This invention relates to printers in general and in particular to tuning a printer by printing patterns which beat against a spatial frequency of components within the printer.
Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume printing press to produce an example of an intended image. These intended images may require several corrections and be reproduced several times to satisfy customers"" requirements, which results in loss of profits. By utilizing pre-press color proofing, time and money can be saved.
One such commercially available image-forming apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708, is an image forming apparatus having half-tone color proofing capabilities. This image forming apparatus is arranged to form an intended image on a sheet of thermal print media by transferring colorant from a sheet of colorant donor material to the thermal print media by applying a sufficient amount of thermal energy to the colorant donor material to form an intended image. This image forming apparatus is comprised generally of a material supply assembly or carousel; lathe bed scanning subsystem, which includes a lathe bed scanning support frame, translation drive, translation stage member, printhead, and imaging drum; and thermal print media and colorant donor material exit transports.
A printer is comprised of numerous subsystems each of which have spatial harmonics, which may cause errors in printing. For example, the drum on which a donor and receiver rest rotates at a certain frequency. A motor, which drives a lead screw on which a printhead is mounted, rotates at a certain frequency. These harmonic frequencies, which are innate to the components of the printer, may cause undesirable banding or artifacts in the image which is printed.
There are many examples of using two frequencies beating against each other to measure time, frequency, and distance. For example:
A Vernier beats two scales against each other. A micrometer uses this to make a finer measurement than either of the two scales alone could produce.
Wagon wheels on television beat against the frame rate. If they match the frame rate they appear to be stopped even though the wagon is moving.
A stroboscope flashes a light at a constant rate. If the item of interest appears stopped it is moving at a multiple rate of the strobe light.
A musical tuner may use a strobing light driven by an input audio frequency with a spinning wheel encoded with a pattern that represents the beat frequency of different musical notes. When the input tone frequency matches the frequency of the pattern on the wheel, the wheel appears to be stopped. If the tone is flat or sharp the pattern walks one way or the other. The direction of the walking pattern indicates flatness or sharpness of the musical note.
A strobe light on the side of a record player indicates that the platter is spinning at the correct speed by beating against the pattern on the platter. If the platter is too fast or too slow the pattern on the platter will walk one way or the other.
Amplitude Modulation beats a carrier frequency against a signal to demodulate the radio signal to audible frequencies.
Printed test targets which are used to calibrate printers do not purposely beat against the spatial harmonics of the printer. Graphic Arts Technical Foundation, GATF, publishes a number of digital test targets. GATF Digital Test Form contains color patches, Modulation Transfer Function (MTF) Targets, text, halftone tints, resolution targets, and checkerboards and lines composed of discrete numbers of pixels. xe2x80x9cGAFT Process Controls Product Catalogxe2x80x9d, 1999, Graphics Art Technology Foundation, 200 Deer Run Road, Sewickley, Pa., 15413-2600. The GATF Systems of Merit Digital Plate Control Target, FIG. 3, consists of checkerboards composed of discrete numbers of pixels, lines composed using discrete numbers of pixels, MTF Targets, and halftone tints with and without dot gain compensation. Similarly the Rochester Institute of Technology Digital Output Resolution Target Version 2.2, FIG. 4, contains horizontal lines, vertical lines, and squares composed of discrete numbers of pixels, along with a spoke wheel MTF target. While all of these test targets measure printer performance, they do not identify the source or component within the printer that is responsible for poor performance.
Briefly, according to one aspect of the present invention a method of tuning a printer by printing a pattern, which beats against a component within the printer comprises designing a first pattern, which has a spatial frequency, which is different from a first component spatial frequency. A first pattern is printed and the resultant banding is measured. To minimize banding a characteristic of the first component is adjusted.
Each printer has numerous subsystems which have spatial harmonics which may contribute to errors in the print. This invention is the technique of perturbing the printer, imaging a test pattern or image that is designed to beat against the spatial harmonic of interest, and using the results to tune the subsystem or select the settings of the printer to reduce the artifact resulting in a better printing system. The principle is the same as that used in a micrometer to measure small distances. Here the printer images a pattern which beats against a component of the printer resulting in a low frequency beat pattern which is visible to the eye.
The present invention uses this technique to adjust the angle of the printhead and to optimize the balance amongst multiple writing channels. The present invention also uses this technique to judge the quality of printer translation subsystems. The present invention also is used to measure printer drum flutter at less than 0.05%. Finally, the present invention is used to select the number of channels to image a given halftone line screen.
The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below.
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