cyan, and black is sequentially transferred to the transfer belt  2040  with specified timing. Then, the transferred toner images are superimposed on top of one another to form a color image. 
     The paper feed tray  2060  stores recording papers therein. The paper feed roller  2054  is arranged near the paper feed tray  2060 , and the paper feed roller  2054  takes a piece of recording paper from the paper feed tray  2060 . Then, the recording paper is fed between the transfer belt  2040  and the transfer roller  2042  with specified timing. Accordingly, the toner image on the transfer belt  2040  is transferred to the recording paper. The recording paper on which the toner image has been transferred is conveyed to the fixing device  2050 . 
     At the fixing device  2050 , heat and pressure are applied to the recording paper to fix the toner on the recording paper. The recording paper on which the toner is fixed is conveyed to the paper output tray  2070  through the ejection roller  2058 . 
     Each of the cleaning units  2031   a,    2031   b,    2031   c,  and  2031   d  removes the residual toner left on the surface of the corresponding photoreceptor drum. The surface of the photoreceptor drum from which the residual toner has been removed moves back to a position where the surface of the photoreceptor drum faces the corresponding charging device. 
     The sensor device  100  may be detached from the printer housing  2200 , and is disposed near the operation panel such that an operator can pick it up with his/her hand. The sensor device  100  is used to identify the brand of the recording paper. 
       FIG. 2  is an external view of the sensor device  100  according to an example embodiment of the present invention. As illustrated in  FIG. 2  as an example, the sensor device  100  is shaped like a quadrangular prism, and has a slit in the insertion direction of recording paper. In the XYZ three-dimensional orthogonal coordinate system according to the present example embodiment, it is assumed that the direction orthogonal to the surface of the recording paper is a Z-axis direction, and that the direction in which the recording paper is inserted into the slit is the +X direction. 
       FIG. 3  illustrates the configuration of a sensor device  100  according to an example embodiment of the present invention. As illustrated in  FIG. 3  as an example, the sensor device  100  includes an optical system  110 , a processing device  130 , a supporting member  140 , a housing  150 , and a plurality of spring members  170 . In  FIG. 3 , the wall on the −Y side of the housing  150  is removed for ease of illustration of the inner structure. 
       FIGS. 4A to 4C  illustrate the housing  150  according to an example embodiment of the present invention. As illustrated in  FIGS. 4A to 4C , a slit is formed on the −X-side surface, and the slit penetrates to the +Y-side surface and the −Y-side surface, In other words, the housing  150  has a slit of prescribed length in the direction in which the recording paper is inserted. 
       FIG. 5  illustrates the XZ cross section of the housing  150  according to an example embodiment of the present invention. As illustrated in  FIG. 5 , the housing  150  has inner space in which the optical system  110 , the processing device  130 , the supporting member  140 , and the spring members  170  are accommodated. 
       FIGS. 6A to 6C  illustrate the supporting member  140  according to an example embodiment of the present invention. As illustrated in  FIGS. 6A and 6B , in the present example embodiment, the supporting member  140  has a supporting surface on the +Z side parallel to the XY surface, and supports the recording paper by that supporting surface. At an edge on the −X side of the supporting surface, a tapered surface (inclined plane surface) is formed to guide the recording paper to the supporting surface. Alternatively, as illustrated in  FIG. 6C , an inclined curved surface may be formed in place of the tapered surface. Such a curved surface has a round surface at an edge on the −X side of the supporting surface. 
       FIG. 7  illustrates the spring members  170  according to an example embodiment of the present invention. As illustrated in  FIG. 7 , the −Z-side edges of the spring members  170  are fixed to the housing  150 , and the +Z-side edges of the spring members  170  are fixed to the −Z side surface of the supporting member  140 . In other words, the supporting member  140  is attached to the housing  150  through the spring members  170 . 
       FIG. 8  illustrates the relative positions of the optical system  110 , the supporting member  140 , and recording paper M when the recording paper M is inserted into the slit, according to an example embodiment of the present invention. As illustrated in  FIG. 3 , the optical system  110  is disposed on the +Z-side of the supporting member  140 . As illustrated in  FIG. 8 , when the recording paper M is inserted into the slit, the supporting member  140  is pressed down by the recording paper M, and the recording paper M is fed between the −Z-side of a dark box  119  of the optical system  110  and the supporting surface of the supporting member  140 . In other words, the recording paper M that is inserted into the slit is on the −Z side of the optical system  110 . 
       FIG. 9  illustrates the configuration of the optical system  110  according to an example embodiment of the present invention. As illustrated in  FIG. 9 , in the present example embodiment, the optical system  110  includes a light source  111 , a collimate lens  112 , three photosensors  113 ,  114 , and  115 , a polarizing filter  116 , and the dark box  119  that encloses these elements. 
     The dark box  119  is a box made of metal such as aluminum, and the surface of the dark box  119  is anodized in black in order to reduce the influence of disturbance light and stray light. The dark box  119  is fixed to the housing  150 . 
       FIG. 10  illustrates the relative positions of the optical system  110  and a slit according to an example embodiment of the present invention. As illustrated in  FIG. 10 , in the present example embodiment, the position of the −Z-side surface of the dark box  119  is equal to the +Z-side surface of the slit of the housing  150 . Note that the position of the −Z-side surface of the dark box  119  in the Z-axis direction is not limited to the above configuration, but may be positioned above the slit of the housing  150 . 
     Before the recording paper M is inserted into the slit, the supporting member  140  is pressed by the spring members  170  towards the +Z-direction, such that the supporting surface of the supporting member  140  softly touches the −Z-side surface of the dark box  119 . 
       FIG. 11  illustrates a surface emitting laser array according to an example embodiment of the present invention. The light source  111  includes a plurality of light-emitting units. Each of the light-emitting units is a vertical cavity-surface emitting laser (VCSEL). In other words, the light source  111  includes a vertical cavity-surface emitting laser array (VCSEL array). In the present example embodiment described above, as illustrated in  FIG. 11 , nine light-emitting units are two-dimensionally arranged in the VCSEL array. 
     The light source  111  is disposed so as to irradiate the recording paper M with the s-polarized linear light.  FIG. 12  illustrates the angle of incidence θ of irradiation light on recording paper, according to an example embodiment of the present invention. In the present example embodiment, the angle of incidence θ is 80°. The light source  111  is switched on and switched off by the processing device  130 . 
     The collimate lens  112  is disposed on the optical path of the light emitted from the light source  111 , and collimates the light. The light that has passed the collimate lens  112  passes through the opening of the dark box  119  to irradiate the recording paper M. Hereinafter, the center of the irradiated area on the surface of the recording paper M is referred to simply as a “irradiation center”. Hereinafter, the light that has passed through the collimate lens  112  may be referred to as irradiation light. 
     Assuming that the light enters the boundary surface of a medium, the plane that includes the incident light beam and the normal line drawn from the point of incidence of the boundary surface is referred to as the incidence plane. When the incident light includes a plurality of light beams, an incidence plane exists for each of the light beams. However, for the purpose of simplification, the incidence plane of the light beam incident on the irradiation center is referred to as the incidence plane of the recording paper M. In other words, the plane that includes the irradiation center and is parallel with the XZ-plane is the incidence plane of the recording paper M. 
     In the present description, the terns “s-polarized light” and “p-polarized light” are used not only for the incident light on the recording paper M but also for the reflection light. This is for the purpose of simplification, and the linearly polarized light whose polarization direction is the same as that of the incident light (i.e., s-polarized light) on an incidence plane is referred to as the s-polarized light, and the linearly polarized light whose polarization direction is orthogonal to the s-polarized light is referred to as the p-polarized light. These terms are used with reference to the polarization direction of the incident light on the recording paper M. 
     The polarizing filter  116  is arranged on the +Z-side of the irradiation center. The polarizing filter  116  transmits the p-polarized light and blocks the s-polarized light. Alternatively, the polarizing filter  116  may be replaced with a polarization beam splitter whose capability is equivalent to that of the polarizing filter  116 . 
       FIG. 13  illustrates the relative positions of the photosensor  113  and the photosensor  114  according to an example embodiment of the present invention. The photosensor  114  is disposed on the +Z-side of the polarizing filter  116 , and receives the light that has passed through the polarizing filter  116 . As illustrated in  FIG. 13 , the angle φ 1  which the surface of the recording paper M forms with a line L 1 , which is drawn through the irradiation center, the center of the polarizing filter  116 , and the center of the photosensor  114 , is 90 degrees. 
     The photosensor  113  is arranged on +X side of the irradiation center, with reference to the X-axis direction. As illustrated in  FIG. 13 , the angle φ 2  which the surface of the recording paper M forms with a line L 2 , which is drawn through the irradiation center and the center of the photosensor  113 , is 170 degrees. 
       FIG. 14  illustrates the arrangement of a third photosensor according to an example embodiment of the present invention. The photosensor  115  is arranged between the photosensor  114  and the photosensor  113 , with reference to the X-axis direction. As illustrated in  FIG. 14 , the angle φ 3  which the surface of the recording paper M forms with a line L 3 , which is drawn through the irradiation center and the center of the photosensor  115 , is 120 degrees. 
     Note that the center of the light source  111 , the irradiation center, the center of the polarizing filter  116 , the centers of the photosensors  113 ,  114 , and  115  are substantially on the same plane. 
     Note that the direction in which the light source  111  and the photosensors  113 ,  114 , and  115  are arranged is not limited to the above configuration. For example, the direction in which the light source  111  and the photosensors  113 ,  114 , and  115  are arranged may be orthogonal to the direction in which the recording paper is inserted, or may be inclined towards the direction in which the recording paper is inserted. 
       FIG. 15A  illustrates surface regular reflection light, and  FIG. 15B  illustrates surface diffuse reflection light. The light reflected from recording paper when the recording paper is irradiated with light is classified into two kinds of reflected light, consisting of the light reflected at the surface of the recording paper, and the light reflected inside the recording paper. Moreover, the light reflected at the surface of the recording paper is classified into two kinds of reflected light, consisting of the light of regular reflection and the light of diffuse reflection. Hereinafter, the light reflected by regular reflection on the recording paper is referred to as “surface regular reflection light”, and the light reflected by diffuse reflection is referred to as “surface diffuse reflection light” for the purpose of simplification, as illustrated in  FIGS. 15A and 15B . 
     The surface of recording paper is composed of plane portions and oblique portions, and the smoothness of the recording paper is determined by the ratio of the plane portions to the oblique portions. The light reflected at a plane portion becomes surface regular reflection light, and the light reflected at an oblique portion becomes surface diffuse reflection light. The surface diffuse reflection light is completely-dispersed reflection light, and it is assumable that the direction of reflection is isotropic. When the smoothness is high, the light quantity of the surface regular reflection light increases accordingly. 
       FIG. 15C  illustrates internal reflection light. When the recording paper is ordinary printing paper, the reflected light from the inside of the recording paper is composed of only diffuse reflection light because multiple scattering occurs inside the fibers of the recording paper. Hereinafter, the reflected light from the inside of the recording paper is referred to as “internal reflection light” for the purpose of simplification, as illustrated in  FIG. 15C . The internal reflection light is also completely-dispersed reflection light in a similar manner to the surface diffuse reflection light, and it is assumable that the direction of reflection is isotropic. 
     The polarization direction of the surface regular reflection light and surface diffuse reflection light towards a photosensor is the same as the polarization direction of the incident light. Note that the polarization direction rotates on the surface of recording paper only when the incident light is reflected on a part of the surface that is inclined in the direction of the rotation with reference to the incident direction. Because the center of the light source, the irradiation center, and the centers of the photosensors are substantially on a same plane, the light whose polarization direction is rotated on the surface of recording paper is not reflected to any of the photosensors. 
     By contrast, the polarization direction of the internal reflection light is rotated with reference to the polarization direction of the incident light. This happens as the light that enters recording paper is transmitted through Fibers and is subject to optical rotation while experiencing multiple scattering, and the polarization direction is rotated. 
       FIG. 16  illustrates the reflected light that enters the polarization filter  116 , according to an example embodiment of the present invention. As illustrated in  FIG. 16 , reflection light including the surface diffuse reflection light and internal reflection light enters the polarizing filter  116 . 
     As the surface diffuse reflection light has the s-polarized light same as that of the incident light, the surface diffuse reflection light that enters the polarizing filter  116  is blocked at the polarizing filter  116 . On the other hand, the internal reflection light has both the s-polarized light and p-polarized light, and the p-polarized light passes through the polarizing filter  116 . In other words, the p-polarized light included in the internal reflection light is received at the photosensor  114 .  FIG. 17  illustrates the light received by the photosensor  114 , according to an example embodiment of the present invention. Hereinafter, the p-polarized light included in the internal reflection light may be referred to as “p-polarized internal reflection light” for the purpose of simplification. In a similar manner, the s-polarized light included in the internal reflection light may be referred to as “s-polarized internal reflection light”. 
     According to the experiments run by the inventor and his associates, it is known that the amount of the p-polarized internal reflection light correlates with the thickness or density of recording paper. This is because the amount of the p-polarized internal reflection light is dependent upon the length of the path where the light passes through the fibers of the recording paper. 
       FIG. 18  illustrates the light received by the photosensor  113 , according to an example embodiment of the present invention. As illustrated in  FIG. 18 , reflection light including the surface regular reflection light, surface diffuse reflection light, and internal reflection light enters the photosensor  113 . At this light receiving position, the amount of the surface diffuse reflection light and internal reflection light is very small compared with the amount of the surface regular reflection light. For this reason, it can be assumed that the amount of the light received at the photosensor  113  is substantially equivalent to the amount of the surface regular reflection light. 
     As illustrated in  FIG. 18 , reflection light including the surface diffuse reflection light and internal reflection light enters the photosensor  115 . At this light receiving position, the amount of the internal reflection light is very small compared with the amount of the surface diffuse reflection light. For this reason, it can be assumed that the amount of the light received at the photosensor  115  is substantially equivalent to the amount of the surface diffuse reflection light. 
     Each of the photosensors transmits an electrical signal to the processing device  130  based on the amount of the received light. 
       FIG. 19  illustrates the configuration of the processing device  130  according to an example embodiment of the present invention. As illustrated in  FIG. 19 , the processing device  130  according to the example embodiment of the present invention includes a central processing unit (CPU)  131 , a read only memory (ROM)  132 , a random access memory (RAM)  133 , a light source drive circuit  134 , a current/voltage converter  135 , and an analog-to-digital (A/D) converter  136 . The power is supplied to the processing device  130  from the printer controller  2090  through a cable, and the processing device  130  is controlled by the printer controller  2090  through a wired interface such as a universal serial bus (USB) or RS-232C. 
     The ROM  132  stores a program described by codes readable by the CPU  131  and various kinds of data used for executing the program. The RAM  133  serves as a working memory. 
     The light source drive circuit  134  transmits a light-source driving signal to the light source Ill in accordance with the instructions provided by the CPU  131 . 
     The current/voltage converter  135  converts the current signal output from the photosensors into a voltage signal. The A/D converter  136  converts the voltage signal output from the current/voltage converter  135  from an analog signal to a digital signal. 
     The CPU  131  identifies the brand of recording paper according to the program stored in the ROM  132 . The printer controller  2090  is notified of the result of identification. 
       FIGS. 20A to 20C  illustrate how the supporting member  140  moves when the recording paper M is inserted into a slit, according to an example embodiment of the present invention.  FIG. 21  illustrates the pressing force exerted by the spring members  170  on the supporting member  140 , according to an example embodiment of the present invention. When the recording paper M is inserted into the slit and the front end of the recording paper M reaches the tapered surface of the supporting member  140 , the supporting member  140  is pressed down towards the −Z side. Then, the spring members  170  contract, and the supporting member  140  moves towards the −Z side by the thickness of the recording paper M. Accordingly, as illustrated in  FIG. 21 , the supporting member  140  is pressed towards the +Z side due to the resilience force of the spring members  170 . Note that a guide is provided for the housing  150  such that the supporting member  140  can smoothly move in the Z-axis direction. 
     As a result, the recording paper M in full contact with the optical system  110 . By so doing, the irradiation position or angle of incidence of the light is fixed with reference to the recording paper M, and a stable amount of reflection light can be obtained. In other words, an improved precision is achieved in identifying the brand of the recording paper M. If the recording paper M is not in full contact with the optical system  110 , the irradiation position or angle of incidence of the light fluctuates, and the brand of the recording paper M cannot be identified with precision. 
     Moreover, when the recording paper M is in full contact with the optical system  110 , the optical system  110  becomes completely closed and the noise generated by disturbance light can be prevented. Further, the light (laser light) can be prevented from leaking to the outside of the sensor device  100 , and the security for an operator is improved accordingly. 
     The intensity of the pressing force applied to the supporting member  140  by the spring members  170  is controlled such that an operator can easily pull out the recording paper M from the sensor device  100 . 
     Here, the holding force applied to the recording paper M by the optical system  110  and the supporting member  140  is controlled such that the recording paper M can smoothly move without being caught in the sensor device  100  when the recording paper M is moved relative to the sensor device  100 . If the pressing force applied to the recording paper M is too great, it may become difficult to smoothly move the recording paper M, or the recording paper M may be damaged when the recording paper N is moved relative to the sensor device  100 . On the other hand, if the pressing force applied to the recording paper M is too weak, a gap is created between the recording paper M and the optical system  110 , and the brand of the recording paper M can not be identified with precision. 
     When very thin recording paper is inserted into a narrow gap, such insertion may fail due to crimping caused to the recording paper. In order to prevent such crimping, a mechanism that increases the gap when recording paper is to be inserted may be provided, but it is likely that such provision leads to complication in structure or an increase in cost. 
       FIG. 22  illustrates the pulling force applied to recording paper when the recording paper is inserted into a slit, according to an example embodiment of the present invention. In the present example embodiment, a slit is provided on the −X-side surface of the housing  150  to which the recording paper is inserted, and the slit penetrates to +Y-side surface and the −Y-side surface that are adjacent to the −X-side surface. Accordingly, in the present example embodiment, as illustrated in  FIG. 22 , even very thin recording paper can be fed between the optical system  110  and the supporting member  140  without causing bending or crimping, by holding the recording paper with both hands and inserting the recording paper into the slit with tension (pulling force) being applied to the right and left sides of the recording paper with reference to the insertion direction. 
     In the present example embodiment, a plurality of brands of recording paper, which arc compatible with the color printer  2000 , are inserted into the slit of sensor device  100 , and the output levels of the photosensors when the light source  111  is switched on are obtained. The results of the above obtaining processes are stored in the ROM  132  of the processing device  130  as “brand-specific output level data”. The brand-specific output level data refers to a database in which the output levels of the photosensors, which are obtained for the already-known different brands of recording paper, are recorded. 
       FIG. 23  illustrates brand-specific output level data according to an example embodiment of the present invention. Here, the output levels of the photosensor  113 , the photosensor  114 , and the photosensor  115  are referred to as S 1 , S 2 , and S 3 , respectively. 
     Next, processes in which the brand of the recording paper M is identified (brand identification processes) when the brand of the recording paper M is unknown are described. 
     Firstly, the operation performed by an operator in the brand identification processes are described. 1. The sensor device  100  is placed on the flat surface arranged near the operation panel. 2. To-be-identified recording paper is inserted into the slit of the sensor device  100  with tension (pulling force) being applied to both sides of the recording paper in the Y-axis direction. 3. A request for an identification process is made through the operation panel. 
     This request for an identification process is sent to the processing device  130  of the sensor device  100  through the operation panel and the printer controller  2090 . 
     Once the request for an identification process is received, the processing device  130  starts a brand identification process.  FIG. 24  is a flowchart of a series of computational algorithms executed by the CPU  131  of the processing device  130  when brand identification processes are performed. 
     In the first step S 401 , a variable to that indicates the value of a time counter is initialized with 0. Here, it is assumed that the variable to is increased by 1 (+1) for every 10 milliseconds (msec) in the interruption process of a timer. 
     In the next step S 403 , a variable m that indicates the number of times the signals output from photosensors have been obtained is initialized with 0. 
     In step S 405 , the light source  111  is switched on. Here, a plurality of light-emitting units are switched on at the same time. 
     In the next step S 407 , whether or not the variable to is equal to or greater than “300” is determined. In other words, whether or not three seconds have passed since the light source  111  was switched on is determined. When this determination is negative, the process shifts to step S 409 . 
     In step S 409 , the signals output from photosensors are obtained. 
     In the next step S 411 , the variable m is increased by 1 (+1). 
     In the next step S 413 , the data obtained in step S 409  is stored in the RAM  133  together with the value of the variable m. Then, the process returns to step S 407 . 
     The processes in steps S 409  to S 413  are repeated until the determination in step S 407  becomes positive. 
     When the determination becomes positive in step S 407 , the process shifts to step S 415 . 
     In step S 415 , the light source  111  is switched off. Assuming that the value of the variable in is “P”, P pieces of data are stored in the RAM  133  for each of the photosensors. 
     In step S 417 , a mean value of P output levels is calculated for each photosensor. The mean values of the output level of the photosensor  113 , the photosensor  114 , and the photosensor  115  arc referred to as S 1 ′, S 2 ′, and S 3 ′, respectively. SI&#39;, S 2 ′, and S 3 ′ are used as measurement data. 
     In step S 419 , the brand-specific output level data stored in the ROM  132  is referred to, and a relevance ratio R is calculated for each brand by using formula (1) below. 
     
       
         
           
             
               
                 
                   
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     In step S 421 , the brand for which the calculated relevance ratio R becomes the greatest is identified as the brand of the recording paper. 
     In step S 423 , the printer controller  2090  is notified of the identified brand of the recording paper. Then, the brand identification process is terminated. 
       FIGS. 25A and 25B  illustrate the relative movement of recording paper according to an example embodiment of the present invention.  FIG. 26  illustrates a plurality of detection positions according to an example embodiment of the present invention. An operator may move at least one of the sensor device  100  and the recording paper M, as illustrated in  FIGS. 25A and 25B , while the light source  111  is being switched on in the brand identification processes. In such cases, different P positions on the recording paper M are used as detection positions, as illustrated in  FIG. 26 . Accordingly, nonuniformity in the state of surface or the density of the fiber of the recording paper is averaged, and measurement can be performed with minimized effect of such nonuniformity. Accordingly, identification accuracy can be improved. Note that the P positions are not necessarily equally spaced. 
     The direction in which the sensor device  100  or the recording paper M is moved is not limited to the Y-axis direction. For example, the direction in which the sensor device  100  or the recording paper M is moved may be the direction in which the recording paper M is pulled Out from the sensor device  100  (i.e., the -X direction in the present example embodiment). 
     When an operator does not move any one of the sensor device  100  and the recording paper M while the light source  111  is being switched on in the brand identification processes, data is obtained P times at a single detection position. 
     The printer controller  2090  controls the display unit of the operation panel to display (he brand of the recording paper identified by the sensor device  100 , and stores the identification results in the RAM of the printer controller  2090 . 
     When the identified brand of the recording paper is displayed on the display unit of the operation panel, an operator returns the sensor device  100  to the originally disposed position. Then, the operator sets the recording paper of the specified brand to the paper feed tray  2060 . The brand of the recording paper displayed on the display unit of the operation panel may be registered into the printer controller  2090  by using the keys on the operation panel. 
     Then, the printer controller  2090  reads the brand of the recording paper from the RAM when a request for a print job is received, and then determines optimal developing conditions and transferring conditions from the development and transfer table for the specified brand of the recording paper. 
     Then, the printer controller  2090  controls the development device and transfer device of each station in accordance with the determined optimal developing conditions and transferring conditions. For example, the printer controller  2090  controls the transfer voltage or the amount of toner. Accordingly, a high-quality image is formed on recording paper. 
     As described above, the sensor device  100  according to the present example embodiment includes the optical system  110 , the processing device  130 , the supporting member  140 , the housing  150 , and the spring members  170 . 
     The housing  150  has a slit into which recording paper is inserted to identify the brand of the recording paper. When recording paper is inserted into the slit, the recording paper is sandwiched between the optical system  110  and the supporting member  140 . The supporting member  140  is attached to the housing  150  through the spring members  170 . When recording paper is fed between the optical system  110  and the supporting member  140 , pressing force is applied by the spring members  170  in the +Z-direction of the supporting member  140 . By so doing, the recording paper is in full contact with the optical system  110 , and the irradiation position or angle of incidence of the light is fixed with reference to the recording paper, and a stable amount of reflection light can be obtained. 
     The CPU  131  of the processing device  130  measures the output levels of the photosensors when the light source  111  of the optical system  110  is switched on to irradiate to-be-identified recording paper with light, and calculates a relevance ratio R by referring to the brand-specific output level data and using the formula (1) described above. Further, the CPU  131  identifies the brand of the to-be-identified recording paper based on the calculated relevance ratio R. 
     As the spring members  170 , inexpensive metal springs may be used. A variety of such inexpensive metal springs are commercially available and easily accessible. Accordingly, a sensor device can be realized at low cost. 
     Because a surface emitting laser array is used as a light source in the present example embodiment, a polarizing filter is not required to obtain linearly polarized light as irradiation light. Further, the use of a surface emitting laser array realizes high-density integration of a plurality of light-emitting units, which was difficult to achieve with the conventional LEDs or is the like. In such cases, a small-sized light source having a plurality of light-emitting units can be realized. Moreover, the laser light can be all condensed to the optical axis of a collimate lens, and a plurality of rays of light can be collimated with a fixed angle of incidence. In other words, a collimate optical system can be realized at low cost. Accordingly, the cost and size of an optical system can be reduced. 
     In the brand identification processes, a plurality of light-emitting units of a surface emitting laser array are switched on at the same time. Accordingly, the signal-to-noise ratio (S/N) improves in the output of the photosensors, and identification accuracy also improves. Moreover, the contrast ratio of the speckle pattern of reflection light decreases by switching on all the light-emitting units at the same time, and the amount of reflection light can be measured more accurately. Accordingly, the identification accuracy improves. 
     Further, the light quantity of the internal reflection light can be increased by switching on the light-emitting units at the same time. Accordingly, it becomes possible to achieve precise separation of the internal reflection light in the optical system  110 . Conventionally, such separation of the light reflected from the inside of recording paper was difficult to achieve. The reflection light from the inside of recording paper includes the information about the inside state of the recording paper. 
     The CPU  131  of the processing device  130  identifies the brand of recording paper based on the signals output from the three photosensors. By taking into consideration the information about the inside state of the recording paper, it becomes possible to improve the level of paper identification such that the level of the recording paper can be determined. Conventionally, such identification of brand was difficult. 
     Moreover, as it is not necessary to combine various kinds of sensors and the configuration of parts is simple according to the present example embodiment, it is possible to achieve a small sensor device at low cost. 
     The sensor device  100  according to the present example embodiment can precisely identify an object with simple configuration. 
     Due to the provision of the sensor device  100 , the color printer  2000  according to the present example embodiment can form a high-quality image without increasing the cost or size. Further, troublesome manual settings or failure in printing due to a setting error, which are still present in the conventional products, can be eliminated according to the present example embodiment. 
       FIG. 27  illustrates a first modification (1) of a sensor device according to an example embodiment of the present invention.  FIG. 28  illustrates a first modification (2) of a sensor device according to an example embodiment of the present invention. In the example embodiment described above, as illustrated in  FIG. 27  and  FIG. 28 , a through hole may be made through the supporting member  140  directly below the opening of the dark box  119  on the −Z-side), and a photosensor  117  may be provided to receive transmission light that has passed through the recording paper M. 
     According to the experiments run by the inventor and his associates, it is known that the intensity distribution of the light that passes through the paper follows Lambert&#39;s law.  FIG. 29  illustrates the characteristics of transmission light according to an example embodiment of the present invention. The light that has entered the paper is dispersed by the fibers of the paper, and has an angle distribution that is not dependent on the angle of incidence, where the peak is at the light intensity of the direction orthogonal to the surface of the paper. For this reason, it is desired that the photosensor  117  be arranged directly underneath the irradiation center. 
     Because the light that passes through the paper is dispersed by the fibers of the paper, the amount of the transmission light decreases when the paper is thick, i.e., when the basis weight is greater. According to the present example embodiment, the identification accuracy of recording paper can be improved by measuring the amount of the transmission light in consideration of the basis weight of the recording paper, as described above. 
     In such cases, a relevance ratio R is calculated by using formula (2) below.  FIG. 30  illustrates brand-specific output level data that corresponds to the first modification of a sensor device, according to an example embodiment of the present invention. Here, as depicted in  FIG. 30 , the output level of the photosensor  117  is referred to as S 4  in the brand-specific output level data, and a mean value of the output levels of the photosensor  117  when to-be-identified recording paper is irradiated with light is referred to as S 4 ′. 
     
       
         
           
             
               
                 
                   
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     In the embodiments described above, cases which the optical system  110  includes three photosensors for receiving reflection light were described. However, no limitation is indicated therein, and the optical system  110  may include any number of photosensors depending on the desired identification accuracy. 
       FIG. 31  illustrates a second modification of the Sensor device  100  according to an example embodiment of the present invention. In  FIG. 31 , the optical system  110  according to the example embodiment described above from which the photosensor  115  is removed is illustrated. In such cases, a relevance ratio R is calculated by using formula (3) below 
     
       
         
           
             
               
                 
                   
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       FIG. 32  illustrates a third modification (1) of the sensor device  100  according to an example embodiment of the present invention. In  FIG. 32 , the optical system  110  according to the example embodiment described above to which a photosensor  120  and a polarizing filter  121  are added is illustrated. As illustrated in  FIG. 32 , the polarizing filter  121  is arranged on the optical path of reflection light including the surface diffuse reflection light and internal reflection light. The polarizing filter  121  transmits the p-polarized light and blocks the s-polarized light. The photosensor  120  is arranged on the optical path of the light that has passed through the polarizing filter  121 . The photosensor  120  receives the p-polarized light included in the internal reflection light. 
       FIG. 33  illustrates a third modification (2) of the sensor device  100  according to an example embodiment of the present invention. As illustrated in  FIG. 33 , for example, the angle φ 4  which the surface of the recording paper M forms with a line L 4 , which is drawn through the irradiation center and the centers of the polarizing filter  121  and photosensor  120 , is 150 degrees. Note that the center of the light source  111 , the irradiation center, the centers of the polarizing filters, the centers of the photosensors are substantially on the same plane. 
     In such cases, a relevance ratio R is calculated by using formula (4) below.  FIG. 34  illustrates brand-specific output level data that corresponds to the third modification of the sensor device  100 , according to an example embodiment of the present invention. Here, as depicted in  FIG. 34 , the output level of the photosensor  120  is referred to as S 5  in the brand-specific output level data, and a mean value of the output levels of the photosensor  120  when to-be-identified recording paper is irradiated with light is referred to as S 5 ′. 
     
       
         
           
             
               
                 
                   
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       FIG. 35  illustrates a sensor for detecting the insertion of recording paper, according to an example embodiment of the present invention. In the example embodiment described above, as illustrated in  FIG. 35 , the sensor device  100  may be provided with a sensor  125  that detects the insertion of the recording paper M. As illustrated in  FIGS. 36A and 36B , the sensor  125  includes a light emitting element  125   a  and a light receiving element  125   b  that are arranged to face each other in the Z-axis direction. When the light emitted from the light emitting element  125   a  is received by the light receiving element  125   b,  it is determined that “no recording paper is inserted”. On the other hand, when the light emitted from the light emitting element  125   a  is not received by the light receiving element  125   b,  it is determined that “recording paper is inserted”. In other words, the sensor  125  is a so-called transmission photo interrupter sensor. However, a different type of sensor may also be used. In other words, any sensor may be used as long as that sensor can detect the insertion of recording paper. 
     In the example embodiment described above, the light source  111  and the related processes may be controlled as follows. The light source  111  is switched on and the output of the photosensors is obtained when it is determined that “recording paper is inserted”. On the other hand, the light source  111  is switched off and data processing is performed when it is determined that “no recording paper is inserted”. As described above, it can be configured such that measurement will be performed only when recording paper is inserted into the slit. Accordingly, measurement is not erroneously performed when no recording paper is inserted. Moreover, it may be configured such that measurement will start automatically without any instruction through the operation panel, or power consumption may be reduced by minimizing the length of time during which the light source  111  is switched on. 
     Moreover, in the example embodiment described above, the sensor device  100  may be provided with an LED that illuminates when it is determined that “recording paper is inserted”. Due to such provision of an LED, an operator can visually notice that recording paper has been inserted. 
     In the example embodiment described above, at least some of the processes performed by a program, which is executed by the CPU  131 , may be performed by hardware, or all of the processes may be performed by hardware. 
     Note that cases in which recording paper is irradiated with the s-polarized light have been described in the example embodiment described above, but no limitation is indicated therein. In other words, recording paper may be irradiated with the p-polarized light. In such a case, however, a polarizing filter that transmits the s-polarized light is used in place of the polarizing filter  116 , and the photosensor  114  receives the s-polarized light included in the internal reflection light. 
       FIG. 37  illustrates a modification of a surface emitting laser array according to an example embodiment of the present invention. As illustrated in  FIG. 37 , at least some of light-emitting units of a surface emitting laser array may be spaced differently from the other light-emitting units. In other words, the space between adjacent light-emitting units may vary. 
     In the example embodiment described above, eases in which the light source  111  includes nine light-emitting units were described. However, no limitation is indicated therein. 
     In the example embodiment described above, a conventional laser diode (LD) may be used in place of the surface emitting laser array. 
     In the example embodiment described above, cases in which the light source  111  emits Linearly polarized light were described. However, no limitation is indicated therein. 
       FIG. 38  illustrates a fourth modification of the sensor device  100  according to an example embodiment of the present invention. In the example embodiment described above, as illustrated in  FIG. 38 , a polarizing filter  126  is provided to obtain s-polarized light from the light emitted from the light source  111 . 
     In the example embodiment described above, a different kind of elastic member may be used in place of the spring members  170 . 
       FIG. 39  illustrates a fifth modification (1) of the sensor device  100  according to an example embodiment of the present invention.  FIG. 40  illustrates a fifth modification (2) of the sensor device  100  according to an example embodiment of the present invention. As illustrated in  FIGS. 39 and 40 , for example, a plurality of rubber members  174  may be used in place of the spring members  170 . Moreover, a porous resin member such as a sponge may be used in place of the spring members  170 . 
       FIG. 41  illustrates a sixth modification (1) of the sensor device  100  according to an example embodiment of the present invention.  FIG. 42  illustrates a sixth modification (2) of the sensor device  100  according to an example embodiment of the present invention. In the example embodiment described above, as illustrated in  FIGS. 41 and 42 , for example, a plurality of magnets  176  may be used in place of the spring members  170 . In the present modification, the repulsive force caused between two magnets that are disposed in the Z-axis direction is used. There is some concern that spring members or rubber members become deteriorated due to aging and become incapable of appropriately maintaining the holding force. For this reason, long-term use of spring members and rubber members is to be avoided. By contrast, repulsive force does not deteriorate easily when magnets are used. For this reason, magnets are suitable for long-term use Magnets may be arranged to use the attractive force between two magnets. 
     Note that cases in which the supporting member  140  is pressed down by recording paper when the recording paper is inserted into the slit of the housing  150  were described in the example embodiment described above, but no limitation is indicated therein. It may be configured such that the dark box  119  is lifted up by recording paper. 
       FIG. 43  illustrates a seventh modification (1) of the sensor device  100  according to an example embodiment of the present invention. In the example embodiment described above, the dark box  119  may be omitted as illustrated in  FIG. 43 . 
       FIG. 44  illustrates a seventh modification (2) of the sensor device  100  according to an example embodiment of the present invention. In the modification described above with reference  FIG. 43 , the housing  150  does the job of the −Z-side surface of the dark box  119 , as illustrated in  FIG. 44 . 
     In the example embodiment described above, it is desired that a condensing lens be provided in front of each of the photosensors. By so doing, the changes in the amount of the light received at photosensors can be reduced. 
     In the example embodiment described above, cases in which the processing device  130  performs brand identification processes were described, but no limitation is indicated therein. For example, the printer controller  2090  may perform brand identification processes. In that case, the brand-specific output level data may be stored in a ROM of the printer controller  2090 . 
     In the example embodiment described above, cases in which the light source  111  illuminates for three seconds in the brand identification processes were described. However, no limitation is indicated therein. The length of time during which the light source  111  illuminates in the brand identification processes may be set through the operation panel. 
     In the example embodiment described above, a start button for the identification process may be provided for the sensor device  100 . In that case, it is not necessary for an operator to input a request for an identification process through the operation panel. 
     In the example embodiment described above, an LED that indicates the on/off of the light source  111  may be provided for the sensor device  100 . Due to such provision of an LED, an operator can visually know the on/off of the light source  111 . 
     In the example embodiment described above, a display unit may be provided for the sensor device  100 . In that case, the processing device  130  controls the display unit to display the identification results. 
     In the example embodiment described above, cases in which the outside shape of the housing  150  of the sensor device  100  is like a block were described. However, no limitation is indicated therein. For example, the outside shape of the housing  150  may have a curve like a mouse. In other words, the housing  150  may have any outside shape as long as the housing  150  has a slit of prescribed length in the direction in which the recording paper is inserted. 
     In the example embodiment described above, an object to be identified by the sensor device  100  is not limited to recording paper. 
     Note that cases in which recording paper is pressed and held such that the relative positions of the recording paper inserted into a slit and a plurality of photosensors are fixed were described in the example embodiment described above, but no limitation is indicated therein. For example, the recording paper may be held by electrostatic force instead of the pressing force, or the recording paper may be held by suction (negative pressure). 
       FIG. 45  illustrates an irradiation area according to an example embodiment of the present invention. In the example embodiment described above, as illustrated in  FIG. 45 , the shape of an irradiation area is elliptic where the major (longer) axis is the X axis and the minor (shorter) axis is the Y axis. The length of the minor axis (BM 1 ) is 2.3 mm, and the length of the major axis (BM 2 ) is 13.4 mm. Preferably, the opening made on the dark box  119  is made along the circumference of the irradiation area and the size is as small as possible, as long as the opening does not cover the irradiation area. 
       FIG. 46  illustrates a desired shape of an opening according to an example embodiment of the present invention. In the example embodiment described above, some margin is provided in consideration of tolerance or adjustment of beam, and a preferred shape of the opening is, as illustrated in  FIG. 46 , a rectangle where the length in the Y-axis direction (K 1 ) is 4 mm and the length in the X-axis direction (K 2 ) is 15 mm. In other words, the opening is shaped to have the longitudinal direction parallel to the direction in which a light source and a plurality of photosensors are arranged (i.e., X-axis direction). 
     If the opening is narrower than the irradiation area and some of the irradiation area is covered by the −Z-side surface of the dark box  119 , the light reflected at the −Z-side surface of the dark box  119  become stray light, and such stray light may be received by a photosensor. In that case, the signal-to-noise ratio (S/N) of the signal output from the photosensor decreases, and the identification accuracy deteriorates. 
     By contrast, if the opening is much wider than the irradiation area, dust or paper dust may enter the dark box  119  and stick to a photosensor. If dust or paper dust sticks to a photosensor, the amount of the light received at the photosensor varies, and the identification accuracy deteriorates. Moreover, if the opening is too wide, bending of recording paper may occur, and such bending makes it difficult to measure the reflected light with precision. 
     If the reduction in identification accuracy is no concern, the opening may be shaped differently from the preferred configuration described above. As long as an opening is arranged on a part of the −Z-side surface of the dark box  119 , recording paper can be held by the −Z-side surface of the dark box  119 . 
       FIG. 47  illustrates a first modification of an opening according to an example embodiment of the present invention. When there is sonic concern that recording paper is caught at an edge of the opening, as illustrated in  FIG. 47  (and  FIGS. 49A and 49B  and  FIGS. 50A and 50B  as will be described later), in the first modification of the opening, the center of the edge that is on the leading-end side of the direction in which the recording paper is inserted (i.e., on the +X side) is positioned on a further leading-end side of the insertion direction than at least one of the two ends of the above edge. Accordingly, it becomes possible to prevent the recording paper from being caught at an edge of the opening. 
       FIG. 48  illustrates the arrangement of a light source and a plurality of photosensors nee to an opening of a modification, according to an example embodiment of the present invention. Note that, as illustrated in  FIG. 48 , the direction in Which light source and a plurality of photosensors are arranged is the Y-axes direction, which is different from the example embodiment described above. 
     In  FIG. 47 , the above-described edge of the opening is convexly curved towards the direction in which the recording paper is inserted, and the center of the above-described edge is positioned on a further leading-end side of the insertion direction than at least one of the two ends of the edge. 
       FIG. 49A  illustrates a second modification of an opening according to an example embodiment of the present invention. As illustrated in  FIG. 49A , in the second modification of the opening, the shape of the opening is elliptic. Moreover, the above-described edge of the opening is convexly curved towards the direction in which the recording paper is inserted, and the center of the above-described edge is positioned on a further leading-end side of the insertion direction than at least one of the two ends of the edge. 
       FIG. 49B  illustrates a third modification of an opening according to an example embodiment of the present invention. As illustrated in  FIG. 49B , in the third modification of the opening, the shape of the opening is semiellptic. Moreover, the above-described edge of the opening is convexly curved towards the direction in which the recording paper is inserted, and the center of the above-described edge is positioned on a further leading-end side of the insertion direction than at least one of the two ends of the edge. 
       FIG. 50A  illustrates a fourth modification of an opening according to an example embodiment of the present invention. As illustrated in  FIG. 50A , in the fourth modification of the opening, the shape of the opening is triangular. Moreover, the above-described edge is composed of two sides of a triangle, and the point of intersection of these two sides is the center of the edge. The center of the above-described edge is positioned on a further leading-end side of the insertion direction than at least one of the two ends of the edge. 
       FIG. 50B  illustrates a fifth modification of an opening according to an example embodiment of the present invention. As illustrated in  FIG. 50B , in the fifth modification of the opening, the opening is shaped like a parallelogram. Moreover, the above-described edge of the opening is a straight line that is inclined with reference to the direction orthogonal to the direction in which the recording paper is inserted, and the center of the above-described edge is positioned on a further leading-end side of the insertion direction than at least one of the two ends of the edge (i.e., the end on the −Y side). 
       FIGS. 51A and 51B  illustrate other methods of preventing recording paper from being caught at an edge of an opening, according to an example embodiment of the present invention. As illustrated in  FIG. 51A  for example, the direction in which light source and a plurality of photosensors are arranged may be inclined with reference to the direction in which the recording paper is inserted. In that case, as illustrated in  FIG. 51B , even if the opening is rectangular, it becomes possible to prevent the recording paper from being caught at an edge of the opening. Moreover, the above-described edge of the opening is the long side of the parallelogram on the +X side, and the center of the above-described edge is positioned on a further leading-end side of the insertion direction than at least one of the two ends of the edge (i.e., the end on the −Y side). 
     In the example embodiment described above, cases in which the color printer  2000  is used as an image forming apparatus were described, but no limitation is indicated therein. For example, a laser printer that forms monochrome images may be used as an image forming apparatus. Moreover, an image forming apparatus other than a printer, for example, a copier, a facsimile, or a multifunction peripheral (MFP) into which these elements are integrated, may be used. 
     In the example embodiment described above, cases in which an image forming apparatus includes four photoreceptor drums were described, but no limitation is indicated therein. For example, a printer for which five photoreceptor drums are provided may be used as an image forming apparatus. 
     In the example embodiment described above, eases of an image forming apparatus in which a toner image is transferred from a photoreceptor drum to a transfer belt and then is transferred to recording paper were described, but no limitation is indicated therein. For example, an image forming apparatus in which a toner image is directly transferred from a photoreceptor drum to recording paper may be used. 
     The sensor device  100  may be applied to an image forming apparatus in which an image is formed by ejecting ink onto recording paper. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of the present invention may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.