US3801197A

US3801197A - Color electrophotographic copying apparatus capable of color balance adjustment

Info

Publication number: US3801197A
Application number: US00247477A
Authority: US
Inventors: M Akiyama; I Yoshii
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1971-04-30
Filing date: 1972-04-25
Publication date: 1974-04-02
Anticipated expiration: 1991-04-02

Abstract

A color electrophotographic copying apparatus which is capable of adjusting color balance. With the apparatus, an original image to be copied is scanned and color-resolved, whereafter one of the color-resolved radiations is adjusted in color balance by a corresponding filter. The particular color-resolved radiation is then converted into an electrical signal, which is corrected and amplified for further adjustment of the color balance. Thereafter, the electrical signal is again converted into the form of radiation, which is scanned to form an electrostatic latent image on a photosensitive medium. The latent image is developed with a color toner corresponding thereto, and then transferred and fixed onto a transfer sheet. Such a cycle of operation is repeated for each of the resolved colors, and the resultant color images are superposed one upon another on the transfer sheet to thereby provide a final copy image with excellent color balance corresponding to the original image.

Description

United States Patent Akiyama et al.

Apr. 2, 1974 COLOR ELECTROPHOTOGRAPHIC COPYING APPARATUS CAPABLE OF COLOR BALANCE ADJUSTMENT Inventors: Mikio Akiyama, Yokohama; Isao Yoshii, Tokyo, both of Japan Filed: Apr. 25, 1972 Appl. No.: 247,477

Foreign Application Priority Data Apr. 30, 1971 Japan 46-29184 [52] US. Cl. 355/4, 355/38 [51] Int. Cl G03g 13/22 [58] Field of Search 355/38, 4

[56] References Cited UNITED STATES PATENTS 2,231,669 2/1941 Hall 178/52 2,947,810 8/1960 Horsley..... 178/6.7 3,467,468 9/1969 Johnson i 355/4 3,130,655 4/1964 Meyer 355/4 T i I 64 74 SIGNAL FOR A 7 GREEN W MAGENTA Primary ExaminerJohn M. Horan Attorney, Agent, or FirmFitzpatrick, Celia, Harper & Scinto A color electrophotographic. copying apparatus which is capable of adjusting color balance. With the apparatus, an original image to be copied is scanned and color-resolved, whereafte'r one of the color-resolved radiations is adjusted in color balance by a corresponding filter. The particular color-resolved radiation is then converted into an electrical signal, which is corrected and amplified for further adjustment of the color'balance. Thereafter, the electrical signal is again converted into the form of radiation, which is scanned to form an electrostatic latent image on a photosensitive medium. The latent image is developed with a color toner corresponding thereto, and then transferred and fixed onto a transfer sheet. Such a cycle of operation is repeated for each of the resolved colors, andthe resultant color images are superposed one upon another on the transfer sheet to thereby provide a final copy image with excellent color balance corresponding to the original image.

ABSTRACT 8- Claims, 3 Drawing Figures I A 5mm. Fo e SHADOW AMPLIFIER B PATENTEUAPR 2 I974 SHEET 1 BF 2 kwm m PATENTEOAPR 2l974 3801 197 SHEET 2 [IF 2 COLOR ELECTROPHOTOGRAPHIC COPYING APPARATUS CAPABLE OF COLOR BALANCE ADJUSTMENT BACKGROUND OF THE INVENTION An electrophotographic technique for providing color copies is disclosed, for example, in U.S. Pat. No. 2,986,466 to E. K. Kaprelian issued on May 30, 1961. According to this patent, photosensitive drums are arranged in response to various colors, and light from an original picture is passed through filters for the various colors so as to form latent images on the respective photosensitive drums. Thereafter, the latent images are developed with corresponding color toners. From the first to last photosensitive drums, the developed images are successively transferred onto a transfer sheet until the image transfer from the last drum is completed, whereupon there is provided a color copy image.

Other types of color electrophotography are shown as by U.S. Pat. No. 2,962,347 to J. Dessauer, issued on Nov. 29, 1960, and U.S. Pat. No. 3,060,019 to S.W.Johnson, issued on October 23, 1962.

All these prior techniques, however, are unsatisfactory in terms of the color reproducibility and the size of the apparatus.

SUMMARY OF THEINVENTION It is a primary object of the present invention to pro vide a color copying apparatus which is simple in structure and reliable in operation.

It is another object of the present invention to provide a color copying apparatus which can produce copy images with excellent color balance.

1t is still another object of the present invention to provide a color copying apparatus which permits the use of a photosensitive medium requiring no panchromatic sensitivity.

It is yet still another object of the present invention to provide a color copying apparatus which is compact and fully automatic as well as low in cost.

According to the present invention, an original image to be copied is scanned for color resolution and optically adjusted in color balance, whereafter the colorresolved light rays are converted into electrical signals, which, in turn, are corrected and amplified in accordance with the characteristics of the photosensitive medium and other factors. Such electrical signals are converted again into light rays and projected on the photosensitive medium to form an electrostatic latent image on the photosensitive medium. The electrophotographic processes advantageously available for the present invention may be those proposed in the applicants copending applications. The electrophotographic copying apparatus of the present invention is based on the process described in copending U.S. Pat.

Application Ser. Nos. 563,899 and 571,538 of the present applicant.

In the process of this invention, fundamentally speaking, the photosensitive plate comprises a base, a photoconductive layer on said base, and a translucent insulating layer on said photoconductive layer. First of all, the surface of the translucent insulating layer is charged (the primary charge), and by making the use of the field thereof, the charge layer is strongly trapped between the photoconductive layer and translucent insulating layer and in the immediate neighborhood thereof. Next, an alternating current corona discharge (the secondary corona discharge) of the polarity opposite to the primary charge, and the illumination of the original image, and carried out simultaneously. Then, light rays are uniformly irradiated on the whole surface of the translucent insulating layer to form the static image of high contrast in accordance with light and dark patterns of the original image on the surface of the translucent insulating layer.

Further, in the apparatus of the present invention, various process means of exposure, charging, development, transfer and cleaning are all based on those disclosed in U.S. Application, Ser. No. 585,091 filed Oct. 7, 1966 by the applicant which matured into U.S. Pat. No. 3,438,706 on Apr. 15, 1969.

The original image is illuminated and scanned by a regular polygonal scanning mirror. This scanning mirror is of the type commonly used in the art of highspeed cameras, but in the present invention it is sufficiently small in size to make full use of the space allotted for an optical system in a conventional copying apparatus. The reflected light from the original image so scanned is resolved into three colors by a dichroic mirror. If desired, the light may be resolved into more colors.

Wedge-shaped filters are movably disposed on't he optical path in a direction perpendicular thereto in order to adjust the color balance between the respective color-resolved light rays. The positions of the respective wedge-shaped filters may appropriately be determined on the paths of the respective color-resolved lights. The color-resolved lights so adjusted are passed to phototubes, which convert such lights into electrical signals, which in turn are suitably corrected and amplified through an amplifier and thereafter converted again into light rays by a discharge tube. The extent of such correctionand amplification is appropriately con trolled in accordance with the characteristic of the photosensitive medium, the densities of developing toners, etc. The light rays thus converted are again scanned and directed to the photosensitive medium, which thus produces an electrophotographic latent image thereon. Such electrostatic latent image is developed with color toner corresponding thereto, and transferred to a transfer sheet. Thereafter, the photosensitive medium is cleaned. The above-described operation takes place repetitively for each of the resolved colors, thus finally providing a color copy of the original image.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become fully apparent from the following detailed description of an embodiment thereof taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of the optical arrangement in the apparatus according to an embodiment of the present invention;

FIG. 2 illustrates the arrangement of various electrophotographic process systems incorporated in the apparatus of the present invention; and

FIG. 3 is a perspective view of a regular polygonal scanning mirror employed in the optical system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Description will now be made of a specific embodiment of the present copying apparatus in which a color original image is subjected to color separation and thereafter development is effected with four color toners such as cyan, magenta, yellow and black, thereby providing a color copy image.

Referring to FIG. 1 of the drawings, a color original to be copied is designated by P and which bears an original image facing downwardly and is movable at a predetermined velocity in a direction vertical with respect to the plane of the drawing sheet.

A pair of white light lamps l are provided for illuminating the image-bearing surface of the original P. A regular polygonal mirror 2 is disposed below the original P for rotation about its own center at a predetermined velocity in the direction as indicated by an arrow. The regular polygonal mirror is best shown in the perspective view of FIG. 3. A mirror system'is designated by 4. A dichroic mirror 34 serves to resolve incident light into three colors. A shaft-mirror 35 is provided to obtain an optical image to be masked with black toner.

Reflective mirrors

36, 37 and 38 are provided for changing the direction of the light paths respectively. Filters 3 are provided for color resolution and for color balance adjustment, the

filters

3,, 3 3 and 3 corresponding to cyan, magenta, yellow and black, respectively.

Further provided are slits 5 for cutting scattered light rays, condenser lenses 6, phototubes 7, a color reproduction correcting amplifier circuit 8, discharge tube 9, a condenser lens 10, and a regularpolygonal mirror 11 of the same shape as and rotatable synchronously with the aforesaid polygonal mirror 2. Instead of using such two

similar mirrors

2 and 11, a single such mirror may be used as a common one. A photosensitive medium 12 is shown as a rotatable drum type, but alternatively it may take the form of flat sheet or endless belt.

Referring to FIG. 2, there is shown an arrangement for carrying out a series of electrophotographic processes with respect to the photosensitive medium, which has an outermost layer 12' of dielectric material in accordance with the type, of the electrophotography described previously. The arrangement includes a primary Charger 13, means 14 for effecting exposure and discharge simultaneously, overall exposure means 15, and developing means 16 for various colors, say, 16, for cyan, 16 for magenta, 16 for yellow and 16 for black. The arrangement further includes an image transfer means 1'7 having pawl means 17 for retaining a transfer sheet 18 until all color images are transferred in a manner to be described. Where the toners in use are of the dry type, heat fixing means 21 may be provided. Cleaning means is generally designated by numeral 22.

In operation, the color original P is illuminated by the white light lamps 1 and moved in the aforesaid direction as the successive portions thereof are scanned laterally thereof (i.e. in the direction perpendicular to the direction of movement of the original P) by the successive reflection surface of the regular polygonal rotatable mirror 2, until the entire surface area of the original is scanned. During this scanning process, light reflected from the successive sides of the regular polygonal mirror 2 is directed to follow a path a so that the light is subjected to color resolution by the dichroic mirror 34. For further thorough color resolution and color balance adjustment, the light rays thus resolved are further passed through filters 3. For example, the cyan ray is passed through the filter 3 for cyan, whereafter the light ray b resulting from such secondary color resolution is passed through the slit 5, for cutting scattered light and through the condenser lens 6, to the phototube 7.

.Thus, the phototube 7 continuously produces a current corresponding to the intensity of the successive cyan rays resulting from the color resolution. This current is suitably corrected and amplified by the color reproduction correcting amplifier circuit 8 in accordance with various factors such as the reflection factor of the original, the characteristics of the illuminating lamps, filters, phototubes, photosensitive medium, toners, the efficiency of the image transfer, etc. Thereafter, the current is converted into a radiation c through the phototube 9. The emitted radiation is scanned by the regular polygonal mirror 1 l rotating synchronously with the regular polygonal mirror 2, and directed to the surface of the photosensitive medium 12 which is rotating synchronously with the movement of the original P while the various electrophotographic process means 13-15 are operative, whereby the photosensitive medium 12, as it passes the overall exposure means 15, produces an electrostatic latent image corresponding to the pattern of the cyan component contained in the picture of the color original P. Subsequently, the electrostatic latent image is developed by the developing means l6 containing a developer of cyanic color and then transferred onto the transfer sheet 18 at the transfer means 17.

The described series of processes may be successively repeated with respect to each of the other resolved colors, i.e. magenta, yellow and masking black by electrically and mechanically changing over the respective color resolving filters and developing the respective latent images with the associated color developing means (16 16 and 16., for magenta, yellow and black, respectively), thus successively providing developed images of the respective resolved colors on the photosensitive medium 12. These different color images are then successively transferred onto the transfer sheet which already carries the cyanic color pattern thereon, thereby providing a complete color print or copy.

The image transfer means herein used comprises a transfer roller 17 and a pawl 17, for retaining a transfer sheet wrapped around the transfer roller. The transfer roller 17 is rotated in synchronism with the movement of the original carriage with the pawl 17, retaining the transfer sheet thereto, and when the four cycles of transfer process have all been completed, the pawl 17, releases its retention to allow the transfer sheet to be discharged through outlet 20.

The heating means 21 serves to heat and fix the toners to the transfer sheet during each transfer cycle if the toners in use are dry ones. If the developer in use is in the form of liquid, the heating means 21 serves to quickly evaporate any carrier liquid and expedite the fixing and-drying of the toner on the transfer sheet.

Where the photosensitive medium in use is in a continuous form such as drum or endless belt, more clearly colored copies may be provided by dividing the imageforming peripheral surfaceof the photosensitive medium into sections corresponding to individual particular colors. More specifically, so long as the illustrated case, the outer peripheral surface of the drum type photosensitive medium 12 may be divided into four sections I to IV so as to correspond to the four colors,

i.e. cyan, magenta, yellow and masking blank, respectively. This will ensure a genuine monochromatic image to be formed in each divisional surface section of the photosensitive drum, thus resulting in a highly clear color copy image with the respective monochromatic images superposed one upon another.

Theoperation of the developing

means

16, 16 with respect to the corresponding' surface sections l IV of the photosensitive medium may be accomplished by a cam mechanismv The cleaning device 22 includes suction type cleaning means 22,- 22 which are operable with respect to the respective surface sections 1 IV to remove any residual developer of distinct colors from such surface sections after image transfer.

Further, in the above-described copying apparatus, enhanced color reproducibility is provided by elaborately designing the various color-resolving filters. More specifically, as shown, the color-resolving filters 3 -3., are wedge-shaped and positioned for movement perpendicularly to the optical path to thereby vary the rate of light transmitting therethrough so as to adjust the color balance of the resultant copy.

At the outset of a copy operation, a standard twocolor test sheet is first copied and the copy sheet thus obtained is juxtaposed with the said standard two-color test sheet on the original carriage of the copying apparatus to electro-optically compare and measure these two sheets with respect to the difference in color balance therebetween. Then, the wedge-shaped filters are automatically adjusted in position in accordance with the output of the measured difference in color balance until such difference becomes zero. Thus, the copying apparatus is now ready to continuously produce copies with high fidelity to the color tones of any color original.

As noted from the foregoing description, the present invention enables an operator to obtain electrophotographic color copies rapidly and automatically on ac count of a considerably simple and compact construction, and also enables the resolved color lights of a color original picture to be temporally converted into electrical signals which may be suitably corrected and amplified in accordance with various factors, such as the reflection factor of the original, the characteristics of the illuminating lamps, filters, phototubes, photosensitive medium, toners, the image transfer efficiency, etc., whereby the color balance of the resultant copies may be readily controlled by an electrical operation to provide color copies with excellent color reproducibility. Furthermore, the radiations emitted for scanning the photosensitive medium may be selected to any suitable wavelength in accordance with the sensitivity range of the photosensitive medium, and this leads to an advantage that the photosensitive medium to be used need not necessarily be panchromatic.-

Thus, according to the present invention, the light relfected from an original image is color-resolved, whereafter the resolved color lights are automatically adjusted in color balance by the wedge-shaped filters, then converted into electrical signals and amplified suitably. This permits the use of a photosensitive medium having a narrow sensitivity range which has heretofore been regarded as unsuitable, and also enables the color balance adjustment to be achieved with much readiness. In these respects, the color electrophotographic copying apparatus embodying the present invention is satisfactory for its intended purposes. Moreover, the copying apparatus of the present invention may find a usage as a simple printer which can not only copy original images but also can vary the color balance or equalize the density of the resultant copies.

It will be apparent to those skilled in the art from the specification, drawings and appended claims that the apparatus of the present invention is applicable for other various purposes than described above.

We claim:

1. A color electrophotographic copying apparatus for forming an electrostatic latent image on a photosensitive medium having photoconductivity, said apparatus comprising:

a. means for illuminating an original image;

b. means for scanning said original image to enable radiation reflected therefrom to be converted into electrical signals, and means for moving said original image, wherein said means for scanning the radiation reflected from said original image comprises a regular polygonal mirror having an axis of rotation extending in the direction of movement of said original image, said regular polygonal mirror being rotatable about said axis of rotation thereof synchronously with the movement of said original image;

c. means including wedge-shaped filter means disposed in the path of said radiation for colorresolving said radiation reflected from said scanned original image for providing a plurality of first separate color-resolved radiations;

(1. means for varying the position of said wedgeshaped filter means to adjust the densities of said color-resolved radiations for the adjustment of the color balance therebetween;

e. means for converting said colorresolved radiations into electrical signals and for amplifying such electrical signals and emitting them again in the form of second radiations for a suitable image reproduction on a photosensitive medium;

f. means for scanning said second radiations to direct them to a precharge photosensitive medium to thereby form electrostatic latent images thereon;

g. developing means for successively imparting color toners to said electrostatic latent images formed on said photosensitive medium by said color-resolved radiations, said color toners corresponding to said electrostatic latent images, respectively;

h. means for transferring said color-developed images to a transfer sheet;

i. means for cleaning any residual toner on said photosensitive medium; and

j. means for retaining said transfer sheet until completion of transferring all the color-resolved images by applying all said process means for each resolved color.

2. Apparatus according to claim 1 wherein said color-resolving means further comprises a dichroic mirror for providing said first separate radiations; and said converting means includes a plurality of light-receiving elements for generating said electrical signals representing said resolved colors, and means for successively selecting said generated signals for coupling to light emitting means for controlling said light emitting means to provide said second radiations.

3. Apparatus according to claim 2, wherein said wedge-shaped filter means comprises a plurality of wedge-shaped filters adjustably positioned between said dichroic mirror and said light-receiving elements to vary the transmissibility of said first color-resolved radiations.

4. Apparatus according to claim 3, wherein an original image having various primary color tonesand a copy thereof are juxtaposed on an original carriage; means are provided for generating input signals from said original image and said copy and for comparing said signals; and means are provided for controlling adjustment of said wedge-shaped filters in response to said comparison until the difference between said input signals becomes zero.

5. A color electrophotographic copying apparatus for forming an electrostatic latent image on a photosensitive medium having photoconductivity, said apparatus comprising: i

a. means for illuminating an original image;

b. means for scanning said original image to enable radiation reflected therefrom to be converted into electrical signals, and means for moving said original and said photosensitive medium synchronously with each other;

. means including wedge-shaped filter means disposed in the path of said radiation for colorresolving said radiation reflected from said scanned original image for providing a plurality of first separate color-resolved radiations; 1

d. means for varying the position of said wedgeshaped filter means to adjust the densities of said color-resolved radiations for the adjustment of the color balance therebetween;

e. means for converting said color-resolved radiations into electrical signals and for amplifying such electrical signals and emitting them again in the form of second radiations for a suitable image reproduction on a photosensitive medium;

f. means for scanning said second radiations to direct them to a precharged photosensitive medium to thereby'form electrostatic latent images thereon, wherein said means for scanning the radiation reflected from said original image and said means for scanning and directing said color-resolved radiations to said photosensitive medium are provided by polygonal mirror means rotatable synchronously with the movement of said originalimage and of said photosensitive medium;

h. means for transferring said color-developed images to a transfer sheet;

. means for cleaning any residual toner on said photosensitive medium; and

. j. means for retaining said transfer sheet until completion of transferring all the color-resolved images by applying all said process means for each' resolved color.

6. Apparatus according to claim 5, wherein said color-resolving means further comprises a dichroic mirror for providing said separate radiations and means are provided wherein said electrical signals representing said resolved colors are successively selected upon completion of a cycle of operation so that such signals are successively emitted in the form of radiations of the respective colors.

7. Apparatus according to claim 6, wherein said wedge-shaped filter means comprises a plurality of wedge-shaped filters adjustably positioned perpendicular to the optical paths of said separate radiations from said dichroic mirror so as to vary the transmissibility of said color-resolved radiations.

8. Apparatus according to claim 7, wherein an original image having various primary color tones and a copy thereof are juxtaposed on an original carriage; means are provided for generating input signals from said original image and said copy and for comparing said signals; and means are provided for controlling adjustment of said wedge-shaped filters in response to said comparison until the difference between said input signals becomes zero.

; own-1n s'm'ncs lAiiCN'? wrun KTIFICATE OF CORRECTION patent 3,801,197 Dated April 2 1974 w fl A Mikio yama and Isa n Vnnhii It is certified that error appears in the above-identified patent and .that said Letters Patent are hereby corrected as shown below:

at Column 3, line 3'3,

the term "shaft-mirror" should read "half-mirror".

Signed and sealed this 1st day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR.

c; Qm-mLL DANN Attesting Officer t w Commissioner of Patents FORM Pro-1050 0- um'ncn s'm'ncs alum-1m.mwugu QERTIFXCATE OF CORRECTION Patent x0. 1,197 Dated pril 2, 1974 Inventor-(S) Mikio yama and Isa Vnclq II 11 .-a.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

at I I column 3, line 33, the term "shaft-mirror" should read "half-mirror".

Signed and sealed this 1st day of October 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. c- MARSHALL DANN Attesting Officer Commissioner of Patents USCCMM-DC 6G376-PG9 0.5. GOVIINHKN! FRINTING OPTIC! \NS/ 0-1436-154

Claims

1. A color electrophotographic copying apparatus for forming an electrostatic latent image on a photosensitive medium having photoconductivity, said apparatus comprising: a. means for illuMinating an original image; b. means for scanning said original image to enable radiation reflected therefrom to be converted into electrical signals, and means for moving said original image, wherein said means for scanning the radiation reflected from said original image comprises a regular polygonal mirror having an axis of rotation extending in the direction of movement of said original image, said regular polygonal mirror being rotatable about said axis of rotation thereof synchronously with the movement of said original image; c. means including wedge-shaped filter means disposed in the path of said radiation for color-resolving said radiation reflected from said scanned original image for providing a plurality of first separate color-resolved radiations; d. means for varying the position of said wedge-shaped filter means to adjust the densities of said color-resolved radiations for the adjustment of the color balance therebetween; e. means for converting said color-resolved radiations into electrical signals and for amplifying such electrical signals and emitting them again in the form of second radiations for a suitable image reproduction on a photosensitive medium; f. means for scanning said second radiations to direct them to a precharge photosensitive medium to thereby form electrostatic latent images thereon; g. developing means for successively imparting color toners to said electrostatic latent images formed on said photosensitive medium by said color-resolved radiations, said color toners corresponding to said electrostatic latent images, respectively; h. means for transferring said color-developed images to a transfer sheet; i. means for cleaning any residual toner on said photosensitive medium; and j. means for retaining said transfer sheet until completion of transferring all the color-resolved images by applying all said process means for each resolved color.

4. Apparatus according to claim 3, wherein an original image having various primary color tones and a copy thereof are juxtaposed on an original carriage; means are provided for generating input signals from said original image and said copy and for comparing said signals; and means are provided for controlling adjustment of said wedge-shaped filters in response to said comparison until the difference between said input signals becomes zero.

5. A color electrophotographic copying apparatus for forming an electrostatic latent image on a photosensitive medium having photoconductivity, said apparatus comprising: a. means for illuminating an original image; b. means for scanning said original image to enable radiation reflected therefrom to be converted into electrical signals, and means for moving said original and said photosensitive medium synchronously with each other; c. means including wedge-shaped filter means disposed in the path of said radiation for color-resolving said radiation reflected from said scanned original image for providing a plurality of first separate color-resolved radiations; d. means for varying the position of said wedge-shaped filter means to adjust the densities of said color-resolved radiations for the adjustment of the color balance therebetween; e. means for converting said color-resolved radiations into electrical signals and for amplifying such electrical signals and emitting them again in the form of second radiations for a suitable image reproduction on a photosensitive medium; f. means for scanning said second radiations to direct them to a precharged photosensitive medium to thereby form electrostatic latent images thereon, wherein said means for scanning the radiation reflected from said original image and said means for scanning and directing said color-resolved radiations to said photosensitive medium are provided by polygonal mirror means rotatable synchronously with the movement of said original image and of said photosensitive medium; g. developing means for successively imparting color toners to said electrostatic latent images formed on said photosensitive medium by said color-resolved radiations, said color toners corresponding to said electrostatic latent images, respectively; h. means for transferring said color-developed images to a transfer sheet; i. means for cleaning any residual toner on said photosensitive medium; and j. means for retaining said transfer sheet until completion of transferring all the color-resolved images by applying all said process means for each resolved color.