This invention pertains to electrophotographic printers and, more particularly, to the calibration in real time of a marking engine in a print system. Electrophotographic print engines have been utilized with both printers and copiers. In a printer, the print engine is typically interfaced with a computer to select and organize fonts or bit map the images. In a copier application, the print engine is interfaced with an input device that scans the image onto the photoconductor drum of the print engine. However, a CCD device could also be utilized in this application in the form of a CCD scanner. In either of the applications, a conventional print engine for a monochrome process would typically feed a single sheet of paper and pass it by the photoconductor drum for an image transfer process and then pass it to a fuser. Thereafter, the completed sheet will be output. Multiple copy print jobs will sequentially feed the paper in a serial manner. The speed of the printer is a function of the speed at which the image can be created, the speed at which the image can be transferred to the paper and the speed of the fuser. As increased output is required, the speed of each of these elements must be increased.
In a monochrome process, only one transfer operation is required. However, in a multipass color process, multiple images must be superimposed on one another on the sheet of paper in a direct transfer system, thus requiring multiple passes of the paper or image carrier through the print engine. In a double transfer system, the image is disposed on an intermediate drum and then the composite image transferred to the paper or image carrier. In a multiple print job on a direct transfer system, this requires each sheet of paper to be printed in a serial manner by passing it through the print engine. For either the monochrome process or the color process, a conventional serial feed print engine has the output thereof defined by the speed of the input device and the speed of the print engine itself.
One technique that has been utilized to increase throughput is a tandem print engine. In a tandem print engine, multiple colors can be disposed on the sheet of paper or the image carrier at different stations that are disposed in serial configuration. In this manner, the speed is the same for one, two, three or four color printing.
When dealing with multiple print engines, there can be a problem that exists with respect to insuring that there is adequate “color balance.” In general, all color devices have a native range of colors in which they operate. This is called the color gamut of that device. Any color that falls within this gamut can be reproduced. Any color that falls outside cannot. This gamut is defined by the hardware of the device and its addressability, and the colorants used. A monitor uses a phosphorus of some given type and is addressed in 8 bits per channel of RGB. This native gamut or range of colors changes for every different device. If it is desirable to reproduce a color on some devices, two things have to occur. First, those devices would have to be able to make that color, meaning, have that given color inside their gamuts. Second, the color would have to be correctly described, or defined as it moves from one device to another. RGB and CMYK are color spaces that devices utilize to define colors. They do not always have a direct translation between them, because they are device dependent. A method is needed to correctly translate between these methods. The analogy is as if one person would speak German and another spoke French, wherein an intermediate or interpreter would be required in order to provide communication.
One method for solving this problem is to use a device independent (or color independent) space. A number of years ago, the CIR created device independent color spaces such as XYZ, Lab, Luv that define color based on the light source they are viewed under, and the color response of the eye. A color independent space is a mathematical way to map device gamuts to see where they intersect. Where they intersect represents the colors they share. It is also the best platform for determining which color to use if gamuts do not intersect. If one of these color spaces is used as a master color space, all colors would be described or defined using the same terms, independent of any device. In this space, all colors are brought to a common ground. Once a color is defined in XYZ space, it can be sent and accurately reproduced on any device whose gamut in XYZ space includes that color. The reproduction of any color is accomplished by correlating the device native gamut to the color independent space.
During a conventional print operation, toner is used up at a rate that is actually defined by the amount of information that is disposed on the given page multiplied by the number of pages. Typically, systems incorporate some type of page counter that, when it exceeds a predetermined number of pages, indicates that the toner is low. This, of course, is reset when a new toner cartridge is disposed in the printer. However, this toner decision is made strictly based upon the number of pages and not the amount of toner actually depleted from the toner cartridge. This is due to the fact that some pages have a very light toner usage compared to others. For example, an image having a large percentage of black area associated therewith will utilize a large amount of toner, whereas a page having very light grey regions will utilize a small amount of toner. As such, the determination of a low toner level in a cartridge is extremely inaccurate.
Other approaches to toner depletion management include the use of toner sensors which sample the amount of toner remaining in the toner cartridge and add toner accordingly. These sampling systems can be based on either optical or magnetic inductance sensing depending on the developer and toner system used. At issue with these methods is the lag time experienced between sensing low toner and the eventual replenishment and proper charging of the replenished toner. During this lag period, reduced image optical density and in the case of color printing engines, color shifts can occur.
While a print job is in progress a single marking engine or a cluster of marking engines may vary in its ability to print with sufficient and consistent ink or pigment application to the imaging substrate materials. The rendered output may show differences in its appearances from the original output sample in such parameters as grey balance, linearity, density level, hue, saturation, lightness, brightness, or contrast. This variation may occur in both grey scale or color marking engines of single or multiple bit depth as well as a cluster of the same or differing engine types. In conventional printing devices, the operator or user, in order to maintain a rendering comparable to the original output sample, must stop or interrupt the print job in progress and make corrections before resuming the print job.