Color printer and exposure head therefor

Compact and inexpensive color printers can be manufactured. An exposure head using linearly-aligned organic EL elements each composed of subpixels each emitting light of R, G, and B is used as a light source. Input color image data are dealt with as color data corresponding to each of the color components R, G, and B. For each color data, each of the organic EL elements is caused to emit light in accordance with line data for one line corresponding to all organic EL elements composing the exposure head, and a photosensitive material is exposed to the light. After completion of the exposure, vertical scan means moves the photosensitive material by a predetermined amount in the vertical scan direction and the photosensitive material is exposed in accordance with image data corresponding to a subsequent line, as has been described above. By repeating this procedure, exposure corresponding to all image data is carried out.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a color printer and an exposure head
 therefor. More specifically, the present invention relates to an exposure
 head and a printer for obtaining color prints by exposing color
 photosensitive material to color light.
 2. Description of the Related Art
 Color printers for obtaining color prints by 2-dimensionally scanning color
 photosensitive material by laser beams of 3 colors namely R (Red), G
 (Green) and B (Blue) have been known.
 This 2-dimensional scan by each of the RGB-color laser beams in such color
 printers is carried out by deflecting the laser beams with a
 photodeflector such as a rotational polygon mirror to cause the beams to
 scan the material in the main scan direction while moving the
 photosensitive material relative to the laser beams in the vertical scan
 direction approximately orthogonal to the main scan direction.
 In the scanning method in such conventional color printers for obtaining
 color prints by the 2-dimensional scan with the 3-color laser beams
 deflected by the rotational polygon mirror or the like, a laser beam
 source for emitting the beams is necessary. Since the laser beam sources
 are expensive, color printers using them are also expensive. Furthermore,
 since the laser beam scanning optical systems are large, color printers
 using them are also large. Moreover, they have difficulty with respect to
 registration of the RGB-color laser beams on the photosensitive material.
 SUMMARY OF THE INVENTION
 The present invention has been created based on consideration of the above
 problems. An object of the present invention is to provide an exposure
 head for a color printer enabling size reduction and cost reduction of the
 color printer and to provide a color printer using such an exposure head.
 An exposure head of the present invention is used to obtain a color print
 by exposing a color photosensitive material to color light and comprises
 linearly-aligned organic EL(electro-luminescent) elements each representing
 a pixel of a color image and composed of a combination of a plurality of
 organic EL's each emitting different color light.
 In other words, the exposure head of the present invention comprises the
 organic EL elements each representing a pixel of a color image and
 composed of the plurality of organic EL's (subpixels) emitting light of R,
 G or B respectively, for example.
 It is preferable for the organic EL elements composing the exposure head to
 be formed integrally on one substrate by using a semiconductor technique.
 A color printer of the present invention comprises the exposure head
 described above for exposing the color photosensitive material and a
 driver for causing each of the organic EL elements to emit light in
 accordance with input color image information, and vertical scan means for
 moving the color photosensitive material relative to the exposure head in
 the vertical scan direction.
 The exposure head of the present invention is formed by aligning the
 organic EL elements. Therefore, a light source for a printer which is
 smaller and less expensive than a laser beam source can be provided.
 Furthermore, if organic EL's having stronger luminance are used, an
 exposure head having higher efficiency can be composed. Moreover, when the
 exposure head is manufactured, if the organic EL elements are integrally
 formed on one substrate by using a semiconductor technique, an exposure
 head having high resolution of more than 1200 dpi, for example, can be
 manufactured easily.
 Therefore, by using this exposure head in the color printer, it becomes
 possible to produce color printers that are smaller and less expensive
 than those using conventional laser beam sources. If high-luminance
 organic EL's are used for the exposure head, high speed printers can be
 manufactured.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Hereinafter, an embodiment of the present invention will be explained with
 reference to the accompanying drawings. FIG. 1 is a diagram showing a
 surface of an exposure head 1 as the embodiment of the present invention.
 As shown in FIG. 1, the exposure head is formed by organic EL elements 2
 aligned linearly. Each of the organic EL elements 2 representing a pixel
 of a color image is composed of subpixels 2R, 2G, and 2B emitting light of
 R, G, and B respectively. The exposure head 1 can be formed by linearly
 aligning each pixel formed separately. However, in this example, the
 exposure head is formed by the organic EL elements unified on one
 substrate by using a semiconductor technique. In this manner, the exposure
 head 1 attains the resolution of 1200 dpi.
 A thin film stacking structure of the subpixels 2R, 2G, and 2B emitting
 light of R, G and B respectively and forming each of the organic EL
 elements 2 used in the present invention is not explained here. As a
 method of constructing the thin film stacking structure, various known
 methods can be used. As a method of manufacturing the high luminance
 organic EL elements, methods proposed in "Next Generation Display Device
 Research Group, 1992, Organic EL Development Strategy, Science Forum
 Publishers" or "Proceedings of Organic EL Electronics Material Meeting,
 Consideration of Achievement and Implementation Strategy of Organic EL, at
 Hotel Tenbo, Ikaho Hot Spring, Gunma Pref. Japan, 1995" can be used, for
 example. For the exposure head of the present invention, not only the
 methods described in these reference materials but also various methods
 having been proposed regarding a structure of an organic EL element can be
 adopted.
 FIG. 2 is a side view showing an outline configuration of a color printer 6
 using the above-described exposure head 1. The color printer 6 comprises a
 head unit 3 including the exposure head 1 for exposing a color
 photosensitive material 5 and a driver 3a unified with the exposure head 1
 for causing each of the organic EL elements 2 to emit light in accordance
 with color image data D input therein, and vertical scan means 4 including
 four rollers 4a for conveying the photosensitive material 5 in a vertical
 scan direction Y while nipping the material 5. The rollers 4a are forced
 to rotate in a direction such that the photosensitive material 5 is
 conveyed in the vertical scan direction Y, by a motor which is not shown
 in FIG. 2.
 The vertical scan means 4 is not limited to the above example. Any means
 which can move the photosensitive material 5 relative to the head unit 3,
 that is, move the exposure head 1 in the vertical scan direction Y, may be
 adopted.
 FIG. 3 is a circuit block diagram showing the head unit 3. The input image
 data D are stored temporarily in frame memories 10R, 10G, and 10B of the
 driver 3a as frame data DF respectively corresponding to the colors R, G
 and B. Image data corresponding to all organic EL elements 2 composing the
 exposure head 1 are read from the frame memories 10R, 10G and 10B to line
 memories 12R, 12G, and 12B respectively. The image data input to the line
 memories 12R, 12G, and 12B are image data (line data) DL corresponding to
 one line of the main scan. The line data DL in the line memories 12R, 12G
 and 12B are respectively input to drive circuits 14R, 14G, and 14B.
 An example of the drive circuit is shown in FIG. 4, regarding the drive
 circuit 14R. The input line data are 8-bit data. The line data DL are
 first input to a shift register (SR) 22 corresponding to the pixel number
 0. Each time a shift clock SCLK is input, the line data are sequentially
 transferred to a subsequent shift register. In this manner, the image data
 corresponding to one line end up being stored in every shift register
 corresponding to all organic EL elements 2 composing the exposure head 1.
 After completion of storage of the line data DL, a latch clock LCLK is
 input to each of latches (LT) 23, and the line data DL are stored in the
 latches 23 each connected to each shift register 22. An output enable
 signal OE is then input to the latches 23 and the line data DL stored in
 each of the latches are input to each pulse width controlling circuit (PW)
 24.
 The pulse width controlling circuit 24 converts the input 8-bit line data
 DL into a signal which represents the line data DL by its pulse width. The
 converted image signal D0 is input to the base of a drive transistor 25
 whose collector is grounded. One terminal of each of the organic EL
 elements (in this example, R0, R1, R2, and R3 for each pixel) is connected
 to the emitter of each of the drive transistors 25. The other terminal of
 each of the organic EL elements R0, R1, R2 and R3 is connected to a power
 supply line 27 via a resistor 26 for setting an operating point.
 Therefore, each of the organic EL elements R0, R1, R2, and R3 emits light
 in accordance with the pulse width. In this manner, the exposure of the
 photosensitive material 5 (see FIG. 2) is controlled in response to the
 line data Dl, that is, in response to the image data DF. As the drive
 transistor 25, a known drive IC for a thermal head can be used.
 Once the exposure for one line has been completed in this manner, the
 vertical scan means 4 (FIG. 1) moves the photosensitive material 5 by a
 predetermined amount in the vertical scan direction Y. The image data DL
 corresponding to a subsequent line are read from the frame memories 10R,
 10G, and 10B to the line memories 12R, 12G, and 12B respectively, as has
 been described above. The above-described exposure is then carried out
 sequentially and exposure corresponding to all the image data D is
 finished. In this manner, the exposure of one sheet of the photosensitive
 material 5 is completed.
 As has been described above, if a color printer is manufactured by using
 the exposure head composed of linearly-aligned organic EL elements each
 composed of subpixels each emitting light of a different color, it is not
 necessary to use a conventional large optical scanning apparatus for its
 main scan optical system. Therefore, very small and inexpensive printers
 can be manufactured. Moreover, if the organic EL elements can emit light
 of high luminance, high speed printing can be carried out.