Optics: measuring and testing – By shade or color
Reexamination Certificate
1999-11-24
2002-02-26
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By shade or color
C356S406000, C356S243500, C250S226000
Reexamination Certificate
active
06351308
ABSTRACT:
This disclosure relates to an improved spectrophotometer color measurement calibration system, particularly for an on-line color measurement system with a spectrophotometer in the output path of a color printer for measuring the colors on printed test sheets, wherein a fully automatic re-calibration system is provided for the spectrophotometer, at little or no additional cost or modification of the color printer, and without requiring any manual operations or operator involvement.
This automatic re-calibration system assists in the effectiveness of such an on-line color measurement system in which a spectrophotometer may be mounted in the paper path of the moving copy sheets in the printer, preferably the output path, without having to otherwise modify the printer, or interfere with or interrupt normal printing, or the movement of the printed sheets in said paper path, and yet provide accurate color measurements of test color patches printed on the moving sheets as they pass the spectrophotometer. That enables a complete closed loop color control of a printer.
In the specific disclosed embodiment below the respective (different output color) LED illuminators of the spectrophotometer are normally sequentially illuminating different color test patches on test sheets in the color printer output path as they pass the spectrophotometer, for detection by a photosensor of the reflections from the respective color test patches of those respective color illuminations. In this exemplary automatic re-calibration system embodiment there is a white tile test standard surface mounted opposite to the spectrophotometer position in the printer output path, i.e., on the other side of the printer output path. These same LED's of the spectrophotometer are sequentially activated in selected inter-sheet gaps (the spaces and times in between printed sheets), and the resultant spectrophotometer photosensor signal output for each activated LED is compared to stored values to provide calibration data for the spectrophotometer. This system allows for frequent automatic re-calibration without having to remove the spectrophotometer from the printer or perform any other manual operations with either the spectrophotometer or any test tiles or other white or color reflectance test surfaces.
However, color measurements, and/or the use of color measurements for various quality or consistency control functions, are also important for many other different technologies and applications, such as in the production of textiles, wallpaper, plastics, paint, inks, etc. This, the disclosed system may have applications in some of those other fields for on-line color testing where these materials or objects to be color tested and/or interspersed test sheets are also moving as sheets in a defined path with spaces therebetween. Thus, although the specific exemplary embodiment herein is of a preferred automatic re-calibration system for an on-line color printer color spectrophotometer, it will be appreciated that this exemplary re-calibration system is not limited to this specific exemplary spectrophotometer or its application as disclosed in this example.
By way of background, studies have demonstrated that humans are particularly sensitive to spatial color variations. Typical full color printing controls, as well as typical color controls in other commercial industries, still typically utilize manual off-line color testing and frequent manual color adjustments by skilled operators. Both the cost and the difficulty of on-line use of prior color measurement apparatus and control systems, and the need for manual re-calibration steps, has heretofore inhibited automation of many of such various commercial color testing and color adjustment systems. The disclosed system addresses both of those concerns.
As used in the patent claims and elsewhere herein unless otherwise specifically indicated, the term “spectrophotometer” may encompass a spectrophotometer, calorimeter, and densitometer, as broadly defined herein. That is, the word “spectrophotometer” is to be given the broadest possible definition and coverage in the claims herein, consistent with the rest of the claims themselves. The definitions or uses of terms vary or differ among various scientists and engineers. However, the following is an attempt to provide some simplified clarifications relating and distinguishing the respective terms “spectrophotometer”, “calorimeter”, and “densitometer”, as they may be used in the specific context of specification examples of providing components for an on-line color printer color correction system, but not as limitations.
A typical “spectrophotometer” measures the reflectance of an illuminated object of interest over many light wavelengths. Typical prior spectrophotometers in this context use 16 or 32 channels measuring from 400 nm to 700 nm or so, to cover the humanly visible color spectra or wavelength range. A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. (This is to measure more closely to what the human eye would see as a combined image of a broad white light spectra image reflectance, but the spectrophotometer desirably provides distinct electrical signals corresponding to the different levels of reflected light from the respective different illumination wavelength ranges or channels.)
A “colorimeter” normally has three illumination channels, red, green and blue. That is, generally, a “calorimeter” provides its three (red, green and blue or “RGB”) values as read by a light sensor or detector receiving reflected light from a color test surface sequentially illuminated with red, green and blue illuminators, such as three different color LED's or three lamps with three different color filters. It may thus be considered different from, or a limited special case of, a “spectrophotometer”, in that it provides output color information in the trichometric quantity known as RGB.
Trichometric quantities may be used for representing color in three coordinate space through some type of transformation. Other RGB conversions to “device independent color space” (i.e., RGB converted to conventional L*a*b*) typically use a color conversion “lookup table” system in a known manner. (Examples are provided in patents cited below, and elsewhere.)
A “densitometer” typically has only a single channel, and simply measures the amplitude of light reflectivity over a range of wavelengths, which may be wide or narrow. The output of the densitometer detector is programmed to give the optical density of the sample. A densitometer is basically “color blind”. For example, a cyan patch and magenta patch could have the same optical densities as seen by a densitometer, but, of course, are different colors.
A multiple LED's reflectance spectrophotometer, as in the example of the embodiment herein, may be considered to belong to a special case of spectrophotometers. (Others, with different respective illumination sources, can be flashed Xenon lamp spectrophotometers, or QH spectrophotometers.) It is a spectrophotometer programmed to give truer reflectance values by using more than 3 channel measurements (e.g., 10 or more channel measurements), with conversion algorithms. That is in contrast to normal calorimeters, which cannot give true, human eye related, reflectance spectra measurements, because they have insufficient measurements for that (only 3 measurements).
As noted, the type of spectrophotometer in the disclosed embodiment is a spectrophotometer especially suitable for being mounted in the printed sheets output path of a color printer to optically evaluate the output sheets as they move past the spectrophotometer. In particular, to measure a limited number of color test patch samples printed by the printer on actual printed sheet output of the printer during regular or selected printer operation intervals (between normal printing runs or print jobs). These color test sheet printing intervals may be at regular timed i
Evans F. L.
Xerox Corporation
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