Patent Publication Number: US-9411288-B2

Title: Toner detection sensor and image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2014-239124 filed on Nov. 26, 2014 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates to toner detection sensors and image forming apparatuses. 
     In image forming apparatuses typified by a digital multifunctional peripheral and the like, an image of a document is read by an image reading unit, and a photoreceptor included in an image forming unit is irradiated with light based on the read image, whereby an electrostatic latent image is formed on the photoreceptor. Thereafter, a developer such as charged toner is supplied onto the electrostatic latent image to form a visible image, which is transferred and fixed onto a sheet of paper fed. The sheet is then discharged to the outside of the apparatus. 
     Image forming apparatuses capable of forming full-color images include those which form a full-color image by overlaying images of respective colors of yellow, cyan, magenta, and black on one another. In this case, toner images of the respective colors are once transferred onto a transfer belt as an intermediate transfer body, and the resultant full-color image is transferred onto a sheet of paper. When forming a full-color image, it is necessary to perform corrections at certain timings so as to maintain high levels of color developing property and color reproducibility. In such corrections, the amount of toner on the transfer body is detected, and the developing bias value, the amount of exposure, the timing of exposure and so on are adjusted to achieve a proper amount of toner. 
     Techniques regarding the sensors which detect the amount of toner are conventionally known. 
     A typical gloss sensor is disclosed, in which a projector emits measurement light having a predetermined incident angle onto an object surface, and light reflected from the object surface is measured by a light receiver at the same angle as the incident angle, to measure the glossiness. In this gloss sensor, the projector emits light of a single wavelength. The projector is provided with a polarizer, through which the light is directed onto the object surface as polarized light in one direction. Light reflected from the object surface is transmitted through a polarization beam splitter, where the light is separated into a reflected light component having the polarized light in the same direction as the measurement light and a reflected light component in a different direction. The reflected light components are received by light-receiving means provided respectively for these components, and the outputs from the two light-receiving means are computed to thereby measure the glossiness. 
     A typical image forming apparatus is disclosed, which includes a recording medium conveyance belt rotatably stretched over a plurality of roller members. In this apparatus, at least one specular reflection detection type optical sensor and at least one specular reflection/diffuse reflection simultaneous detection type optical sensor are disposed to face an intermediate transfer body, and at least one specular reflection detection type optical sensor is disposed to face the recording medium conveyance belt or a second image carrier. In this apparatus, the at least one specular reflection detection type optical sensor disposed to face the recording medium conveyance belt or the second image carrier is used for black toner adhesion amount control, while the at least one specular reflection/diffuse reflection simultaneous detection type optical sensor disposed to face the intermediate transfer body is used for remaining color toner adhesion amount control. Further, in this apparatus, the at least one specular reflection/diffuse reflection simultaneous detection type optical sensor and the at least one specular reflection detection type optical sensor disposed to face the intermediate transfer body are used for color alignment control. 
     SUMMARY 
     In an aspect of the present disclosure, a toner-amount detection sensor detects the amount of toner of a toner visible image formed on a surface of a transfer body. The toner-amount detection sensor includes a light-emitting element, a first light-receiving element, and a toner-amount calculating unit. The light-emitting element emits light toward the surface of the transfer body at a predetermined incident angle. The first light-receiving element is disposed on a side opposite to the light-emitting element with respect to a plane normal to the surface of the transfer body. The first light-receiving element receives light reflected from the surface of the transfer body. The toner-amount calculating unit calculates the amount of toner from the quantity of the reflected light received by the first light-receiving element. The sensor has a relationship of A 1 &lt;A 2 &lt;1.5A 1  where A 1  represents the predetermined incident angle with respect to the plane normal to the surface of the transfer body, and A 2  represents an angle of disposition of the first light-receiving element with respect to the plane normal to the surface of the transfer body. 
     In another aspect of the present disclosure, an image forming apparatus includes an image forming unit which forms a visible image with toner and includes a toner-amount detection sensor for detecting the amount of toner of the toner visible image formed on a surface of a transfer body. The toner-amount detection sensor includes a light-emitting element, a first light-receiving element, and a toner-amount calculating unit. The light-emitting element emits light toward the surface of the transfer body at a predetermined incident angle. The first light-receiving element is disposed on a side opposite to the light-emitting element with respect to a plane normal to the surface of the transfer body. The first light-receiving element receives light reflected from the surface of the transfer body. The toner-amount calculating unit calculates the amount of toner from the quantity of the reflected light received by the first light-receiving element. The sensor has a relationship of A 1 &lt;A 2 &lt;1.5A 1  where A 1  represents the predetermined incident angle with respect to the plane normal to the surface of the transfer body, and A 2  represents an angle of disposition of the first light-receiving element with respect to the plane normal to the surface of the transfer body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows an appearance of a digital multifunctional peripheral in the case where an image forming apparatus according to an embodiment of the present disclosure is applied to the digital multifunctional peripheral; 
         FIG. 2  is a block diagram showing the configuration of the digital multifunctional peripheral in the case where the image forming apparatus according to the embodiment of the present disclosure is applied to the digital multifunctional peripheral; 
         FIG. 3  is an appearance diagram schematically showing the configuration of an image forming unit; 
         FIG. 4  is an appearance diagram schematically showing the configuration of a toner-amount detection sensor according to an embodiment of the present disclosure; 
         FIG. 5  is a graph diagram showing the relationship between the reflectance of the surface of a transfer belt and the reflection angle of incident light; 
         FIG. 6  is a graph diagram showing the relationship between the amount of toner and the output of the toner-amount detection sensor in the case of detecting the toner amount of a black toner visible image; 
         FIG. 7  is a graph diagram showing the relationship between the amount of toner and the output of the toner-amount detection sensor in the case of detecting the toner amount of a yellow toner visible image; and 
         FIG. 8  is an appearance diagram schematically showing the configuration of a toner-amount detection sensor according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described below. First, the configuration of a digital multifunctional peripheral in the case where an image forming apparatus according to an embodiment of the present disclosure is applied to the digital multifunctional peripheral will be described.  FIG. 1  schematically shows an appearance of the digital multifunctional peripheral.  FIG. 2  is a block diagram showing the configuration of the digital multifunctional peripheral. 
     Referring to  FIGS. 1 and 2 , the digital multifunctional peripheral  11  includes: a control unit  12 ; an operation unit  13 ; an image reading unit  14 ; a paper setting unit  19 ; an image forming unit  15 ; a discharge tray  30 ; a hard disk  16 ; a facsimile communication unit  17 ; and a network interface unit  18  for connecting with a network  25 . The control unit  12  is responsible for overall control of the digital multifunctional peripheral  11 . The operation unit  13  includes a display screen  21  for displaying information originated from the digital multifunctional peripheral  11  side and content input by a user. The operation unit  13  causes a user to set image forming conditions, such as the number of copies and gradation, and to turn the power on or off. The image reading unit  14  includes an auto document feeder (ADF)  22  which automatically feeds a document that has been set, to a reading section. The image reading unit  14  reads an image of a document. The paper setting unit  19  includes a manual feed tray  28  allowing a user to manually set a sheet thereon, and a paper cassette group  29  capable of storing sheets of paper different in size. One or more sheets of paper to be supplied to the image forming unit  15  are set on the paper setting unit  19 . The image forming unit  15  forms an image on the basis of image data of the read image or on the basis of image data received via the network  25 . The sheet of paper on which an image has been formed by the image forming unit  15  is discharged onto the discharge tray  30 . The hard disk  16  stores, among others, the image data received, and the image forming conditions input. The facsimile communication unit  17 , which is connected to a public line  24 , performs facsimile transmission and reception. Although the digital multifunctional peripheral  11  further includes a dynamic random access memory (DRAM) to and from which image data is written and read, and other components, the illustration and description thereof will be omitted. The arrows in  FIG. 2  indicate flows of control signals as well as data related to control and images. In the present embodiment, as shown in  FIG. 1 , the paper cassette group  29  is composed of three paper cassettes  23   a ,  23   b , and  23   c.    
     The digital multifunctional peripheral  11  operates as a copier by forming an image in the image forming unit  15  using the image data of the document read by the image reading unit  14 . The digital multifunctional peripheral  11  operates as a printer by forming an image and printing the image on a sheet of paper in the image forming unit  15  using image data received from a computer  26   a ,  26   b ,  26   c  connected to the network  25 , via the network interface unit  18 . That is, the image forming unit  15  operates as the printing unit which prints a requested image. Further, the digital multifunctional peripheral  11  operates as a facsimile machine by forming an image in the image forming unit  15  via the DRAM using the image data received from the public line  24  through the facsimile communication unit  17 , or by transmitting the image data of the document read by the image reading unit  14  to the public line  24  through the facsimile communication unit  17 . As such, the digital multifunctional peripheral  11  has a plurality of functions related to image processing, such as a copying function, function as a printer, facsimile function, etc. The digital multifunctional peripheral  11  further has a function enabling detailed settings for each of the above functions. 
     An image forming system  27  according to an embodiment of the present disclosure includes the digital multifunctional peripheral  11  having the above-described configuration, and a plurality of computers  26   a ,  26   b ,  26   c  connected to the digital multifunctional peripheral  11  via the network  25 . In this embodiment, three computers  26   a - 26   c  are shown by way of example. Each of the computers  26   a - 26   c  is able to issue a print request to the digital multifunctional peripheral  11  via the network  25  for printing. The digital multifunctional peripheral  11  and the computers  26   a - 26   c  may be connected via wire using a local area network (LAN) cable or the like, or they may be connected wirelessly. Another digital multifunctional peripheral or a server may be connected in the network  25 . 
     The configuration of the image forming unit  15  included in the digital multifunctional peripheral  11  will be described below in more detail.  FIG. 3  is a cross-sectional view showing the schematic configuration of the digital multifunctional peripheral  11  according to an embodiment of the present disclosure. In  FIG. 3 , the hatching of the members are omitted for ease of understanding.  FIG. 3  shows the cross-sectional view of the digital multifunctional peripheral  11  taken along a plane that extends in the up-and-down direction. 
     Referring to  FIG. 3 , the image forming unit  15  includes a toner image forming device  33  which includes four toner image forming units  32   a ,  32   b ,  32   c , and  32   d  corresponding respectively to four colors of yellow, magenta, cyan, and black and including photoreceptors  31   a ,  31   b ,  31   c , and  31   d , respectively. The image forming unit  15  also includes a laser scanner unit (LSU)  34  which exposes the four toner image forming units  32   a - 32   d  to light on the basis of the image read by the image reading unit  14 , a transfer belt  35  as the intermediate transfer body to which the toner visible images formed by the toner image forming units  32   a - 32   d  are temporarily transferred before being transferred onto a sheet, and a transfer belt cleaning unit  37  which uses a blade or the like to remove any toner remaining on the transfer belt  35 . The LSU  34  is shown schematically with a dot chain line. The transfer belt cleaning unit  37  is also shown schematically. The image forming unit  15  is a so-called four-unit tandem type developing system. 
     The transfer belt  35  is endless, and the visible images formed by the yellow, magenta, cyan, and black toner image forming units  32   a - 32   d  are transferred onto the transfer belt  35  as it rotates in one direction by a driving roller  36   b  and a driven roller  36   a . The rotational direction of the transfer belt  35  is shown by an arrow D 1  in  FIG. 3 . Of the toner image forming units  32   a - 32   d , the yellow toner image forming unit  32   a  is disposed most upstream and the black toner image forming unit  32   d  is disposed most downstream in the rotational direction of the transfer belt  35 . The transfer belt cleaning unit  37  is disposed upstream of the yellow toner image forming unit  32   a.    
     The toner visible images transferred on the transfer belt  35  are transferred onto a sheet fed, and fixed onto the sheet by a fixing unit (not shown). The sheet with the image fixed thereon is discharged to the outside of the digital multifunctional peripheral  11 , specifically onto the discharge tray  30 . After the toner visible images are transferred onto the sheet, any toner remaining on the transfer belt  35  is removed by the transfer belt cleaning unit  37 . The process of forming a next image is then carried out. 
     The digital multifunctional peripheral  11  is capable of monochrome printing using only the black toner image forming unit  32   d . The digital multifunctional peripheral  11  is also capable of color printing using at least one of the yellow toner image forming unit  32   a , the magenta toner image forming unit  32   b , and the cyan toner image forming unit  32   c.    
     Here, the control unit  12  included in the digital multifunctional peripheral  11  performs corrections on the densities, positions, and color shifts of the visible images formed on the transfer belt  35  by the toner image forming units  32   a - 32   d , at the timing when the number of printed sheets has reached a predetermined number, specifically once per every 1000 sheets of printed images, at the timing when the drive time has reached a predetermined time, and further at the timing when the environment has changed, specifically when the temperature or humidity has changed abruptly, as well as at the timing when a part of the units constituting the digital multifunctional peripheral  11  is replaced. At the time of regular maintenance, for example, the image forming unit  15  forms patch images on the transfer belt  35  for use in correcting the toner visible images. The image forming unit  15  uses the patch images to change, among others, the amounts of toner to be applied to the transfer belt  35  and the timing and intensity of laser light to be emitted by the LSU  34 , thereby adjusting and correcting the toner densities, color shifts, and the like. It should be noted that the patch images formed are not transferred onto a sheet; they are removed from the surface  38  of the transfer belt  35  by the transfer belt cleaning unit  37 . 
     For such corrections, a toner-amount detection sensor is used which detects the amount of toner of a patch image formed on the transfer belt  35 . That is, the image forming unit  15  includes the toner-amount detection sensor  41  which measures the amount of toner of a toner visible image transferred onto the transfer belt  35 . 
     A description will now be made about the configuration of the toner-amount detection sensor  41  according to an embodiment of the present disclosure.  FIG. 4  schematically shows the configuration of the toner-amount detection sensor  41  of the embodiment. In  FIG. 3 , the toner-amount detection sensor  41  is shown schematically with a two-dot chain line. 
     Referring to  FIGS. 1 to 4 , the toner-amount detection sensor  41  is disposed downstream of the black toner image forming unit  32   d . The toner-amount detection sensor  41  includes: a light-emitting element  42  which emits light toward the transfer belt  35 ; a first light-receiving element  43  which receives light reflected from the surface  38  of the transfer belt  35 ; a second light-receiving element  44  which is provided separately from the first light-receiving element  43  and receives light reflected from the surface  38  of the transfer belt  35 ; and a toner-amount calculating unit  45  which calculates the amount of toner from the quantities of the reflected light received by the first light-receiving element  43  and the second light-receiving element  44 . As an example of the light-emitting element  42 , an infrared light-emitting diode, for example, is adopted. As an example of the first light-receiving element  43  and the second light-receiving element  44 , infrared light-receiving elements, for example, are adopted. 
     The light-emitting element  42  emits light  46   a , such as infrared light, in a diagonally upper left direction shown by an arrow E 1  in  FIG. 4 , toward the surface  38  of the transfer belt  35  or a toner visible image  39  thereon. The light  46   a  is emitted at an incident angle A 1  shown in  FIG. 4 . This angle A 1  is an angle made between a plane  48  normal to the surface  38  of the transfer belt  35 , indicated by a dot chain line in  FIG. 4 , and the irradiation direction of the light  46   a . In the present embodiment, the angle A 1  is also the angle of disposition of the light-emitting element  42  with respect to the plane  48 . It is preferable that the angle A 1  is relatively small, from the standpoint of making as small as possible the changes of the output values from the first and second light-emitting elements  43  and  44  with respect to the change in distance between the light-emitting element  42  and the object to be measured. For example, the angle A 1  preferably falls within the range of at least 10° and less than 12°, and specifically it is set to 11°. 
     The first light-receiving element  43  is disposed on a side opposite to the light-emitting element  42  with respect to the plane  48  normal to the surface  38  of the transfer belt  35 . The first light-receiving element  43  receives light  46   b  which is reflected at an angle close to the specular reflection angle from either one or both of the toner visible image  39  and the surface  38  of the transfer belt  35 , in a diagonally lower left direction indicated by an arrow E 2  in  FIG. 4 . In the case where the toner visible image  39  completely covers the surface  38  of the transfer belt  35 , the light  46   b  reflected from the toner visible image  39  alone is received. In the case where no toner visible image  39  has been formed on the surface  38  of the transfer belt  35 , the light  46   b  reflected from the surface  38  of the transfer belt  35  alone is received. In the case where the toner visible image  39  has not completely covered the surface  38  of the transfer belt  35  and the amount of toner of the toner visible image  39  is small, then the light  46   b  reflected from both the toner visible image  39  and the surface  38  of the transfer belt  35  is received. The light  46   b  is received at an angle A 2  shown in  FIG. 4 . In the present embodiment, the angle A 2  is an angle of disposition of the first light-receiving element  43  with respect to the plane  48 . The direction of specular reflection, which is reflected specularly at the angle A 1 , is indicated by a broken line  47  for reference. 
     The second light-receiving element  44  is disposed on the same side as the light-emitting element  42  with respect to the plane  48  normal to the surface  38  of the transfer belt  35 . The second light-receiving element  44  receives diffuse reflection light  46   c  from either one or both of the toner visible image  39  and the surface  38  of the transfer belt  35 , in a diagonally lower right direction indicated by an arrow E 3  in  FIG. 4 . In the case where the toner visible image  39  completely covers the surface  38  of the transfer belt  35 , the diffuse reflection light  46   c  from the toner visible image  39  alone is received. In the case where no toner visible image  39  has been formed on the surface  38  of the transfer belt  35 , the diffuse reflection light  46   c  from the surface  38  of the transfer belt  35  alone is received. In the case where the toner visible image  39  has not completely covered the surface  38  of the transfer belt  35  and the amount of toner of the toner visible image  39  is small, then the diffuse reflection light  46   c  from both the toner visible image  39  and the surface  38  of the transfer belt  35  is received. The diffuse reflection light  46   c  is received at an angle A 3  shown in  FIG. 4 . In the present embodiment, the angle A 3  is an angle of disposition of the second light-receiving element  44  with respect to the plane  48 . 
     The toner-amount detection sensor  41  irradiates the transfer belt  35  having a toner visible image  39  formed on its surface  38  with light  46   a  in the direction shown by the arrow E 1  in  FIG. 4 . The light  46   a  impinges on and is reflected from either one or both of the toner visible image  39  and the surface  38  of the transfer belt  35 . Of the reflected light, light  46   b  reflected at an angle close to the specular reflection angle is received by the first light-receiving element  43  disposed at the angle A 2  with respect to the plane  48 . Of the reflected light, diffuse reflection light is received by the second light-receiving element  44  disposed at the angle A 3  with respect to the plane  48 . The first light-receiving element  43  and the second light-receiving element  44  each output a current according to the quantity of the received light. The toner-amount calculating unit  45  converts the respective currents output from the first light-receiving element  43  and the second light-receiving element  44  into voltages, and calculates the amount of toner on the basis of those voltage values. In this manner, the toner-amount detection sensor  41  detects the amount of toner. 
     Here, the sensor is configured to have a relationship of A 1 &lt;A 2 &lt;1.5A 1  where A 1  represents a predetermined incident angle with respect to the plane  48  normal to the surface  38  of the transfer belt  35 , and A 2  represents the angle of disposition of the first light-receiving element  43  with respect to the plane  48  normal to the surface  38  of the transfer belt  35 . It is preferable that the angle A 2  falls within the range of at least 12° and less than 18°, and it is set to 13°, for example. 
     The sensor is also configured to have a relationship of A 3 &gt;A 1  where A 3  represents the angle of disposition of the second light-receiving element  44  with respect to the plane  48  normal to the surface  38  of the transfer belt  35 . That is, the second light-receiving element  44  is disposed in such a manner that the angle of disposition thereof is larger than that of the first light-receiving element  43 . In the present embodiment, the angle A 3  is set to 25°. It should be noted that in the case where the second light-receiving element  44  is disposed on the side opposite to the light-emitting element  42  with respect to the plane  48  normal to the surface  38  of the transfer belt  35 , it is set to have a relationship of A 3 &gt;2A 1 . 
     With the above configuration, the light reflected from the surface  38  of the transfer belt  35  when no toner visible image  39  is formed thereon can be received in large quantity. In the case where the toner visible image  39  has not completely covered the surface  38  of the transfer belt  35  and the amount of toner of the toner visible image  39  is small as well, the quantity of the light that impinges on and is reflected from the surface  38  of the transfer belt  35 , transmitted through the toner layer, can be detected with accuracy. It is thus possible to accurately detect the amount of toner. 
     This will now be described.  FIG. 5  is a graph diagram showing the relationship between the reflectance of the surface  38  of the transfer belt  35  and the reflection angle of incident light. The position with the indication of 0% located at the center  51  in  FIG. 5  shows the irradiation position of light. In  FIG. 5 , concentric semicircles are drawn about the center  51 , at the positions corresponding respectively to the reflectance of 25%, 50%, 75%, and 100%. A solid line  52   a  represents incident light, and a solid line  52   b  represents specularly reflected light. The line corresponding to the plane normal to the reflected plane is indicated by a solid line  53 . A broken line  54  represents the reflectance of the surface  38  of the transfer belt  35  within a range of certain reflection angles. 
     Referring to  FIG. 5 , the angle between the solid lines  52   a  and  53  corresponds to the above-described angle A 1 , which is set to 30°. The angle between the solid lines  52   b  and  53  also corresponds to the above-described angle A 1 , which is 30° here. The point of intersection between the solid line  52   b  and the broken line  54  indicates the reflectance when the incident light is reflected specularly, which is about 75%. The reflectance increases gradually as the reflection angle increases from the angle A 1 . In this case, the reflectance becomes almost 100% when the reflection angle becomes 40° as indicated by the solid line  52   c . This reflection angle corresponds to the maximum reflectance. Thereafter, the reflectance decreases gradually with increasing reflection angle, and at the reflection angle of 45° as indicated by the solid line  52   d , the reflectance becomes about 75%, which is approximately the same as the reflectance of the specular reflection. Thus, in terms of the angles A 1  and A 2 , the relationship of A 1 &lt;A 2 &lt;1.5A 1  at least ensures that the light can be received in a position where the reflectance is higher than in the position of the specular reflection. Accordingly, with such a configuration, the light reflected from the surface  38  of the transfer belt  35  when no toner visible image  39  is formed thereon can be received in large quantity. Further in the case where the toner visible image  39  has not completely covered the surface  38  of the transfer belt  35  and the amount of toner of the toner visible image  39  is small, the quantity of light that impinges on and is reflected from the surface  38  of the transfer belt  35 , transmitted through the toner layer, can be detected with accuracy. It is thus possible to accurately detect the amount of toner. 
     This is conceivably for the following reasons. The surface  38  of the transfer belt  35  is covered with a very thin layer of certain coating agent for improving the toner transfer efficiency, protecting the surface  38  of the transfer belt  35 , and other purposes. The incident light refracts or scatters depending on the type of the coating agent, the thickness of the coating layer, and the like. Such refraction or scattering of the incident light may possibly cause the above-described tendency that the reflectance becomes greater at angles larger than the specular reflection angle. Examples of the coating agent include polyamide resin, polyamide-imide resin, polyimide resin, and polycarbonate resin. 
     Therefore, for example with the angle A 1  of 30°, the angle A 2  may be set to be larger than 30° and less than 45°. This enables light to be received within the range where the reflectance is higher than in the case of the specular reflection. Specifically, the angle A 2  is set to 35° or 40°. For this angle A 2 , an arbitrary value may be selected within the above-described range of larger than 30° and less than 45°, depending on the material of the transfer belt  35  and the like. For example, in the case where the transfer belt  35  is formed of resin including at least one selected from the group of polyamide-imide resin, polyimide resin, and polycarbonate resin, the angle A 2  may be set to 35°. In the case where the transfer belt  35  is formed of rubber including at least one of urethane rubber and hydrin rubber, the angle A 2  may be set to 40°. 
     When the reflection angle is further increased from 45° shown by the solid line  52   d , the angle falls outside the range delimited by the broken line  54 , as shown by the solid line  52   e . When the second light-receiving element  44  which receives diffuse reflection light is disposed at an angle larger than that angle, it can receive the diffuse reflection light efficiently, without being affected by the specular (or near-specular) reflection. The angle between this solid line  52   e  and the solid line  53  is indicated by 2A 1 , which is 60° here. 
     Regarding the diffuse reflection light, in the case where the second light-receiving element  44  is disposed on the same side as the light-emitting element  42  with respect to the plane  48  normal to the surface  38  of the transfer belt  35 , it would hardly be affected by the specular (or near-specular) reflection. Therefore, the device configuration may be determined to have a relationship of A 3 &gt;A 1 . That is, the second light-receiving element  44  may be disposed on the side opposite to the first light-receiving element  43  with respect to the location of the light-emitting element  42 . 
       FIG. 6  is a graph diagram showing an approximate relationship between the amount of toner and the output of the toner-amount detection sensor  41  in the case of detecting the toner amount of a black toner visible image  39 .  FIG. 7  is a graph diagram showing an approximate relationship between the amount of toner and the output of the toner-amount detection sensor  41  in the case of detecting the toner amount of a yellow toner visible image  39 . The approximate relationships between the amount of toner and the output of the toner-amount detection sensor  41  in the case of detecting the toner amount of a cyan toner visible image  39  and in the case of detecting the toner amount of a magenta toner visible image  39  are identical to the approximate relationship between the amount of toner and the output of the toner-amount detection sensor  41  in the case of detecting the toner amount of a yellow toner visible image  39 , and therefore, the description thereof will be omitted. 
     In each of  FIGS. 6 and 7 , the vertical axis represents the output value of the toner-amount detection sensor  41 , and the horizontal axis represents the amount of toner. On the vertical axis, the value increases toward the upper side of the paper plane. On the horizontal axis, the value increases toward the right side of the paper plane. In  FIG. 6 , the upper solid line  56   a  represents the output value which is output on the basis of the quantity of light received by the first light-receiving element  43  in the case where the angle A 2  is 40°, and the lower solid line  56   b  represents the output value which is output on the basis of the quantity of light received by the second light-receiving element  44  in the case where the angle A 2  is 40°. In  FIG. 6 , the upper broken line  57   a  represents the output value which is output on the basis of the quantity of light received by the first light-receiving element  43  in the case where the angle A 2  is 30°, and the lower broken line  57   b  represents the output value which is output on the basis of the quantity of light received by the second light-receiving element  44  in the case where the angle A 2  is 30°. In  FIG. 7 , the upper solid line  58   a  represents the output value which is output on the basis of the quantity of light received by the first light-receiving element  43  in the case where the angle A 2  is 40°, and the lower solid line  58   b  represents the output value which is output on the basis of the quantity of light received by the second light-receiving element  44  in the case where the angle A 2  is 40°. In  FIG. 7 , the upper broken line  59   a  represents the output value which is output on the basis of the quantity of light received by the first light-receiving element  43  in the case where the angle A 2  is 30°, and the lower broken line  59   b  represents the output value which is output on the basis of the quantity of light received by the second light-receiving element  44  in the case where the angle A 2  is 30°. 
     Referring first to  FIG. 6 , in the case of the black toner visible image  39 , when the amount of toner is very small and nearly 0, the output value based on the quantity of light received by the first light-receiving element  43  when the angle A 2  is 40° takes a large value as compared to the output value based on the quantity of light received by the first light-receiving element  43  when the angle A 2  is 30°. As such, in the case where the amount of toner is small, the quantity of the reflected light is larger when the angle A 2  is 40°, shown by the solid line  56   a , as compared to when the angle A 2  is 30°, shown by the broken line  57   a.    
     It should be noted that the solid line  56   b  represents the output value based on the quantity of light received by the second light-receiving element  44  when the angle A 3  is 60°, and the broken line  57   b  represents the output value based on the quantity of light received by the second light-receiving element  44  when the angle A 3  is 60°. The output values are almost the same. 
     Accordingly, the toner-amount detection sensor  41  can accurately detect the amount of toner over a wider output value range, from the state of no toner, or, the state where the surface  38  of the transfer belt  35  is detected with no toner visible image  39  formed thereon, to the state where a small amount of toner is detected with the toner only slightly covering the surface  38  of the transfer belt  35 . That is, while the sensor output values ultimately converge to almost the same value in the solid line  56   a  and the broken line  57   a  as the amount of toner increases, the sensor output value when the amount of toner is 0 can be increased in the solid line  56   a . This ensures accurate detection of the amount of toner. 
     Referring next to  FIG. 7 , in the case of the yellow toner visible image  39  as well, when the amount of toner is very small and nearly 0, the output value based on the quantity of light received by the first light-receiving element  43  when the angle A 2  is 40° takes a large value as compared to the output value based on the quantity of light received by the first light-receiving element  43  when the angle A 2  is 30°. As such, in the case where the amount of toner is small, the quantity of the reflected light is larger when the angle A 2  is 40°, shown by the solid line  58   a , as compared to when the angle A 2  is 30°, shown by the broken line  59   a.    
     It should be noted that the solid line  58   b  represents the output value based on the quantity of light received by the second light-receiving element  44  when the angle A 3  is 60°, and the broken line  59   b  represents the output value based on the quantity of light received by the second light-receiving element  44  when the angle A 3  is 60°. The output values are almost the same. 
     Therefore, according to the toner-amount detection sensor  41  with the above configuration, the light reflected from the surface  38  of the transfer belt  35  can be received in large quantity. It is thus possible to accurately detect the amount of toner. Further, according to the digital multifunctional peripheral  11  with the above configuration, the quality of the image formed can be improved, as the apparatus includes the toner-amount detection sensor  41  which can accurately detect the amount of toner. 
     In the embodiment described above, the toner-amount detection sensor  41  includes the second light-receiving element which receives diffuse reflection light. The configuration of the sensor, however, is not limited thereto; the second light-receiving element may be omitted if necessary. This can simplify the device configuration. 
     Further, in the embodiment described above, the light-emitting element may be configured to emit polarized light having a prescribed wavelength. In this case, of the reflected light, polarized light having the prescribed wavelength may be dispersed and received, and the amount of toner may be detected on the basis of the received light. 
       FIG. 8  shows a toner-amount detection sensor according to another embodiment of the present disclosure. Referring to  FIG. 8 , this toner-amount detection sensor  61  includes: a light-emitting element  62  which emits light toward a surface  38  of a transfer belt  35  or a toner visible image  39 ; a first polarization unit  63   a  which disperses the light from the light-emitting element  62  into polarized light of P- and S-polarization and emits the P-polarized light component toward the surface  38  of the transfer belt  35  or the toner visible image  39 , as shown by an arrow F 1 ; a first polarized-light-receiving element  64   a  which receives the S-polarized light that was dispersed by the first polarization unit  63   a  in the direction shown by an arrow F 2 ; a second polarization unit  63   b  which receives the light in the direction shown by an arrow F 3  from the surface  38  of the transfer belt  35  and/or the toner visible image  39  formed thereon, and disperses the received light into polarized light of P- and S-polarization; a second polarized-light-receiving element  64   b  which receives the S-polarized light that was dispersed by the second polarization unit  63   b  in the direction shown by an arrow F 4 ; a third polarized-light-receiving element  65  which receives the P-polarized light that has passed through the second polarization unit  63   b ; and a toner-amount calculating unit (not shown) which calculates the amount of toner from the quantity of the reflected light received by the third polarized-light-receiving element  65  or the like. 
     Here, the sensor is configured to have a relationship of B 1 &lt;B 2 &lt;1.5B 1  where B 1  is the angle of incidence with respect to a plane  48  normal to the surface  38  of the transfer belt  35 , or, the angle of disposition of the light-emitting element  62  with respect to the plane  48  normal to the surface  38  of the transfer belt  35 , and B 2  is the angle of disposition of the third polarized-light-receiving element  65  with respect to the plane  48  normal to the surface  38  of the transfer belt  35 . 
     With the above configuration, polarized light of P- and S-polarization can be used to detect the amount of toner on the basis of the quantities of those lights. In this case as well, it is of course possible to provide a light-receiving element which receives diffuse reflection light, and the amount of toner may be detected on the basis of the received diffuse reflection light. 
     It should be noted that in the embodiment described above, polyimide resin was used as the material for the resin transfer belt. The material of the transfer belt, however, is not limited thereto; it may be, for example, any of polyamide-imide resin, polyimide resin, and polycarbonate resin. Further, although urethane rubber was used as the material for the rubber transfer belt, not limited thereto, hydrin rubber may be used as well. That is, at least one of polyamide resin, polyamide-imide resin, polyimide resin, polycarbonate resin, urethane rubber, and hydrin rubber may be included as the material for the transfer belt. 
     Further, in the above embodiment, the angle A 1  may be set to an angle other than that mentioned above. 
     In the above embodiment, an infrared light-emitting diode was adopted as an example of the light-emitting element, and infrared light-receiving elements were adopted as an example of the first and second light-receiving elements. The elements, however, are not limited thereto; a light-emitting element which emits light having another wavelength, such as visible light, and first and second light-receiving elements which receive light having the other wavelength may be used as well. 
     Further, although the transfer belt as an intermediate transfer body was used as the transfer body in the above embodiment, not limited thereto, the present disclosure is applicable to the case where the transfer body is a photoreceptor or the like. Further, in the case where the transfer body has a curved surface, the plane normal to the surface of the transfer body, as shown in  FIG. 4 , is represented by a normal line to the curved surface. 
     It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present disclosure is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     The toner-amount detection sensor and the image forming apparatus according to the present disclosure are applicable particularly advantageously to the case where an improvement in image quality of the image formed is required.