The quality or acceptability of a color copy is a function on how the human eye and mind receives and perceives the colors of the original and compares it to the colors of the copy. The human eye has three color receptors that sense red light, green light, and blue light. These colors are known as the three primary colors of light. These colors can be reproduced by one of two methods, additive color mixing and subtractive color mixing, depending on the way the colored object emits or reflects light.
In the method of additive color mixing, light of the three primary colors is projected onto a white screen and mixed together to create various colors. A well known exemplary device that uses the additive color method is the color television. In the subtractive color method, colors are created from the three colors yellow, magenta and cyan, that are complementary to the three primary colors. The method involves progressively subtracting light from white light. Examples of subtractive color mixing are color photography and color printing. Also, it has been found that electrophotographic printing machines are capable of building up a full subtractive color image from cyan, magenta, yellow and black. They can produce a subtractive color image by one of three methods. One method is to transfer the developed image of each color on an intermediary, such as a belt or drum, then transferring all the images superimposed on each other on a sheet of copy paper. A second method involves developing and transferring an image onto a sheet of copy paper, then superimposing a second and subsequent images onto the same sheet of copy paper. The third method which will be discussed, infra, in detail involves superimposing developed images on each other on the same photoconductive surface.
In a typical, monochrome electrophotographic printing machine, the surface of a rotating belt or drum is electrically charged; the surface is selectively discharged by light from an original document to be copied to record a charge pattern corresponding to the original document; toner is electrically attracted to the charge pattern; toner is transferred from the charge pattern to the sheet of copy paper; the toner is permanently fused to the sheet of copy paper; and the remaining toner is cleaned from the photoconductive surface.
Color copies can be produced by repeating the monochrome electrophotographic printing machine process for different colors. This can be accomplished by using four development stations containing cyan, magenta, yellow and black toners. A subtractive color image can be produced by utilizing the Recharge, Expose, and Develop (READ) process. In this process, the light reflected from the original is first converted into an electrical signal by a raster input scanner (RIS), subjected to image processing, then reconverted into a light, pixel by pixel, by a raster output scanner (ROS) which exposes the charged photoconductive surface to record a latent image thereon corresponding to the substractive color of one of the colors of the appropriately colored toner particles at a first development station. The photoconductive surface with the developed image thereon is recharged and re-exposed to record a latent image thereon corresponding to the subtractive primary of another color of the original. This latent image is developed with appropriately colored toner. This process (READ) is repeated until all the different color toner layers are deposited in superimposed registration with one another on the photoconductive surface. The multi-layered toner image is transferred from the photoconductive surface to a sheet of copy paper. Thereafter, the toner image is fused to the sheet of copy paper to form a color copy of the original.
In a monochrome electrophotographic printing process, suitable controls maintain a substantially constant relationship between exposure and developed mass per area of photoconductive surface. Multi-pass/multi-transfer color systems have this same property since the photoconductive surface is cleaned between passes, i.e. between successive exposures. In the REaD color process, the photoconductive surface is not cleaned between exposure steps. It has been found that between successive exposures, the amount of developed mass of toner on the photoconductive surface for a selected exposure level is a function of the amount of toner previously developed on the photoconductive surface. Basically, three factors contribute to the amount of developed mass of toner on the photoconductive surface. First, the toner backscatters or absorbs some of the incident light, thus decreasing exposure at the photoconductive surface. Second, the voltage drop across the developed toner layer cannot be photo-discharged, and, thus, detracting from the total amount of voltage available for development. Third, the dielectric thickness of the previously developed toner reduces the amount of charge which can be deposited before the electric field driving the development process collapses to zero.
FIG. 9 shows graphs of developed mass per area (DMA) as a function of position for line screens of various duty cycles of an electrophotographic printer utilizing REaD process color employing a ROS having the beam intensity set to optimize line screen performance on a bare photoreceptor. The intensity of the ROS beam is set in such a way that images deposited on a previously bare photoreceptor (left hand column) are relatively "faithful"--i.e., the duty cycle of the developed lines is similar to the duty cycle of the ROS beam. However, the same beam intensity on a previously developed area produces lines which are less well developed and narrower. The ultimate result of this effect is a loss in color gamut because the second toner layer can never fully develop.
FIG. 10 also shows graphs of DMA as a function of position for line screens of various duty cycles for an electrographic printer utilizing REaD process color employing a ROS having the beam intensity set to optimize performance on a previously toned photoreceptor. By increasing the beam intensity over what was used in FIG. 9, one can recover, approximately, both the height and width of the developed lines. The color gamut is now restored, but imaging on a bare photoreceptor is no longer faithful. The lines are broadened by the increased beam intensity resulting in premature saturation. This results in a loss of halftone levels which can result in a harsh looking image, Moire' patterns or even contours which are visible shifts in color between adjacent stops in the halftone screen. FIG. 11 is a summary of FIGS. 9 and 10 in the form of DMA vs. duty cycle curves. FIG. 11 shows the effects of a ROS beam intensity in a REaD process color set to optimize line screen performance on a bare photoreceptor and when a ROS beam intensity is set to optimize performance on a previously toned photoreceptor.
Various techniques for reproducing color documents and correcting color images have hereinbefore been devised as illustrated by the following disclosures, which may be relevant to certain aspects of the present invention:
U.S. Pat. No. 4,236,809 Patentee: Kermisch issued: Dec. 2, 1980 PA1 U.S. Pat. No. 4,403,848 Patentee: Snelling issued: Sep. 13, 1983 PA1 U.S. Pat. No. 4,599,285 Patentee: Haneda et al. issued: Jul. 8, 1986 PA1 U.S. Pat. No. 4,679,929 Patentee: Haneda et al. issued: Jul. 14, 1987 PA1 U.S. Pat. No. 4,791,455 Patentee: Yamamoto et al. issued: Dec. 13, 1988 PA1 U.S. Pat. No. 4,809,038 Patentee: Yamamoto et al. issued: Feb. 28, 1989 PA1 U.S. Pat. No. 4,833,503 Patentee: Snelling issued: May 23, 1989 PA1 U.S. Pat. No. 4,839,722 Patentee: Barry et al. issued: Jun. 13, 1989 PA1 U.S. Pat. No. 4,927,724 Patentee: Yamamoto et al. issued: May 22, 1990 PA1 U.S. Pat. No. 4,929,978 Patentee: Kanamori et al. issued: May 29, 1990 PA1 U.S. Pat. No. 4,941,003 Patentee: Takeada et al. issued: Jul. 10, 1990 PA1 U.S. Pat. No. 4,949,125 Patentee: Yamamoto et al. issued: Aug. 14, 1990 PA1 U.S. Pat. No. 4,953,012 Patentee: Abe issued: Aug. 28, 1990 PA1 U.S. Pat. No. 5,023,632 Patentee: Yamamoto et al. issued: Jun. 11, 1991 PA1 U.S. Pat. No. 5,066,989 Patentee: Yamamoto issued: Nov. 19, 1991 PA1 U.S. Pat. No. 5,079,115 Patentee: Takashima et al. issued: Jan. 7, 1992
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. 4,236,809 discloses a method in which an optical latent image is corrected for tone or color in real time by a parallel raster exposure arrangement. A raster input scanner (RIS) generates electrical raster image signals representative of the original. A processor converts electrical correction signals according to prescribed scheme, i.e., in parallel or additive mode from the raster image signals. A raster output scanner (ROS) generates a raster latent image generated in registration with the optical latent image in response to the electrical correction signals.
U.S. Pat. Nos. 4,403,848, 4,599,285, 4,679,929, 4,791,455, 4,809,038, 4,833,504, 4,927,724, 4,941,003, 4,949,125, 5,023,632, 5,066,989 and 5,079,155 discloses various methods of forming color copies, where a first image is formed and developed on a photoconductive surface, the steps above are repeated to superimpose a plurality of toner images on the photoconductive surface, and the toner images is transferred on a copy sheet by one step.
U.S. Pat. No. 4,839,722 discloses a color correction apparatus for a color copier, where the apparatus utilizes a 3-dimensional look-up table of pigment density values addressed by primary color values. The look-up table is created by printing a plurality of pigment bars in response to known input density signals to a laser beam for each of three color pigments used in the system.
U.S. Pat. No. 4,929,978 discloses a color correction method for color copier where a set of color patches of respectively different sample colors is printed using a set of printing data values, the color patches are then scanned and analyzed to obtain color patch input data values by the color copier, and each of all of the possible input color data values that can be produce by the scanner/analyzer section of the color copier is then related to one of the color patch input data values which is closest thereto in a 3-dimensional color space.
U.S. Pat. No. 4,953,012 discloses an image processing system which can produce a color image by developing the image on a photoconductive surface and transferring an image onto a sheet of copy paper, then superimposing a second and subsequent images onto the same sheet of copy paper. A processor processes the image signal input by the first input device to reproduce a color image from the color components and includes a half-tone processing section for half-tone processing of the image signal. A discriminator discriminates the presence of a specific color component, half-tone portion, and line image portions in the image signal in accordance with the result of discrimination of the specific color component. A selector selects a predetermined sequence for processing by the processor of the image signal having the specific color component to reproduce a color image with the specific color component in accordance with the result of discrimination by said discrimination means.