Patent Publication Number: US-2007121176-A1

Title: Image forming apparatus having an optical unit and an image reading device capable of preventing deterioration of image reading accuracy

Description:
PRIORITY STATEMENT  
      This patent specification is based on and claims priority under 35 U.S.C. §119 of Japanese patent application, No. JP2005-341992 filed on Nov. 28, 2005 in the Japanese Patent Office, the entire contents of which are incorporated herein by reference.  
     BACKGROUND  
      1. Field  
      The present invention generally relates to an optical unit, an image reading device and/or image forming apparatus using the same. More particularly, it may relate to an image forming apparatus that includes an optical unit and an image reading unit which prevent deterioration of reading accuracy of an image, even if a holding member, which support a lens unit and an image pick-up device, is thermally expanded.  
      2. Discussion of the Background  
      In an image reading device provided in a copier, a facsimile, a multi-functional machine and so forth, light is emitted on a document surface from a light source, and the reflected light from the document is entered in a lens unit through a plurality of mirrors. The lens unit forms an image on an image pick-up device such as a CCD line sensor or the like so as to convert the reflected light to an electric signal. Accordingly, the document is being read.  
      In order to accurately image the light reflected from the document surface, focused by the lens unit, on an element surface of the image pick-up device, highly precise positioning of a distance between the lens unit and the image pick-up device will be needed. In a related art image reading apparatus, according to Japanese Patent Laid-Open Application Publication No. JP2005-217500, for example, a supporting member, which supports the lens unit and the image pick-up device, is provided with a protruding member so that the lens unit and the image pick-up device are accurately positioned.  
      However, in such a related art image reading apparatus, due to a rise of the temperature of devices surrounding the image pick-up device or the image reading apparatus, the supporting member which supports the lens unit and the image pick-up device may expand. As a result, the positional relationship between the lens unit and the image pick-up device may be altered, thereby causing a problem such as a decrease in a Modulation Transfer Function (hereinafter referred to as MTF) which ultimately deteriorates reading accuracy of the image.  
     SUMMARY  
      In view of the foregoing, it is an object of at least one example embodiment of the present invention to provide a novel image forming apparatus which includes an optical unit which effectively prevents deterioration of an image reading accuracy when a holding member which holds a lens unit and an image pick-up device is thermally expanded.  
      In one example embodiment, a novel optical unit may include an image pick-up device, a lens unit and a holding member. The image pick-up device may convert a reflected light reflected from an original document to an electric signal, after the light is irradiated on the original document from a light source. The lens unit may image the reflected light on the image pick-up device. The holding member may hold the image pick-up device and lenses. The direction of a fluctuation of a focal length of the lens unit caused by heat and a direction of the fluctuation of a distance between the image pick-up device and the lens unit caused by heat may agree with each other.  
      In an example embodiment of the above-mentioned optical unit, the direction of the fluctuation of the focal length of the lens unit caused by heat and the direction of the fluctuation of the distance between the image pick-up device and the lens unit caused by heat may be a direction of increase.  
      In an example embodiment of the above-mentioned optical unit, a difference between the fluctuation of the focal length of the lens unit caused by heat, and the fluctuation of the distance between the image pick-up device and the lens unit caused by heat may be less than an effective focal depth of the lens unit.  
      In an example embodiment of the above-mentioned optical unit, the holding member may be made of a sheet metal.  
      In an example embodiment of the above-mentioned optical unit, the sheet metal member may be made of a black steel plate.  
      In an example embodiment of the above-mentioned optical unit, the lens unit may read full-color including three wavelengths of Red, Green and Blue, and the difference between the fluctuation of the focal length of all three wavelengths of RGB of the lens unit caused by heat, and the fluctuation of the distance between the image pick-up device and the lens unit is less than the effective focal depth of the lens unit.  
      In an example embodiment of the above-mentioned optical unit, in the wavelength in which a peak width of an MTF curve is the narrowest, among three wavelengths of RGB read by the lens unit, the holding member may hold the image pick-up device and the lens unit at a position where the difference between the fluctuation of the focal length of the lens unit caused by heat, and the fluctuation of the distance between the image pick-device and the lens unit caused by heat may be less than the effective focal depth of the lens unit.  
      In one example embodiment, an image reading apparatus may include the above-mentioned optical unit.  
      In one example embodiment, an image forming apparatus may include an image forming mechanism which may form an image on a recording medium, and the above-mentioned image reading apparatus. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description of example embodiments when considered in connection with the accompanying drawings, wherein:  
       FIG. 1  is a schematic diagram illustrating an image forming apparatus according to a first example embodiment of the present invention;  
       FIG. 2  is a perspective view illustrating an optical unit to which a cover is mounted according to the first example embodiment of the present invention;  
       FIG. 3  is a perspective view illustrating the optical unit of  FIG. 2  from which the cover is removed;  
       FIG. 4  is a top view of the optical unit;  
       FIG. 5  is a lateral cross-sectional view illustrating the optical unit;  
       FIG. 6  is a cross-sectional view illustrating a lens unit of an image reading apparatus of the first example embodiment of the present invention;  
       FIG. 7  is a diagram for explaining a relationship between an MTF of each wavelength of RGB, and a distance between a lens and a CCD; and  
       FIG. 8  is a lateral cross-sectional view illustrating an optical unit according to a second example embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS  
      In describing example embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. For the sake of simplicity of drawings and descriptions, the same reference numerals are given to materials and constituent parts having the same functions, and descriptions thereof will be omitted unless otherwise stated. Example embodiments of the present invention are now explained below with reference to the accompanying drawings. In the later described comparative example, example embodiment, and alternative example, the same reference numerals will be given to constituent elements such as parts and materials having the same functions, and the descriptions thereof will be omitted. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, particularly to  FIG. 1 , an image forming apparatus according to an example embodiment of the present invention is described.  
       FIGS. 1 through 8  illustrate an optical unit, an image reading apparatus and an image forming apparatus of a first example embodiment of the present invention. In the example embodiment, an example in which the image forming apparatus is applied to a copier is shown. However, the image forming apparatus may not be limited to the copier. As long as the image forming apparatus includes an image reading apparatus, the image forming apparatus may be applied to apparatuses such as facsimiles, multifunctional apparatuses and so forth.  
      First, a description will be given of a structure of a copier serving as an image forming apparatus. In  FIG. 1 , a copier  10  serving as an image forming apparatus includes an automatic document feeder (hereinafter referred to as an ADF)  11 , a paper feed unit  12 , an image reading unit  13  serving as an image reading apparatus and an image forming unit  14  serving as an image forming unit.  
      The ADF  11  includes a contact glass  15 , a document tray  16 , a separation paper feed mechanism  17 , a conveyance belt  18 , a paper ejecting mechanism  19  and a catch tray  20 . In the ADF  11 , an original document placed on the document tray  16  is transported onto the contact glass  15  by the separation paper feed mechanism  17  which is equipped with various rollers such as a paper feed roller, a separation roller and so forth. The document being read is conveyed from the contract glass  15  by the conveyance belt  18 , and then is ejected to the catch tray  20  by the paper ejecting mechanism  19 .  
      In a case where both sides of the document are read, the document is sent back to the contact glass  15  by a branch mechanism and the conveyance belt  18  so as to read a surface which has not been read.  
      The paper feed unit  12  includes paper feed cassettes  21   a  and  21   b , and a paper feed mechanism  22 . The paper feed cassettes  21   a  and  21   b  store recording paper of different sizes. The paper feed mechanism  22  is equipped with various rollers which convey the recording paper stored in the paper feed cassettes  21   a  and  22   b  to a position where an image is formed.  
      The image reading unit  13  includes a first carriage  31 , a second carriage  32  and an optical unit  33 . As will be later described, when reading the document being transported onto the contact glass  15 , the first carriage  31  and the second carriage  32  are secured at a lower left of the contact glass  15 . When reading the document placed on the contact glass  15 , the first carriage  31  and the second carriage  32  are moved in left and right directions in  FIG. 1  at a lower part of the contact glass  15 .  
      The image forming unit  14  includes an exposure device  23 , a plurality of photoreceptor drums  24 , a plurality of developing units  25 , a transfer belt  26  and a fixing device  27 . The exposure device  23  forms a write signal based on a read signal loaded in the optical unit  33 . The write signal generated by the exposure apparatus  23  is formed on the surfaces of the photoreceptor drums  24 . Toners of different colors are filled in the respective developing units  25 . The developing units  25  supply the toners of different colors to each of the respective photoreceptor drums  24  so as to transform the write signal to a visible image. The visible images formed on the photoreceptor drums  24  are transferred in sequence so that a color image is formed. The transfer belt  26  transfers the color image to the recording paper supplied from the paper feed unit  12 . The fixing unit  27  fixes the color image to the recording paper.  
      A light source  34  and a first mirror  35  are mounted inside the first carriage  31 . The light source  34  is formed of a halogen lamp or the like and is configured to irradiate the document passing the contact glass  15  or the document placed thereon. Both end portions of a front side and a rear side of the first mirror  35  in  FIG. 1  are supported by the first carriage  31  such that the light reflected from the document surface enters the first mirror  35 .  
      In the second carriage  32 , a second mirror  36  and a third mirror  37  are mounted. Both end portions of a front side and a rear side of the second mirror  36  and the third mirror  37  in  FIG. 1  are supported by the second carriage  32  such that the light reflected from the first mirror  35  is sequentially reflected on the second mirror  36  and the third mirror  37 .  
      Furthermore, a lens unit  38  and an image pick-up device  39  such as a CCD line sensor are mounted in the optical unit  33 . A plurality of device surfaces are aligned in a scanning direction in the image pick-up device  39 . After the light reflected from the third mirror  37  is focused by the lens unit  38 , the light is imaged on the image pick-up device  39  and is converted to an analogue image signal corresponding to image data of the document surface.  
      A wire of a not-shown publicly known operating mechanism is provided to the first carriage  31  and the second carriage  32 , and is slidably attached to a traveling rail which constitutes a part of a main frame of the image reading unit  13 . The wire extends from side to side in the image reading unit  13  in  FIG. 1 .  
      In a case where the document placed on the contact glass  15  is read, the operating mechanism is operated in accordance with a signal requesting for image reading for each line, sent from a not-shown host computer of the copier  10  so as to move the wire from side to side in  FIG. 1 . Accordingly, the first carriage  31  and the second carriage  32  move along the document surface at a speed ratio of 2:1, respectively, while irradiating the light on the document surface on the contact glass  15  by the light source  34 .  
      Therefore, the document surface is optically scanned in a subscanning direction. After the reflected light is sequentially reflected in the order of the first mirror  35 , the second mirror  36  and the third mirror  37 , the reflected light is imaged on the surface of the image pick-up device  39  through the lens unit  38 . Accordingly, the document surface is read.  
       FIGS. 2 through 5  illustrate a structure of the optical unit  33 . In  FIGS. 2 through 5 , a tabular supporting bracket or a supporting member  40  is formed of a sheet metal member and is mounted on a traveling rail  13   a  of the image reading unit  13  by bolts and the like. As described above, the first carriage  31  and the second carriage  32  are slidably mounted on the traveling rail  13   a.    
      A sidewardly open U-shaped holding bracket  41 , which is formed of a black steel plate as a sheet metal member, is attached to the supporting bracket  40 . The holding bracket  41  is fastened to the supporting bracket  40  by two sets of bolts  42   a  and  42   b . The bolts  42   a  and  42   b  are configured to separately be placed in an optical axis direction of the reflected light from the document surface, reflected from the third mirror  37 .  
      The lens unit  38  is secured to the holding bracket  41  by a bracket  43  having a semicircular shape. Both end portions of the bracket  43  are secured to the holding bracket  41  by the bolts  43   a.    
      The image pick-up device  39  is mounted on one end portion or the tip portion of the holding bracket  41 . The image pick-up device  39  is adhered to protruding members  41   a  and  41   b  which are adhered to the tip of one end portion of the holding bracket  41  so that the image pick-up device  39  is firmly held by the holding bracket  41 .  
      Furthermore, with reference to  FIG. 3 , an opening  41   c  is formed on one end portion or the tip portion of the holding bracket  41 . The device surface of the image pick-up device  39  adhered to the protruding members  41   a  and  41   b  faces the lens unit  38  through the opening  41   c . Therefore, the reflected light from the document focused on the lens unit  38  securely enters the device surface of the image pick-up device  39 .  
      A circuit board  46  is installed on the image pick-up device  39 . The circuit board  46  includes an analogue/digital converter, a binarization circuit, a multivalued circuit, a gradation processing circuit, a variable power circuit, an edit processing circuit and so forth. The circuit board  46  is configured such that the read data of the document, which is converted to an analogue signal by the image pick-up device  39 , is converted to a digital image signal by the analogue/digital converter. Then, the binarization processing, multivalued processing, gradation processing, variable power processing and edit processing are performed so as to generate image data. The image data being generated is sent to the image forming unit  14 .  
      As shown in  FIG. 2 , on the holding bracket  41 , a cover  47  is mounted at a position between the lens unit  38  and the image pick-up device  39 . The image pick-up device  39  is shaded with the cover  47 .  FIG. 3  illustrates the optical unit  33  before the cover  47  is mounted.  
      On the other end of the holding bracket  41 , a shading correction member or a shading correction panel  48  is provided. On the shading correction member  48 , an opening  48   a  for shading correction is provided at a position facing the lens unit  38 . The opening  48   a  is formed such that both end portions of the opening  48   a  are broader in width than the central portion thereof.  
      Accordingly, the reflected light from the original document, which is reflected by the third mirror  37 , is focused on the lens unit  38  after the image irregularity is corrected by the shading correction member  48 . Subsequently, the reflected light from the original document is imaged on the image pick-up device  39 .  
      On the holding bracket  41 , openings  49  for determination of a position of the image pick-up device  39  and the lens unit  38  are provided. The openings  49  are formed between the bolts  42   a  and  42   b . The openings  49  are configured such that pins or the like of a not-shown assembling equipment may be inserted in the openings  49 . When the optical unit  33  is assembled, the pins are inserted in the openings  49  so as to adjust the positions of an area from the openings  49  to the image pick-up device  39 . Accordingly, the position of the image pick-up device  39  and the lens unit  38  is determined, and the image pick-up device  39  is adhered to the protruding members  41   a  and  41   b.    
      In the optical unit  33  structured in an above described manner, due to the emission of heat from the image pick-up device  39 , and the emission of heat from the inside of the image reading unit  13  or the copier  10 , the holding bracket  41  formed of the black steel plate may thermally be expanded. The amount of the thermal expansion of the holding bracket  41  is substantially small at a place between the bolts  42   a  and  42   b  which serve as a securing portion to secure the supporting bracket  40 . However, the amount of the thermal expansion of the holding bracket  41  is relatively large on the outside of the bolts  42   a  and  42   b . Consequently, due to the thermal expansion of the holding bracket  41 , the image pick-up device  39  may move in an arrow direction in  FIG. 5 . That is, the image pick-up device  39  may move away from the lens unit  38 . In other words, in a case where the distance between the image pick-up device  39  and the lens unit  38  is determined in a cold period during which the holding bracket  41  is not thermally expanded, and the temperature of the holding bracket  41  increases causing the thermal expansion thereof, the distance between the image pick-up device  39  and the lens unit  38  increases.  
       FIG. 6  is a cross sectional view illustrating the lens unit  38 . The lens unit  38  includes a plurality of lenses  51 ,  52 ,  53 ,  54 ,  55  and  56 . Converging lenses are used for the lenses  51 ,  52 ,  55  and  56 . Diverging lenses are used for the lenses  53  and  54 . The lens  52  and the lens  53  are adhered each other. The lens  54  and the lens  55  are also adhered each other. The lens unit  38  has a lens structure consisting of four groups of lenses using six lenses.  
      Reference is now made to TABLE 1 and TABLE 2 to explain the temperature dependency of lenses of different materials. TABLE 1 shows the temperature dependency of BACD4 which is a plastic lens manufactured by Hoya Corporation. TABLE 2 shows the temperature dependency of a glass lens which uses ZEONEX-E48R® manufactured by ZEON Corporation as a material.  
               TABLE 1                          TEMPERATURE COEFFICIENT OF REFLACTIVE       INDEX (×10 −6 /K)                         (° C.)   (Δn/ΔT)rel.   (Δn/ΔT)abs.               −40/−20   2.4   0.2       −20/0    2.5   0.6        0/+20   2.6   1.0       +20/+40   2.6   1.2       +40/+60   2.7   1.5       +60/+80   2.8   1.7                  
 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
               
               
                 TEMPERATURE AND WAVELENGTH DEPENDENCY 
               
               
                 OF REFRACTIVE INDEX 
               
            
           
           
               
               
               
            
               
                   
                 Wavelength (nm) 
                 Abbe 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 435.835 
                 486.133 
                 546.075 
                 587.562 
                 656.273 
                 785.1 
                 Number 
               
               
                   
                 (g) 
                 (F) 
                 (e) 
                 (d) 
                 (C) 
                 (L.D780) 
                 V/d 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Temper- 
                  0 
                 1.5396 
                 1.5343 
                 1.5300 
                 1.5277 
                 1.5250 
                 — 
                 56 
               
               
                 ature 
                 25 
                 1.5369 
                 1.5317 
                 1.5273 
                 1.5251 
                 1.5224 
                 — 
                 56 
               
               
                 (° C.) 
                 40 
                 1.5352 
                 1.5299 
                 1.5257 
                 1.5234 
                 1.5207 
                 1.5174 
                 57 
               
               
                   
                 60 
                 1.5329 
                 1.5276 
                 1.5234 
                 1.5211 
                 1.5184 
                 1.5152 
                 57 
               
               
                   
                 80 
                 1.5308 
                 1.5253 
                 1.5214 
                 1.5189 
                 1.5164 
                 1.5132 
                 58 
               
               
                   
               
            
           
         
       
     
      As shown in TABLE 1 and TABLE 2, when the temperature rises, the refractive index of the plastic lens and the glass lens normally decreases. In a case where the plastic lens and the glass lens are used for the converging lens so as to form a part of or all of the constituent elements of the lens unit  38 , when the temperature is high, the refractive index decreases so that the focusing strength is weakened, compared with a case in which the temperature is low. Consequently, the focal length of the lens unit  38  becomes longer than that of the case in which the temperature is low.  
      On the other hand, in a case where the plastic lens and the glass lens are used for the diverging lens, the refractive index decreases so that the focal length of the lens unit  38  becomes shorter than that of a case in which the temperature is low. As shown in TABLE 1 and TABLE 2, when comparing the plastic lens with the glass lens, the temperature dependency of the plastic lens is greater than that of the glass lens, and the fluctuation of the refractive index due to a rise in the temperature becomes greater. The power of the lens, that is, the refracting strength of the lens becomes greater when the radius of the lens is small. Therefore, if the combination of the lens shape such as a diverging lens, converging lens and the radius of the lenses, and the material of the lens is set, it may be possible to select either the focal length is near or far, when the temperature of the lens rises. Furthermore, it may be possible to select an amount of the fluctuation of the focal length.  
      Therefore, in the lens unit  38  of the example embodiment, the lens  51 , which is a converging lens, is made of plastic having a high temperature dependency, while other lenses  52 ,  53 ,  54 ,  55  and  56  are made of glass having a low temperature dependency. Accordingly, the lens unit  38  is structured such that, in a case where the temperature rises, the focal length is extended. Furthermore, each radius of the lenses  51 ,  52 ,  53 ,  54 ,  55  and  56  is configured such that, in a case where the temperature rises, the amount of shift in the focal length corresponds to the amount of expansion of the holding bracket  41  due to heat.  
      The material and the radius of the lenses  51 ,  52 ,  53 ,  54 ,  55  and  56  may be configured such that the difference between the amount of shift in the focal length in a case where the temperature rises, and the amount of expansion of the holding bracket  41  due to heat falls within an effective focal depth. The effective focal depth herein refers to a region which is considered to be practically in-focus, and has the same meaning as a depth of field.  
      In the example embodiment, plastic and glass are used as materials for the lenses  51 ,  52 ,  53 ,  54 ,  55  and  56 . However, without limiting the materials to plastic and glass, lenses made of more than two different materials having different temperature dependencies may be assembled and used in the optical unit  33 .  
      Next, with reference to  FIG. 7 , a description will be given of an effect of the shift in the distance between the lens unit  38  and the image pick-up device  39  to the Modulation Transfer Function (hereinafter referred to as MTF) of each wavelength of RGB. In  FIG. 7 , the fluctuation of the distance between the lens unit  38  and the image pick-up device  39  due to heat is indicated by an arrow. In the example embodiment, RGB refers to the three primary colors, Red, Green and Blue. The distance between the lens unit  38  and the image pick-up device  39  is determined in such a manner that the distance is appropriate for all of the wavelengths of RGB during the cold period. In other words, the distance between the lens unit  38  and the image pick-up device  39  is determined such that the desired characteristics are satisfied. However, when the distance between the lens unit  38  and the image pick-up device  39  changes (in  FIG. 7 , the distance increases), the MTF may deviate from a predetermined value. As a result, image information with the RGB balance being out of balance may be obtained. Furthermore, in all of the three wavelengths of RGB, it may be difficult for the lens unit  38  to accurately form an image on the image pick-up device  39 , after the distance between the lens unit  38  and the image pick-up device  39  changes due to a temperature fluctuation.  
      Thus, according to the example embodiment of the present invention, the distance between the lens unit  38  and the image pick-up device  39  is set such that, in any of the wavelengths of RGB, the difference between the displacement amount of the distance between the lens unit  38  and the image pick-up device  39 , and the displacement amount of the focal length of the lens unit  38  falls within the focal depth of the lens unit  38 . Accordingly, the lens unit  38  and the image pick-up device  39  are assembled.  
      In any of the wavelengths of RGB, ideally, the difference between the displacement amount of the distance between the lens unit  38  and the image pick-up device  39 , and the displacement amount of the focal length of the lens unit  38  falls within the focal depth of the lens unit  38 . However, the adjustment for this particular purpose is difficult to perform. Thus, the lens unit  38  and the image pick-up device  39  are assembled such that the difference between the displacement amount of the distance between the lens unit  38  and the image pick-up device  39 , and the displacement amount of the focal length of the lens unit  38  falls within the focal depth of the lens unit  38  in the wavelengths which the fluctuation of the distance between the lens unit  38  and the image pick-up device  39  most affects, that is, the wavelengths in which the peak width of the MTF is the narrowest.  
      The optical unit  33  mounted to the image reading unit  13  reads an original document when the original document is copied. At this time, light is irradiated on the document surface from the light source  34 . After the reflected light from the document surface is reflected on the first mirror  35 , the second mirror  36  and the third mirror  37  in sequence, the reflected light is imaged on the image pick-up device  39  through the lens unit  38 . In a case where the number of document sheets to be read is large, for example, the temperature of the image pick-up device  39  rises. Consequently, the holding bracket  41  is affected by the heat from the image pick-up device  39 , and therefore expands. Furthermore, the focal length of the optical unit  33  is affected by the heat from the image pick-up device  39 , and therefore becomes long. The displacement amount of the focal length of the optical unit  33  is set so as to correspond to the expansion amount of the holding bracket  41  as described above. Accordingly, in the optical unit  33 , even if the temperature rises, an image is appropriately formed on the image pick-up device  39 .  
      According to the example embodiment, a direction of the fluctuation of the focal length of the lens unit  38  caused by the temperature fluctuation and a direction of the fluctuation of the distance between the image pick-up device  39  and the lens unit  38  caused by the temperature fluctuation are configured to be the same direction. Accordingly, even if the temperature fluctuates, the lens unit  38  may be able to form an image on the image pick-up device  39  so that a problem such as a decrease in the MTF due to a displacement of the imaging position may be prevented. Accordingly, favorable image information may be obtained. Therefore, even if the holding bracket  41  which holds the lens unit  38  and the image pick-up device  39  is thermally expanded, deterioration of the image reading accuracy may be prevented.  
      Both the direction of the fluctuation of the focal length of the lens unit  38  caused by the heat and the direction of the fluctuation of the distance between the image pick-up device  39  and the lens unit  38  are configured to be the direction of increase. Consequently, the focal length of the lens unit  38  normally increases due to the heat generated from the optical unit  33 , the image reading unit  13  equipped with the optical unit  33  and the image forming unit  14 . The distance between the image pick-up device  39  and the lens unit  38  also increases. Therefore, even if the holding bracket  41  which holds the lens unit  38  and the image pick-up device  39  is thermally expanded, the deterioration of the image reading accuracy may be prevented.  
      According to the example embodiment, the difference between the fluctuation of the focal length of the lens unit  38  caused by the heat and the fluctuation of the distance between the image pick-up device  39  and the lens unit  38  caused by the heat is configured to be less than the effective focal depth of the lens unit  38 . Accordingly, even if there is a difference between the focal length of the lens unit  38  and the distance between the image pick-up device  39  and the lens unit  38 , the difference falls within a range of the effective focal depth of the lens unit  38 . Therefore, even if the temperature fluctuates, the lens unit  38  may practically be able to form an image on the image pick-up device  39 , thereby preventing a decrease in the MTF which may cause a problem to the extent of image characteristics.  
      According to the example embodiment, the holding bracket  41  is made of the sheet metal member so that the expansion caused by the heat may be suppressed to approximately the half the amount when compared with a case in which the holding bracket  41  is made of a resin mold member. Therefore, the expansion of the holding bracket  41  caused by the effect of heat may be prevented.  
      Furthermore, according to the example embodiment, the holding bracket  41  is formed of a black steel plate so that the flair light which adversely affects the quality of the read image may be suppressed.  
      According to the example embodiment, the lens unit  38  reads full color including the three wavelengths of RGB. The difference between the fluctuation of the focal length of all three wavelength of RGB of the lens unit  38  caused by the heat and the fluctuation of the distance between the image pick-up device  39  and the lens unit  38  caused by the heat is configured to be less than the effective focal depth of the lens unit  38 . Accordingly, in the optical unit  33  which reads full color and may not be able to obtain a favorable read image if the MTF of any one of the wavelengths decreases, the three wavelengths of RGB of the lens unit  38  may be imaged on the image pick-up device  39  even if the temperature fluctuates. Therefore, a problem such as the decrease in the MTF caused by the displacement of the imaging position may be prevented so that favorable image information may be obtained.  
      According to the example embodiment, in the wavelength in which the peak width of the MTF curve is the narrowest among three wavelengths of RGB read by the lens unit  38 , the holding bracket  41  is configured to hold the image pick-up device  39  and the lens unit  38  at the position where the difference between the fluctuation of the focal length of the lens unit  38  caused by the heat, and the fluctuation of the distance between the image pick-device  39  and the lens unit  38  caused by the heat is less than the effective focal depth of the lens unit  38 . Consequently, the holding bracket  41  holds the image pick-up device  39  and the lens unit  38  so as to correspond to the wavelength in which the peak width of the MTF curve is the narrowest among three wavelengths of RGB. Therefore, the effect to the image information caused by the decrease in the MTF of the wavelength with the peak width being the narrowest may be minimized, thereby obtaining favorable image information.  
      Thus, by providing the optical unit  33  having such a structure described above to the image reading unit  13  and the copier  10 , the copier  10  which favorably forms an image in the image reading unit  13  and on a recording sheet may be attained.  
      Reference is now made to  FIG. 8  to illustrate an optical unit, an image reading apparatus and an image forming apparatus of a second example embodiment of the present invention. The same reference numerals used in the first embodiment will be given to constituent elements such as parts and materials having the same functions, and the descriptions thereof will be omitted.  
      In  FIG. 8 , the lens unit  38  is held by a first holding bracket  71  made of a black steel plate through the bracket  43 . The image pick-up device  39  is adhered to a second holding bracket  72  made of a black steel plate in the same manner as that of the first example embodiment. The first holding bracket  71  is secured to the supporting bracket  40  by the bolts  42   a  and  42   b . The second holding bracket  72  is secured by a bolt  42   c , while a sliding member provided on the second holding bracket  72  maintains a certain gap between the supporting bracket  40  and the second holding bracket  72 . Accordingly, the image pick-up device  39  may always correctly face relative to the lens unit  38 .  
      In the optical unit  33  structured in the above described manner, the first holding bracket  71  and the second holding bracket  72 , both of which are formed of the black steel plate, thermally expand due to the heat from the image pick-up device  39  as well as the heat from the inside of the image reading unit  13  or from the inside of the copier  10 . Since the first holding bracket  71  is positioned between the bolts  42   a  and  42   b  which serve as a securing portion to secure the supporting bracket  40 , the displacement amount of the lens unit  38  is small. However, the displacement amount of the image pick-up device  39  is large, because the position of the second bracket  72  is determined by the bolt  42   c  and the sliding member  72   a . Mainly due to the effect of the thermal expansion of the holding bracket  72 , the image pick-up device  39  comes closer to the lens unit  38  as shown by an outline arrow in  FIG. 8 . In other words, in a case where the distance between the image pick-up device  39  and the lens unit  38  is determined in a cold period during which the first holding bracket  71  and the second holding bracket  72  are not thermally expanded, the distance between the image pick-up device  39  and the lens unit  38  decreases if the temperature of the first holding bracket  71  and the second holding bracket  72  increases causing the first holding bracket  71  and the second holding bracket  72  to thermally expand.  
      In the lens unit  38  of the example embodiment, a group of diverging lenses formed of the lenses  52  and  53  is made from plastic having a high temperature dependency. Other lenses  51 ,  54 ,  55  and  56  are made of glass having a low temperature dependency. Accordingly, the lens unit  38  is configured such that, in a case where the temperature rises, the focal length thereof may become short. Furthermore, each radius of the lenses  51 ,  52 ,  53 ,  54 ,  55  and  56  is configured such that in a case where the temperature rises, the displacement amount of the focal length corresponds to the fluctuation amount of the distance between the lens unit  38  and the image pick-up device  39  caused by the heat.  
      The optical unit  33  of the second example embodiment structured in the above described manner, similarly to the first example embodiment, reads the original document when the original document is copied. At this time, light is irradiated on the document surface from the light source  34 . After the reflected light from the document surface is sequentially reflected on the first mirror  35 , the second mirror  36  and the third mirror  37 , the reflected light is imaged on the image pick-up device  39  through the lens unit  38 . In a case where the number of sheets of the document to be read is large, for example, the temperature of the image pick-up device  39  rises. Consequently, the first holding bracket  71  and the second holding bracket  72  are affected by the heat from the image pick-up device  39 , and therefore expand. The distance between the lens unit  38  and the image pick-up device  39  becomes short.  
      Furthermore, the heat from the image pick-up device  39  affects the focal length of the lens unit  38  causing the focal length thereof to extend. The fluctuation amount of the focal length of the optical unit  33  is set so as to correspond to the displacement amount of the image pick-up device  39  caused mainly by the thermal expansion of the second supporting bracket  72 . Accordingly, in the optical unit  33 , even if the temperature rises, an image is appropriately formed on the image pick-up device  39 .  
      Thus, in the second example embodiment of the present invention, the direction of the fluctuation of the focal length of the lens unit  38  caused by the temperature fluctuation and the direction of the fluctuation of the distance between the image pick-up device  39  and the lens unit  38  caused by the temperature fluctuation are configured to be the same direction. Accordingly, even if the temperature fluctuates, the lens unit  38  may still be able to form an image on the image pick-up device  39  so that a problem such as a decrease in the MTF due to a displacement of the imaging position may be prevented. Accordingly, favorable image information may be obtained. Therefore, even if the first holding bracket  71  and the second holding bracket  72 , which hold the lens unit  38  and the image pick-up device  39 , are thermally expanded, the deterioration of the image reading accuracy may be prevented.  
      As described above, the optical unit, the image reading apparatus and the image forming apparatus of example embodiments of the present invention may prevent the deterioration of the reading accuracy of an image, even if the holding members which hold the lens unit and the image pick-up device are thermally expanded. The optical unit, the image reading apparatus and the image forming apparatus of the example embodiments of the present invention may be applied to an optical unit which forms an image on the image pick-up device and converts the reflected light to electric signals after the reflected light from the document is focused by the lenses, to the image reading apparatus such as a scanner using the optical unit, and to the image forming apparatus including the image reading apparatus.  
      Embodiments of this invention may be conveniently implemented using a conventional general purpose digital computer programmed according to the teachings of the present specification, as will be apparent to those skilled in the computer art. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art. Embodiments of the present invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be readily apparent to those skilled in the art.  
      Any of the aforementioned methods may be embodied in the form of a system or device, including, but not limited to, any of the structure for performing the methodology illustrated in the drawings.  
      Further, any of the aforementioned methods may be embodied in the form of a program. The program may be stored on a computer readable media and is adapted to perform any one of the aforementioned methods, when run on a computer device (a device including a processor). Thus, the storage medium or computer readable medium, is adapted to store information and is adapted to interact with a data processing facility or computer device to perform the method of any of the above mentioned embodiments.  
      The storage medium may be a built-in medium installed inside a computer device main body or removable medium arranged so that it can be separated from the computer device main body. Examples of the built-in medium include, but are not limited to, rewriteable non-volatile memories, such as ROMs and flash memories, and hard disks. Examples of the removable medium include, but are not limited to, optical storage media such as CD-ROMs and DVDs; magneto-optical storage media, such as MOs; magnetism storage media, such as floppy disks (trademark), cassette tapes, and removable hard disks; media with a built-in rewriteable non-volatile memory, such as memory cards; and media with a built-in ROM, such as ROM cassettes.  
      Example embodiments being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.