GB2091893A - Colour copying - Google Patents

Abstract

A colour original is conveyed successively over slits 2a, b and c which are illuminated by independent blue, green and red light sources respectively, at least the blue and green light sources (e.g. metal halide lamps) being capable of emitting light of a narrow wavelength band, and a photoconductive sheet is conveyed successively in the opposite direction through respective colour processing sections a, b and c, each including charging, exposing and developing stations 11-13, and a single fixing station 14. Each developing station 13 includes a sheet- supporting plate 23 and a developing electrode 22 having a developer applicator slit 28, the gap between plate 23 and electrode 22 (through which the photoconductive sheet passes) being 0.1 - 1mm. <IMAGE>

Description

SPECIFICATION Colour copying machine The present invention relates to a colour copying machine and, more particularly, to a colour copying machine of electrofax type which makes use of photosensitive papers having a photo-sensitive surface layer.

Various types of colour copying machines using electrophotography are known, such as ordinary paper transfer types and electrofax types. These colour copying machines, however, are still unsatisfactory so far as the quality of picture is concerned and in the performance of the machine.

These known colour copying machines give a copy having poor colour quality because of the following reasons. If colour correction by masking, which is commonly adopted in multi-colour printing, is to be used in the electrophotographic process, it is necessary to employ a highly complicated construction resulting in an increased cost of the copying machine. Thus in colour copying using electrophotography, it is necessary in order to keep down cost to adopt the simplest image forming method in which separated images of three elementary colours are superposed without any colour correction.

The image reproduced by such a process inevitably is inferior in cleanliness of colour and quality of the colour image.

It is also to be noted that these known copying machines require a comparatively long time for the formation of the colour image.

In the known colour copying machines, light emitted from a set of white sources is separated by a colour separation filter and the separated light is made to scan the surface of the original by means of an illumination slit for exposure thereby to form an electrostatic latent image on the photosensitive member.

The latent image is then developed and is transferred to an image carrier which runs in synchronism with the movement of the illumination slit and the photosensitive member, thus completing the formation of a monochromatic image. The same process is repeated three or four times with different combinations of colour separation filter and developer, so that a multi-colour image of three or four colours is formed on the image carrier.

The length of time required for the formation of each colour image is usually several tens of seconds.

Thus, the application of this type of known machine to an industrial use for conventional colour printing imposes problems concerning capacity and cost.

Accordingly, an object of the present invention is to provide a colour copying machine which can produce a colour copy at high speed with improved accuracy in colour as compared to known machines making use of an ordinary wide-band spectrum light source, without adopting colour correction by masking.

To this end, according to one aspect of the present invention, a light source of a narrow band of wavelenghts is used as the light source of each colour in the colour separation exposure, in order to avoid the darkening of colour attributable to the unnecessary absorption colour possessed by colour materials such as pigments, dyestuffs or the like.

More particularly, in the blue-light exposure which is affected by the unnecessary colour absorption from magenta and cyanic colour materials, as well as in the green-light exposure affected by the unnecessary colour absorption from cyanic colour material, such a light source is used as is capable of emitting a light of wave length which can maximize the colour separation efficiency. A typical example of such a light source is a metal halide lamp.

The narrow-band exposure is not always necessary for red-colour exposure, because the unnecessary colour absorption from the colour material is not so serious in the red colour region. It is also possible to add an independent ordinary white colour light source, to the three independent light sources which include narrow-band light sources for blue and green and a light source for red.

According to a second aspect of the invention, a plurality of pairs of rollers are used as means for conveying the photosensitive paper to the process for visualizing the optic image projected by the optical system.

Generally speaking, in the formation of a multi-colour image by Carlson's method, the length of travel of the photosensitive member to the stationary part of the machine is extremely large because the process including charging, exposure and development is repeated three or four times. The image quality is largely affected by the precision of superposition of decomposed colour images. This means that the precise control of the position of the photosensitive member during its travel is a key to producing a higher quality copy image.

In the apparatus of the type using the same photosensitive member repeatedly such as a plain paper copier, the photosensitive member itself can be fixed in the form of a drum or a belt in the apparatus. It is, therefore, comparatively easy to overcome the above-described problem in this type of apparatus.

However, in the coated paper copier, it is difficult to adopt the paper conveying system employing a plurality of pairs of rollers, which system is most commonly used in most ordinary monochromatic line (light) copiers, because it is extremely difficult to precisely control the position of the copying paper in relation to the stationary part of the apparatus.

It has been proposed to use a carrier in the form of a drum, belt or plate for carrying the paper to achieve precise control of the paper. This countermeasure, however, imposes a different problem. Namely, the use of such a carrier not only complicates the apparatus thus raising the cost but also makes it difficult to adopt the method for simultaneously charging both sides of the paper by corona discharges of different polarities, the method most advantageously used in a coated paper copier.

Under these circumstances, the present invention makes it possible to adopt the paper conveying system incorporating pairs of rollers, by arranging the optical system in such a manner as to permit the formation of multi-colour superposed image in quite a short distance of travel of the photosensitive paper.

The third feature of the invention is as follows. In order to obtain high colour quality, particularly to reproduce gradation of neutral tints with high fidelity, it is necessary to adopt a liquid development system having good gradation reproducibility and a non-transfer type electrofax type system in combination. In this case, however, it is necessary to arrange three to four sets of charging, exposure and development means linearly within the picture angle of a single lens, so that it is not possible to obtain a sufficiently large space for the installation of the developing systems. The characteristic feature of the invention resides in the use of a novel high-speed liquid developing device to realize sufficient development with a soft liquid developer of comparatively slow developing speed, under the above-mentioned restriction of the space available for the installation.

According to the invention, the above-described features are effectively combined to provide a multi-colour copying machine capable of making a copy of a multi-colour image in a short period of time and having a high accuracy in colour, particularly a capability of tone reproduction.

In order to evaluate the advantages of the invention, the present inventors have confirmed an improvement in the percent colour separation efficiency.

The percent colour-separation efficiency D is generally expressed by the following equation: D = 100 Dw Du Dw - Du where, Dw is the density of the necessary colour, while Du represents the density of the unnecessary colour.

For reproducing a colour patch consisting of three colours of, for example, yellow (Y), magenta (M) and cyan (C), it is necessary that only the Y image appears while the densities of M and C images are null in the blue-colour exposure. However, as a matter of fact, the M and C colour materials exhibit a small absorption of blue light, so that the densities of the M and C images are not nullified completely.

With this knowledge, the present inventors have conducted the following experiments.

Experiment I {Conventional Case) 1. Conditions A: light source blue light source: 400 to 500 nm green light source: 500 to 600 nm red light source: 600 nm or higher Each light source was composed of a xenon lamp and an interference film filter.

B: photosensitive paper The photosensitive paper used had a layer of photosensitive electrophotographic material consisting of photosensitive titanium dioxide dispersed in a binding resin.

2. D separated images of Kodak colour patch were formed under the above-stated conditions to obtain the following percent colour-separation efficiences: Y M C blue colour - 37 73 green colour 82 - 11 red colour 93 90 Experiment11 (Embodiment of Invention) 1. Conditions A: light source (narrow band exposure light source) blue light source: 417 nm (Ga Lamp) green light source: 535 nm (TI Lamp) red light source: Same as experiment I B: Photosensitive paper Same as Experiment I.

2. D Separated colour patch image wave formed in the same manner as Experiment I to obtain the following percent colour-separation efficiences: Y M C blue colour - 54 72 green colour 80 - 41 red colour 93 90 From the results of the experiments shown above, it will be seen that, according to the invention, a remarkable increase of the M percent colour-separation efficiency in the blue light exposure from conventional value of 37% to 54%, as well as of the C percent colour-separation efficiency in green light exposure from conventional value of 1 1% to 41%, is achieved advantageously.

The invention may be put into practice in various ways and a number of specific embodiments will be described to illustrate the invention with reference to the accompanying examples and drawings in which: Figure lisa schematic sectional view of a colour copying machine; Figure 2 is a perspective view of the developing section of the colour copying machine illustrated in Figure 1; Figure 3 is a perspective view of the developing unit used in the developing section illustrated in Figure 2; Figure 4 is a perspective view of a photosensitive sheet supporting plate which is a part of the developing unit shown in Figure 3; Figure 5 is a sectinal view of the developing unit in one exemplified form; Figure 6 is a sectional view of the developing unit in another exemplified form which only shows the portion which differs from the developing unit shown in Figure 5; and Figure 7 is a sectional view of a nozzle hole portion formed in the photosensitive sheet supporting plate.

Referring first to Figure 1, a colour copying machine in accordance with an embodiment of the invention has an original holder 1 a adapted to be slid on slits formed in the top wall of a box-like frame.

More specifically, there are provided a slit 2a for blue light illumination, slit 2b for green light illumination and a slit 2c for red light illumination, adapted to be illuminated by a blue light source (of narrow wavelength bads) 3a, green light source of narrow wavelength bands 3b and a red light source of narrow wavelength bands 3c, respectively. The light rays from the light sources (3a, 3b, 3c) are reflected upon the original on the original holder 1a through the respective slits 2a, 2b and 2c. The reflected rays are introduced again through the slits and reach respective exposure positions 12a, 12b, 12c on the path of travel of the photosensitive sheet, via a Dach mirror 4, a lens 5, a reflecting mirror 6 and respective exposure slits 7a, 7b, 7c. These slits 7a, 7b, 7c may be provided with filters as required.A container cassette 8 for storing a multiplicity of photosensitive sheets is disposed beneath the end wall of the box-like frame 1. The photosensitive sheets are fed one by one from the cassette 8 by a paper feed roller 9, and are made to move, in synchronsim with the movement of the original holder 1a, along the path of travel past the exposure positions 12a, 12b and then 1 2c, by means of pairs of conveying rollers 10 arranged along the feed path.

Corona-discharge type charging units 1 1a, 11 b, 1 1c are disposed at the upstream side (right-hand) of the respective exposure positions 12a, 12b and 12c, while, at the downstream side of the same, there are provided high speed type liquid developing units 13a, 13b, 13c. Afixing unit 14 is disposed at the downstream side of the developing unit 1 3c. The photosensitive paper after passing through the fixing unit 14 is introduced to an outlet tray 19 by the conveying rollers 10.

The colour copying machine of this embodiment operates in the manner explained hereinunder.

First of all, a coloured original is placed on the original holder 1 a with its image bearing surface to be copied downwards directed toward the illumination slits 2a, 2b and 2c. As the colour copying machine is operated, the original holder 1 a is slid from the position A shown in chain lines in Figure 1 in the direction indicated by the arrow and stops as it arrives at the position B also shown in chain lines. During this sliding of the original holder lathe original surface is swept by the blue, green and red light from the light sources 3a, 3b, and 3c through the slits 2a, 2b, and 2c, respectively.

As stated before, the rays of respectively coloured light reflected by the original are made to focus at the exposure positions 12a, 12b, 12c, respectively, on the path of travel of the photosensitive sheet. Before entering each exposure position, the photosensitive copy sheet is electrostatically charged on both its faces (top and bottom) by corona discharges of different polarities by the appropriate one of the charging units 11 a, 11 b or 11 c, so that a latent image is formed as a result of the exposure. The photosensitive sheet is then made to pass through the respective developing unit 1 3a, 1 3b or 1 3c so that the latent image is developed to form a colour image of the colour corresponding to the exposure position.After leaving the developing unit 13c of the final stage, now having a developed three-colour image, the copy sheet is conveyed to the fixing unit 14 where the developed image is fixed. Finally, the developed copy sheet is conveyed to the tray 19.

As will be understood from Figure 1, the illumination slits 2a, 2b, 2c and the exposure positions 1 2a, 1 2b, 1 2c are located within the picture angle of the lens 5. It is, therefore, possible to make the original to be swept by the blue, green and red lights in a single stroke or pass of the original holder 1a. Also, the exosure and development of all the colours are achieved in the single pass of the original holder. This permits a remarkable reduction in the length of time required for the colour copying. It is also to be noted that, since light sources capable of emitting light of narrow wavelength bands suitable for colour separations of blue, green and red colours are used in accordance with the spectro-reflection factor measurement of the colour materials used in the original, it is possible to obtain a multi-colour copy of high equality.

A detailed description of the construction of the developing units 13a, 1 3b, 1 3c schematically shown in Figure 1 will now be given with specific reference to Figures 2 to 7.

Figure 2 is a perspective view of an essential part of an individual developing unit 13a, 13b or 13c. A hollow casing 21 constituting the outer wall of the developing head is united with a developing electrode 22 and the end of a developing liquid supply pipe 30. The housing 21 can have any other cross-sectional shape than the illustrated rectangularform, e.g. a semicircular form, oval form and so forth, but is required to have a volume large enough to accommodate the supply pipe 30. A supporting plate 23 for supporting a latent-image carrying photosensitive sheet is disposed in close proximity to and beneath the developing electrode 22 so as to extend parallel to the latter. The gap between the electrode 22 and the supporting plate 23 is usually selected to be in the range between 0.1 and 1 mm.The photosensitive copy sheet 25 carrying an electrostatic latent image on its upper surface is conveyed on the upper surface of the supporting plate 23 in the direction of the arrow by means of conveyor rollers or the like, so as to pass through the developing electrode 22 and the supporting plate 23, and is conveyed out of the developing unit by means of conveyor rollers or the like.

The developing liquid is fed to the supply pipe 30 by means of a pump from a separately installed tank, and is supplied to the developing region after forming a stabilized flow in the developing head. The developing liquid thus used is collected through apertures formed in the supporting plate or from one lateral side of the latter.

Figure 3 is a perspective view showing the internal structure of the developing head. The side 26 of the developing liquid supply pipe 30 is provided with a plurality of holes 27 so that the liquid is supplied uniformly over the entire region of the developing head. The free end (not shown) of the pipe is completely closed. The liquid discharged from the holes 27 then fills the developing head and is discharged in the form of a liquid film of a uniform thickness through a slit 28 (see Figure 5 for more detail) formed in the developing electrode 22 down to the developing region defined beneath the latter.

Figure 4 is a perspective view of the photosensitive sheet supporting plate 23 as used in the developing unit shown in Figure 2. The portions of the supporting plate 23, which are in the vicinity of the slit 28, i.e. the sheet insertion guiding portion 31, as well as the leading head portion as viewed in the direction of movement of the sheet, is provided with a multiplicity of apertures 32.

We refer now to Figure 5 which is a schematic sectional view in greater detail of one embodiment of the essential region around the developing head and the developing region. The holes 27 formed at the end of the developing liquid supply pipe, accommodated by the developing head, are not directly opposed to the slit 28, so that the developing liquid is supplied toward the wall of the housing 21 or to the portion of the electrode 22 other than the portion containing the slit 28. The developing liquid temporarily fills the developing head and is discharged from the slit 28 downwardly, in the form of a liquid film of uniform thickness and at a uniform velocity or rate. The photosensitive sheet supporting plate 23 is disposed in close proximity to and parallel with the developing electrode 22.Since the portions of this plate other than the portion just beneath the slit 28 are provided with a multiplicity of apertures, it is possible to obtain a smooth flow of the liquid to fill the developing region (as shown by cross-hatching) beneath the slit, while avoiding the inclusion of air bubbles and local concentration of the liquid. This in turn permits the image carrying sheet 25 to be contacted by a sufficiently large amount of developing liquid instantaneously, to reduce or avoid the incidence of various development failures such as insufficient initial development, uneven development density, edge effect and white blank.

Figure 6 is a schematic sectional view of another embodiment of the developing head, having an improved shape of the inner surface of the housing, as well as an improved positional relationship between the inner surface of the housing and the slit.

Figure 7 shows an example of the aperture formed in the photosensitive sheet supporting plate in section.

In this case, the aperture 28 is inclined forwardly as viewed in the direction of movement of the image carrying sheet. The angle of inclination is generally selected to be in the range between 30 and 90 , e.g. 30 to 45owe As will be understood from the foregoing description, in the colour copying machine of the present invention, the copy sheet surface carrying the electrostatic latent image is made to pass along a path defined in the close proximity to the developing electrode, and is caused to be contacted by a uniform flow of the developing liquid. It is, therefore, possible to obtain a copy of a high image quality, without suffering unfavourable insufficient initial development, development density fluctuation and so forth. In addition, since the sheet is caused to be contacted with a sufficiently large amount of liquid at a time, the development is completed in quite a short period of time. This in turn permits a high-speed operation of the copying machine, as well as a reduced size of the same.

Wet type development using liquid developer is used most suitably for obtaining a copy of high image quality with a good gradation and spot characteristics, particularly a colour picture of high image quality.

Wet development, however, tends to suffer various troubles such as edge effects. In order to reduce the tendency for such defects to occur, it is necessary to position the developing electrode as close as possible to the latent image surface and to supply sufficiently large amount of developing liquid to the narrow space between the image surface and the developing electrode.

The arrangement of the developing unit of the invention described above well meets these demands and makes it possible to complete in quite a short period of several seconds the development of an image having continuous gradation which has taken a considerably longer time in the prior art systems.

Claims (10)

1. A colour copying apparatus for producing copies of a colour picture original from photosensitive paper provided on its surface with a photosensitive layer, the light sources at least for blue and green light being sources which emit light of narrow wavelength bands and are independent light sources and the conveying means for conveying the copy sheet comprise a plurality of pairs of rollers and the liquid development means for developing the exposed image are high-speed type liquid developing means.

2. A colour copying machine having an original holder adapted to slide on a frame, light source means adapted to emit lights for scanning the original surface, the light source means including a plurality of independent light sources for blue, green and red colour exposures, capable of emitting lights of narrow wavelength bands, being used as the light sources at least for blue and green colour exposures, means for conveying a photosensitive sheet in synchronization with the sliding of the original holder along a predetermined path, optical means for focussing the lights reflected by the original on three different points arranged along the path, electrostatic charging means and developing means disposed at the upstream and downstream sides of each focal point or region as viewed in the direction of movement of the photosensitive sheet, and fixing means for fixing the visualized image, so that colour-separated images are successively formed and developed on the photosensitive sheet and then fixed.

3. A colour copying apparatus for producing copies of a colour picture original including; a box-like frame; an original holder slidably mounted on the said box-like frame and adapted to hold the said original; light source means adapted to apply light to the said original held by the said original holder, the said light source means including independent light sources for emitting blue, green and red light; a plurality of slits corresponding to the independent light sources and adapted to permit the light from the said light sources to pass therethrough toward the said original; a lens having a picture angle within which the said slits are disposed; a photosensitive medium adapted to pass a plurality of focal points or regions where the light rays reflected by the said original and passed through the said slits are focussed by said lens; means for conveying the said photosensitive medium along a path on which the said focal points are arranged, in synchronism with the movement of the said original; a plurality of charging means disposed at the upstream sides of the respective focal points; a plurality of liquid developing means disposed at the downstream sides of the respective focal points; and fixing means for fixing the visible image produced as a result of the development, the said photosensitive medium being a sheet of photosensitive paper provided on its surface with a photosensitive layer; the said light sources at least for blue and green light being sources which emit light of narrow wavelength bands and being independent light sources, the said conveying means comprising a plurality of pairs of rollers and the said liquid development means being high-speed type liquid developing means.

4. A colour copying machine as claimed in Claim 1,2 or 3 in which the said light sources for blue and green light are metal halide lamps.

5. A colour copying machine as claimed in any one of Claims 1 to 4 in which the said charging means are adapted to charge the said photosensitive paper from both sides of the said paper simultaneously by corona discharges of different polarities.

6. Liquid developing means for a colour copying machine which includes a developing head having a planar developing electrode provided with a slit for discharging the said developing liquid, the said slit being formed in the portion of the said electrode near the inlet guide portion within the developing region; a hollow housing accommodating the said slit, and a developing liquid supply pipe opening to the inside of the said housing; and a photosensitive sheet supporting plate disposed in close proximity to the lower end of the said electrode, the said supporting plate being provided with a multiplicity of apertures at its portions other than the portion confronting the said electrode.

7. Liquid developing means as claimed in Claim 6, in which the said developing liquid supply pipe is provided at its portion not confronting the said slit with a plurality of holes for discharging the developing liquid.

8. Liquid developing means as claimed in Claim 6 substantially as specifically described herein with reference to Figure 2,3,4 and 5 or Figure 2,3,4, 5 and 7, or Figure 2,3,4, 5, and 6 or Figure 2, 3,4, 5, 6 and 7.

9. A colour copying machine as claimed in any one of Claims 1 to 5 in which the liquid developing means are as claimed in any one of Claims 6 to 8.

10. A colour means as claimed in Claim 3 substantially as specifically described herein with reference to Figurel,2,3,4andsorsand7,or5and6or5,Sand7.