Abstract:
An endoscope including: a light source for emitting light; a solid state imaging unit comprising a plurality of photoelectric conversion elements for accumulating signal charges corresponding to an incidence light amount, transfer units for transferring signal charges accumulated in the photoelectric conversion elements, and a plurality of color filters formed above the photoelectric conversion elements; and a transmission tube accommodating the light source and the solid state imaging unit, wherein the color filters include red, green and blue color filters, and the number of red photoelectric conversion elements upon which light transmitted through the red color filters are incident is larger than the number of green photoelectric conversion elements upon which light transmitted through the green color filters are incident and the number of blue photoelectric conversion elements upon which light transmitted through the blue color filters are incident. The endoscope can obtain a high quality image.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is based on and claims priority of Japanese Patent Application No. 2004-156509 filed on May 26, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     A) Field of the Invention  
         [0003]     The present invention relates to an endoscope for color imaging.  
         [0004]     B) Description of the Related Art  
         [0005]      FIG. 3A  is a block diagram showing a main portion of a solid image pickup device assembling a solid state imaging unit, and  FIGS. 3B and 3C  are schematic plan views showing the structure of a solid state imaging unit.  FIG. 3D  is a cross sectional view showing a portion of a pixel arrangement unit of a solid state imaging unit.  FIGS. 3E and 3F  are schematic plan views showing layouts of a color filter layer of three primary colors, red (R), green (G) and blue (B).  FIG. 3G  is a flow chart briefly illustrating image data processing.  
         [0006]     Referring to  FIG. 3A , the structure of a solid state image pickup device will be described. A solid state imaging unit  51  generates signal charges corresponding to an amount of light incident upon each pixel and supplies an image signal corresponding to the generated signal charges. A drive signal generator  52  generates drive signals (transfer voltage, etc.) for driving the solid state imaging unit  51  and supplies them to the solid state imaging unit  51 . An analog front end (AFE)  53  adjusts a gain in accordance with a change in the level of an input signal supplied from the solid state imaging unit  51 , to maintain constant the level of an output signal. A digital signal processor (DSP)  54  processes an image signal supplied from the analog front end  53 , such as recognition process, data compression and network control, and outputs the processed image data. A timing generator (TG)  55  generates timing signals for the solid state imaging unit  51 , drive signal generator  52  and analog front end  53 , to control the operations thereof.  
         [0007]     Solid state imaging units are mainly divided into CCD types and MOS types. In the CCD type, charges generated in a pixel is transferred by charge coupled devices (CCD). In the MOS type, charges generated in a pixel are amplified by a MOS transistor and output. Although not limitative, the following description will be made by using a CCD type as an example.  
         [0008]     The drive signal generator  52  includes, for example, a V driver for generating a vertical CCD drive signal. Signals supplied from the drive signal generator  52  to the solid state imaging unit  51  are a horizontal CCD drive signal, a vertical CCD drive signal, an output amplifier drive signal and a substrate bias signal.  
         [0009]     As shown in  FIG. 3B , the solid state imaging unit is constituted of: a plurality of photosensitive units  62  disposed, for example, in a matrix shape; a plurality of vertical CCD units  64 , a horizontal CCD unit  66  electrically connected to the vertical CCD units  64 ; and an amplifier circuit unit  67 , connected to an output terminal of the horizontal CCD unit  66 , for amplifying an output charge signal from the horizontal CCD unit  66 . A pixel arrangement unit  61  is constituted of the photosensitive units  62  and vertical CCD units  64 .  
         [0010]     The photosensitive unit  62  is constituted of a photosensitive element, e.g., a photoelectric conversion element (photodiode) and a read out gate. The photoelectric conversion element generates signal charges corresponding to an incidence light amount and accumulates them. The accumulated signal charges are read via the read out gate to the vertical CCD unit  64  and transferred in the vertical CCD unit (vertical transfer channel)  64  toward the horizontal CCD unit  66  (in a vertical direction). Signal charges transferred to the bottom end of the vertical CCD unit  64  are transferred in the horizontal CCD unit (horizontal transfer channel)  66  in a horizontal direction, amplified by the amplifier circuit unit  67  and output to an external.  
         [0011]     The photosensitive units  62  are disposed in a square matrix layout at a constant pitch in the row and column directions as shown in  FIG. 3B , or disposed in a honeycomb layout in the row and column directions by shifting every second units, for example, by a half pitch.  
         [0012]      FIG. 3C  is a schematic plan view of a solid state imaging unit having the pixel interleaved layout. The pixel interleaved layout has photosensitive units  62  disposed in a first square matrix layout and photosensitive units  62  disposed in a second square matrix layout at positions between lattice points of the first square matrix layout. Vertical CCD units (vertical transfer channels)  64  are disposed in a zigzag way between photosensitive units  62 . Although this layout is called a pixel interleaved layout, the photosensitive unit  62  of most pixel interleaved layouts is octangular.  
         [0013]     As shown in  FIG. 3D , formed in a p-type well  82  formed in a semiconductor substrate  81 , e.g., an n-type silicon substrate, are a photoelectric conversion element  71  made of an n-type impurity doped region, a p-type read gate  72  disposed next to the photoelectric conversion element, and a vertical transfer channel  73  of made of an n-type region disposed next to the read out gate. A vertical transfer electrode  75  is formed above the vertical transfer channel  73 , with a gate insulating film  74  being interposed therebetween. A p-type channel stop region  76  is formed between adjacent photoelectric conversion elements  71 .  
         [0014]     The channel stop region  76  is used for electrically isolating the photoelectric conversion elements  71 , vertical transfer channels  73  and the like. The gate insulating film  74  is a silicon oxide film formed on the surface of the semiconductor substrate  81 , for example, by thermal oxidation. The vertical transfer electrode  75  is constituted of first and second vertical transfer electrodes made of, for example, polysilicon. The first and second vertical transfer electrodes may be made of amorphous silicon. An insulating silicon oxide film  77  is formed on the vertical transfer electrode  75 , for example, by thermally oxidizing polysilicon. The vertical CCD unit  64  is constituted of the vertical transfer channel  73 , upper gate insulating film  74  and vertical transfer electrode  75 .  
         [0015]     A light shielding film  79  of, e.g., tungsten, is formed above the vertical transfer electrode  75 , with the insulating silicon oxide film  77  being interposed therebetween. Openings  79   a  are formed through the light shielding film  79  at positions above the photoelectric conversion elements  71 . A silicon nitride film  78  is formed on the light shielding film  79 .  
         [0016]     Signal charges corresponding to an incidence light amount generated in the photoelectric conversion element  71  are read via the read out gate  72  into the vertical transfer channel  73  and transferred in the vertical transfer channel  73  in response to a drive signal (transfer voltage) applied to the vertical transfer electrodes  75 . As described above, the light shielding film  79  has the openings  79   a  above the photoelectric conversion elements  71  and prevents light incident upon the pixel arrangement unit  61  from entering the region other than the photoelectric conversion elements  71 .  
         [0017]     A planarized layer  83   a  made of, e.g., borophosphosilicate glass (BPSG) is formed above the light shielding film  79 . On this planarized surface, a color filter layer  84  is formed which is three primary colors: red (R), green (G) and blue (B). Another planarized layer  83   b  is formed on the color filter layer  84 . On the planarized layer  83  having a planarized surface, micro lenses  85  are formed, for example, by melting and solidifying a photoresist pattern of micro lenses. Each micro lens  85  is a fine hemispherical convex lens disposed above each photoelectric conversion element  71 . The micro lens  85  converges incidence light to the photoelectric conversion elements  71 . Light converged by one micro lens  85  passes through the color filter layer  84  of one of the red (R), green (G) and blue (B) and becomes incident upon one photoelectric conversion element  71 . Therefore, the photoelectric conversion elements include three types of photoelectric conversion elements: photoelectric conversion elements upon which light passed through the red (R) color filter layer  84  becomes incident; photoelectric conversion elements upon which light passed through the green (G) color filter layer  84  becomes incident; and photoelectric conversion elements upon which light passed through the blue (B) color filter layer  84  becomes incident.  
         [0018]     In the specification and claims, “above” the photoelectric conversion element or the semiconductor substrate on which the photoelectric conversion elements are formed, intended to mean “at a higher position” in the above-described structure of the solid state imaging unit.  
         [0019]      FIG. 3E  shows an example of the layout of color filters of three primary colors, red (R), green (G) and blue (B) of a solid state imaging unit having photoelectric conversion elements  71  disposed in the square matrix shape. Green (G) filters are disposed in a checkered pattern, and a row having green (G) filters and red (R) filters disposed alternately and a row having green (G) filters and blue (B) filters disposed alternately are alternately disposed along the column direction, to form the color filter layer of three primary colors (Bayer layout). In this layout, the pixel number ratio of red (R), green (G) and blue (B) pixels is 1:2:1.  
         [0020]      FIG. 3F  shows an example of the layout of the color filters of three primary colors, red (R), green (G) and blue (B) of a solid state imaging unit having photoelectric conversion elements  71  disposed in the honeycomb layout.  
         [0021]     Red (R) and blue (B) filters are disposed in a checkered pattern above the photosensitive units disposed in a first square matrix shape, and green (G) filters are disposed above the photosensitive units disposed in a second square matrix shape at positions between lattice points of the first square matrix shape (pixel interleaved array (PIA)). Also in this layout, the pixel number ratio of red (R), green (G) and blue (B) pixels is 1:2:1.  
         [0022]     In the layouts of three primary colors shown in  FIGS. 3E and 3F , the number of pixels, upon which light passed through the green (G) color filters becomes incident, is largest (for example, refer to Japanese Patent Laid-open Publication No. HEI-10-262260).  
         [0023]     Most of image pickup elements for general photographing, such as video cameras, digital still image cameras and cameras of portable phones, have the pixel number ratio of red (R), green (G) and blue (B) of 1:2:1. This is because green components of general images contribute more to the resolution of human eyes.  
         [0024]     With reference to  FIG. 3G , brief description will be made on an example of image data processing by the digital signal processor (DSP)  54 .  
         [0025]     Digital data output from the analog front end (AFE)  53  is supplied to the digital signal processor (DSP)  54 . The supplied data is first subjected to interpolation calculation which calculates full resolution image data of each of red (R), green (G) and blue (B). Data after the interpolation calculation is thereafter subjected to a gamma process, a spatial filtering process and a tone adjustment process to thereby output image data.  
         [0026]     With the interpolation calculation, data of each of red (R), green (G) and blue (B) is formed for the pixel layout of the square matrix shape shown in  FIG. 3E , and data of each of red (R), green (G) and blue (B) at each pixel position and at a middle position between adjacent pixels is formed for the pixel layout of the honeycomb shape shown in  FIG. 3F .  
         [0027]     If a medical endoscope using a solid state imaging unit with a pixel number ratio of red (R), green (G) and blue (B) of 1:2:1 is used for photographing organs or tissues in a human body, it is difficult to obtain an image of high resolution and good color reproduction. This is because there are large red (R) color components in a body.  
       SUMMARY OF THE INVENTION  
       [0028]     An object of this invention is to provide an endoscope capable of obtaining a high quality image.  
         [0029]     According to one aspect of the present invention, there is provided an endoscope comprising: a light source for emitting light; a solid state imaging unit comprising a plurality of photoelectric conversion elements for accumulating signal charges corresponding to an incidence light amount, transfer units for transferring signal charges accumulated in the photoelectric conversion elements, and a plurality of color filters formed above the photoelectric conversion elements; and a transmission tube accommodating the light source and the solid state imaging unit, wherein the color filters include red, green and blue color filters, and the number of red photoelectric conversion elements upon which light transmitted through the red color filters are incident is larger than the number of green photoelectric conversion elements upon which light transmitted through the green color filters are incident and the number of blue photoelectric conversion elements upon which light transmitted through the blue color filters are incident.  
         [0030]     This endoscope has an excellent resolution of red color components and is suitable for photographing a good quality image of the interior of a living body having large red color components.  
         [0031]     According to the present invention, it is possible to provide an endoscope capable of obtaining a high quality image. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]      FIG. 1A  is a schematic plan view showing the outline of a tip portion of an optical magnification electronic scope (endoscope) for observing precisely an upper digestive tract,  FIG. 1B  is a perspective view showing the tip portion of the scope and a tube connected to the tip portion, and  FIGS. 1C and 1D  are schematic diagrams showing an observation optical system of the scope.  
         [0033]      FIGS. 2A and 2B  are schematic plan views showing the layouts of color filters of three primary colors of red (R), green (G) and blue (B) of a solid state imaging unit used by an optical magnification electronic scope for observing precisely an upper digestive tract.  
         [0034]      FIG. 3A  is a block diagram showing a main portion of a solid image pickup device assembling a solid state imaging unit,  FIGS. 3B and 3C  are schematic plan views showing the structure of a solid state imaging unit,  FIG. 3D  is a cross sectional view showing a portion of a pixel arrangement unit of a solid state imaging unit,  FIGS. 3E and 3F  are schematic plan views showing layouts of a color filter layer of three primary colors, red (R), green (G) and blue (B), and  FIG. 3G  is a flow chart briefly illustrating image data processing. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]      FIG. 1A  is a schematic plan view showing the outline of a tip portion of an optical magnification electronic scope for observing precisely an upper digestive tract,  FIG. 1B  is a perspective view showing the tip portion of the scope and a tube connected to the tip portion, and  FIGS. 1C and 1D  are schematic diagrams showing an observation optical system of the scope.  
         [0036]     Referring to  FIG. 1A , the tip portion of an optical magnification electronic scope for observing precisely an upper digestive tract, is of generally the circular shape having a diameter of, e.g., 10.8 mm. This tip portion is constituted of a light source  11  with two light output openings, an observation optical system  12 , a nozzle  13  and a forceps opening  14 . The light source  11  includes a light emission source, a light guide (fiber) and light output openings. The electronic scope is used, for example, as a photogastroscope.  
         [0037]     The light source  11  emits white light with light in the infrared range being cut, through the two light output openings and illuminates, e.g., the inner wall of a human gaster. The observation optical system  12  includes a solid state imaging unit similar to the solid state imaging unit described with reference to  FIGS. 3B  to  3 D (with a difference between the layouts of color filters, as will be later described). The observation optical system  12  receives mainly light emitted from the light source  11  and reflected from the inner wall of the gaster, and forms an image which is sent to an observer. The observation optical system  12  will be later described in detail. The nozzle  13  jets out gas or liquid such as washing liquid and dye liquid for facilitating observation of a diseased part. A pair of forceps protrudes through the forceps opening  14  which has a diameter of, e.g., 2.8 mm.  
         [0038]     Referring to  FIG. 1B , the pair of forceps  14   a  is moved in and out through the forceps opening  14 . The pair of forceps  14   a  has a tip portion which can perform an open/close operation like blades of a pair of scissors, and can hold a target member. By operating the pair of forceps  14   a,  it becomes possible to observe minutely a diseased part, pick up cells of the diseased part or cut the diseased part.  
         [0039]     The light source  11 , observation optical system  12 , nozzle  13  and forceps  14   a  are accommodated in a tube  15 , e.g., near the end portion thereof. For example, the tube  15  is guided into the interior of a body from a mouth to make the end portion reach a position near a diseased part. The tube  15  near the end portion is made flexible so that the observation optical system  12  and the like can be positioned nearer to the diseased part and the operability of the scope can be improved. A full length of the tube  15  is, e.g., 1400 mm. A manipulation apparatus is coupled to the end of the tube  15  opposite to the side where the observation optical system  12  and the like are disposed. The manipulation apparatus can operate the light source  11 , observation optical system  12 , nozzle  13  and forceps  14   a.  Image data from the observation optical system  12  is transmitted via the inside of the tube  15 . The tube  15  is a mechanical and electrical transmission tube.  
         [0040]     With reference to  FIG. 1C , description will be made on the observation optical system  12 . The observation optical system  12  is constituted of an objective lens  21 , a prism  22 , a semiconductor chip  23  and a wiring board  26 . Light  20  emitted from the light source  11  and reflected from, e.g., the inner wall of a gaster, becomes incident upon the objective lens  21 , is bent generally a right angle by the prism  22 , and becomes incident upon the semiconductor chip  23 . The semiconductor chip  23  has a solid state imaging unit such as that described with reference to  FIGS. 3B  to  3 D, and pads  24   a.  These pads  24   a  of the semiconductor chip  23  are wire-bonded to pads  24   b  of the wiring board  26  on which a driver circuit and the like and wirings are formed. Lead wires  25  are connected to the pads  24   b  on the wiring board  26 . The lead wires  25  extend in the tube  15  along its extension direction. The semiconductor chip  23  and wiring board  26  are supported on a support plate  27 .  
         [0041]     Referring to  FIG. 1D , light  20  becomes incident upon the objective lens  21 , changes its propagation direction at the prism  22  and becomes incident upon the photoelectric conversion elements in the light reception unit  23   a  of the solid state imaging unit in the semiconductor chip  23 . As described earlier, one of color filters of three primary colors is disposed above each photoelectric conversion element. The light  20  transmits through one of color filters of red (R), green (G) and blue (B) and becomes incident upon the photoelectric conversion element which generates and accumulates signal charges. The signal charges are transferred in the solid state imaging unit, processed in the manner described with reference to  FIGS. 3A and 3G , and output as image data. The image data is sent to an external via the lead wires  25 .  
         [0042]     The semiconductor chip  23  is disposed in such a manner that its principal surface (on which photoelectric conversion elements are formed) of, e.g., a rectangular shape is set vertical to the cross section of the tube  15  and a longitudinal direction of the principal surface is set parallel to the extension direction of the tube  15 . With this arrangement, the scope can be made compact. In order to set the principal surface of the semiconductor chip  23  vertical to the cross section of the tube  15 , the propagation direction of incidence light is changed by the prism  22 .  
         [0043]      FIGS. 2A and 2B  are schematic plan views showing the layouts of color filters of three primary colors of red (R), green (G) and blue (B) of a solid state imaging unit used by an optical magnification electronic scope for observing precisely an upper digestive tract.  FIG. 2A  shows an example of the layout of a solid state imaging unit whose photoelectric conversion elements are disposed in the square matrix shape, and  FIG. 2B  shows an example of the layout of a solid state imaging unit whose photoelectric conversion elements are disposed in the honeycomb shape.  FIG. 2A  corresponds to  FIG. 3E , and  FIG. 2B  corresponds to  FIG. 3F .  
         [0044]     In the layout of color filters of three primary colors shown in  FIG. 2A , red (R) filters are disposed in a checkered pattern, and a row having red (R) filters and green (G) filters disposed alternately and a row having red (R) filters and blue (B) filters disposed alternately are alternately disposed along the column direction, to form the color filter layer of three primary colors. As compared to the layout shown in  FIG. 3E , the red (R) filter and the green (G) filter are exchanged. In the layout shown in  FIG. 2A , the pixel number ratio of red (R), green(G) and blue (B) is 2:1:1.  
         [0045]     In the layout of color filters of three primary colors shown in  FIG. 2B , green (G) and blue (B) filters are disposed in a checkered pattern above the photosensitive unit disposed in a first square matrix shape, and red (R) filters are disposed above the photosensitive units disposed in a second square matrix shape at positions between lattice points of the first square matrix shape. As compared to the layout shown in  FIG. 3F , the red (R) filter and the green (G) filter are exchanged. In the layout shown in  FIG. 2B , the pixel number ratio of red (R), green(G) and blue (B) is 2:1:1.  
         [0046]     By using the color filters having the layout shown in  FIG. 2A  or  2 B, the resolution of red (R) color components can be increased so that a good quality image of the interior of a living body (such as a fine blood vessel) can be photographed. This contributes to high quality medical care.  
         [0047]     With the color filters having the layout shown in  FIG. 2A  or  2 B, the image signal processing described with reference to  FIG. 3G  is executed by considering the pixel number ratio of red (R), green (G) and blue (B) of 2:1:1. In the interpolation calculation process shown in  FIG. 3G , R interpolation is performed by a method similar to conventional G interpolation, and G interpolation is performed by a method similar to conventional R/B interpolation. The other processes shown in  FIG. 3G  are executed by a method similar to the method for the case of the pixel number ratio of red (R), green (G) and blue (B) of 1:2:1.  
         [0048]     An image may be formed by three primary colors R/G/B, Y/Cr/Cb signals or both.  
         [0049]     In order to maintain a white balance, color filters of all three primary colors R/G/B are used.  
         [0050]     As compared to the solid state imaging unit having the color filter layout of  FIG. 3E  or  3 F, the solid state imaging unit having the color filter layout of  FIG. 2A  or  2 B can obtain a proper image without any practical problem although the resolution of green (G) color components is reduced.  
         [0051]     Although the pixel number ratio of red (R) is set to 50% for the layouts shown in  FIGS. 2A and 2B , the pixel number ratio of red (R) may be increased to photograph the interior of a living body having a large amount of red (R) color components. An endoscope having the solid state imaging unit with a red pixel number ratio larger than 50% may be realized. The number of pixels with red (R) color filters is set larger than the number of pixels with green (G) color filters and the number of pixels with blue (B) color filters to increase the resolution of red (R) color components. With this arrangement, a good quality image of a part containing large red color components can be photographed.  
         [0052]     The position of the color filter layer is not limited to that shown in  FIG. 3D  if only the color filter layer is disposed above the photoelectric conversion elements.  
         [0053]     As compared to the solid state imaging unit whose photoelectric conversion elements are disposed in the square matrix shape, the solid state imaging unit whose photoelectric conversion elements are disposed in the honeycomb layout has a larger light reception area per pixel, and color data is obtained not only at each pixel position but also at the intermediate positions of adjacent pixels so that a high resolution can be obtained and a more detailed image can be obtained with the same chip size. It is expected that the solid state imaging unit of the honeycomb layout is suitable for use with an endoscope for observing the interior of a living body.  
         [0054]     The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It will be apparent to those skilled in the art that other various modifications, improvements, combinations, and the like can be made.  
         [0055]     The embodiments are suitable for use with a medical endoscope.