Patent Publication Number: US-6710796-B1

Title: Optical printer with color filter and optical printing method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical printer with a color filter and optical printing method, and more particularly, relates to an optical printer with a color filter and optical printing method in which printing light is applied to color photosensitive material to print a color image, and in which such an image can be printed efficiently. 
     2. Description Related to the Prior Art 
     An optical printer is a known device in which printing light of plural colors is applied to color photosensitive material for printing a color image thereto. The optical printer has a printing head, which includes a light source of white light and a color separation filter unit. The color separation filter unit is constituted by plural color filters of red, green and blue colors for converting light from the light source into the printing light of the red, green and blue colors. The printing head has a length of one line included in the color image, and is disposed to extend in a main scan direction. To record an entire frame of the color image, the printing head is moved relative to the color photosensitive material in the sub scan direction. 
     JP-A 11-127409 (corresponding to WO 99/21055) discloses a combined structure in which the optical printer is incorporated in an electronic still camera. An instant photo film is used as the color photosensitive material in the optical printer, which operates according to frame-sequential printing. The frame-sequential printing is a method in which colors are recorded frame after frame to obtain the color image. At first, a red one of the color filters is set in a light path. The printing head is moved in the sub scan direction, to record one red frame by use of red printing light. Then a green one of the color filters is set in the light path. The printing head is moved in the sub scan direction, to record one green frame by use of green printing light. After this, a blue one of the color filters is set in the light path. The printing head records one blue frame by use of blue printing light, to obtain the color image. Spreader rollers are operated to break a processing solution pod contained in the instant photo film, spread processing solution over an exposure surface to process the color image. The instant photo film is ejected at the same time. 
     The color separation filter unit has a plate shape in which the color filters are arranged adjacently to one another. The color separation filter unit is slid to set a selected one of the color filters in the light path. 
     However, the optical printer for the frame-sequential printing has shortcomings in efficiency. The instant photo film cannot be advanced toward the outside of the optical printer after the completion of three-color exposure. The printing head must be moved during recording. A space must be kept enough for moving the printing head. Also, a head moving mechanism is required for the printing head, and may cause the optical printer to have a considerably large size. Furthermore, a problem of the frame-sequential printing in the full color lies in very long time required for printing, as the printing head is moved back and forth for frames of three colors. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, an object of the present invention is to provide an optical printer and optical printing method in which printing light is applied to color photosensitive material to print a color image, and in which such an image can be quickly printed with a simple structure. 
     In order to achieve the above and other objects and advantages of this invention, an optical printer records a color image by exposing color photosensitive material extending two-dimensionally in a main scan direction and a sub scan direction. The optical printer comprises a printing head, which includes a light source and a color separation filter unit. The color separation filter unit is disposed in a light path of light from the light source, for color separation of the light from the light source, the color separation filter unit having a tubular shape, having a central axis extending substantially parallel with the main scan direction, being rotatable about the central axis, and into which the light from the light source travels through an entrance position along the light path, and out of which the light from the light source travels through an exit position along the light path. The color separation filter unit includes plural color filters, arranged about the central axis, having selectivity of a wavelength, for converting the light from the light source into printing light of respectively single colors, the plural color filters being so disposed that a single one thereof is in the entrance position or the exit position in a manner irrespective of a rotational position with reference to the central axis. Plural transmission portions or openings are arranged alternately with the plural color filters. 
     Furthermore, a filter drive mechanism rotates the color separation filter unit. A light amount adjustor adjusts a light amount of the printing light by each of pixels. A controller causes the filter drive mechanism to set in the light path a color filter among the plural color filters associated with a respective color to be recorded, and for operating the light amount adjustor according to density of the pixels of the respective color while the color filter is set in the light path. 
     The printing head is stationary. Furthermore, a feeder feeds the color photosensitive material in the sub scan direction, to cause the printing head to record the color image to the color photosensitive material. 
     The exit position is opposed to the entrance position with reference to the central axis. A total number of the plural color filters and the transmission portions or openings is 6(2n+1) where n is an integer equal to or more than zero, and the transmission portions or openings are opposed to respectively the plural color filters with reference to the central axis. 
     According to another preferred embodiment, furthermore, a reflector is disposed in the color separation filter unit, for reflecting light from the entrance position to cause the light to travel to the exit position. 
     The reflector is so disposed that the light from the entrance position is reflected substantially at a right angle to travel to the exit position. A total number of the plural color filters and the transmission portions or openings is 12(2n+1) where n is an integer equal to or more than zero. 
     The color separation filter unit further includes at least one ND filter for reducing the light amount of the printing light of at least one of the single colors. 
     The at least one ND filter is plural ND filters for constituting the plural transmission portions. 
     According to another preferred embodiment, the at least one ND filter is plural ND filters disposed to lie on respectively the plural color filters. 
     The color separation filter unit has a cylindrical shape. 
     According to another preferred embodiment, the color separation filter unit has a prismatic shape. 
     The light amount adjustor comprises a micro shutter, disposed in the light path, and of which transmittance density is variable by each of the pixels. 
     According to another preferred embodiment, the light source includes an array of plural light-emitting elements. 
     The light amount adjustor comprises a light-emitting element driver for driving the plural light-emitting elements by applying energy thereto according to the pixel density. 
     Furthermore, an encoder plate is secured to the color separation filter unit, has color information disposed in association with the plural color filters, for representing colors of the plural color filters. A phase detecting sensor reads the color information to recognize a color of one of the plural color filters set in the light path. The controller controls the light amount adjustor according to signal from the phase detecting sensor. 
     The plural color filters are disposed with respectively a first central angle about the central axis in the color separation filter unit. The color information is disposed with respectively a second central angle about a central axis in the encoder plate, the second central angle being smaller than the first central angle. The controller sends an image signal to the light amount adjustor, and the image signal starts being output after a portion of respectively the plural color filters comes in the light path, and finishes being output before an entirety of respectively the plural color filters comes out of the light path. 
     According to another preferred embodiment, the filter drive mechanism includes a stepping motor. The controller controls the stepping motor to set in the light path one of the plural color filters having a color of the printing light of which the light amount is to be adjusted. 
     The color photosensitive material is an instant photo film. 
     According to another aspect of the invention, an optical printer records a color image by exposing a color photosensitive material. The optical printer comprises a printing head, which includes a light source and a color separation filter unit. The color separation filter unit is disposed in a light path of light from the light source, for color separation of the light from the light source. The color separation filter unit includes plural color filters, arranged in a manner adjacent to one another in a plate shape, having selectivity of a wavelength, set in the light path sequentially, for converting the light from the light source into printing light of respectively single colors. At least one ND filter, disposed to lie on at least one of the plural color filters, for reducing the light amount of the printing light of at least one of the single colors. 
     The color photosensitive material extends two-dimensionally in a main scan direction and a sub scan direction, the color separation filter unit extends substantially parallel with the main scan direction, and the plural color filters are arranged in an arranging direction crosswise to the main scan direction. Furthermore, a filter sliding mechanism slides the color separation filter unit in the arranging direction. 
     The plural color filters are included in a single piece. 
     According to still another aspect of the invention, an optical printing method of recording a color image by exposing a color photosensitive material comprises the following steps. Light from a light source is converted into printing light of plural colors by filtering. A light amount of the printing light of the plural colors is controlled according to an image signal. The light amount of the printing light of at least one of the plural colors is reduced by filtering. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent from the following detailed description when read in connection with the accompanying drawings, in which: 
     FIG. 1 is an explanatory view schematically illustrating an optical printer; 
     FIG. 2 is a vertical section illustrating a printing head with an instant photo film; 
     FIG. 3A is a perspective, partially broken, illustrating a color separation filter unit and a light source; 
     FIG. 3B is a cross section illustrating the color separation filter unit; 
     FIG. 3C is a cross section illustrating another color separation filter unit having openings; 
     FIGS. 4A-4F are cross sections illustrating the color separation filter unit of FIGS. 3A and 3B at six phase angles in sequence, where FIG. 4A illustrates a state in which a red color filter is in an entrance position, FIG. 4B illustrates a state in which a green color filter is in an exit position, FIG. 4C illustrates a state in which a blue color filter is in the entrance position, FIG. 4D illustrates a state in which the red color filter is in the exit position, FIG. 4E illustrates a state in which the green color filter is in the entrance position, FIG. 4F illustrates a state in which the blue color filter is in the exit position; 
     FIG. 5 is a cross section illustrating another color separation filter unit with nine color filters; 
     FIG. 6 is a block diagram schematically illustrating circuits in the optical printer of FIG. 1; 
     FIG. 7 is a side elevation illustrating an encoder disk for use with the color separation filter unit; 
     FIG. 8 is a block diagram schematically illustrating a motor control circuit with relevant circuits; 
     FIGS. 9A and 9B are timing charts illustrating waveforms of respectively a reference frequency signal and a reference phase signal; 
     FIGS. 10A and 10B are flow charts illustrating a printing operation; 
     FIG. 11 is a block diagram schematically illustrating another preferred optical printer with a stepping motor in a printing head; 
     FIG. 12 is a vertical section illustrating a printing head in the optical printer of FIG. 11; 
     FIG. 13 is a vertical section illustrating another printing head with a reflector between a color separation filter unit and a light source; 
     FIGS. 14A-14F are cross sections illustrating another color separation filter unit with ND filters behind the color filters; 
     FIG. 15A is a cross section illustrating a color separation filter unit having an ND filter with a space behind each color filter; 
     FIG. 15B is a cross section illustrating a color separation filter unit similar to that of FIG. 15A but having the openings; 
     FIGS. 16A-16F are cross sections illustrating another color separation filter unit with ND filters between the color filters; 
     FIGS. 17A-17F are cross sections illustrating another color separation filter unit in a shape of a hexagonal prism; 
     FIG. 17G is a cross section illustrating a color separation filter unit similar to that of FIGS. 17A-17F but with the openings; 
     FIGS. 18A-18F are cross sections illustrating another color separation filter unit with the ND filters behind the color filters; 
     FIG. 18G is a cross section illustrating a color separation filter unit similar to that of FIGS. 18A-18F but with the openings; 
     FIGS. 19A-19F are cross sections illustrating another color separation filter unit with the ND filters between the color filters; 
     FIG. 20 is a vertical section illustrating another preferred printing head in which a reflector is contained in a color separation filter unit; 
     FIG. 21 is a vertical section illustrating a printing head similar to that of FIG. 20 but with a reflector between a color separation filter unit and a light source; 
     FIG. 22 is a vertical section illustrating a printing head similar to that of FIG. 20 but in which an LCD micro shutter is incorporated; 
     FIG. 23 is a vertical section illustrating a printing head similar to that of FIG. 22 but with a reflector between a color separation filter unit and a light source; 
     FIGS. 24A-24F are cross sections illustrating the color separation filter unit in the printing head of FIGS. 20-23; 
     FIGS. 25A-25F are cross sections illustrating a color separation filter unit similar to that of FIGS. 24A-24F but with the ND filters behind the color filters; 
     FIGS. 26A-26F are cross sections illustrating a color separation filter unit with the ND filters between the color filters; 
     FIGS. 27A-27F are cross sections illustrating a color separation filter unit in a shape of a hexagonal prism; 
     FIGS. 28A-28F are cross sections illustrating a color separation filter unit with the ND filters behind the color filters; 
     FIGS. 29A-29F are cross sections illustrating a color separation filter unit with the ND filters between the color filters; 
     FIG. 30 is an explanatory view illustrating another preferred printing head with a slidable color separation filter unit; 
     FIG. 31A is an explanatory view illustrating the color separation filter unit of FIG. 30; and 
     FIG. 31B is an explanatory view illustrating another color separation filter unit in which an ND filter is smaller. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE PRESENT INVENTION 
     In FIG. 1, an optical printer  11  is depicted, and includes a printing head  12  and spreader rollers  13 . The optical printer  11  is loaded with a cartridge  16  or instant photo film pack containing a stack of plural instant photo films  14  as color photosensitive material. The instant photo films  14  are a well-known mono-sheet type including red, green and blue sensitive layers overlaid on a support, and have a processing solution pod  14   a  for containing processing solution. After printing, the instant photo film  14  being exposed is squeezed by the spreader rollers  13  to break the processing solution pod  14   a , from which the processing solution is spread to an exposure surface of the instant photo film  14 . The instant photo film  14  is ejected through an exit slot  11   a  upon spreading of the processing solution. The instant photo film  14  after the ejection comes to have a positive image in the exposure surface at a lapse of one minute or a few minutes. 
     The printing head  12  records a color image by applying printing light of red, green and blue colors to the instant photo film  14 . The printing head  12  has a length enough for one line of an image, and longitudinally extends in a main scan direction. The printing head  12  emits printing light of the red, green and blue colors for an exposure line by line. When color recording of one line is completed, the instant photo film  14  is fed by a feeder having a feeder motor  126  by an amount of one line, and is subjected to recording of the following line. Recording and feeding of one line are repeated to record a color image of one frame. An exposed portion of the instant photo film  14  is gradually advanced to the spreader rollers  13 , and processed. 
     In FIG. 2,  3 A and  3 B, the printing head  12  is a combination of a light source  21 , a color separation filter unit  22 , a SELFOC lens array  23 , a reflector  24  and a liquid crystal display (LCD) micro shutter  26  included in a light amount adjustor. Light emitted directly by the light source  21  is passed through the color separation filter unit  22  and separated into printing light of any one of the blue, green and red colors. The printing light passes the SELFOC lens array  23 , the reflector  24 , the LCD micro shutter  26  and an opening  12   a  and is applied to the instant photo film  14  for exposure. 
     The light source  21  is constituted by a light-emitting element  27  for emitting white light. An example of the light-emitting element  27  is a light-emitting diode. The light-emitting element  27  has a length of one line. Note that light from the light-emitting element  27  may be what has a color different from white but includes light of the red, green and blue colors. Also, a fluorescent lamp or other suitable light source may be used instead of the light-emitting element. 
     The LCD micro shutter  26  consists of an array of plural liquid crystal display (LCD) segments, each of which corresponds to one pixel. The LCD micro shutter  26  is controllable for changes in transmittance per each of the LCD segments, and adjusts an amount of the printing light passed through or blocks the printing light. The LCD segments are controlled according to an image signal. 
     The SELFOC lens array  23  includes a great number of SELFOC lenses arranged in one train for converging light at one point. The number of the SELFOC lenses is that of pixels. The SELFOC lens array  23  produces printing light by each pixel, and prevents the light from being spread to positions of adjacent pixels. Note that light-shielding members are disposed in the case in a suitable manner, and cause printing light to exit from the opening  12   a.    
     The color separation filter unit  22  has a tubular shape, and has a central axis  22   a  extending perpendicularly to a light path. The color separation filter unit  22  is supported in a rotatable manner about the central axis  22   a . A peripheral surface of the color separation filter unit  22  is equally divided about the central axis  22   a , and is provided with red, green and blue color filters  31 ,  32  and  33  and transparent plates  36 ,  37  and  38  or transmission portions. The color filters  31 - 33  have selectivity of the wavelength, and converts white light into printing light of each one color. The transparent plates  36 - 38  do not have selectivity of the wavelength, and allows light to pass without a change. A filter drive motor  34  drives the color separation filter unit  22  to rotate. When the color separation filter unit  22  rotates, the color filters  31 - 33  are inserted into the light path selectively. 
     The peripheral face of the color separation filter unit  22  is divided into six portions to dispose the color filters  31 - 33  and the transparent plates  36 - 38  alternately. In other words, the transparent plates  36 - 38  are opposed to respectively the color filters  31 - 33 . An entrance position is defined as a position where light from the light source  21  enters the color separation filter unit  22 . An exit position is defined as a position where the light exits from the color separation filter unit  22 . When each of the color filters  31 - 33  is located in the entrance position, one of the transparent plates  36 - 38  is located in the exit position. Also, when each of the color filters  31 - 33  is located in the exit position, one of the transparent plates  36 - 38  is located in the entrance position. As a result, light passing through the color separation filter unit  22  passes only one of the color filters  31 - 33  for the red, green and blue colors. The light from the light source  21  is converted by the color separation filter unit  22  into printing light of one of the red, green and blue colors. 
     Let r 1  be a central angle of each of the color filters  31 - 33 . Let r 2  be a central angle of each of the transparent plates  36 - 38 . The central angles r 1  and r 2  are approximately 60 degrees for the color filters  31 - 33  and the transparent plates  36 - 38  to have an equal size. The central angles r 1  and r 2  are determined according to the total number of the color filters  31 - 33  and the transparent plates  36 - 38 . It is to be noted that the central angle r 2  may not be equal to the central-angle r 1 . For example, it is possible that r 1 &lt;r 2 . 
     Note that, in the present embodiment, the color filters  31 - 33  and the transparent plates  36 - 38  are originally separate pieces assembled and fixed by use of a pair of caps or rings fitted to their ends. Alternatively, it is possible to produce the color filters  31 - 33  and the transparent plates  36 - 38  from a single sheet. At first, a transparent sheet is wound into a cylindrical shape, on which three colors are printed in three portions of a rectangular shape. Then a pair of caps or rings are fitted on ends of the cylindrical sheet. 
     A preferable example of material for producing the color filters  31 - 33  and the transparent plates  36 - 38  is a plastic film, sheet or plate. For the colors of the color filters  31 - 33 , the film, sheet or plate may be coated with suitably selected pigment, dye or any coloring material. Also, film, sheet or plate may be glass or the like. 
     All the periphery of the color separation filter unit  22  is constituted by the combination of the color filters  31 - 33  and the transparent plates  36 - 38 , and does not have an opening. Thus, there is little resistance of air to the color separation filter unit  22  during rotation. There occurs little noise in the rotation. Note that openings  112  in FIG. 3C can be formed instead of the transparent plates  36 - 38  for passage of light. This is effective in reducing the cost as the transparent plates  36 - 38  are not used. 
     Note that, in the present embodiment, the color filters  31 - 33  are originally separate pieces assembled and fixed by use of a pair of caps or rings fitted to their ends, to result in forming the openings  112  defined between the color filters  31 - 33 . Alternatively, it is possible to produce the color filters  31 - 33  in a single sheet being transparent or of a certain color. To be precise, three colors are printed on a flat sheet in three portions of a rectangular shape. Then the openings  112  are formed by cutting operation between the colored portions. The sheet is wound cylindrically, and provided with a pair of caps or rings on its ends. 
     In FIGS. 4A-4F, an image of one line is exposed in a sequence of red, green and blue during rotation of the color separation filter unit  22 . Each time that the color separation filter unit  22  makes one rotation, two lines are recorded. In FIG. 4A, the red color filter  31  is set in an entrance position, which is defined as a position of entry of light from the light source  21  into the color separation filter unit  22 . The light from the light source  21  is passed through the red color filter  31  to become red printing light, which passes the transparent plate  37  set in an exit position, which is defined as a position of exit of the light from the color separation filter unit  22 . 
     When the color separation filter unit  22  is rotated in the counterclockwise direction by 60 degrees, the green color filter  32  is inserted in the light path. In FIG. 4B, the green color filter  32  is disposed in the exit position. Light from the light source  21  enters the color separation filter unit  22  through the transparent plate  38 , and passes the green color filter  32  to exit from the color separation filter unit  22 . The light becomes green printing light upon passage through the green color filter  32 . In FIG. 4C, the color separation filter unit  22  rotates further by 60 degrees, to set the blue color filter  33  in the entrance position. Light from the light source  21  enters the color separation filter unit  22  through the blue color filter  33 , becomes blue printing light, and passes the transparent plate  36  to exit from the color separation filter unit  22 . Consequently printing light of the red, green and blue colors operates for an exposure, to record an image of one line. In FIGS. 4D,  4 E and  4 F, the color separation filter unit  22  further makes half a rotation, while another line is recorded. In this half rotation, the color filters  31 - 33  are inserted in the light path in a different manner related to the entrance position and exit position. 
     In the present embodiment, the peripheral face of the color separation filter unit  22  is divided into the six having the color filters  31 - 33  and the transparent plates  36 - 38 . However, the peripheral face of the color separation filter unit  22  may be divided into a number 6(2n+1) of portions, where n is an integer equal to or more than zero (0). 
     In FIG. 5, another preferred color separation filter unit  121  is depicted, in which the periphery is divided into  18  portions (n=1). The color separation filter unit  121  has the three red color filters  31 , the three green color filters  32 , the three blue color filters  33  and the nine transparent plates  36 ,  37  and  38 . While the color separation filter unit  121  makes one rotations, six lines are recorded. 
     In FIG. 6, a control unit  41  controls the printing head  12 . The control unit  41  is constituted by a controller  42 , a memory  43 , a light source driver  44 , an image data processor circuit  46 , an LCD driver  47 , a motor control circuit  48 , a motor driver  49  and a feeder motor driver  124  included in a feeder. The light source driver  44  drives the light source  21  according to a control signal from the controller  42 . The image data processor circuit  46  converts image data from the memory  43  to an image signal, which is sent to the LCD driver  47 . Also, a display panel  51  of the printer is driven by the image data processor circuit  46  to display a simulated image to be printed. 
     According to the image signal, the LCD driver  47  drives the LCD micro shutter  26  by each LCD segment. Transmittance of each LCD segment changes to control intensity of printing light. Timing of driving the LCD micro shutter  26  is synchronized with the phase of the color separation filter unit  22  by means of a timing signal output by a phase detecting sensor  52 . 
     The feeder motor  126  is driven by the feeder motor driver  124 . The feeder motor  126  operates to feed the instant photo film  14 , and consists of a stepping motor. The feeder motor driver  124  sends a drive pulse to the feeder motor  126  upon receiving a detection signal from the phase detecting sensor  52 . Thus, the feeder motor  126  is synchronized with the phase of the color separation filter unit  22 . Each time that the color separation filter unit  22  makes half a rotation, the instant photo film  14  is fed by one line. 
     The filter drive motor  34  is a DC servo motor, and is driven by a driving voltage applied by the motor driver  49 . A rotating amount of the filter drive motor  34  is controlled by the driving voltage. The rotating speed and phase of the filter drive motor  34  is controlled by the motor control circuit  48 . A rotational angle detector circuit  53  is incorporated in the filter drive motor  34 , and sends a signal to the motor control circuit  48  to control the number of rotations. 
     In FIG. 7, a phase detecting encoder  54  includes an encoder disk  56  provided with slits  57 ,  58  and  59 . The slits  57 - 59  are color information having discernability and operate for detecting the phase of the color separation filter unit  22 . An output shaft  34   a  of the filter drive motor  34  as a central axis is provided with the phase detecting encoder  54  secured thereto, so the phase detecting encoder  54  rotates with the filter drive motor  34 . The color separation filter unit  22  is directly connected with the filter drive motor  34 , and rotates at the same rotational speed as the latter. 
     The slit  57  is formed in a manner corresponding to the red color filter  31  and the transparent plate  37  opposed to the red color filter  31 . Similarly, the slit  58  is formed in a manner corresponding to the green color filter  32  and the transparent plate  38  opposed to the green color filter  32 . The slit  59  is formed in a manner corresponding to the blue color filter  33  and the transparent plate  36  opposed to the blue color filter  33 . The slits  57 - 59  are provided with different widths and positions in a radial direction for discernment by means of the phase detecting sensor  52 . 
     The phase detecting sensor  52  consists of a photo interrupter, and includes a light source and two photo receptors  52   a  and  52   b  for receiving light from the light source. The photo receptors  52   a  and  52   b  discern the slits  57 - 59 . The phase detecting sensor  52  is secured to a position for detecting each one of the slits  57 - 59  corresponding to an inserted one of the color filters  31 - 33  upon insertion into the light path. 
     When the slit  57  for the red color filter  31  passes the phase detecting sensor  52 , both of the photo receptors  52   a  and  52   b  receives light from a light source. When the slit  58  for the green color filter  32  passes the phase detecting sensor  52 , light to the photo receptor  52   a  is blocked. The photo receptor  52   b  receives light. When the slit  59  for the blue color filter  33  passes the phase detecting sensor  52 , light to the photo receptor  52   b  is blocked. The photo receptor  52   a  receives light. Thus, one of the color filters  31 - 33  set in the light path is recognized. The phase of the color separation filter unit  22  is detected. 
     While the phase detecting sensor  52  generates the phase detecting signal, the LCD micro shutter  26  is driven according to an image signal for each color of the color filters  31 - 33  being inserted. Note that time during which the phase detecting sensor  52  generates the phase detecting signal is determined according to the angle r 3  of the range of the slits  57 - 59 . The angle r 3  of the slits  57 - 59  is approximately 50 degrees, and smaller than the angles r 1  and r 2  of the ranges of the color filters  31 - 33  and the transparent plates  36 - 38 . 
     After rotation of the color separation filter unit  22  inserts one of the color filters in the light path, the LCD micro shutter  26  starts being driven upon an image signal of the color of the one color filter. Before the entirety of the one color filter is moves away from the light path, outputting the image signal is completed to discontinue the driving of the LCD micro shutter  26 . Thus, no mixture in the color occurs, because the color of the color filters is exactly set as the color of the image signal sent to the LCD driver  47  at the time of changing over the color filters. 
     In FIG. 8, the motor control circuit  48  includes a frequency comparator  40 , a phase comparator  45 , a reference frequency signal generator  50  and a reference phase signal generator  55 . As illustrated in FIG. 9A, the reference frequency signal generator  50  generates a reference pulse of a regular width at a regular period. The number of reference pulses generated while the color separation filter unit  22  makes one rotation is predetermined. The number of the reference pulses is compared by the frequency comparator  40  with the pulse number output by the rotational angle detector circuit  53 , to keep constant the rotational speed of the filter drive motor  34 . 
     As depicted in FIG. 9B, the reference phase signal generator  55  generates a reference phase pulse having a width of the angle of the slits  57 - 59  or approximately 50 degrees. While the color separation filter unit  22  makes one rotation, the reference phase signal generator  55  generates two reference phase pulses. A pulse output by the phase detecting sensor  52  is compared with the reference phase pulse by the phase comparator  45 , to control the phase of the color separation filter unit  22 . Thus, the color of the image signal to be sent to the LCD driver  47  is caused by synchronization to coincide with the colors of the color filters  31 - 33  to be inserted in the light path. 
     The operation of the above embodiment is described with reference to flow charts of FIGS. 10A and 10B. At first, the cartridge  16  is set. A command signal for printing is input. Image data is read from the memory  43 , and is converted to an image signal by the image data processor circuit  46 . At the same time, the filter drive motor  34  is actuated to rotate the color separation filter unit  22 . With the start of driving the filter drive motor  34 , the reference frequency signal generator  50  operates to control the rotational speed. 
     An image signal of a first line is transferred to the LCD driver  47 . A phase detecting signal is sent by the phase detecting sensor  52  to the LCD driver  47 . When the phase detecting sensor  52  detects the slit  59  for the blue color filter  33 , an exposure of the one first line is started. 
     When the phase detecting sensor  52  detects the slit  57 , the LCD micro shutter  26  is driven according to the red image signal. Light emitted directly by the light source  21  passes the red color filter  31  to become red printing light, which passes the LCD micro shutter  26  to expose the instant photo film  14 . Exposure continues while the phase detecting sensor  52  detects the slit  57 . 
     The color separation filter unit  22  rotates to move the red color filter  31  away from the light path. Instead, the green color filter  32  is set in the light path. When the phase detecting sensor  52  detects the slit  58 , the LCD micro shutter  26  is driven according to the green image signal. The angle r 3  of the slits  57 - 59  is predetermined smaller than the angles r 1  and r 2  of the color filters  31 - 33  and the transparent plates  36 - 38 . Shortly after setting the green color filter  32  in the light path, the slit  58  is detected to start driving the LCD micro shutter  26 . Thus, no mixture in the colors occurs. Light from the light source  21  passes the green color filter  32  to become green printing light, which passes the LCD micro shutter  26  to come incident upon the instant photo film  14 . Exposure of the instant photo film  14  continues while the phase detecting sensor  52  detects the slit  58 . 
     Furthermore, the color separation filter unit  22  rotates to set the blue color filter  33  in the light path. Upon detection of the slit  59 , the LCD micro shutter  26  is driven according to the blue image signal. Blue printing light passes the LCD micro shutter  26  to expose the instant photo film  14 . 
     When the exposure of one line is completed, the mechanism with the feeder motor  126  feeds the instant photo film  14  by the line. Then another line starts being exposed. When the exposure of the two lines is completed, the color separation filter unit  22  is caused to make one rotation. Similarly, line images are recorded line after line, until an image of a frame is recorded. During feeding of the instant photo film  14 , the spreader rollers  13  squeeze the processing solution pod  14   a . An exposed portion of the instant photo film  14  is subjected to processing by spreading of processing solution to an exposure surface. When the exposure of the frame is completed, the instant photo film  14  is exited from the optical printer  11 . 
     In the above embodiments, the LCD segments in the LCD micro shutter  26  are controlled for transmittance. Furthermore, it is possible to control the LCD segments in the LCD micro shutter  26  time-sequentially for changing light amounts for pixels. To be precise, a pixel to be recorded with a middle density is exposed with a controlled middle light amount by keeping open the LCD segment during time shorter than a full time length that is predetermined for recording the highest density. 
     In FIGS. 11 and 12, another preferred embodiment is depicted, in which a light-emitting element array  61  as light source is incorporated in a printing head  60 . The light-emitting element array  61  includes light-emitting elements in a number enough for the number of pixels. The light-emitting element array  61  is driven by a light-emitting element driver  62 , and controlled for intensity of light to emit, the light-emitting element driver  62  constituting a light amount adjustor. The light from the light-emitting element array  61  passes the color separation filter unit  22  and becomes printing light of any of the red, green and blue colors, and then passes the SELFOC lens array  23  to come incident upon the instant photo film  14 . The use of the light-emitting element array  61  is advantageous as the LCD micro shutter is unnecessary to reduce the number of the parts. 
     In the present embodiment, a stepping motor  127  instead of a DC servo motor is incorporated as a filter drive mechanism for driving the color separation filter unit  22 . The phase of the color separation filter unit  22  can be controlled by drive pulses. This is advantageous as no control circuit is required for the motor. 
     In FIG. 13, another preferred printing head  63  is illustrated, in which the light source  21  is directed downwards. A reflector  64  is disposed in the light path, and makes it possible to dispose the light source  21  in a suitable orientation. It is to be noted that the positions of the SELFOC lens array  23  and the color separation filter unit  22  may be changed to each other. 
     In FIGS. 14A-14F, another preferred color separation filter unit  65  is depicted, in which ND filters  66  are incorporated for reducing a light amount. The ND filters  66  are attached to respectively the color filters  31 - 33 . Accordingly, the exposure amount is adjusted to regularize density of the blue, green and red colors even though the relative sensitivity of the blue, green and red sensitive layers is different in the instant photo film  14 . 
     Note that the ND filters  66  may be attached to an outer face of the color filters  31 - 33 . In FIG. 15A, an embodiment is illustrated in which the ND filters  66  are disposed behind the color filters  31 - 33  with a space without direct contact. In FIG. 15B, an embodiment is illustrated in which the openings  112  are formed instead of the transparent plates  36 - 38  for transmission of light. 
     In the above embodiments, all the color filters  31 - 33  are provided with the ND filters  66 . However, the ND filters  66  may be attached to filters of only one or two colors selected from the color filters  31 - 33 , to reduce light amounts of only the one or two colors. Thus, differences in the density between three colors can be compensated for by means of the ND filters  66  easily in connection with the photo film, color separation filter unit and light source. 
     If there are differences between relative photo sensitivity of photosensitive layers of the instant photo film  14 , it is possible to use ND filters only for color filters of the color of a photosensitive layer with higher relative photo sensitivity. The light amount of the same color can be reduced exclusively. Thus, density of the three colors can be well-balanced. Also, if there are differences in transmittance of the color filters  31 - 33 , it is possible to use ND filters only for color filters with higher transmittance. Thus, density of the three colors can be well-balanced. Furthermore, if there are differences in brightness in spectral distribution of the light source, it is possible to use ND filters only for color filters of the color with higher spectral brightness. Thus, density of the three colors can be well-balanced. 
     However, it is preferable in a color separation filter unit to have a transparent filter or transparent plate without reduction of a light amount if the filter unit does not have the ND filters  66 . This is in consideration of balance of the weight of the color separation filter unit. 
     In FIGS. 16A-16F, a preferred embodiment is depicted, in which a color separation filter unit  67  includes the ND filters  66  arranged alternately with the color filters  31 - 33 . The ND filters  66  operate as transmission portions without selectivity of a wavelength. Also, FIGS. 17A-17F illustrate a color separation filter unit  71  in a shape of a polygonal prism. The peripheral face of the color separation filter unit  71  is divided into six portions, and provided with transparent plates  128  or transmission portions. Furthermore, the total number of the color filters and the transmission portions can be 6(2n+1) in the manner described above. In FIG. 17G, an embodiment is depicted, in which the openings  112  are formed instead of the transparent plates  128 . 
     In FIGS. 18A-18F, a preferred color separation filter unit  77  is illustrated, in which ND filters  76  are attached to color filters  72 ,  73  and  74 . Also, FIG. 18G illustrates a color separation filter having the openings  112 . In FIGS. 19A-19F, a color separation filter unit  78  has the color filters  72 - 74  and the ND filters  76  arranged alternately with the color filters  72 - 74 . 
     In FIGS. 20 and 21, other preferred printing heads  81  and  82  are depicted, in which the light-emitting element array  61  is incorporated and a color separation filter unit  85  is provided with a reflector  83 . In FIG. 20, a light-emitting surface of the light-emitting element array  61  is directed horizontally. In FIG. 21, the light-emitting surface of the light-emitting element array  61  is directed downwards. In FIG. 22 and 23, printing heads  86  and  87  have the light source  21  and the LCD micro shutter  26  instead of the light-emitting element array  61 . 
     The color separation filter unit  85  to be contained in the printing heads  81 ,  82 ,  86  and  87  has the reflector  83  as illustrated in FIGS. 24A-24F. The reflector  83  reflects light from an entrance position of the color separation filter unit  85  with approximately a right angle toward an exit position. The periphery of the color separation filter unit  85  is divided into  12  portions in which color filters  88 ,  89  and  91  are arranged alternatively with the openings. Thus, a single one of the color filters  88 ,  89  and  91  is located in either one of the entrance position and exit position in a manner irrespective of the rotational position of the color separation filter unit  85 . Note that the number of portions into which the periphery of the color separation filter unit  85  is divided may be different from 12, and may be 12(2n+1) where n is an integer equal to or more than zero. In the present embodiment, the printing head can have a small size as the reflector  83  is contained in the color separation filter unit  85 . 
     In FIGS. 25A-25F, a color separation filter unit  94  has ND filters  92  attached directly to the color filters  88 ,  89  and  91 . Note that, as described above, the ND filters  92  may be attached to filters of only one or two colors selected from the color filters  88 ,  89  and  91 . Also, the ND filters  92  may be disposed behind the color filters  88 ,  89  and  91  with a space without direct contact. In FIGS. 26A-26F, a color separation filter unit  95  includes the ND filters  92  arranged alternately with the color filters  88 ,  89  and  91 . 
     In FIGS. 27A-27F, a color separation filter unit  96  has a shape of a polygonal prism. The periphery of the color separation filter unit  96  is divided into 12 portions. Note that the number of portions into which the periphery of the color separation filter unit  96  is divided may be 12(2n+1) where n is an integer equal to or more than zero, which is similar to the embodiment of FIGS. 24A-24F. In FIGS. 28A-28F, a color separation filter unit  97  includes ND filters  98  attached to respectively the color filters. Note that the ND filters  98  may be disposed with a space to the color filters without a direct contact. In FIGS. 29A-29F, a preferred color separation filter unit  99  includes the color filters and the ND filters  98  arranged alternatively with the color filters. 
     It is to be noted that the openings  112  are formed for transmission of light in the color separation filter unit  85  of FIGS. 24A-24F, the color separation filter unit  94  of FIGS. 25A-25F, the color separation filter unit  96  of FIGS. 27A-27F, and the color separation filter unit  97  of FIGS. 28A-28F. Furthermore, transparent plates may be secured in place of the openings. 
     Note that, in any of the above embodiments, the optical printer is a line printer. Alternatively, a printer of the invention may be a serial printer in which a printing head is movable in a width direction of the instant photo film to record each line. 
     In the present invention, ND filters may be attached to a color separation filter unit of a type different from the rotatable type. In FIG. 30, another preferred printing head  100  is depicted. A color separation filter unit  104  is constituted by color filters  101 ,  102  and  103  arranged in a manner of a single plate. One or more ND filters  106  are attached to the color separation filter unit  104 . A filter sliding mechanism  132  slides the color separation filter unit  104  to insert the color filters  101 - 103  in the light path selectively. 
     The color separation filter unit  104  is formed as a single transparent plastic plate, in which coatings are applied to its surface to determine the color filters  101 - 103 . The ND filters  106  are attached to a lower surface of the color filters  101 - 103 . It is to be noted that the ND filters  106  may be disposed behind the color filters  101 - 103  with a space without direct contact. 
     A head moving mechanism  130  slides the printing head  100  in parallel with an exposure surface of the instant photo film  14 . At first, the red color filter  101  is set in the light path. The printing head  100  moves by an amount of one frame, to take an exposure with red printing light. Then the green color filter  102  is set in the light path. The printing head  100  moves by an amount of the one frame, to take an exposure with green printing light. Finally, the blue color filter  103  is set to expose the frame with blue printing light. The ND filters  106  are operated to regularize density of the three colors easily. 
     In FIG. 31A, the ND filters  106  are attached to positions of only two of the color filters. In FIG. 31B, the ND filters  106  are attached to a position of only one of the color filters. Selection of only one or two of the color filters for attachment of the ND filters  106  is determined in consideration of the relative photo sensitivity of photosensitive layers of the instant photo film  14 , transmittance of the color filters  101 - 103 , or brightness in spectral distribution of the white color light source, as described above. 
     In any of the above embodiments, the printing light of the three colors are controlled by pixels to record one line at each time of an exposure. However, the liquid crystal display (LCD) micro shutter  26  or the light-emitting element array  61  may have a structure to record two or more lines in each exposure. 
     Although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.