Optics: measuring and testing – By shade or color
Reexamination Certificate
2000-03-23
2002-05-07
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By shade or color
C356S419000, C356S425000, C250S226000
Reexamination Certificate
active
06384918
ABSTRACT:
The subject disclosure relates to an improved color measurement apparatus and system. In particular, to a spectrophotometer which is more accurate and much less sensitive to variations in spacing between the spectrophotometer and the surface of the substrate being color evaluated by the spectrophotometer, by displacement insensitive optics, so that direct contact between the spectrophotometer and the test surface is not required. Yet, as disclosed, this may be provided at little or no additional cost, in a compact and low cost spectrophotometer. This improvement allows or provides non-contact color measurement systems in which the surface can be tested while it is moving, and/or with only limited restraints on the position or movement of the test surface in the direction towards or away from the spectrophotometer; and/or provides more accurate color measurements from different areas of uneven test surfaces.
This enables such an improved spectrophotometer to be part of a color measurement system in which it may be mounted in a color printer, preferably in the paper path of the moving copy sheets, without having to otherwise modify the printer, or interfere with or interrupt normal printing, and yet accurately provide measurements of colors printed on the moving copy sheets as they pass the spectrophotometer. The disclosed improved spectrophotometer is thus especially suitable for an on-line color control and correction systems for color printers, for measuring the colors of moving image substrates, especially print sheets printed with selected color test patches. It enables a complete closed loop color control of a printer. However, the present invention is not limited thereto. 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. 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 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, colorimeter, 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 “colorimeter” 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, which illuminate the target with narrow band or monochromatic light. (Others, with wide band illumination sources, can be flashed Xenon lamp spectrophotometers, or QH spectrophotometers.) It is a spectrophotometer programmed to give more detailed 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 3 channel colorimeters, which cannot give true, human eye related, reflectance spectra measurements, because they have insufficient measurements for that (only 3 measurements).
It is well known to use conventional color filters of different colors for each of respectively different color LED's, as in the exemplary spectrophotometer embodiment herein. It is well known to use such color filters to exclude secondary emissions from LED's, and/or to further narrow the output spectra of LED illumination sources. Such color filters are believed to be used for that purpose in some “Accuracy Microsensors” LED based commercial products, for example. Thus, it will be further appreciated by those skilled in this art that such color filters are not needed for LEDs with sufficiently narrow bandwidths or for those which do not have secondary emissions that need to be suppressed. Therefor, color filters need not be employed in other embodiments of the subject spectrophotometer.
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 imprinted 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 each machine “cycle-up”, or as otherwise directed by the system software.
It is, however, as disclosed in the embodiment herein, also particularly advantageous to provide dual-mode color test sheets/banner sheets, in which multiple color patches of different colors are printed on otherwise blank areas of each, or selected, banner, cover, or other inter-document or print job separator sh
Hubble, III Fred F.
Kubby Joel A.
Evans F. L.
Xerox Corporation
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