Abstract:
The present invention enables to increase the speed of recognizing characters on an undefined-size postal matter such as a flat mail. The flat mail often has an address area different in color or pattern from the flat mail itself. The present invention provides an image input apparatus including pickup means of monochromatic-mode CCD  31  having a fine resolution capable of recognizing characters and pickup means of color-mode CCD  30  having a rough resolution only sufficient for detecting an address area. These pickup means are mounted in a unitary block so that images are taken in from approximately identical positions and a color image is used to detect an address area and a monochromatic image is used to recognize characters, thereby significantly increasing the speed of flat mail classification. An illumination unit  44  is forcibly cooled so as to enable to use a large-output lamp tube  75  capable of illuminating a wide range to assure the character recognition.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image input apparatus and in particular, to an image input apparatus for inputting a monochromatic image and a color image of a portion of a postal matter to which light is applied from a light source, thereby recognizing the postal matter for the area division.  
           [0003]    2. Description of the Related Art  
           [0004]    This type of image input apparatus is generally mounted on a so-called flat apparatus for recognizing postal matters having a size out of a predetermined range (hereinafter, referred to as a flat mail classification apparatus). The conventional flat mail classification apparatus uses an image input apparatus for reading an entire image on a flat mail surface as a monochromatic image of a fine resolution capable of recognizing characters, so that an address is recognized from the image information for the area division of the flat mail.  
           [0005]    However, a flat mail has a large area to be processed as compared to a normal mail size and it is often a case that a complicated design or characters are printed beside the address. Accordingly, when a format of a high resolution is used, the information amount of the monochromatic image becomes enormous, requiring a plenty of time for recognizing only the address from that information.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to provide an image input apparatus for use in a flat mail classification apparatus for detecting an address area by using a color information and performing a character recognition to that area, thereby enabling to obtain a high-speed flat mail classification.  
           [0007]    The present invention provides an image input apparatus comprising: feed means for feeding an object in a constant direction; illumination means having a light source for applying light rays toward the object fed by the feed means in the constant direction and a filter for cutting heat rays from the light source while allowing visible light rays to pass through; monochromatic and color pickup means for picking up the object illuminated by the light rays with a linear field of view intersecting the feed direction; and cooling means for forced cooling by generating an air flow between the illumination means, the light source, and the filter, wherein the monochromatic pickup means has a high resolution appropriate for a character recognition and the color pickup means has a low resolution appropriate for a character area detection.  
           [0008]    Images are taken in from the same position by the monochromatic pickup means having a fine resolution sufficient for a character recognition and the color pickup means having a rough resolution sufficient for an address area detection. The address area is detected by the color image and a character recognition process is performed only to that area by using the monochromatic image of the high resolution, thereby enabling to obtain a high-speed character recognition, increasing the speed of the flat mail classification. The address area often has a color different from the flat mail itself or other pattern. Accordingly, the address area detection based on the rough color information is advantageous.  
           [0009]    Moreover, since the illumination means is provided with the cooling means for forcibly cooling the light source and the filter by generating an air flow, it is possible to use a high-output light source accompanied by heat rays, reducing the CCD exposure time for assuring the necessary S/N ratio. This further increases the speed of the flat mail classification. Furthermore, since it is possible to obtain a sufficient illumination depth of luminance, it is possible to cope with flat mails having different thickness values.  
           [0010]    Especially when the illumination means  1  is realized by a single unit, it is preferable that the illumination means, the feed means, and the pickup means be arranged in such a manner that the illumination unit has its optical axis forming an acute angle against the feed direction of the feed means and the angle is different from an angle defined by the optical axis of the monochromatic and color optical axis of the pickup means and the feed direction of the feed means.  
           [0011]    This provides an advantage that it is possible to prevent halation caused by the light rays from the illumination means which is mirror-reflected directly from an object on the feed means to be introduced into the pickup means.  
           [0012]    Moreover, when the illumination means is realized by two illumination units, they are arranged so as to sandwich the linear field of view of the pickup means.  
           [0013]    Similarly as described above, it is possible to prevent halation caused by the light rays from the illumination means which is mirror-reflected directly from the object on the feed means and introduced into the pickup means.  
           [0014]    The illumination means may use as the light source a high pressure sodium lamp.  
           [0015]    Although the high-pressure sodium lamp is accompanied by heat rays, the heat rays may be removed by a heat ray cut filter and the heated lamp itself can be sufficiently cooled by an air flow.  
           [0016]    The pickup means preferably has the monochromatic optical path and the color optical path which are approximately vertical to the feed direction of the feed means and are positioned in the proximity to each other along the feed direction of the feed means.  
           [0017]    When the monochromatic optical path is in the proximity to the color optical path, it is possible to reduce an address area detection error to a value that can be ignored. This eliminates need to perform an operation required for matching the fields of view of the monochromatic image and the color image taken in, thereby enabling to perform a character recognition based on the address area detection at an increased speed.  
           [0018]    Furthermore, it is possible to constitute the configuration such that the pickup means has its monochromatic optical path and color optical path which are overlapped in the first half of the optical path from the pickup surface to the focusing means and then branched by optical path branching means so as to be introduced to monochromatic focusing means and color focusing means of the pickup means, respectively.  
           [0019]    Moreover, the pickup means may include a plurality of mirrors for refracting the optical paths so as to prolong both of the chromatic optical path and the color optical path within a limited space.  
           [0020]    When the optical path is prolonged, it is possible to suppress magnification fluctuations caused by movement of the pickup surface, thereby enabling to cope with flat mail thickness fluctuation, which in turn improves the character recognition accuracy.  
           [0021]    Furthermore, the pickup means preferably includes a polarization filter and a infrared ray cut filter which are arranged in the monochromatic optical path, and a color correction filter, a polarization filter and an infrared ray cut filter which are arranged in the color optical path.  
           [0022]    The polarization filter removes the mirror reflection from a flat mail wrapped in a material easily causing a mirror reflection such as vinyl and increases the substantial contrast of the image to improve the character recognition accuracy. Moreover, the infrared ray cut filter cuts off near infrared rays remaining in the reflected light and increases the contrast of characters written with ink such as a ball-point pen, thereby improving the character recognition accuracy. Furthermore, the color correction filter serves for rough correction of the color balance.  
           [0023]    The pickup means has its monochromatic field of view and color field of view having an identical width, and only their resolutions are different.  
           [0024]    More specifically, the monochromatic pickup means has a high resolution appropriate for a character recognition while the color pickup means has a resolution low but sufficient for an address area detection. This enables to reduce the processing time required for the address area detection and the character recognition as well as to improve the character recognition accuracy.  
           [0025]    The pickup means preferably includes independent focusing means, photo-electric conversion means, video signal processing means, and image data transfer means for each of the monochromatic pickup and the color pickup.  
           [0026]    This configuration enables to perform the image processing of the monochromatic image and the color image substantially simultaneously, thereby enabling to reduce the processing time required for the address area detection and the character recognition. This significantly increases the flat mail classification work.  
           [0027]    The photo-electric conversion means includes a linear array CCD consisting of light receiving elements arranged in a straight line for monochromatic image pickup and a linear array CCD consisting of light receiving elements arranged in a straight line for color image pickup, and further includes shading means for shading a part of the monochromatic and color linear array CCD.  
           [0028]    Thus, by shading a part of the monochromatic and color linear array CCD, it is possible to obtain pickup means having a linear field of view by utilizing ordinary linear array CCD available on market. Moreover, by performing pickup with a linear field of view, it is possible to reduce the data transfer amount and increase the speed of the entire processing flow. It should be noted that in principle a portion of the CCD which is transferred later is shaded.  
           [0029]    Moreover, the monochromatic-mode video signal processing means may include envelope detection means and analog/digital conversion means; and the color-mode video signal processing means includes analog/digital conversion means and image correction means.  
           [0030]    The envelope detection means detects a background luminance as a reference level so as to digitize the monochromatic image. This enables to obtain a clear image by substantially eliminating the illumination irregularities due to the illumination means and the exposure shortage (shading) at a peripheral portion of the lens, thereby improving the character recognition accuracy. Moreover, the color-mode vide signal processing means is provided with the image correction means for performing a correction process to the image data converted into a digital data. This enables to perform the light distribution correction and the white correction (white balance adjustment), thereby assuring the address area detection.  
           [0031]    Furthermore, the envelope detection means may include correction means for correcting distortion of a video signal when the object caused mirror reflection.  
           [0032]    This correction means includes: a function to detect a luminance increase of a background due to a mirror reflection of an object and to reduce a time constant of the envelope and a function to detect that the background luminance has returned to a normal state and return the time constant of the envelope to a previous state. When a mirror reflection is caused in the object to increase the luminance, the reference level is increased to compress the contrast, which is disadvantageous for character recognition. However, since this period of time can be reduced by reducing the time constant, it is possible to improve the character recognition accuracy as a whole.  
           [0033]    Moreover, the image correction means for color pickup may include correction data generation means for calculating a correction data with respect to a reference signal which has been converted into a digital signal by the photo-electric means and the analog/digital conversion means using a reference plate as the object and writing the calculation result into a storage device.  
           [0034]    This enables to maintain correction data related to the light distribution correction and the white correction, thereby eliminating need to frequency calculate correction data. Thus, it is possible to significantly improve the entire work efficiency. In order to always have an optimal correction data, it is preferable to periodically calculate the correction data and write the calculation result in a storage device.  
           [0035]    Furthermore, the image correction means for the color pickup may include correction data read-out means for reading out the correction data from the storage device using the object image data which has been converted into a digital signal by the analog/digital conversion means, as a part of an address value of the storage device.  
           [0036]    With this configuration, it is possible to read out a correction data from the storage device, corresponding to an address of the object image data, i.e., a portion of the pickup image. This enables to automatically perform image corrections such as the light distribution correction and the white correction.  
           [0037]    The correction data generation means includes a calculation function to perform a smoothing process to remove stains on the reference plate and noise of a circuitry from the reference signal for a curve approximation process of the illumination light distribution characteristic and to unify the reference values after the correction for respective colors, thereby simultaneously performing the light distribution correction and the white correction.  
           [0038]    With this function, it is possible to eliminate noise caused by stains on the reference plate, irregularities of the CCD sensitivity and the light reception characteristics, aberrations of the optical system arranged in the pickup means, and the like, thereby enabling to obtain an appropriate correction data.  
           [0039]    Moreover, the calculation function may include an error detection function for detecting irregularities of the light distribution characteristic obtained by the curve approximation to detect an error of the reference plate and the circuitry and output an error signal.  
           [0040]    With this error detection function, it is possible to detect a significant failure in the reference plate and the circuitry, thereby preventing an error due to use of an improper correction data.  
           [0041]    The smoothing process for noise removal may be performed by a median processing and the curve approximation process of the light distribution characteristic may be performed by a three-dimensional spline interpolation.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0042]    [0042]FIG. 1 is a perspective view of a configuration of a first embodiment of the present invention.  
         [0043]    [0043]FIG. 2 is a perspective view showing a configuration of an illumination unit of FIG. 1.  
         [0044]    [0044]FIG. 3 is a block diagram showing an outline of a circuit configuration of FIG. 1.  
         [0045]    [0045]FIG. 4 is a block diagram showing a configuration of a power source unit of FIG. 1.  
         [0046]    [0046]FIG. 5 is a block diagram showing a configuration of monochromatic video signal processing circuit of FIG. 3.  
         [0047]    [0047]FIG. 6 ( a ) shows a waveform indicative of the relationship between a video signal and an envelope follow-up signal in case of a normal diffused light; and FIG. 6 ( b ) shows a waveform indicative of the relationship between a vide signal and an envelope follow-up signal in case of a regularly reflected light.  
         [0048]    [0048]FIG. 7 is a block diagram showing a configuration of a color video signal processing circuit of FIG. 2.  
         [0049]    [0049]FIG. 8 explains input/output to/from a ROM of FIG. 7.  
         [0050]    [0050]FIG. 9 explains the method for determining the light distribution correction data.  
         [0051]    [0051]FIG. 10 shows an air flow in the illumination unit  4 .  
         [0052]    [0052]FIG. 11 is a lock diagram showing a configuration of an optical system according to a second embodiment of the present invention.  
         [0053]    [0053]FIG. 12 is a perspective view showing a configuration of a third embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0054]    Description will now be directed to preferred embodiments of the present invention with reference to the attached drawings. Firstly, explanation will be given on a first embodiment in which both of a monochromatic optical path and a color optical path of pickup means are arranged approximately orthogonal to the feed direction of feed means and in proximity to each other along the feed direction of the feed means, with reference to FIG. 1 to FIG. 10.  
         [0055]    In FIG. 1, a postal matter  2  such as a flat mail is placed on a feed belt  1  and moved at a constant speed. A photo-electric sensor  3  detects a passing of a tip end of the postal matter  2  and outputs the detection signal via a power source unit  6  to a recognition unit  11 . A rotary encoder  9  generates a pulse having a period proportional to the speed of the feed belt  1  and outputs the pulse to the power source unit  6 . A movement amount of the postal matter  2  after passing the photo-electric sensor  3  is recognized by the number of pulses output from the rotary encoder  9 . More specifically, the image taken-in start timing is set in advance as a predetermined number of output pulses from the rotary encoder  9 . A counter is resent upon detection of passing of the postal matter  2 . When the counter has counted the aforementioned set value, an image take-in signal is output from the power source unit  6  to the recognition unit  11 . An air supply unit  7  supplies air for cooling via an air duct  8  to the illumination unit  4  and to a camera unit  5 . The illumination unit  4  as the illumination means applies an intense light ray over the entire surface of the postal matter  2 . The illumination unit  4  is specifically realized by two units arranged to sandwich a monochromatic and color camera field of view  12 .  
         [0056]    Here, explanation will be given on the configuration of the illumination unit  4  with reference to FIG. 2. The illumination unit  4  includes an illumination module  15  having a built-in light source and an illumination box  16 . The air for cooling is supplied into this illumination box  16  from the air supply unit  7  of FIG. 1 via the air duct  8 , so as to cool the light source in the illumination module, the atmosphere, and a heat ray cut glass  17  arranged in front of the light source. The cooling air is exhausted from air exhaust holes  18 . The heat ray cut glass  17  serves as a filter having a uniform and preferable transmittance in the visible range and a transmittance almost 0 in the range from the near infrared rays to the infrared rays. Thus, lowering of the luminance of the light applied to the feed belt  1  is suppressed to a possible minimum value and the heat ray application is reduced.  
         [0057]    An external glass  19  has a uniform and preferable transmittance in the range from the visible rays to the infrared rays and will not absorb remaining heat rays transmitting through the heat ray cut glass  17 . That is, the external glass  19  itself is not heated and there is no danger of seizure by external dusts or deterioration of durability by an external shock.  
         [0058]    Next, with reference to FIG. 3, explanation will be given on a circuit configuration of the first embodiment of the present invention. As shown in FIG. 3, the camera unit  5  as the pickup means has a configuration as follows with respect to the reflected light  20  and  21  from the surface of the postal matter  2 .  
         [0059]    For the reflected light  21 , the camera unit  5  includes: a monochromatic linear ray type CCD  31  for receiving an image focused by a monochromatic-mode lens  28 ; a CCD circuit  34  for photo-electric conversion; a CCD drive circuit  35  for generating a CCD drive signal; a video signal processing circuit  37  for amplifying an electric signal from the CCD circuit, normalizing the signal level, and converting the signal into a digital signal; and an image data transfer circuit  39  for parallel-serial converting the video signal into an optical signal and transmitting the signal to a character recognition processor  41  of the recognition unit  11 .  
         [0060]    For the reflected light  20 , the camera unit  5  includes: a color linear ray type CCD  30  for receiving an image focused by a color lens  27 ; a CCD circuit  32  for photo-electric conversion; a CCD drive circuit  33  for generating a CCD drive signal; a video signal processing circuit  36  for amplifying an electric signal from the CCD circuit  32 , normalizing the signal level, converting the signal into a digital signal, and outputting a data which has been subjected to a light distribution/color correction processing; and a color image data transfer circuit  38  for parallel-transferring the video signal to outside. The video signal processing circuit  36  is a part of the image correction means.  
         [0061]    Moreover, the camera unit  5  includes a correction error display LED (light emission diode)  44  for receiving a correction error signal from the recognition unit  11  and displaying the correction error.  
         [0062]    As shown in FIG. 4, the power source unit  6  includes: a mail top and bottom signal generation circuit  46  for externally outputting a signal of the photo-electrical sensor with a predetermined delay according to the pulse generated by the rotary encoder  9 ; an alarm signal generation circuit  47  for detecting various alarms including a lamp fan  45  and outputting it to outside; a power source  48 ; an hour meter  49  for displaying an accumulated time of electricity application for maintenance; and a power source breaker  50 .  
         [0063]    The recognition unit  11  includes: a character recognition block  41  for performing a character recognition processing for the monochromatic image data from the monochromatic image data transfer circuit  39 ; an address area detection block  42  for performing an address area detection processing for the color image data from the color image data transfer circuit  38  and transmitting the detected address area information  43  to the character recognition block  41 ; and a light distribution/color correction data generation block  40  for performing a light distribution/color correction to a reference signal transmitted from the color video signal processing circuit  36  upon generation of a correction data, and storing the corrected data in the video signal processing circuit  36 .  
         [0064]    Next, explanation will be given on the operation of the first embodiment. As shown in FIG. 1, the postal matter  2  in an inclined state is fed on the feed belt  1  at a constant speed of 1.5 m/s. The image take-in timing by the signal from the transmitting type photo-electric sensor  3  arranged in front of the camera field of view  12  is fed via the power source unit  6  to the recognition unit  11 .  
         [0065]    The postal matter  2  which has passed by the photoelectric sensor  3  and reaches the camera field of view  12  is picked-up as follows while passing through the camera field of view.  
         [0066]    Two of the illumination units  4  illuminate a field of view with a necessary depth of field (for example, 50 mm) and a uniform light distribution while maintaining a constant luminance. The illumination units  4  are arranged so as to illuminate with approximately 45 degrees to sandwich the camera optical axis. This arrangement reduces the shade by protrusions and indentation of the object and mirror reflection due to a vinyl envelope.  
         [0067]    As shown in FIG. 3, the reflected light  20  (or  21 ) from the surface of the postal matter  2  is reflected twice by a mirror  22  for refracting the optical path and then focused by the lens  27  (or  28 ) on the CCD  30  (or  31 ).  
         [0068]    The mirror  22  is used to prolong the optical path within a limited space and suppress the image magnification fluctuation. In this embodiment, two of the mirrors  22  are arranged in such a manner that the optical path of the reflected light  21  for taking-in as a monochromatic image and an optical path of the reflected light  20  for taking-in as a color image are almost symmetric at right and left and the optical paths are changed their directions four times, respectively, so as to save the space. Moreover, since the light from the illumination unit  4  has a strong intensity, there is an advantage that even when the thickness of the flat male fluctuates to a certain degree, the fluctuation of the luminance on its surface is small.  
         [0069]    As shown in FIG. 3, in front of each of the lenses  27  and  28 , there are arranged a shading correction plate  26 , a polarization filter  24 , and an infrared ray cut filter  25 . Furthermore, a color correction filter  23  is arranged in front of the color-mode lens  27 .  
         [0070]    The shading correction plate  26  has a hemispherical shape, for example and reduces the light in the center portion of a light flux and corrects the shading (luminance irregularity) due to the characteristic of the lens itself. It is possible to utilize a neutral density filter in which concentration is reduced from the center toward the outer circumference.  
         [0071]    The infrared ray cut filter  25  cuts off the infrared rays in the near infrared rays remaining in the illumination light so as to enhance the contrast of a character written by an ink reflecting near infrared light rays such as ball-point pen.  
         [0072]    The polarization filter  24  assures an image contrast by suppressing the mirror reflection from a postal matter wrapped in a vinyl material or the like easily causing a mirror reflection.  
         [0073]    The color correction filter  23 , in case of a high pressure sodium lamp of a high luminance orange color, passes the colors from blue to green as they are and reduces the light of only the red component. This color correction filter  22  is arranged for roughly adjusting the color balance. It should be noted that the color correction includes not only the adjustment of the color balance but also the correction of the color temperature.  
         [0074]    For example, the monochromatic-mode lens  28  is selected to have a magnification of resolution 8.6 lines/mm in the scan direction and the color-mode lens  27  is selected to have a magnification of resolution 4.3 lines/mm in the scan direction, and both of the lenses are selected to take in the field of view 300 mm. Then, in the monochromatic CCD  31 , 2580 pixels (=8.6 (pixels/mm)×300 mm) are actually used, and in the color CCD  30 , 1290 pixels (=4.3 (pixels/mm)×300 mm) are actually used. It should be noted that the “scan direction” is the vertical direction in FIG. 1 and this is the linear field of view in the pickup means, which orthogonally intersects the feed direction of the feed belt  1 . The resolution of 8.6 lines/mm in the monochromatic CCD  31  is sufficient for a character recognition. Moreover, the resolution of 4.3 lines/mm in the color CCD  30  is sufficient for the area detection.  
         [0075]    The CCD  31  (or  30 ) converts the light focused by the lens  28  (or  27 ) into an electric signal.  
         [0076]    For cost reduction, a general-purpose CCD having the number of pixels greater than the aforementioned number actually used is used. In order to increase the resolution in the feed direction, the exposure time is set to the all pixel transfer time or below. Here, in order to avoid deterioration of the image quality due to a start of next exposure during feed of a pixel exposed, a shading plate  29  as shading means is used so as not to expose pixels other than the one actually used.  
         [0077]    Here, a light leak is caused by diffraction through a space between the shading plate and the CCD and a slight exposure occurs at the end portion of the shading plate. To cope with this, in addition to actually used pixels, a part of reserved pixels adjacent to the actually used pixels is also transferred as an empty transfer. As a result, the actual exposure time is a sum of the time required for the actually used pixels added by the pixels of the empty transfer. The reserved pixels subjected to the light leak are processed as empty data (margin) and no exposure overlap is caused.  
         [0078]    The video signal output form the monochromatic CCD circuit  34  is converted by A/D in the monochromatic-mode video signal processing circuit  37 .  
         [0079]    As shown in FIG. 5, a reference level  52  for digitizing by the A/D conversion circuit  53  uses an output signal from a background following signal generation circuit  51  for generating a paper surface level envelope. This enables to correct luminance irregularities on the papers surface due to illumination irregularities and lens shading and to obtain an image of uniform luminance. The background following signal generation circuit  51  constitutes a part of the envelope detection means.  
         [0080]    As shown in FIG. 6 ( a ), when a normal diffused light is taken in, the envelope following signal  58  following the video signal  59  to serve as the A/D conversion reference level  52  has a time constant sufficiently long not to follow black levels  57  scattered with respect to the paper surface level  56  and having a size almost identical to the size of characters. This enables to take in an image while almost maintaining the contrast between the paper surface and the black characters.  
         [0081]    However, as shown in FIG. 6 ( b ), when a mirror reflection light passes through the polarization filter arranged not at a proper angle and is taken in, the video signal  61  has a level  62  by far greater than the paper surface level. In the case of the normal envelope following signal  58 , after the light ray from the object returns to a diffused light of the normal paper surface level, it takes quite a time to be lowered to the paper surface level due to the long time constant. During this period of time, the reference level  52  is kept greater than the paper surface level  56  and accordingly the paper level itself has a contrast and appears like black. In the resultant image, the contrast in a dark portion is compressed and the contrast between the paper surface and the black characters is lowered.  
         [0082]    To avoid this, when the envelope signal exceeds a certain critical level  55  higher than the paper surface level, it is detected by a comparator  54  and the mode is switched to reduce the time constant of the envelope. With this circuit configuration, when a mirror reflection light is incident, the envelope following signal  60  can be lowered to the normal paper surface level faster, thereby enabling to suppress the contrast deteriorated portion in the image to a shorter period of time. When the paper surface level  56  is lowered, a similar detection is performed to return to the normal time constant. This is the function of the correction means to cope with the mirror reflection.  
         [0083]    The video signal output from the color CCD circuit  32  is A/D converted in the color-mode video signal processing circuit  36 .  
         [0084]    As shown in FIG. 7, if the correction mode selector switch  66  is off, the image signal which has been digitized by the A/D conversion circuit  63  is input as an address value of a ROM  65  (storage device) writable in combination with a pixel counter value by the pixel counter  
         [0085]    and a correction data which has been written in beforehand is output.  
         [0086]    For example, as shown in FIG. 8, the counter value  67  of the pixel counter  64  is a 21-bit ROM address  69  consisting of most significant 11 bits for the counter value  67  of the pixel counter  64  and least significant 10 bits for the image input signal  68 , and an 8-bit output signal  71  uniquely correspond to the ROM address via a correction data curved surface  70  is output.  
         [0087]    This correction data curved surface  70  is determined by generation of light distribution/white correction data. For this, firstly, the correction mode selector switch  66  of the color-mode video signal processing circuit  36  in FIG. 7 is turned off. A particular white reference plate is arranged in the camera field of view  12  on the feed belt  1  which has been stopped and the image of the reference plate is taken in by the color-mode CCD  30 . The video signal subjected to the A/D conversion is transmitted as a reference signal for performing light distribution/white correction to the light distribution/color correction data generation block  40  of the recognition unit  11 .  
         [0088]    The light distribution/color correction data generation block  40  is supplied with, for example, data of several tens of lines. Firstly, a median processing is performed for smoothing between the several tens of lines for each of the colors so as to obtain a light distribution data  73  of a single-line scan after removing an electric noise for each line scan. Furthermore, as shown in FIG. 9, a median processing is performed to the line data so as to remove stains on the reference plate and distortion between pixels and obtain a curve  74  for smoothly approximating the light distribution configuration by using a spline interpolation.  
         [0089]    If the light orientation approximation curve  74  obtained here has a maximum value equal to or greater than a predetermined first value or equal to or smaller than a predetermined second value or the ratio of the maximum value against a minimum value is 80% or below, a correction error is detected and the correction error display LED  44  of the camera unit  5  indicates that an error has occurred. This error detection function enables to know whether the reference plate is arranged not properly, the illumination lamp has been deteriorated by time and should be explained, or the lamp itself is out of the reference.  
         [0090]    Then, a factor for matching the light distribution approximation curve  74  with the reference value  72 , i.e., a factor for the light distribution correction and white correction is determined by a calculation function (program) of the correction data generation means. All data input are multiplied by the factor (correction data) to obtain a corresponding output data, which is written as a one-line LUT (look up table) to the ROM  65 , thereby determining the correction data curved surface  70 . This correction data is inherent to a position of each pixel and as has been described with respect to the ROM  65 , on the LUT, each of the correction data (values on the correction data curved surface  70 ) is related via the pixel counter value to the input data to which this correction data is to be applied.  
         [0091]    More specifically, for a pixel on the light distribution approximation curve  74 , the following calculation is performed.  
         [0092]    (output data)=(reference value  72 )/(light distribution approximation curve  74 )*(input data)  
         [0093]    It should be noted that the input data may be any of all 10-bit values; the reference value  72  is a constant 8-bit value; the light distribution approximation curve  74  is 10-bit value on the curve; the output data is an 8-bit value calculated at the right side. However, as shown in FIG. 8, if the output data exceeds the reference value  72 , the reference value  72  is used as the output data regardless of the input data. This processing is repeated each time an image data is taken in by correction data read-out means (program).  
         [0094]    Here, by matching the reference value  72  of the respective colors, the white correction is performed simultaneously with the light distribution correction. The correction data to be written into the ROM  65  as the LUT and output may be, in addition to the 8 bits×3 colors, 4 bits×3 colors or 2 bits×3 colors.  
         [0095]    By periodically performing the writing of this correction data into the ROM  65 , it is possible to absorb the lowering of the color temperature of the illumination, the change of the light distribution by the time, and the like.  
         [0096]    Moreover, the illumination unit used in the image input apparatus according to the present invention needs to illuminate a large volume range and accordingly, needs to use a high-power light source. For example, in case of a high pressure sodium lamp, two 150 W lamps are required. This causes a secondary heat due to heat radiation directly from the lamps and from the heated environment. Especially when a postal matter  2  made from a material easily absorbing heat is halted on the feed belt  1  for a long period of time, the postal matter  2  may be deteriorated. Moreover, the illumination unit  4 , especially the lamp tube  76  (light source) may reduce its service life, shortening the frequent lamp exchange cycle to lower the maintenance condition.  
         [0097]    In order to evade this, air is fed through the air duct  8  to the illumination unit  4  so that the illumination unit  4  is forced to be cooled.  
         [0098]    [0098]FIG. 10 shows an air flow for the illumination unit  4 . A part of the air introduced from an air intake hole  77  of the illumination box  16  goes through the interior of the illumination module  15  to cool the lamp tube  76 , the heat ray cut glass  17 , and the module  15  and another part of the air goes by the exterior of the illumination module  15  to cool the heat ray cut glass  17  and the exterior of the illumination module  15 . The air warmed is exhausted out of the illumination  4  through the air exhaust hole  18 .  
         [0099]    The heat ray cut glass  17  inside can cut and absorb the heat ray and the window glass  19  outside has a sufficiently low absorption of the heat ray. Accordingly, the secondary heat radiation due to temperature increase of the outer window glass  19  itself absorbing the remaining heat is small. With this cooling configuration, it is possible to suppress the heat radiation onto the feed belt  1  by the illumination unit  4  and to improve the maintenance condition of the illumination unit  4  itself.  
         [0100]    It should be noted that the aforementioned values of the field of view, resolution, feed speed, and the like are given only as an example and the present invention is, not to be limited to these values.  
         [0101]    Next, with reference to FIG. 11, explanation will be given on a second embodiment in which a monochrome optical path and a color optical path are identical in a first half of the optical paths from the pickup surface to the focusing means and are branched by optical path branching means arranged in the middle of the optical path to monochrome focusing means and color focusing means.  
         [0102]    In this second embodiment, the camera field of view  12  for monochrome and that for color with respect to the postal matter  12  are completely matched unlike the first embodiment. As shown in FIG. 11, the optical system configuration in the camera unit  5  is different from that of the first embodiment.  
         [0103]    In FIG. 11, the camera optical axis  13  is vertical to the feed belt surface  1  and common to monochrome and color in the first half of the optical path from the pickup surface to the focusing means. A part of the reflected light from the postal matter  2  passes through a half-penetrating mirror  101  to become a penetrating light  20 , which is reflected by four mirrors  22  to prolong the optical path before reaching the color-mode lens  27 . Moreover, the other part of the reflected light from the surface of the postal matter  2  is reflected by the half-penetrating mirror  101  constituting the optical path branching means and becomes a reflected light  21 , which is reflected by three mirrors  22  for prolonging the optical path before being focused by the monochromatic-mode lens  28 .  
         [0104]    Thus, the monochromatic and color fields of view are completely matched with each other. Accordingly, it is possible to perform the address area detection more accurately. In the first embodiment, when the postal matter  2  has a large thickness, the difference between the monochromatic field of view and the color field of view is increased. That is, the thickness of the postal matter  2  fed is limited. In contrast to this, in the second embodiment, the monochromatic optical axis and the color optical axis are completely matched and the axis is approximately vertical to the surface of the postal matter  2 , assuring to cope with the thickness change of the postal matter  2 .  
         [0105]    Next, with reference to FIG. 12, explanation will be given on a third embodiment of the present invention.  
         [0106]    In the third embodiment, the camera unit  5 , the illumination unit  5 , the air supply unit  7 , and the power source unit  6  of FIG. 1 are made into a unitary block. The configuration of the second embodiment is characterized in that the illumination means is realized by a single illumination unit  4 , the optical axis  203  of the illumination unit  4  forms an acute angle against the feed direction  14  of the feed means, and the angle  204  is different from an angle  205  defined by the monochromatic and color optical axis  13  and the feed direction  14  of the feed means.  
         [0107]    As shown in FIG. 12, the module  15 , the camera unit  5 , and power source unit  6  similar to those of the first embodiment are built in a unitary frame  201 . Here, the illumination module  15  is contained in a space defined by a partition  202 . It should be noted that in FIG. 12, only the mirror  22 , the monochromatic-mode lens  28 , and the color-mode lens  27  are shown, omitting the other components of the camera unit  5  and the power source unit  6 .  
         [0108]    In the third embodiment, the optical axis  203  of the illumination light and the optical axis  13  of the camera are at an acute angle against the feed direction  14 ; and the angle  204  defined by the optical axis  203  of the illumination light and the feed direction  14  is smaller than the angle  205  defined by the camera optical axis  13  and the feed direction  14 . This configuration can suppress an image deterioration such as halation due to a mirror reflection by a postal matter wrapped in a material easily causing a mirror reflection such as vinyl.  
         [0109]    According to the image input apparatus of the present invention, a flat mail address area is detected by color information and the character recognition processing is performed only to that area, thereby enabling to classify flat mails with a high speed and a high accuracy.  
         [0110]    Moreover, cooling means is provided for forcibly cooling the light source of the illumination means by generating an air flow. Accordingly, it is possible to use a high-output light source accompanies by heat rays such as a high pressure sodium lamp. The CCD exposure time for assuring a necessary S/N ratio can be reduced and the speed of a flat mail classification procedure is further increased. Furthermore, since it is possible to obtain an illumination depth of sufficient luminance, it is possible to handle flat mails having different thickness values.  
         [0111]    When the illumination means is realized by a single member, the illumination means, the feed means, and the pickup means are arranged in such a manner that the optical axis of the illumination means forms an acute angle against the feed direction of the feed means and the angle is different from an angle defined by the monochromatic and color optical axis of the pickup means and the feed direction of the feed means. On the other hand, when the illumination means is realized by two members, the illumination members are arranged so as to sandwich the linear field of view of the pickup means. This enables to prevent halation or the like generated by light rays from the illumination means reflected by mirror reflection directly from an object on the feed means to enter the pickup means.  
         [0112]    Furthermore, the monochromatic and color optical path are in proximity to each other or completely matched with each other. Accordingly, there is no operation is required for matching the monochromatic image and the color image taken in, thereby enabling to perform the address area detection and the character recognition with a higher speed.  
         [0113]    Moreover, a plurality of mirrors are arranged for refracting the optical path in the pickup means, so as to prolong the monochromatic optical path and the color optical path within a limited space. This suppresses a magnification change caused by movement of the pickup surface, thereby enabling to cope with the flat mail thickness fluctuations and improve the character recognition accuracy.  
         [0114]    Since the polarization filter and the infrared ray cut filter are arranged and the color correction filter is arranged on the color-optical path, it is possible to perform a character recognition process with a sufficient contrast even when the flat mail is wrapped in a material easily causing a mirror reflection and/or has an address written with ink such as a ball-point pen.  
         [0115]    Furthermore, since the monochromatic pickup means has a resolution sufficiently high for a character recognition while the color pickup means has a low resolution sufficient for an area detection, it is possible to achieve both of reduction of the processing time required for the area detection and the character recognition and the accuracy of the character recognition.  
         [0116]    Moreover, for monochromatic pickup and color pickup, separate focusing means, photo-electrical conversion means, video signal processing means, and image data transmission means are arranged. Accordingly, it is possible to perform the monochromatic image processing and the color image processing substantially simultaneously, thereby enabling to significantly reduce the processing time required for the area detection and the character recognition.  
         [0117]    The photo-electrical conversion means is constituted by a linear array CCD, a part of which is shaded by the shading means so as to obtain a necessary linear field of view. Accordingly, it is possible to use CCD available on market to constitute the pickup means. This enables to reduce the cost of the apparatus. Moreover, since pickup is performed with a linear field of view, the data amount to be transferred can also be reduced, thereby enabling to increase the speed of the entire processing flow.  
         [0118]    Furthermore, a monochromatic image is digitized by detecting a background luminance as a reference level by the envelope detection means. This enables to substantially eliminate the illumination irregularities of the illumination means and shading of the lens arranged in the pickup means, thereby enabling to obtain a clear image data and improve the character recognition accuracy. Moreover, the light distribution correction and the white correction are performed by the image correction means, which enables to properly perform the processing related to the area detection.  
         [0119]    Moreover, the envelope detection means includes correction means for correcting distortion of a video signal when an object caused a mirror reflection, so that this correction means adjusts the time constant of the envelope. Accordingly, even when a mirror reflection increases the background luminance to lower the character contrast, the contrast can rapidly be corrected to a normal state, thereby improving the character recognition accuracy as a whole.  
         [0120]    Furthermore, the image correction means includes: the correction data generation means for writing a correction data obtained using the reference plate as an object in the storage apparatus, and the correction data read-out means for reading out correction data for each image portion from the storage apparatus utilizing the address value corresponding to the object image. Accordingly, it is possible to automatically perform the light distribution correction and the white correction during an image take-in. This assures the area detection required for the character recognition.  
         [0121]    Moreover, when an apparent error is present in the light distribution characteristic and the white balance, an error signal is output. Thus, it is possible to prevent generation of an error due to use of an improper correction data.  
         [0122]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristic thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.  
         [0123]    The entire disclosure of Japanese Patent Application No. 2000-057537 (Filed on Mar. 2nd, 2000) including specification, claims, drawings and summary are incorporated herein by reference in its entirety.