Patent Publication Number: US-2019171152-A1

Title: Paper sensor device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to paper sensor devices and image-forming devices including paper sensor devices. 
     Related Art 
     Various types of paper (recording paper) such as high-quality paper, recycled paper, and coated paper, as well as heavy and thin paper, are used in image-forming devices such as copying machines, printers, facsimile machines, and multifunction printers having these functions. Damp paper may also be used in some operating environments. 
     It is necessary to properly set image-forming conditions such as transfer current, fusing pressure, fusing temperature, and fusing time in accordance with the type, water content ratio, and/or other properties of the paper in order to improve the quality of the image formed by the image-forming device. For these purposes, image-forming devices have been developed that are equipped with sensors that detect (determine) paper properties. 
     As an example, Japanese Unexamined Patent Application Publication, Tokukai, No. 2006-53398 (“Patent Document 1”) discloses an image-forming device including a sensor that: projects, onto paper, light having such a wavelength that water can absorb the light; and calculates the water content of the paper on the basis of light reflected off the paper. 
     Japanese Unexamined Patent Application Publication, Tokukai, No. 2007-145590 (“Patent Document 2”) discloses a paper sensor device that: projects light onto paper and recognizes the type of the paper on the basis of light reflected off the paper. The paper sensor device includes, in a light projection target position, a pressing plate that prevents protrusion of paper. Light is projected onto the pressing plate with no paper between the pressing plate and the paper sensor device, so that the sensor can be calibrated on the basis of light reflected off the paper. 
     Both Patent Documents 1 and 2 deal with measurement of reflection of light projected onto a sheet of paper. Some other documents deal with measurement of transmission of light projected onto a sheet of paper. The term, “measurement light,” may be used in the following description to collectively refer to the light that is projected onto, and either reflected off or transmitted through, a sheet of paper. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In Patent Document 1, light is projected onto a sheet of paper being transported to measure the light reflected off the sheet. Patent Document 1, however, is short of considering adverse effects of, for example, flapping, tilting, and warping of the sheet during transport. Precision of measurement of reflected light therefore falls, and the water content is not calculated with high precision. Patent Document 1 is capable of maintaining high levels of measurement precision if the sheet is stopped during measurement. This approach, however, leads to another problem that it takes more time to complete printing in response to a print command input from the user (hereinafter, “printing time”). 
     Meanwhile, in Patent Document 2, the pressing plate is capable of preventing protrusion of paper. If the sheet of paper warps toward a paper sensor device, however, precision of measurement falls similarly, and the type of the sheet is not recognized with high precision. 
     The present disclosure, made in view of these problems, has an object to provide a paper sensor device capable of measuring measurement light with high precision to detect a paper property with high precision without adding to printing time and also to provide an image-forming device including such a paper sensor device. 
     Solution to the Problems 
     To address the problems, the present disclosure, in an aspect thereof, is directed to a paper sensor device including: a light-emitter; a photodetector configured to receive measurement light projected by the light-emitter and then either transmitted or reflected by a sheet of paper, the paper sensor device detecting a paper property based on the measurement light; and a transporting unit configured to transport the sheet of paper while sandwiching the sheet of paper between a transport roller and an opposing member, either or both of the transport roller and the opposing member having a window on a sandwiching face thereof where the sheet of paper is sandwiched, wherein: the light-emitter projects light via the window(s) onto a sandwiched portion of the sheet of paper being transported; and the photodetector receives the measurement light via the window(s). 
     Advantageous Effects of the Invention 
     This arrangement achieves the advantage of providing a paper sensor device capable of measuring measurement light with high precision to detect a paper property with high precision without adding to printing time. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic illustration of a paper sensor device in accordance with Embodiment 1, (a) and (b) of  FIG. 1  representing a first state and a second state of the paper sensor device respectively. 
         FIG. 2  is a schematic perspective view of a paper sensor device in accordance with Embodiment 1. 
         FIG. 3  is a block diagram of a configuration of major components of an image-forming device in accordance with Embodiment 1, the image-forming device including a paper sensor device in accordance with Embodiment 1. 
         FIG. 4  is a circuit diagram of an example configuration of an amplification circuit  14  provided in an image-forming device in accordance with Embodiment 1. 
         FIG. 5  is a flow chart for a printing process in an image-forming device in accordance with Embodiment 1. 
         FIG. 6  is a block diagram of a configuration of major components of an image-forming device in accordance with Variation Example 1 of Embodiment 1. 
         FIG. 7  is a flow chart for a printing process in an image-forming device in accordance with Embodiment  2 . 
         FIG. 8  is a schematic illustration of a paper sensor device in accordance with Embodiment 3, (a) and (b) of  FIG. 8  representing a first state and a second state of the paper sensor device respectively. 
         FIG. 9  is a schematic perspective view of a paper sensor device in accordance with Embodiment 3. 
         FIG. 10  is a block diagram of a configuration of major components of an image-forming device in accordance with Embodiment 3. 
         FIG. 11  is a schematic illustration of a paper sensor device in accordance with Embodiment 4, (a) and (b) of  FIG. 11  representing a first state and a second state of the paper sensor device respectively. 
         FIG. 12  is a schematic illustration of a paper sensor device in accordance with Embodiment 5, (a) and (b) of  FIG. 12  representing a first state and a second state of the paper sensor device respectively. 
         FIG. 13  is a schematic perspective view of a paper sensor device in accordance with Embodiment 5. 
         FIG. 14  is a schematic illustration of a paper sensor device in accordance with Embodiment 5, (a) and (b) of  FIG. 14  representing a first state and a second state of the paper sensor device respectively. 
         FIG. 15  is a schematic perspective view of two transport rollers provided in a paper sensor device in accordance with Embodiment 5. 
         FIG. 16  is a block diagram of a configuration of major components of an image-forming device in accordance with Embodiment 5. 
         FIG. 17  is a diagram of a common example structure of image-forming devices including a paper sensor device in accordance with Embodiments 1 to 6. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
     The following will describe an embodiment of the present disclosure in reference to  FIGS. 1 to 6 and 17 . The present embodiment describes an image-forming device that is included in a copying machine, printer, facsimile machine, or multifunction printer having these functions in order to detect (determine) the thickness (grammage) of a sheet of paper as a paper property and set printing conditions on the basis of the detection. 
     Overview of Image-forming Device  100   
       FIG. 17  is a diagram of an example structure of an image-forming device  100  in which there is provided a paper sensor device  2  in accordance with the present embodiment.  FIG. 17  also represents image-forming devices  100 A to  100 E in which there are provided paper sensor devices  2 A to  2 E (detailed later) respectively in accordance with Embodiments 2 to 6. 
     Referring to  FIG. 17 , the image-forming device  100  includes a yellow-image-forming station  101 Y, a magenta-image-forming station  101 M, a cyan-image-forming station  101 C, and a black-image-forming station  101 B. 
     The four image-forming stations  101 Y to  101 B are disposed along a transport path of a sheet of paper P between a paper feeder  102  and a fuser  103 . Under the four image-forming stations  101 Y to  101 B is there disposed an endless transport belt  104  for electrostatically attracting and transporting the sheet of paper P thereon. There are provided four transfer rollers  105 , one for each of the four image-forming stations  101 Y to  101 B, inside the transport belt  104 . 
     The four image-forming stations  101 Y to  101 B have the same structure, and each of them includes a photoreceptor drum  111 . Around each photoreceptor drum  111  are there provided a charge roller  112 , an exposure device  113 , a development device  114 , a different one of the transfer rollers  105 , and a cleaner device  115 . Each development device  114  in the image-forming stations  101 Y to  101 B contains a developer that contains toner of an associated color. 
     The charge roller  112  uniformly charges the surface of the photoreceptor drum  111 . The exposure device  113  exposes the surface of the photoreceptor drum  111  to light to form an electrostatic latent image. The development device  114  supplies toner to the electrostatic latent image to form a toner image. The transfer roller  105  applies a bias voltage (transfer voltage) from the backside of the transport belt  104  to transfer the toner age formed on the surface of the photoreceptor drum  111  onto the sheet of paper P transported by the transport belt  104 . The cleaner device  115  collects residual toner from the surface of the photoreceptor drum  111 . 
     The paper feeder  102  supplies sheets of paper P. The sheet of paper P may be, for example, high-quality paper, recycled paper, thin paper, heavy paper, or coated paper. The fuser  103  squeezes the sheet of paper P between a belt and a roller to apply suitable heat (fusing temperature) and pressure (fusing pressure) to dissolve toner and thereby fuse a toner image onto the sheet of paper P. 
     The sheet of paper P fed from the paper feeder  102  is attracted and transported on the transport belt  104  and passed below the four image-forming stations  101 Y to  101 B, during which the toner images formed by the image-forming stations  101 Y to  101 B are transferred one by one onto the sheet of paper P. The transferred toner images are fused on the sheet of paper P by the fuser  103 . 
     In the image-forming device  100  structured as above, the paper sensor device  2  is disposed, for example, between the paper feeder  102  and the transport belt  104 .  FIG. 17  shows an electrophotographic printer, which may alternatively be an inkjet or another type of printer. 
     Structure of Paper Sensor Device  2   
       FIG. 1  is a schematic illustration of the paper sensor device  2  in accordance with the present embodiment, (a) and (b) of  FIG. 1  representing a first state and a second state of the paper sensor device  2  respectively. The first state is a state where measurements are made on reference light. The second state is a state where light is projected onto a sheet of paper and measurements are made on measurement light.  FIG. 2  is a schematic perspective view of the paper sensor device  2 . 
     As shown in (a) and (b) of  FIG. 1 , the paper sensor device  2  includes a light-emitter  3 , a photodetector  4 , and a transporting unit  9 . The light-emitter  3  projects light L 0 . The photodetector receives light L 0  and L 1  to measure the intensity (amount of light) of the received light. In the present embodiment, the photodetector  4  measures the intensity of transmitted light L 1  that is part of light L 0 , projected onto the sheet of paper P by the light-emitter  3 , that is transmitted by the sheet of paper P. A controller  12  (detailed later; see  FIG. 3 ) then detects the thickness of the sheet of paper as a paper property on the basis of a result of the measurement. 
     The light-emitter  3  is a light-emitting element and may be, for example, an LED (light emitting diode). The light-emitter  3  is not necessarily an LED and may alternatively be, for example, another type of light source such as a laser beam source. In addition, the light-emitter  3  may be configured to only emit emission light L 0  of a single particular wavelength and may be configured to concurrently emit emission light L 0  of plural wavelengths. 
     The photodetector  4  is a light-receiving sensor (light-receiving element) and may be, for example, a photodiode. The photodetector  4  is not necessarily a photodiode and may alternatively be, for example, a phototransistor or a photo IC. 
     The transporting unit  9  includes a transport roller  5  and a paper guide  6  (opposing member). A sheet of paper is sandwiched between the transport roller  5  and the paper guide  6  during transport. The paper guide  6  is a guide for transporting the sheet of paper P and so arranged as to face the transport roller  5 . 
     The transport roller  5  is disposed such that it can be moved up/down between an upper, retracted position and a lower, transport position by a moving mechanism (not shown). The transport roller  5  is separated from the paper guide  6  when there is no sheet of paper between the transport roller  5  and the paper guide  6 . As a sheet of paper P arrives between the transport roller  5  and the paper guide  6 , the transport roller  5  drops to sandwich the sheet of paper P between the transport roller  5  and the paper guide  6  for transport. The paper guide  6  is not necessarily fiat and may be curved. 
     In the present embodiment, the light-emitter  3  is disposed inside the transport roller  5 , and the photodetector  4  is disposed on a face opposite the sandwiching face where a sheet of paper is sandwiched. The light-emitter  3  is disposed so as to be capable of projecting emission light L 0  from the outer circumferential surface of the transport roller  5 . To describe it in more detail, as shown in  FIG. 2 , there is provided a hole  5   a  in the transport roller  5  from its outer circumferential surface toward its center so that the light-emitter  3  can be disposed inside the hole  5   a.  The light-emitter  3  has a light-emitting face directed at the opening (window) of the hole  5   a.  Throughout the following description, the fixing of an element inside the solid transport roller  5  in this manner will be described as the “embedding” of the element. 
     The paper guide  6  has a window  7  enabling passage therethrough of (emission light) L 0  (see (a) and (b) of  FIG. 1 ) projected by the light-emitter  3  embedded in the transport roller  5 . The photodetector  4  is disposed facing the window  7  in order to receive the light passing through the window  7 . The window  7  needs only to transmit the wavelengths of the light emitted by the light-emitter  3 . The window  7  may be either a mere hole formed in the paper guide  6  or a hole fitted with glass or a like transparent member. The hole  5   a  has an opening (window) structured similarly, possibly fitted with glass or a like transparent member. 
     As shown in (a) of  FIG. 1 , the first state of the paper sensor device  2  is defined as a state in which there exists no sheet of paper P between the light-emitter  3  and the photodetector  4  with the light-emitter  3  pointing in the direction of the photodetector  4  as a result of rotation of the transport roller  5 . Meanwhile, as shown in (b) of  FIG. 1 , the second state of the paper sensor device  2  is defined as a state in which there exists a sheet of paper P between the light-emitter  3  and the photodetector  4  with the light-emitter  3  pointing in the direction of the photodetector  4  as a result of rotation of the transport roller  5 . 
     In the first state, emission light L 0  emitted by the light-emitter  3  strikes the photodetector  4  via the window  7 , and the photodetector  4  receives this light as shown in (a) of  FIG. 1 . In contrast, in the second state, sonic of emission light L 0  is absorbed by the sheet of paper P or scattered by the surface of the sheet of paper P, and the rest of the emission light, Which provides transmitted light L 1 , strikes the photodetector  4  via the window  7  as shown in (b) of  FIG. 1 . The photodetector  4  receives this light. 
     Referring to  FIG. 2 , the transport roller  5 , configured in this manner, includes roller electrodes (first electrodes)  10   a,    10   b  disposed at or near its ends. The roller electrodes  10   a ,  10   b  are connected to the light-emitter  3  by conductive wires inside the transport roller  5 . The roller electrodes  10   a,    10   b  are in contact with respective external electrodes (second electrodes)  15   a,    15   b  provided outside the transport roller  5 . The external electrodes  15   a,    15   b  slide respectively on the roller electrodes  10   a,    10   b  as a result of rotation of the transport roller  5 . The external electrodes  15   a,    15   b  are connected to a constant current source  11  (detailed later; see  FIG. 3 ). This structure maintains the roller electrodes  10   a,    10   b  in contact with the external electrodes  15   a,    15   b  even when the transport roller  5  is rotating, to externally supply electric current to the light-emitter  3  (element) disposed inside the transport roller  5 . 
     Configuration of Major Components of Image-forming Device  100   
       FIG. 3  is a block diagram of a configuration of major components of the image-forming device  100 , Referring to  FIG. 3 , the image-forming device  100  includes, for example, the constant current source  11 , the controller  12 , an A/D (analog/digital) converter  13 , and an amplification circuit  14 , as well as the light-emitter  3  and the photodetector  4  both of which are included in the paper sensor device  2 . 
     The constant current source  11 , in the present embodiment, outputs a constant current to the light-emitter  3  at all times so that the light-emitter  3  can emit light with a fixed intensity. Alternatively, the constant current source  11  may output a constant current so that the light-emitter  3  can emit light with a fixed intensity, only when the light-emitter  3  is turned to face the photodetector  4  as a result of rotation of the transport roller  5 . As further alternatives, the constant current source  11  may be constructed from a constant-voltage power source connected in series with the light-emitter  3  and a fixed resistor and may be built around a constant-current IC. 
     The controller  12  controls the light-emitter  3 , the photodetector  4 , and the transporting unit  9 . Controlling the transporting unit  9  is equivalent to controlling the transport of the sheet of paper P and the rotation of the transport roller  5 . The controller  12  additionally determines a paper property (thickness in this example) of the sheet on the basis of a signal from the A/D converter  13  that indicates the intensity of the light received by the photodetector  4 . The present embodiment is described taking the thickness (grammage) of the sheet as an exemplar paper property. Other examples include the brand name, water content ratio, and surface smoothness of the sheet. The controller  12  determines at least one of these properties. The controller  12  includes a memory  12   a  and a calculating unit  12   b.  The controller  12  may be built, for example, around a microcomputer. 
     The A/D converter  13  converts the output voltage of the amplification circuit  14  to a digital signal for output to the controller  12 . If the controller  12  is a microcomputer, the A/D converter  13  may be, for example, an A/D converter that is a part of the microcomputer. 
     The amplification circuit  14  converts a photocurrent from the photodetector  4  (photodiode) to a voltage in proportion to the photocurrent for output to the A/D converter  13 . Such an amplification circuit  14  can be built, for example, around an operational amplifier with a negative feedback resistor being connected as shown in  FIG. 4 .  FIG. 4  is a circuit diagram of an example configuration of the amplification circuit  14 . 
     The constant current source  11 , the controller  12 , the A/D converter  13 , and the amplification circuit  14  may be included in the paper sensor device  2  or included in the image-forming device  100  separately from the paper sensor device  2 . In the latter case, the functions of the controller  12  may be assigned to a control device in the image-forming device  100 . 
     Printing Process in Image-Forming Device  100   
     Next, referring to  FIG. 5 , the flow of a process will be described in which the thickness of the sheet as a paper property is determined and an image is printed in accordance with the determined thickness.  FIG. 5  is a flow chart for a printing process in the image-forming device  100  in accordance with the present embodiment. 
     The controller  12  awaits an input of a print command from the user (S 1 ). The controller  12  rotates the transport roller  5  in response to an input of a print command. When the light-emitter  3  points in the direction of the photodetector  4  (first state) before the sheet of paper P reaches the transport roller  5 , a measurement is made on reference. During the measurement, the transport roller  5  may be stopped. Alternatively, a measurement may be made on reference when the light-emitter  3  has come to point in the direction of the photodetector  4  while the transport roller  5  is being rotated. Light L 0  emitted by the light-emitter  3  is received (as reference light) by the photodetector  4  in the first state. The output of the photodetector  4  is converted to a voltage by the amplification circuit  14  and then to a digital value V 0  by the A/D converter  13 . The controller  12  stores V 0  in the memory  12   a.    
     Next, the controller  12  awaits the sheet of paper P arriving and the light-emitter  3  pointing in the direction of the photodetector  4  (second state) (S 3 ). As the second state is reached, the controller  12  takes a measurement on the sheet of paper P (S 4 ). In the second state, the photodetector  4  receives light L 1  passing through the sheet of paper P for conversion to a voltage by the amplification circuit  14 . The A/D converter  13  then outputs a digital value V 1 . The controller  12  stores V 1  in the memory  12   a.    
     Measurement may be performed only once, but preferably performed twice or more times on the sheet of paper P to obtain an average of measurements made on a plurality of portions of the sheet. More preferably, measurement may be performed every time the light-emitter  3  comes to point in the direction of the photodetector  4  while the transport roller  5  is rotating, in order to obtain an average of measurements as V 1 . These techniques would reduce errors that may occur in the measurement due to variations in thickness (paper property) from one portion to another in the transport direction of the single sheet of paper P. 
     The inventors of the present invention have observed that the sheet of paper P varies greatly in thickness from one portion to another that are separated by a distance less than 1 cm. It is therefore preferable to perform measurement on a plurality of portions of the sheet of paper P that are separated from each other by approximate intervals of 3 cm. In other words, the transport roller  5  preferably has a diameter of approximately 1 cm (a circumference of approximately 3 cm). 
     The calculating unit  12   b  in the controller  12  retrieves, from the memory  12   a , reference V 0  obtained by measurement in step S 2  and measured value V 1  obtained by measurement made on the sheet of paper P in step S 4  to calculate a ratio V 1 /V 0  (S 5 ). The memory  12   a  in the controller  12  contains threshold values in advance on the basis of V 1 /V 0  measurements made on various sheets of paper P. The manufacturer of the image-forming device  100  may, for example, prepare and store this data in the memory  12   a  in the form of a database. The calculating unit  12   b  compares these threshold values with the ratios V 1 /V 0  calculated in step S 5  to determine the thickness of the sheet of paper (S 6 ). As an example, if V 1 /V 0  is from 0 to 0.1, the paper is determined to be heavy paper; if V 1 /V 0  is from 0.1 to 0.3, the paper is determined to be normal paper; if V 1 /V 0  is from 0.3 to 0.5, the paper is determined to be thin paper; and if V 1 /V 0  is from 0.5 to 1, it is determined that no sheet of paper P has arrived (due to paper jamming or another error). 
     The image-forming device  100  specifies image-forming (printing) conditions in accordance with this determination (S 7 ) and forms (prints) an image on the sheet of paper P (S 8 ). Examples of the image-forming conditions (printing conditions) specified by the controller  12  include the transfer current and voltage applied to transfer toner to the sheet of paper P, the transport speed of the sheet of paper P when the toner is fused onto the sheet of paper P (fusing time), and the temperature of the heating roller (fusing temperature) and the pressure of the pressure roller (fusing pressure) when the sheet of paper P is squeezed in the fuser  103 . When the sheet of paper P is heavy paper, the controller  12  increases the fusing temperature or time over a thin sheet of paper P. 
     The smoothness, brand name, and other properties of the sheet of paper can be recognized if the memory  12   a  contains, in the form of a database, threshold values for determining such properties of the sheet of paper on the basis of V 1 /V 0 . For example, if the sheet of paper P has a rough surface, the controller  12  increases transfer current and fusing pressure over paper with a flat and smooth surface. 
     V 0  (reference light) may not be measured repeatedly. For example, V 0  may be measured only once during the manufacture of the image-forming device  100 , and the measurement be stored for later use. As another alternative, V 0  may not at all be measured. A database of values of V 1  only, instead of V 1 /V 0 , may be prepared so that a paper property can be determined only from a V 1  value. 
     Advantages 
     The configuration described above measures a property of a portion (“measuring portion”), of the sheet of paper P, that is sandwiched between the transport roller  5  and the paper guide  6  (sandwiched portion). The configuration can therefore reduce adverse effects of flapping, tilting, and warping of the sheet of paper P and measure the intensity of transmitted light L 1  (measurement light) with high precision. 
     The incorporation of the paper sensor device  2  in the image-forming device  100  enables automation of the process from the measurement of the intensity of transmitted light to determine a paper property of the sheet of paper P to the specification of image-forming conditions and the formation of an image. The configuration also shares the transport roller  5  with the image-forming device  100 , thereby allowing for reduction in size and cost. 
     The paper sensor device  2  is capable of measurement with high precision even when the sheet of paper P is being transported. The configuration can therefore reduce printing time taken from a print command input by the user to actual printing over cases where the sheet of paper P needs to be stopped for measurement. 
     Variation Example 1 
     The light-emitter  3  is embedded in the transport roller  5  in Embodiment 1 described above. Alternatively, the photodetector  4  may be embedded in the transport roller  5 . The light-emitter  3  generates heat and may be disposed on the paper guide  6  for better heat dissipation. 
     Attention should be paid to the fact that the photodetector  4  itself can only produce a very low output current, which is susceptible to noise. Therefore, if the photodetector  4  is embedded in the transport roller  5 , it is preferable that the amplification circuit  14  and the A/D converter  13  as well as the photodetector  4  be embedded in the transport roller  5 , as shown in  FIG. 6 , to enable the output of measurements in the form of digital signals.  FIG. 6  is a block diagram of a configuration of major components of the image-forming device  100  in accordance with Variation Example 1. 
     In this configuration, the roller electrodes  10   a,    10   b,  disposed at or near the ends of the transport roller  5 , are again maintained in contact with the external electrodes  15   a,    15   b  even when the transport roller  5  is rotating. This structure enables the external supply of power to the amplification circuit  14  and the A/D converter  13  and the external extraction of digital signals from the A/D converter  13 . The external electrodes  15   a,    15   b  are connected to the controller  12  in this example. 
     Two pairs of electrodes do not need to be provided between the transport roller  5  and the controller  12  in the same configuration. There may be provided power sources for the amplification circuit  14  and the A/D converter  13  and electrodes for the output of the digital signals. 
     The output of the A/D converter  13  may be transmitted to the outside of the transport roller  5  in a contactless manner by electromagnetic waves in the configuration. More specifically, there may be additionally provided an electromagnetic wave transmitter embedded in the transport roller  5  and an electromagnetic wave receiver disposed outside the transport roller  5 . This structure enables the output of the A/D converter  13  to be transmitted to the outside of the transport roller  5  by the transmitter and received by the receiver disposed outside the transport roller  5  for further transmission to the controller  12 . 
     Conversely, there may be provided an electromagnetic wave receiver embedded inside the transport roller  5  and an electromagnetic wave transmitter disposed outside the transport roller  5 . This structure enables reception of measurement timings and user commands via electromagnetic waves. 
     This use of a transmitter and a receiver enables contactless communications with elements embedded in the transport roller  5 . That in turn eliminates high contact resistance and improper contacts that could be caused, for example, by dirt and grime between the roller electrodes  10   a,    10   b  and the external electrodes  15   a,    15   b.    
     Variation Example 2 
     Drive current is supplied to the light-emitter  3  embedded in the transport roller  5 , in Embodiment 1 described above, by disposing the roller electrodes  10   a,    10   b  in contact with the external electrodes  15   a,    15   b  connected to the constant current source  11 . Alternatively, there may be provided a coil embedded in the transport roller  5  and an AC magnetic field generator outside the transport roller  5 . In this structure, the external AC magnetic field electromagnetically induces voltage across the coil inside the transport roller  5 . This voltage is rectified and used to drive the light-emitter  3 . It is preferable in this structure that the constant current source  11  be also disposed inside the transport roller  5  to drive the light-emitter  3  with a constant current. 
     This structure again enables contactless supply of power. That eliminates high contact resistance and improper contacts that could be caused, for example, by dirt and grime between the roller electrodes  10   a,    10   b  and the external electrodes  15   a,    15   b.    
     Embodiment 2 
     The following will describe another embodiment of the present disclosure in reference to  FIG. 7 . For convenience of description, members of the present embodiment that have the same function as members of the previous embodiment are indicated by the same reference numerals, and description thereof is omitted. 
     The light-emitter  3  in the paper sensor device  2 A in accordance with the present embodiment (see  FIGS. 1 and 3 ) includes a plurality of light sources of different wavelengths (“n” light sources; n≥2). The controller  12  selectively turns on one of these light sources in the light-emitter  3  at a time to control wavelength. This arrangement enables, for example, selective output of an absorption peak wavelength for water over other wavelengths. That in turn makes it possible to determine the water content ratio of the sheet of paper P as a paper property. 
     There are provided a pair of roller electrodes  10   a,    10   b  and a pair of external electrodes  15   a,    15   b  connected to the constant current source  11  in Embodiment 1 described above. In contrast, in the present embodiment, there are provided n+1 roller electrodes (not shown), one for grounding and n for current supply to the respective light sources. The same applies to the external electrodes connected to the constant current source  11 . Alternatively, there may be provided a plurality of roller electrodes, two for power supply and the rest for signals for controlling turning on/off of the light sources. The same applies to the external electrodes connected to the constant current source  11 . In the latter structure, one of the light sources can be selectively turned on via the control signal in the transport roller  5 . 
     There may be provided the same number of constant current sources  11  as the light sources so that the controller  12  can selectively turn on/off one of the light sources. Alternatively, there may be provided a single constant current source  11  the output current of which is fed to one of the light sources selected by a switch controllable by the controller  12 , to control wavelength for the light-emitter  3 . 
     Next, referring to  FIG. 7 , a description will be given of a printing process in which the water content ratio of a sheet of paper is determined as a paper property and an image is printed in accordance with a result of the determination.  FIG. 7  is a flow chart for a printing process in the image-forming device  100 A (see  FIG. 17 ) in accordance with the present embodiment. 
     In  FIG. 7 , steps S 2 ′ and S 4 ′ to S 6 ′ replace steps S 2  and S 4  to S 6  in the flow chart in  FIG. 5 , which is a difference from the image-forming device  100  in accordance with Embodiment 1. 
     Reference V 0 (λi) is measured in step S 2 ′ for each wavelength i (i=1,2, . . . , and n) in the first state. Specifically, first, only a first one of the light sources is turned on, and the resultant output V 0 (λ1) of the A/D converter  13  is stored in the memory  12   a  of the controller  12 . Next, the first light source is turned off. A second one of the light sources is then turned on, and the resultant output V 0 (λ2) of the A/D converter  13  is stored in the memory  12   a  of the controller  12 . V 0 (λ3) and subsequent voltages are similarly stored in the memory  12   a  of the controller  12 . 
     In step S 4 ′, as the second state is reached, and measurement values V 1 (λ1), V 1 (λ2), . . . , and V 1 (λn) obtained by measurement on the sheet of paper P are similarly stored in the memory  12   a.  In step S 5 ′, the calculating unit  12   b  of the controller  12  retrieves V 0 (λ1), V 0 (λ2), . . . , and V 0 (λn) and V 1 (λ1), V 1 (λ2), . . . , and V 1 (λn) from the memory  12   a  and calculates V 1 (λi)/V 0 (λi) for each wavelength i. The memory  12   a  of the controller  12  stores a relationship between V 1 (λi)/V 0 (λi) and water content ratio, for example, in the form of table or mathematical expression in advance on the basis of measurements of water content ratios of various sheets of paper P. This database is used in step S 6 ′ to determine the water content ratio of the sheet of paper P. 
     In the description above, the water content ratio is calculated through selective output of an absorption peak wavelength for water over other wavelengths. Parameters other than water content ratio may be used. The thickness, grammage, surface smoothness, and other properties of the sheet of paper P can be detected using the paper sensor device  2 A by selecting a proper wavelength. 
     The image-forming device  100 A specifies image-forming conditions in step S 7  in accordance with a result of the calculation in S 6 ′ and in step S 8  forms an image on the sheet of paper P. If the sheet of paper P has a high water content, the controller  12  reduces transfer current over cases where the sheet of paper P has a low water content. 
     Embodiment 3 
     The following will describe another embodiment of the present disclosure in reference to  FIGS. 8 to 10 . For convenience of description, members of the present embodiment that have the same function as members of any of the previous embodiments are indicated by the same reference numerals, and description thereof is omitted. Measurement is made on transmitted light in Embodiments 1 and 2, whereas measurement is made on reflected light in the present embodiment. 
       FIG. 8  is a schematic illustration of the paper sensor device  2 B in accordance with the present embodiment, (a) and (b) of  FIG. 8  representing the first and second states of the paper sensor device  2 B respectively.  FIG. 9  is a schematic perspective view of the paper sensor device  2 B. 
     As shown in (a) and (b) of  FIG. 8  and  FIG. 9 , the paper sensor device  2 B includes the light-emitter  3  and the photodetector  4  both disposed in the transport roller  5 . The paper sensor device  2 B further includes a reflector (reflecting body)  17  outside the transport roller  5  in a direction that differs from the direction in which the light-emitter  3  and the photodetector  4  faces the sheet of paper P. The reflector  17  is used to measure reference light (reference). The light-emitter  3  projects light onto the reflector  17 , and its reflection is received by the photodetector  4 . In the example of  FIG. 8 , the reflector  17  is disposed opposite the transport roller  5  from a paper guide  6 ′. 
     The photodetector  4  is disposed so as to be capable of receiving reflection of emission light L 0 . More particularly, a hole  5   b  is provided adjacent to the hole  5   a  in the transport roller  5  as shown in  FIG. 9 . The hole  5   b  extends from the outer circumferential surface of the transport roller  5  toward its center and contains the photodetector  4  embedded therein.  FIG. 9  shows the light-emitter  3  and the photodetector  4  next to each other When traced along the rotation axis of the transport roller  5 . The light-emitter  3  and the photodetector  4  may be combined into a single unit to be disposed in a single hole. The paper guide  6 ′ has no window (opening)  7  in the present embodiment. Alternatively, a window (opening)  7  may be formed in the paper guide  6 ′ to enable additional light-emitters and photodetectors to be embedded, so that many more paper properties can be measured (e.g., the water content and thickness of a sheet of paper can be simultaneously measured). 
     As shown in (a) of  FIG. 8 , in the first state, emission light L 0  is reflected by the reflector  17 , producing reflected light Lr 0  received by the photodetector  4 . Referring next to (b) of  FIG. 8 , emission light L 0  is reflected, absorbed, or scattered by the sheet of paper P in the second state, producing reflected light Lr striking the photodetector  4 . The photodetector  4  receives reflected light Lr 0  from the reflector  17  in the first state and receives reflected light Lr from the sheet of paper P in the second state. 
     Referring to  FIG. 9 , the roller electrodes  10   a,    10   b,  . . . are disposed at or near the ends of the transport roller  5  in such a manner that the roller electrodes  10   a,    10   b,  . . . are in contact with the external electrodes  15   a,    15   b,  . . . respectively. These electrodes are used to supply power and output measurements. 
       FIG. 10  is a block diagram of a configuration of major components of the image-forming device  100 B. Referring to  FIG. 10 , the amplification circuit  14  and the A/D converter  13  as well as the photodetector  4  are embedded in the transport roller  5  in the image-forming device  100 B as in Variation Example 1 of Embodiment 1. This structure is capable of high precision measurement with low noise. 
     In the first state, the paper sensor device  2 B, configured in this manner, measures reflected light Lr 0  coming from the reflector  17  and uses a resultant A/D converter output as V 0 . In contrast, in the second state, the paper sensor device  2 B measures reflected light Lr coming from the sheet of paper P and uses a resultant A/D converter output as V 1 . 
     The light-emitter  3  and the photodetector  4  in this example are disposed next to each other when traced along the rotation axis of the transport roller  5  (perpendicular to the page of  FIG. 8 ). This is however a mere example, and other positional relationships are also possible. The positional relationship of the light-emitter  3  and the photodetector  4  may be designed in a suitable manner by selecting such a structure, measurement position, and timing that the photodetector  4  can receive emission light L 0  projected onto the reflector  17  or the sheet of paper P as reflected light Lr 0  or Lr with high precision. 
     The reflector  17  in this example is described as being located opposite the transport roller  5  from the paper guide  6 ′. The reflector  17  however needs only to be disposed in such a location outside the transport path of the sheet of paper P that the reflector  17  can face the light-emitter  3  and the photodetector  4  embedded in the transport roller  5 . 
     Advantages 
     Embodiment 3 achieves similar advantages to those achieved by Embodiment 1. 
     In the structure of Patent Document 2, a sheet of paper comes into contact with a reflecting portion (measuring portion) of the bottom face of the pressing plate. The reflecting portion therefore quickly collects dirt and grime. A dirty reflecting portion will lead to a variation in reference reflectance and may cause an error in the measurement made on the sheet of paper. 
     In contrast, in the present embodiment, the light-emitter  3  and the photodetector  4  are rotated together with the transport roller  5 . This structure allows the reflector  17  to be disposed in a location where the sheet of paper P does not pass, which renders the reflector  17  less likely to collect paper powder, toner, and other undesirable objects than a reflector  17  disposed in the transport path of the sheet of paper P. The reference can hence be measured with high precision, which in turn improves the precision of measurement made on the sheet of paper. 
     If the arrangements described earlier in Variation Examples 1 and 2 of Embodiment 1 are adopted in a suitable manner in the structure in which measurement is made on reflected light as described in the present embodiment, similar advantages are achieved. 
     Embodiment 4 
     The following will describe another embodiment of the present disclosure in reference to  FIG. 11 . For convenience of description, members of the present embodiment that have the same function as members of any of the previous embodiments are indicated by the same reference numerals, and description thereof is omitted. The light-emitter  3  and the photodetector  4  are disposed on the back of the paper guide  6 ′, and the transport roller  5  serves also as a replacement for the reflector (reflecting body)  17 , in the present embodiment. 
       FIG. 11  is a schematic illustration of the paper sensor device  2 C in accordance with the present embodiment, (a) and (b) of  FIG. 11  representing the first and second states of the paper sensor device  2 C respectively. As shown in (a) and (b) of  FIG. 11 , the paper sensor device  2 C includes the light-emitter  3  and the photodetector  4  both disposed on the back of the paper guide  6  which has the window  7 . The paper sensor device  2 C uses the transport roller  5  as the reflector  17 . 
     As shown in (a) of  FIG. 11 , in the first state, emission light L 0  is reflected by the transport roller  5 , producing reflected light Lr 0  received by the photodetector  4 . Referring next to (b) of  FIG. 11 , emission light L 0  is reflected, absorbed, or scattered by the sheet of paper P in the second state, producing reflected light Lr striking the photodetector  4 . The photodetector  4  receives reflected light Lr 0  from the transport roller  5  in the first state and receives reflected light Lr from the sheet of paper P in the second state. 
     The image-forming device  1000  in accordance with the present embodiment includes such a paper sensor device  2 C. The image-forming device  100 C determines a paper property by the same method and prints by the same printing process as in Embodiment 3, and description thereof is omitted. 
     Advantages 
     This structure eliminates the need to separately provide the reflector  17 , which reduces the number of components. The structure also makes it unnecessary to transfer electric power and signals from the transport roller  5  to the outside or vice versa. That in turn obviates the need for the roller electrodes  10   a,    10   b,  the external electrodes  15   a,    15   b,  and other special arrangements described in Variation Examples 1 and 2 of Embodiment 1 for electrically connecting elements in the transport roller  5  to the outside. 
     Embodiment 5 
     The following will describe another embodiment of the present disclosure in reference to  FIGS. 12 and 13 . For convenience of description, members of the present embodiment that have the same function as members of any of the previous embodiments are indicated by the same reference numerals, and description thereof is omitted. The transporting unit  9  in the present embodiment includes a hollow transport roller  20  that includes a cylindrical rotator  21  having an outer circumferential surface serving as a sandwiching face. 
       FIG. 12  is a schematic illustration of the paper sensor device  2 D in accordance with the present embodiment, (a) and (b) of  FIG. 12  representing the first and second states of the paper sensor device  2 D respectively.  FIG. 13  is a schematic perspective view of the transporting unit  9  in the paper sensor device  2 D. 
     As shown in (a) and (b) of  FIG. 12 , in the transporting unit  9  in the paper sensor device  2 D, the transport roller  20  includes the cylindrical rotator  21  having an outer circumferential surface serving as a sandwiching face. The cylindrical rotator  21  is driven to rotate in the transporting unit  9 . This rotation transports the sheet of paper P sandwiched between the cylindrical rotator  21  and the paper guide  6 ′. The cylindrical rotator  21  in the transport roller  20  is lifted and lowered by a moving mechanism (not shown) between an upper, retracted position and a lower, transport position. When the sheet of paper P is yet to reach the gap between the cylindrical rotator  21  and the paper guide  6 ′, the cylindrical rotator  21  is separated from the paper guide  6 ′. As the sheet of paper P reaches the gap between the cylindrical rotator  21  and the paper guide  6 ′, the cylindrical rotator  21  is lowered to transport the sheet of paper P sandwiched between the cylindrical rotator  21  and the paper guide  6 ′. 
     The light-emitter  3  and the photodetector  4  are disposed in the cylindrical rotator  21  in such a manner as not to rotate with the cylindrical rotator  21 . Specifically, the light-emitter  3  and the photodetector  4  are fixed to a supporting unit  23  inserted in the cylindrical rotator  21  as shown in  FIG. 13 . The supporting unit  23  fixes and supports the light-emitter  3  and the photodetector  4  and includes conductive wires that transmit drive current to the light-emitter  3  and output signals from the photodetector  4 . 
     The cylindrical rotator  21  has a window  22  formed and includes a reflector  17  formed on the inner circumferential surface thereof. Similarly to the window  7 , the window  22  needs only to transmit the wavelengths of the light emitted by the light-emitter  3 . The window  22  may be either a mere hole formed in the cylindrical rotator  21  or a hole fitted with glass or a like transparent member. 
     As shown in (a) of  FIG. 12 , in the first state, emission light L 0  is reflected by the reflector  17 , producing reflected light Lr 0  received by the photodetector  4 . Referring next to (b) of  FIG. 12 , emission light L 0  is reflected, absorbed, or scattered by the sheet of paper P in the second state, producing reflected light Lr striking the photodetector  4 . The photodetector  4  receives reflected light Lr 0  from the reflector  17  in the first state and receives reflected light Lr from the sheet of paper P in the second state. 
     The image-forming device  100 D in accordance with the present embodiment includes such a paper sensor device  2 D. The image-forming device  100 D determines a paper property by the same method and prints by the same printing process as in Embodiment 3, and description thereof is omitted. 
     Advantages 
     This structure isolates the reflector  17  not only from the transport path of the sheet of paper P, but also from the internal space of the image-forming device  100 D, which in turn can effectively prevent the reflector  17  from collecting dirt and grime. The structure also makes it unnecessary to transfer electric power and signals from the transport roller  5  to the outside or vice versa. That in turn obviates the need for the roller electrodes  10   a,    10   b,  the external electrodes  15   a,    15   b,  and other special arrangements described in Variation Examples 1 and 2 of Embodiment 1 for electrically connecting elements in the transport roller 5 to the outside. 
     Embodiment 6 
     The following will describe another embodiment of the present disclosure in reference to  FIGS. 14 to 16 . For convenience of description, members of the present embodiment that have the same function as members of any of the previous embodiments are indicated by the same reference numerals, and description thereof is omitted. 
       FIG. 14  is a schematic illustration of the paper sensor device  2 E in accordance with the present embodiment, (a) and (b) of  FIG. 14  representing the first and second states of the paper sensor device  2 E respectively.  FIG. 15  is a schematic perspective view of two transport rollers  5 ,  5 ′ provided in the transporting unit  9  in the paper sensor device  2 E.  FIG. 16  is a block diagram of a configuration of major components of the image-forming device  100 . 
     As shown in (a) and (b) of  FIG. 14  and  FIG. 15 , the transporting unit  9  in the paper sensor device  2 E includes the transport roller (second transport roller)  5 ′ in place of the paper guide  6  in the paper sensor device  2  in accordance with Embodiment 1 shown in  FIG. 1 . The photodetector  4  is embedded in the transport roller  5 ′. Specifically, the photodetector  4  is disposed in a hole  5 ′ a  formed in the transport roller  5 ° from its outer circumferential surface toward its center. The photodetector  4  has a light-receiving face directed at the opening (window) of the hole  5 ′ a.    
     In the transporting unit  9 , the transport roller (first transport roller)  5  and the transport roller  5 ′, which constitute a pair of rollers, rotate to sandwich and transport the sheet of paper P in a nip region where the transport rollers  5 ,  5 ′ come in contact with each other. The transport rollers  5 ,  5 ′ are structured so that the light-emitter  3  and the photodetector  4  face each other as a result of the rotation of the transport rollers  5 ,  5 ′. 
     Referring to  FIG. 15 , the transport roller  5 ′ includes roller electrodes (first electrodes)  10 ′ a ,  10 ′ b , . . . disposed at or near its ends, similarly to the transport roller  5 . The transport roller  5 ′ is structured so that external electrodes (second electrodes)  15 ′ a ,  15 ′ b , . . . slide respectively on the roller electrodes  10 ′ a ,  10 ′ b , . . . . The external electrodes (second electrodes)  15 ′ a ,  15 ′ b , . . . are connected to the controller  12  so that the measurements optically obtained by the photodetector  4  can be transmitted. 
     If the photodetector  4  is embedded in the transport roller  5 ′, it is preferable to employ any of the configurations described in Variation Examples 1 and 2 of Embodiment 1 as shown in  FIG. 16 . In the example of  FIG. 16 , the amplification circuit  14  and the A/D converter  13  are embedded in the transport roller  5 ′, and the results of the measurement by the photodetector  4  are outputted in the form of digital signals. 
     The image-forming device  100 E in accordance with the present embodiment includes such a paper sensor device  2 E. The image-forming device  100 E determines a paper property by the same method and prints by the same printing process as in Embodiments 1 and 2, and description thereof is omitted. 
     Advantages 
     In this configuration, measurements are made on the sheet of paper P being sandwiched in the nip region between the transport rollers  5 ,  5 ′. The configuration can therefore further reduce adverse effects of flapping, tilting, and warping of the sheet of paper P and take measurements with higher precision. 
     The transport rollers  5 ,  5 ′, as a pair of rollers, may be in contact with each other with a prescribed pressure at all times. This arrangement eliminates the need for lifting and lowering the transport roller  5  (or the transport roller  5 ′) before and after the sheet of paper P reaches the nip region, thereby rendering it unnecessary to provide a mechanism that lifts and lowers the transport roller  5 . 
     Variation Example 3 
     Embodiment 6 described above includes the transport roller  5 ′ in place of the paper guide  6  provided in the paper sensor device  2  in accordance with Embodiment 1, and the photodetector  4  is embedded in the transport roller  5 ′. As an alternative example, Embodiment 6 may include the transport roller  5 ′ in place of the paper guide  6 ′ provided in the paper sensor device  2 B in accordance with Embodiment 3 shown in  FIG. 8 . As a further alternative, Embodiment 6 may include the transport roller  5 ′ in place of the paper guide  6  provided in the paper sensor device  2 C in accordance with Embodiment 4 shown in  FIG. 11 , and the light-emitter  3  and the photodetector  4  may be embedded in the transport roller  5 ′. As yet another alternative, Embodiment 6 may include the transport roller  5 ′ in place of the paper guide  6 ′ provided in the paper sensor device  2 D in accordance with Embodiment 5 show in  FIG. 12 . 
     SUMMATION 
     The present disclosure, in aspect 1 thereof, is directed to a paper sensor device including: a light-emitter; a photodetector configured to receive measurement light projected by the light-emitter and then either transmitted or reflected by a sheet of paper, the paper sensor device detecting a paper property based on the measurement light; and a transporting unit configured to transport the sheet of paper while sandwiching the sheet of paper between a transport roller and an opposing member, either or both of the transport roller and the opposing member having a window on a sandwiching face thereof where the sheet of paper is sandwiched, wherein: the light-emitter projects light via the window(s) onto a sandwiched portion of the sheet of paper being transported; and the photodetector receives the measurement light via the window(s). 
     This structure includes a transporting unit that sandwiches and transports a sheet of paper. The transporting unit includes a transport roller and an opposing member either or both of which has/have a window on a sandwiching face thereof where the sheet of paper is sandwiched. The light-emitter projects light via the window(s) onto a sandwiched portion of the sheet of paper being transported. The photodetector receives via the window(s) the measurement light, which is transmission or reflection of the projected light. In other words, the sandwiched portion of the sheet of paper is used as a measuring portion where a paper property is detected. 
     The sandwiched portion of the sheet of paper does not flap, tilt, or warp even while being transported. Therefore, a paper property can be detected with high precision based on the measurement light by using the sandwiched portion as the measuring portion. 
     In aspect 2 of the present disclosure, the paper sensor device of aspect 1 is configured such that the photodetector receives reference light that is produced from light emitted by the light-emitter in a first state and receives the measurement light transmitted or reflected by the sheet of paper in a second state, to detect the paper property based on the measurement light and the reference light. 
     In this structure, reference light is received that is produced from the tight emitted by the light-emitter. The structure therefore can eliminate, for example, variations of the intensity of the light emitted by the light-emitter in order to detect a paper property with higher precision. 
     In aspect 3 of the present disclosure, the paper sensor device of aspect 2 is configured such that: the light-emitter and the photodetector are located separately, one inside the transport roller and the other opposite the sandwiching face of the opposing member; and the photodetector, as a result of rotation of the transport roller, faces the light-emitter to receive the reference light emitted by the light-emitter in the first state where there exists no sheet of paper between the light-emitter and the photodetector and to receive the measurement light transmitted by the sheet of paper in the second state where the sheet of paper exists between the light-emitter and the photodetector. 
     In this structure, the light-emitter and the photodetector are located separately, one inside the transport roller and the other opposite the sandwiching face of the opposing member, The photodetector can still receive both the reference light and the measurement light when the photodetector comes to face the light-emitter as a result of rotation of the transport roller. 
     In aspect 4 of the present disclosure, the paper sensor device of aspect 2 further includes a reflecting body configured to measure the reference light, wherein: the light-emitter and the photodetector are located together either inside the transport roller or opposite the sandwiching face of the opposing member; and the photodetector receives the reference light reflected by the reflecting body in the first state where the light emitted by the light-emitter is projected onto the reflecting body and receives the measurement light reflected by the sheet of paper in the second state where the light emitted by the light-emitter is projected onto the sheet of paper. 
     In this structure, the light-emitter and the photodetector are located together either inside the transport roller or opposite the sandwiching face of the opposing member. The photodetector can still receive the reference light when both the photodetector and the light-emitter face the reflecting body and receive the measurement light when both the photodetector and the light-emitter face the sheet of paper. 
     In aspect 5 of the present disclosure, the paper sensor device of aspect 4 is configured such that the reflecting body is located outside a transport path of the sheet of paper. 
     In this structure, the reflecting body is located outside a transport path of the sheet of paper. The reflecting body is therefore unlikely to collect paper powder and toner. The structure can restrain measurement precision from falling due to a dirty reflecting body. 
     In aspect 6 of the present disclosure, the paper sensor device of aspect 4 is configured such that: the light-emitter and the photodetector are located opposite the sandwiching face of the opposing member; and the transport roller doubles as the reflecting body. 
     In this structure, the transport roller doubles as the reflecting body. That eliminates the need to provide a separate reflecting body, which reduces the number of components. 
     In aspect 7 of the present disclosure, the paper sensor device of aspect 5 is configured such that: the light-emitter and the photodetector are disposed so as to rotate with the transport roller; and the reflecting body is located on an outer circumference surface of the transport roller in a direction that differs from a direction in which the light-emitter and the photodetector face the sheet of paper. 
     Using this structure, the reflecting body can be readily located outside a transport path. 
     In aspect 8 of the present disclosure, the paper sensor device of aspect 5 is configured such that: the transport roller includes a cylindrical rotator having an outer circumferential surface that provides the sandwiching face; the light-emitter and the photodetector are located inside the cylindrical rotator in such a manner that the light-emitter and the photodetector do not rotate with the cylindrical rotator; the cylindrical rotator includes the window(s); and the reflecting body is located on an inner circumferential surface of the cylindrical rotator. 
     Using this structure, the reflecting body can be readily located outside a transport path. The structure can isolate the reflecting body not only from the transport path of the sheet of paper, but also from the internal space of an image-forming device or a like device in which the paper sensor device is provided. That can effectively prevent the reflecting body from collecting dirt and grime. 
     In aspect 9 of the present disclosure, the paper sensor device of any one of aspects 1 to 8 is configured such that: the transport roller serves as a first transport roller; and the opposing member serves as a second transport roller that, together with the first transport roller, serves as a pair of rollers. 
     In this structure, measurements are made on the sheet of paper being sandwiched in the nip region between the first and second transport rollers. The structure can therefore further reduce adverse effects of flapping, tilting, and warping of the sheet of paper and take measurements with higher precision. In addition, if the first and second transport rollers are pressed in contact with each other in a suitable manner, there is no need for a mechanism that lifts and lowers the first transport roller, which is needed if the transporting unit is a paper guide. 
     In aspect 10 of the present disclosure, the paper sensor device of any one of aspects 1 to 5 and 7 to 9 is configured such that either of the light-emitter and the photodetector is located at least inside the transport roller, the paper sensor device further including: a first electrode on an outer circumferential surface of the transport roller, the first electrode being electrically connected to an element inside the transport roller; and a second electrode outside the transport roller, the second electrode sliding on the first electrode as a result of rotation of the transport roller. 
     This structure readily enables, for example, the supply of power and the extraction of outputs to/from elements inside the rotating transport roller. 
     In aspect 11 of the present disclosure, the paper sensor device of any one of aspects 1 to 5 and 7 to 10 is configured such that the light-emitter is located at least inside the transport roller, the paper sensor device further including: a coil inside the transport roller; and an AC magnetic field generator outside the transport roller, wherein an element inside the transport roller operates on electric power generated by an electromotive force electromagnetically induced in the coil by an AC magnetic field generated by the AC magnetic field generator. 
     This structure eliminates the need to supply power from the outside of e transport roller. 
     In aspect 12 of the present disclosure, the paper sensor device of any one of aspects 1 to 5 and 7 to 11 is configured such that the photodetector is located at least inside the transport roller, the paper sensor device further including: an electromagnetic wave transmitter inside the transport roller; and an electromagnetic wave receiver outside the transport roller. 
     This structure enables contactless communications with elements embedded in the transport roller. That can in turn eliminate high contact resistance and improper contacts in configurations in which electrodes are provided in contact with each other. 
     In aspect 13 of the present disclosure, the paper sensor device of any one of aspects 1 to 5 and 7 to 12 is configured such that the light-emitter is located at least inside the transport roller, the paper sensor device further including: an electromagnetic wave receiver inside the transport roller; and an electromagnetic wave transmitter outside the transport roller. 
     This structure enables contactless communications with elements embedded in the transport roller. That can in turn eliminate high contact resistance and improper contacts in configurations in which electrodes are provided in contact with each other. 
     In aspect 14 of the present disclosure, the paper sensor device of any one of aspects 1 to 13 is configured such that the light-emitter includes a plurality of types of light sources having different peak wavelengths. 
     This structure enables detection of water content ratio and other paper properties. 
     In aspect 15 of the present disclosure, the paper sensor device of any one of aspects 1 to 14 is configured such that the light-emitter and the photodetector measure the measurement light at least at two portions of each sheet of paper. 
     This structure enables high precision detection of a paper property even in the presence of variations in a sheet of paper. 
     In aspect 16 of the present disclosure, the paper sensor device of any one of aspects 1 to 15 is configured such that the paper property includes at least one of a brand name, thickness, grammage, water content ratio, and surface smoothness of the sheet of paper. 
     The present disclosure, in aspect 17 thereof, is directed to an image-forming device including the paper sensor device of any one of aspects 1 to 16, the image-forming device specifying an image-forming condition based on a result of detection carried out by the paper sensor device. 
     In aspect 18 of the present disclosure, the image-forming device of aspect 17 is configured such that the image-forming condition is at least one of a voltage level applied across a transfer unit, a current level supplied to the transfer unit, a pressure applied to the sheet of paper by a fuser, a temperature at which the fuser heats the sheet of paper, and a velocity at which the fuser transports the sheet of paper. 
     The present invention is not limited to the description of the embodiments above and may be altered within the scope of the claims. Embodiments based on a proper combination of technical means disclosed in different embodiments are encompassed in the technical scope of the present invention. Furthermore, a new technological feature may be created by combining different technological means disclosed in the embodiments. 
     REFERENCE SIGNS LIST 
       2 ,  2 A,  2 B,  2 C,  2 D,  2 E Paper Sensor Device 
       3  Light-emitter 
       4  Photodetector 
       5 ,  5 ′,  20  Transport Roller 
       6 ,  6 ′ Paper Guide (Opposing Member) 
       9 ,  9 B Transporting unit 
       7 ,  22  Window 
       17  Reflector 
       10   a,    10   b  Roller Electrode 
       11  Constant Current Source 
       12  Controller 
       12   a  Memory 
       12   b  Calculating unit 
       13  A/D Converter 
       14  Amplification Circuit 
       15   a,    15   b  External Electrode 
       21  Cylindrical Rotator 
       23  Supporting unit 
       100 ,  100 A,  100 B,  100 D,  100 E Image-forming Device