Patent Publication Number: US-2006017762-A1

Title: Device to determine a print medium type, image forming apparatus having the same, and method of determining a print medium type

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority from Korean Patent Application No. 2004-57279, filed on Jul. 22, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present general inventive concept relates to an ink-jet printer, and more particularly, to a device to determine a print medium type, an ink-jet printer having the same, and a method of determining a print medium type.  
      2. Description of the Related Art  
      In general, an ink-jet printer is a device in which ink is ejected from a printhead to a print medium to form an image. The printhead is mounted in a carrier that is moved along a widthwise direction of the print medium, which is transferred in a lengthwise direction thereof. There are several different types of print media including a paper (normal paper), a photopaper, a transparent paper, such as an overhead projector (OHP) film, or an ink-jet only paper having a coated print side to increase an ink absorption ability. In order to print a clear image on a variety of print media described above, a printing method should be set according to the type of print medium. Thus, a device for determining a print medium type and for discriminating among the different types of print media becomes necessary for use in the ink-jet printer.  
       FIG. 1  is a schematic diagram illustrating a conventional device for determining a print medium type in an ink-jet printer. Referring to  FIG. 1 , the conventional device for determining a print medium type discriminates among the types of print media according to differing optical reflection characteristics of the types of print media The conventional device for determining a print medium type comprises a media sensor  1  having a light-emitting unit  2 , a first light-receiving unit  3 , and a second light-receiving unit  4 . The light-emitting unit  2  emits light incident on a print side of a print medium P, which is transferred along a direction indicated by an arrow during operation. The first light-receiving unit  3  is disposed in a first position to sense light that is properly reflected at a reflection angle that is equal to an incident angle of the emitted light. The second light-receiving unit  4  is disposed in a second position between the light-emitting unit  2  and the first light-receiving unit  3  to sense light that is reflected according to a disturbance in the print side of the print medium R  
      In the conventional device for determining a print medium type, light is emitted by the light-emitting unit  2 , an intensity of properly reflected light and an intensity of light reflected according to a disturbance in the print side of print medium P are sensed by the first and second light-receiving units  3  and  4 , respectively. The type of print medium is determined according to a ratio of the intensities of the light that is properly reflected and the light that is reflected according to a disturbance in the print side of the print medium P.  
      However, since the media sensor  1  of the conventional device for determining a print medium type requires a pair of light-receiving units  3  and  4  and the light-emitting unit  2  as described above, a print medium type cannot be determined by a general sensor having a single light-emitting unit and a single light-receiving unit. Thus, the conventional device for determining a print medium type is not effective.  
     SUMMARY OF THE INVENTION  
      The present general inventive concept provides a device to determine a print medium type including a media sensor having a single light-receiving unit, an ink-jet printer having the same, and a method of determine a print medium using the same.  
      Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.  
      The foregoing and/or other aspects and advantages of the present general inventive concept may be achieved by providing a device to determine a print medium type, comprising a media sensor including a single light-emitting unit to emit a first incident light onto a print medium and a single light-receiving unit to sense an intensity of light reflected from the first incident light by the print medium, and a print medium-determining unit to compare the sensed intensity of a first reflected light reflected from the first incident light with a first predetermined reference value and to determine the print medium accordingly. The first incident light may have an intensity that is stepwise increased.  
      The first predetermined reference value may be 60-90% of a maximum intensity of the first incident light.  
      The light-emitting unit of the media sensor may emit a second incident light onto the print medium. The device may further comprise a moving unit to move the media sensor along a widthwise direction of the print medium so that the media sensor emits the second incident light at a plurality of positions along the width of the print medium and senses a plurality of corresponding intensities of a second reflected light reflected from the second incident light by the print medium at each of the plurality of positions, a C-calculating unit to calculate a distribution factor C according to the plurality of corresponding intensities of the second reflected light, wherein the print medium-determining unit compares the distribution factor C with a second predetermined reference value to determine the print medium accordingly. The second incident light may have an intensity that is fixed at a predetermined value. The print medium-determining unit can determine whether the print medium is a photopaper according to the comparison of the intensity of the first reflected light with the first reference value and determine whether the print medium is a transparent paper, an ink-jet only paper, or a normal paper according to the comparison of the distribution factor C with the second reference value.  
      The plurality of corresponding intensities of the second reflected light may be combined into a plurality of groups of intensities having between 3 and 8 intensity values.  
      The light-emitting unit of the media sensor may be configured to emit the second incident light onto the print medium. The device may further comprise the moving unit to move the media sensor along a widthwise direction of the print medium so that the media sensor emits the second incident light at a plurality of positions along the width of the print medium and senses a plurality of corresponding intensities of light reflected from the second incident light by the print medium at each of the plurality of positions, a V 1 -calculating unit to calculate a first variance V 1  of the plurality of corresponding intensities by filtering the sensed plurality of corresponding intensities with respect to a first frequency area, and an ink-jet only paper-determining unit to determine that the print medium is an ink-jet only paper if the first variance V 1  is less than a boundary value V 1   m  of the first variance V 1 . The second incident light may have an intensity that is fixed at a predetermined value.  
      The first frequency area may be between 50 Hz and 100 Hz.  
      The light-emitting unit of the media sensor may be configured to emit the second incident light onto the print medium. The device may further comprise the moving unit to move the media sensor along a widthwise direction of the print medium so that the media sensor emits the second incident light at a plurality of positions along the width of the print medium and senses a plurality of corresponding intensities of light reflected from the second incident light by the print medium at each of the plurality of positions, a V 2 -calculating unit to calculate a second variance V 2  of the plurality of corresponding intensities by filtering the sensed plurality of corresponding intensities with respect to a second frequency area, and a transparent paper-determining unit to determine that the print medium is a transparent paper if the second variance V 2  is larger than a boundary value V 2   m  of the second variance V 2 . The second incident light may have an intensity that is fixed at a predetermined value.  
      The second frequency area may be between 0 Hz and 5 Hz.  
      The foregoing and/or other aspects and advantages of the present general inventive concept may also be achieved by providing an image forming apparatus comprising a printer to print an image on a print medium and a device to determine a print medium type, the device to determine the print medium type comprising a media sensor including a single light-emitting unit to emit a first incident light onto a print medium and a single light-receiving unit to sense an intensity of light reflected from the first incident light by the print medium, and a print medium-determining unit to compare the intensity of the light reflected from the first incident light with a first predetermined reference value and to determine the print medium accordingly. The first incident light may have an intensity that is stepwise increased.  
      The first predetermined reference value may be 60-90% of a maximum intensity of the first incident light.  
      The light-emitting unit of the media sensor may be configured to emit a second incident light having an intensity fixed at a predetermined value onto the print medium. The apparatus may further comprise a moving unit to move the media sensor along a widthwise direction of the print medium so that the media sensor emits the second incident light at a plurality of positions along the width of the print medium and senses a plurality of corresponding intensities of light reflected from the second incident light by the print medium at each of the plurality of positions, a C-calculating unit to calculate a distribution factor C according the plurality of corresponding intensities of the second reflected light, wherein the print medium-determining unit compares the distribution factor C with a second predetermined reference value to determine the print medium as a transparent paper, an ink-jet only paper, or a normal paper.  
      The plurality of corresponding intensities of the second reflected light may be combined into a plurality of groups of intensities having between 3 and 8 intensity values.  
      The light-emitting unit of the media sensor may be configured to emit the second incident light onto the print medium. The apparatus may further comprise the moving unit to move he media sensor along a widthwise direction of the print medium so that the media sensor emits a second incident light at a plurality of positions along the width of the print medium and senses a plurality of corresponding intensities of light reflected from the second incident light by the print medium at each of the plurality of positions, a V 1 -calculating unit to calculate a first variance V 1  of the plurality of corresponding intensities by filtering the sensed plurality of corresponding intensities with respect to a first frequency area, and an ink-jet only paper-determining unit to determine that the print medium is an ink-jet only paper if the first variance V 1  is less than a boundary value V 1   m  of the first variance V 1 . The second incident light may have an intensity that is fixed at a predetermined value.  
      The first frequency area may be between 50 Hz and 100 Hz.  
      The light-emitting unit of the media sensor may be configured to emit the second incident light onto the print medium. The apparatus may further comprise the moving unit to move the media sensor along a widthwise direction of the print medium so that the media sensor emits the second incident light at a plurality of positions along the width of the print medium and senses a plurality of corresponding intensities of light reflected from the second incident light by the print medium at each of the plurality of positions, a V 2 -calculating unit to calculate a second variance V 2  of the plurality of corresponding intensities by filtering the sensed plurality of corresponding intensities with respect to a second frequency area, and a transparent paper-determining unit to determine that the print medium is a transparent paper if the second variance V 2  is greater than a boundary value V 2   m  of the second variance V 2 . The second incident light may have an intensity that is fixed at a predetermined value.  
      The second frequency area may be between 0 Hz and 5 Hz.  
      The foregoing and/or other aspects and advantages of the present general inventive concept may also be achieved by providing a method of determining a print medium type, the method comprising performing a primary sensing operation to sense an intensity of a first reflected light reflected from a print medium by emitting a first incident light onto the print medium, and comparing the intensity of the first reflected light with a first predetermined reference value and determining the print medium accordingly. The first incident light may have an intensity that is stepwise increased.  
      The first predetermined reference value may be 60-90% of a maximum intensity of the first incident light.  
      The method may further comprise performing a secondary sensing operation to sense a plurality of intensities of a second reflected light reflected from each of a plurality of positions along the print medium by emitting a second incident light at the plurality of positions along the width of the print medium, calculating a distribution factor C according to the sensed plurality of intensities of the second reflected light, and comparing the distribution factor C with a second predetermined reference value and determining that the print medium is a transparent paper, an ink-jet only paper, or a normal paper accordingly. The second incident light may have an intensity that is fixed at a predetermined value.  
      The plurality of intensities of the second reflected light may be combined into a plurality groups of intensities having between 3 and 8 intensity values.  
      The method may further comprise performing the secondary sensing operation to sense an intensity of the second reflected light reflected from each of a plurality of positions along the print medium by emitting the second incident light at the plurality of positions along the width of the print medium, calculating a first variance V 1  of the plurality of intensities of the second reflected light by filtering the sensed plurality of intensities of the second reflected light with respect to a first frequency area, and determining that the print medium is an ink-jet only paper if the first variance V 1  is less than a boundary value V 1   m  of the first variance V 1 . The second incident light may have an intensity that is fixed at a predetermined value.  
      The first frequency area may be between 50 Hz and 100 Hz.  
      The method may further comprise performing the secondary sensing operation to sense a plurality of intensities of the second light reflected from each of a plurality of positions along the print medium by emitting the second incident light at the plurality of positions along the width of the print medium, calculating a second variance V 2  of the plurality of intensities of the second reflected light by filtering the sensed plurality of intensities of the second reflected light with respect to a second frequency area, and determining that the print medium is a transparent paper if the second variance V 2  is greater than a boundary value V 2   m  of the second variance V 2 . The second incident light may have an intensity that is fixed at a predetermined value.  
      The second frequency area may be between 0 Hz and 5 Hz.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a schematic diagram illustrating a conventional device for determining a print medium type in an ink-jet printer;  
       FIG. 2  is a schematic diagram illustrating a device to determine a print medium type according to an embodiment of the present general inventive concept;  
       FIG. 3  is a schematic diagram illustrating a portion of an ink-jet printer having the device to determine a print medium type of  FIG. 2  according to an embodiment of the present general inventive concept;  
       FIG. 4  is a conceptual diagram illustrating a microprocessor of the device to determine a print medium type of  FIG. 2  according to an embodiment of the present general inventive concept;  
       FIG. 5  is a conceptual diagram illustrating a microprocessor of the device to determine a print medium type of  FIG. 2  according to another embodiment of the present general inventive concept;  
       FIG. 6  is a flowchart illustrating a method of determining a print medium type according to an embodiment of the present general inventive concept;  
       FIG. 7  is a flowchart illustrating a method of determining a print medium type according to another embodiment present general inventive concept;  
       FIG. 8  is a graph illustrating intensities of reflected light sensed by emitting a second incident light at 4000 positions along a width of a print medium;  
       FIG. 9  illustrates a method of calculating a distribution factor C;  
       FIG. 10  is a graph illustrating a distribution factor C with respect to each print medium;  
       FIG. 11  is a graph illustrating a first variance V 1  with respect to each print medium; and  
       FIG. 12  is a graph illustrating a second variance V 2  with respect to each print medium.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept while referring to the figures.  
      Referring to  FIG. 2 , a device to determine a print medium type according to an embodiment of the present general inventive concept comprises a media sensor  10 , an analog to digital converter  15 , and a microprocessor  20 .  
      The media sensor  10  includes a light-emitting unit  11  to emit a first incident light having a stepwise increasing intensity or a second incident light having an intensity fixed at a predetermined value onto a print medium P, and a light-receiving unit  12  to sense an intensity of first and second reflected light reflected from the first and second incident light. As illustrated in  FIG. 2 , the media sensor  10  is positioned above the print medium P, which is transferred along an X-direction during operation. The light-emitting unit  11  may include a light-emitting diode (LED). A signal input into the LED is adjusted by pulse width modulation (PWM) so that an intensity of incident light of the light-emitting unit  11  can be at a constant level that can be increased or decreased. The light-emitting unit  11  emits the incident light toward the print medium P at an incidence angle The light-receiving unit  12  is disposed at a position in the media sensor  10  that is directly above a light incidence position (i.e., where the light emitted by the light-emitting unit  11  is incident on the print medium P). In other words, the light-receiving unit  12  and the light incidence position form a line that is perpendicular to the print medium P. This arrangement is illustrated in  FIG. 2  by a first dotted line arrow that is perpendicular to the print medium P. However, as indicated by a second dotted line arrow in  FIG. 2  having a non-right angle with respect to the print medium P, the light-receiving unit  12  may alternatively be disposed at a second position in the media sensor  10  that is at an angle with respect to the light incidence position and the print medium P. The second position is indicated by a dotted line square  12  in the media sensor  10 . An angle between the second dotted line arrow and the print medium P may be equal to the incidence angle of the light emitted by the light-emitting unit  11 . As described above, since the media sensor  10  comprises the single light-emitting unit  11  and the single light-receiving unit  12 , an alignment sensor of an ink-jet printer may be used as the media sensor  10 .  
      Referring to  FIG. 3 , the media sensor  10  is installed on a carrier  52  in which a printhead  50  of an ink-jet printer is mounted. The carrier  52  is guided by a guide  54  and is reciprocated by a moving unit (not shown) in a widthwise direction of the print medium P, (i.e., a Y-direction). Thus, the media sensor  10  that is installed on the carrier  52  may be reciprocated in the Y-direction. As described above, the media sensor  10  moves in the widthwise direction of the print medium P, emits a second incident light at a plurality of positions set at equal intervals along the width of the print medium P, and senses a plurality of corresponding intensities of the second reflected light by the print medium P.  
      A Y-direction position of the media sensor  10  can be determined by a position encoder. Since the position encoder determines a Y-direction position of the printhead  50  used during printing operations, an additional position encoder is not needed to determine the Y-direction position of the media sensor  10 .  
      The ADC  15  converts an analog signal that indicates the intensities of the first or second reflected light sensed by the light-receiving unit  12  into a digital signal.  
       FIG. 4  is a conceptual diagram illustrating a microprocessor  20  of the device to determine a print medium type of  FIG. 2  according to an embodiment of the present general inventive concept. Referring to  FIG. 4 , the microprocessor  20  comprises a photopaper-determining unit  20 - 10 , a C-calculating unit  20 - 11 , and a print medium-determining unit  20 - 12  that determines the print medium type according to a distribution factor C that indicates a distribution of the plurality of corresponding intensities of the second reflected light for each of the different types of print media.  
      When the first incident light is emitted onto the print medium P by the light-emitting unit  11  ( FIG. 2 ) and an intensity of the first reflected light reflected by the print medium P to the light-receiving unit  12  reaches a first reference value, the photopaper-determining unit  20 - 10  determines that the print medium P is a photopaper. The first reference value may be set to 60-90% of a maximum intensity of the first incident light which the light-emitting unit  11  of the media sensor  10  emits. However, if the first reference value is set to be less than 60% of the maximum intensity of the first incident light, the print medium P may be incorrectly determined as another type of print medium P. For example, the print medium P may be incorrectly determined as a transparent paper instead of the photopaper. Additionally, if the first reference value is set to be greater than 90% of the maximum intensity of the first incident light, the print medium P may not be able to be determined to be the photopaper, since the photopaper may be incapable of reflecting 90% of the first incident light to the light-receiving unit  12  (see  FIG. 2 ).  
       FIG. 8  is a graph illustrating intensities of the second reflected light sensed by emitting the second incident light at 4000 positions set along a width of a print medium, and  FIG. 9  illustrates a method of calculating the distribution factor C.  
      The distribution factor C is determined after emitting the second incident light at 4000 positions along the width of the print medium P (see  FIG. 2 ) and sensing corresponding intensities of the second reflected light reflected from the second incident light. In other words, the light-emitting unit  11  (see  FIG. 2 ) of the media sensor  10  (see  FIG. 2 ) emits the second incident light at 4000 positions and the light-receiving unit  12  (see  FIG. 2 ) of the media sensor  10  measures the corresponding intensities of the second reflected light.  FIG. 8  illustrates the graph including the intensities measured by the light-receiving unit  12  of the media sensor  10  that correspond to the second incident light emitted at the 4000 corresponding positions. In particular  FIG. 8  illustrates the measured intensities for the different types of print media usable with the present general inventive concept.  
      For example, as illustrated in  FIG. 8 , the photopaper reflects the largest amount of the second incident light to the light-receiving unit  12  and has the highest average intensity of the second reflected light of all the different types of print media. The transparent paper has the second highest average intensity of second reflected light. The normal paper and the ink-jet only paper have similar average intensities of the second reflected light. Since the normal paper and the ink-jet only paper have similar average intensities, it may be difficult to discriminate between the two using only the average value of the measured intensities of the second reflected light. As described below with reference to  FIGS. 6, 8 , and  9 , the normal paper can be discriminated from the ink-jet only paper according to the distribution factor C of the intensities of the second reflected light. Alternatively, as described below with reference to  FIGS. 7 and 8 , the normal paper can be discriminated from the ink-jet only paper by filtering a frequency of the intensities of the second reflected light and determining a variance.  
      As illustrated in  FIG. 9 , the distribution factor C can be defined for each of the different types of print media by plotting groups of adjacent intensities of the second reflected light as a scatter line “G” with respect to a straight line “A” representing an average intensity over the 4000 positions along the width of the print medium P. A number of times in which groups of adjacent intensities of the second reflected light represented by the scatter line “G” cross the average intensity line “A” over the 4000 positions is equal to the distribution factor C. More specifically, the intensities of the second reflected light are combined into groups of intensities having a predetermined size. For example, the groups of intensities may include three adjacent intensities (e.g., intensity at a second position, intensity at a third position, and intensity at a fourth position). An average value of the intensities of second reflected light of each group of the predetermined size (i.e., three) is calculated as a representative intensity value and the scatter line plot “G” is graphed by connecting the representative intensity value of each group by a line. The line “A” having an average value of the intensities of second reflected light over the 4000 positions is also plotted. The number of times that “G” crosses “A” is determined to be a value of the distribution factor C. The C-calculating unit  20 - 11  (see  FIG. 4 ) calculates the value of C by performing an arithmetic operation.  
      Thus, data about adjacent intensities of the second reflected light are combined into a plurality of groups having a predetermined size. The representative intensity value of each group is then calculated.  
      For example, a first value, a second value, a third value, a fourth value, . . . , and a 4000-th value are combined into groups having three intensity values. The groups of adjacent intensities may include a group having a combination of a first intensity value, a second intensity value, a third intensity value, a group having a combination of a second intensity value, a third intensity value, a fourth intensity value, . . . , and a group having a combination of a 3998-th intensity value, a 3999-th intensity value, 4000-th intensity value. The representative intensity value of each group is then calculated. Next, the representative intensity value of each group is plotted, and adjacent plots are connected to one another to form the scatter line plot “G” in  FIG. 9 . Also as illustrated in  FIG. 9 , the line “A” is plotted according to the average value of intensities of the second reflected light. Next, as indicated by intersection points  1 ,  2 ,  3 ,  4 , and  5  in  FIG. 9 , C can be calculated by adding the number of times that “G” crosses “A.” 
      Although  FIGS. 8 through 11  describe the second reflected light being emitted at 4000 positions along the width of the print medium P to determine a type of print medium, it should be understood that the print medium can be determined by emitting light at less than 4000 positions or more than 4000 positions. Additionally, although  FIGS. 8 and 9  illustrate that the intensities of the second reflected light are combined into groups of three adjacent intensities, other predetermined sizes of groups may alternatively be used.  
      If the predetermined size of the groups is equal to or less than 2, large variations occur in “G.” Accordingly, all values of the distribution factor C corresponding to the different types of print media increase, a difference in the values of the distribution factor C between the different types of print media is reduced, and errors may occur in discrimination among the different types of print media. On the other hand, if the predetermined size of the groups is equal to or greater than 9, variations in “G” are smaller. Accordingly, all values of the distribution factor C corresponding to the different types of print media decrease, a difference in the values of the distribution factor C between the different types of print media is reduced, and errors may occur in discrimination among the different types of print media. Accordingly, the predetermined size of the groups may be set between 3 and 8 intensity values.  
      The print medium-determining unit  20 - 12  (see  FIG. 4 ) that determines the type of print medium according to the distribution factor C determines that the print medium P is a transparent paper if the distribution factor C is less than or equal to a first boundary value C 1 . If the distribution factor C is greater than a second boundary value C 2 , which is greater than the first boundary value C 1 , the print medium-determining unit  20 - 12  determines that the print medium P is an ink-jet only paper. In addition, if the distribution factor C is greater than the first boundary value C 1  and less than or equal to the second boundary value C 2 , the print medium-determining unit  20 - 12  determines that the print medium P is a normal paper.  
       FIG. 10  is a graph illustrating a value of the distribution factor C with respect to the different types of print media when intensities of the second reflected light are combined into groups of three intensity values and the distribution factor C is calculated with respect to a normal paper, an ink-jet only paper, a photopaper, and a transparent paper. Referring to  FIG. 10 , the distribution factor C of the ink-jet only paper exceeds 2000, the distribution factor C of the normal paper and the photopaper fall between 1000 and 2000, and the distribution factor C of the transparent paper is less than 1000.  
      Since the device to determine a print medium type can determine whether the print medium P is the photopaper by determining whether the intensity of the light reflected to the light-receiving unit  12  from the first incident light is greater than the first reference value, which is a percentage of the maximum intensity of the first incident light, the distribution factor C may only need to be calculated when the print medium P can not be determined to be the photopaper. If the print medium P is determined not to be the photopaper, the print medium P is determined as one of the normal paper, the transparent paper, and the ink-jet only paper according to the distribution factor C using the second incident light reflected at the plurality of positions along the width of the print medium P Thus, if the first boundary value C 1  is set to 1000 and the second boundary value C 2  is set to 2000, the print medium P can be properly determined as the normal paper, the ink-jet only paper, or the transparent paper.  
       FIG. 6  illustrates a method of determining a print medium type according to an embodiment of the present general inventive concept. The method of  FIG. 6  will now be described with reference to  FIGS. 2 and 4 .  
      If the print medium P (see  FIG. 2 ) is fed into an ink-jet printer in operation S 1 , the media sensor  10  (see  FIG. 2 ) senses the print medium P in a primary sensing process of operation S 2 . In the primary sensing process of operation S 2 , the media sensor  10  emits the first incident light onto the print medium P and senses an intensity of a first reflected light, which is light reflected from the first incident light by the print medium P If the intensity of the first reflected light reaches a first predetermined reference value in operation S 3 , the photopaper-determining unit  20 - 10  (see  FIG. 4 ) determines that the print medium P is a photopaper in operation S 4 . However, if the intensity of the first reflected light does not reach the first reference value, the media sensor  10  senses the print medium P according to a secondary sensing process in operation S 5 . In the secondary sensing process of operation S 5 , the media sensor  10  emits a second incident light at a plurality of positions along the width of the print medium P onto the print medium P and senses an intensity of the second reflected light reflected from each of the positions along the width of the print medium P. The first incident light may have an intensity that is increased stepwise, and the second incident light has an intensity that is fixed at a predetermined value.  
      The C-calculating unit  20 - 11  (see  FIG. 4 ) calculates the distribution factor C by performing an arithmetic operation in operation S 6 - 11 , and the print medium-determining unit  20 - 12  (see  FIG. 4 ) determines the print medium P according to the distribution factor C, by comparing the distribution factor C with the first and second boundary values C 1  and C 2 . If the distribution factor C is less than or equal to the first boundary value C 1  in operation S 6 - 12 , the print medium-determining unit  20 - 12  determines that the print medium P is the transparent paper in operation S 6 - 13 . If the distribution factor C is greater than the second boundary value C 2  in operation S 6 - 14 , the print medium-determining unit  20 - 12  determines that the print medium P is the ink-jet only paper in operation S 6 - 15 . If the distribution factor C is greater than the first boundary value C 1  and less than or equal to the second boundary value C 2 , the print medium determining unit  20 - 12  determines that the print medium P is the normal paper in operation S 6 - 16 .  
       FIG. 5  is a conceptual diagram illustrating a microprocessor  30  of the device to determine a print medium type according to another embodiment of the present general inventive concept. Referring to  FIG. 5 , the microprocessor  30  comprises a photopaper-determining unit  20 - 20 , a V 1 -calculating unit  20 - 21 , an ink-jet only paper-determining unit  20 - 22 , a V 2 -calculating unit  20 - 23 , and a transparent/normal paper-determining unit  20 - 24 .  
      When the first incident light is emitted onto the print medium P by the light-emitting unit  11  ( FIG. 2 ) and the intensity of the first reflected light (i.e., an intensity of light reflected from the first incident light reflected by the print medium P to the light receiving unit  12  ( FIG. 2 )) reaches a first reference value, the photopaper-determining unit  20 - 20  determines that the print medium P is a photopaper. The first reference value may be set to 60-90% of the intensity of maximum incident light which the light-emitting unit  11  of the media sensor  10  emits.  
      The V 1 -calculating unit  20 - 21  calculates V 1 , which is a first variance of a value calculated by filtering the intensities of the second reflected light (see  FIG. 8 ) sensed by the light-receiving unit  12  of the media sensor  10  with respect to a first frequency area.  
      The variance can be defined as an average value of a square of a standard deviation. Since one of ordinary skill should be able to calculate the first variance V 1  from this definition, a detailed description thereof will not be provided.  
      The ink-jet only paper-determining unit  20 - 22  determines that the print medium P is an ink-jet only paper if the first variance V 1  is less than a boundary value V 1   m . Referring to  FIG. 8 , it can be determined that a graph on which the intensities of the second reflected light are plotted illustrates more high frequency components when the print medium is the ink-jet only paper than when the print medium is the normal paper, the photopaper, or the transparent paper.  FIG. 11  is a graph illustrating the first variance V 1  for the different types of print media determined by filtering the graph of the intensities of the second reflected light in an intermediate frequency area (i.e., an area between 50 Hz and 100 Hz). Referring to  FIG. 11 , it can be determined through several repeated experiments that the first variance V 1  when the print medium is the ink-jet only paper is substantially lower than the first variance V 1  when the print medium is the transparent paper, the photopaper, or the normal paper. Thus, when a first frequency area is between 50 Hz and 100 Hz (i.e., the intermediate frequency area) and the boundary value V 1   m  is set to 1.5, it can be correctly determined whether the print medium P is the ink-jet only paper.  
      The V 2 -calculating unit  20 - 23  calculates a second variance V 2 , which is a second variance of a value calculated by filtering the intensities of the second reflected light sensed by the light-receiving unit  12  of the media sensor  10  with respect to a second frequency area. Since one or ordinary skill in the art should be able to calculate the second variance V 2  from this definition, a detailed description thereof will not be provided.  
      The transparent/normal paper-determining unit  20 - 24  determines that the print medium P is the transparent paper if the second variance V 2  is greater than a boundary value V 2   m . In addition, if the second variance V 2  is less than or equal to the boundary value V 2   m , the transparent/normal paper-determining unit  20 - 24  determines that the print medium P is the normal paper.  
      Referring to  FIG. 8 , it can be determined that a graph on which the intensities of the second reflected light are plotted illustrates more low frequency components when the print medium is the transparent paper or the photopaper than when the print medium is the normal paper or the ink-jet only paper.  FIG. 12  is a graph illustrating the second variance V 2  for the different types of print media determined by filtering the graph of the intensities of the second reflected light in a low frequency area (i.e., an area between 0 Hz and 5 Hz). Referring to  FIG. 12 , it can be determined through several repeated experiments that the second variance V 2  when the print medium is the normal paper or the ink-jet only paper is substantially lower than the second variance V 2  when the print medium is the transparent paper or the photopaper.  
      V 2  need only be calculated when the print medium P is not determined to be the photopaper or the ink-jet only paper. If the print medium is not determined to be the photopaper or the ink-jet only paper, the second variance V 2  is calculated, and the print medium is determined using the second variance V 2 . Thus, when the boundary value V 2   m  is set to 12, it can be correctly determined whether the print medium P is the transparent paper or the normal paper.  
       FIG. 7  is a flowchart illustrating a method of determining a print medium type according another embodiment of the present general inventive concept. The method of  FIG. 7  will now be described with reference to  FIGS. 2 and 5 .  
      If the print medium P (see  FIG. 2 ) is fed into an ink-jet printer in operation S 1  the media sensor  10  (see  FIG. 2 ) senses the print medium P in a primary sensing process of operation S 2 . In the primary sensing process of operation S 2 , the media sensor  10  emits a first incident light having an intensity increased stepwise onto the print medium P and senses an intensity of a first reflected light, which is light reflected from the first incident light by the print medium P. If the intensity of the first reflected light reaches a first predetermined reference value in operation S 3 , the photopaper-determining unit  20 - 20  (see  FIG. 5 ) determines that the print medium P is a photopaper in operation S 4 . However, if the intensity of the first reflected light does not reach the first reference value, the media sensor  10  senses the print medium P according to a secondary sensing process in operation S 5 . In the secondary sensing process of operation S 5 , the media sensor  10  emits a second incident light at a plurality of positions along the width of the print medium P onto the print medium P and senses an intensity of a second reflected light reflected from each of the positions along the width of the print medium P. The second incident light has an intensity that is fixed at a predetermined value.  
      The V 1 -calculating unit  20 - 21  (see  FIG. 5 ) calculates V 1  by performing an arithmetic operation in operation S 6 - 21 , and the ink-jet only paper-determining unit  20 - 22  (see  FIG. 5 ) determines whether the print medium is the ink-jet only paper by comparing the first variance V 1  with the boundary value V 1   m  of the first variance V 1 . If the first variance V 1  is less than the boundary value V 1   m  in operation S 6 - 22 , the ink-jet only paper-determining unit  20 - 22  determines that the print medium P is the ink-jet only paper in operation S 6 - 23 . If the first variance V 1  is greater than or equal to the boundary value V 1   m , a V 2 -calcuating unit  20 - 23  (see  FIG. 5 ) calculates a second variance V 2  by performing a second arithmetic operation in operation S 6 - 24 . The transparent/normal paper-determining unit  20 - 24  (see  FIG. 5 ) determines whether the print medium is the normal paper or the transparent paper by comparing the second variance V 2  with a boundary value V 2   m  of the second variance V 2 . If the second variance V 2  is greater than the boundary value V 2   m  in operation S 6 - 25 , the transparent/normal paper-determining unit  20 - 24  determines that the print medium P is the transparent paper in operation S 6 - 26 . If the second variance V 2  is less than or equal to the boundary value V 2   m , the transparent/normal paper-determining unit  20 - 24  determines that the print medium P is the normal paper in operation S 6 - 27 .  
      As described above, in the device to determine a print medium type, an image forming apparatus having the same device, and a method of determining the print medium type according to the present general inventive concept, a print medium can be determined using a sensor having a single light-emitting unit and a single light-receiving unit such that compatibility of parts is improved and costs are reduced. Specifically, an alignment sensor of an ink-jet printer can be used as a media sensor such that the number of sensors is reduced, thereby further reducing costs. Although the description above refers to the different types of print media including a photopaper, a normal paper, an ink-jet only paper, and a transparent paper, it should be understood that other types of print media may be used with the present general inventive concept. For example, relationships among distribution factors and variances of additional types of print media may be determined experimentally.  
      Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.