Optics: measuring and testing – By shade or color
Reexamination Certificate
2002-02-28
2004-08-31
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By shade or color
Reexamination Certificate
active
06784994
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to color printing and copying, and more particularly to accurately reproducing and measuring color regardless of the substrate on which it is produced.
BACKGROUND OF THE INVENTION
In order to reproduce color prints such as for printing, photography or copying, the spectrum of light that emerges from the printed colors is determined. The print is illuminated and the reflected light emerging therefrom is detected. A portion of the light directed to the surfaces of the print is absorbed and a certain amount of the light is transmitted through the colorant and reflected back through the colorant by the substrate. In the prior art, measurements are made of the spectral distribution of the illuminant (or more precisely of the reflection from an unprinted substrate), and the spectral reflectance of the printed substrate. The difference is assumed to be due to absorption by the colorant on the substrate.
However, in addition to the effect of the color of the substrate, it is known that the final apparent color of a print also depends on other characteristics of the substrate on which the color is printed or spread. This is true whether the color data for printing is computer generated or generated by scanning a physical image. Thus, when copies are made on both glossy and matte substrates, with the same colorant thickness, their apparent color is different.
Another problem in the printing field is matching the OD of a printed color with some desired value of color. In general, as the color saturation increases reflective methods become less sensitive and less accurate.
FIG. 1
illustrates a standard apparatus and methodology for measuring colors printed on a sheet. A light source
12
illuminates a sheet having a colored layer
14
printed on a sheet
16
at some angle to the normal to the sheet. A detector
18
which views the surface generally from a direction normal to the surface, receives light which passes through layer
14
and which is diffusely reflected
13
from the surface of sheet
16
. Light source
12
is set at an angle so as to avoid specular reflection from the surface of color layer from affecting the color measurement. Diffuse reflection
15
from the surface of color layer
14
, does affect the measurement. However, this measurement of the diffuse reflection mimics the apparent optical density seen by an observer, since the observer also views this diffusely reflected light. A series of filters is used to separate the color reaching the detector into spectral components, which breakdown is used to determine the apparent OD of each of the process colors required to reproduce the color or to enable preparation of a specially mixed color.
When the OD of a single patch of process or specially mixed color is being measured, a series of filters are sequentially placed between sheet
16
and detector
18
. Each of these filters corresponds to one of the process colors and selectively passes the spectral band absorbed by that process color. The identity of the process color being tested can be determined from the filter which gives the lowest output for detector
18
. The OD is determined from the amplitude of the light which reaches the detector with the color filter associated with the particular process color. Here again, the effects of specular reflection from the colored layer is avoided, but there is an effect of the diffuse reflection therefrom on the measurement. Of course, if the identity color being measured is known, a priori, as in an in-line densitometer, the measurement may be made immediately with the correct filter.
SUMMARY OF THE INVENTION
An aspect of some preferred embodiments of the invention is related to more accurate production of printed images, independently of the gloss of the substrate on which the images are being printed.
An aspect of some preferred embodiments of the invention is related to more accurate copying of printed images, while reducing the effect of gloss from the color accuracy.
An aspect of some preferred embodiments of the invention relates to compensation for the characteristics of the surface being printed upon.
In general, if the apparatus of
FIG. 1
is used to measure the color spectrum of the colors of the printed surface and this measured spectrum is used to compute the percentage of coverage of primary colors for printing, or the color components used to mix a special color, the printed image will have a somewhat different color than that of the original image. Furthermore, this effect will depend on the finish of the master image and of the copy, and may exist even if the master and the copy have the same finish.
In accordance with a preferred embodiment of the invention, the effect of diffuse reflection from the surface of a color layer is separated from the effect of light that is diffusely reflected from the substrate after passing the color layer by which the color is printed or otherwise formed. The two components are preferably separately taken into account for both measurement purposes and for computation of the amounts of color that are to be printed (either as process colors or as color components of a special colorant), to achieve a required apparent color and optical density (OD).
In a preferred embodiment of the invention, a value “S” is determined that is dependent primarily on the gloss of the print. This factor corresponds to the diffuse light that is reflected from the surface of the color layer. In general it can be considered to be equal to the ratio of the light measured from the printed substrate and the light measured from the underlying unprinted substrate. The glossier the print (often directly related to the gloss of the underlying substrate) the smaller S, since for glossy prints the specular reflection is high, but the diffuse reflection is low. The gloss of the print closely relates to the gloss of the substrate because of the relative thinness of the printed ink, especially for liquid inks and toners.
In a preferred embodiment of the present invention, to determine S, any color is thickly printed on a substrate. Preferably, the ink thickness is such that the diffused scatter from the surface of the printed color, in the wavelength band in which the color absorbs light, is much larger than the light that passes through the color layer, strikes the substrate and is reflected back to the detector or light sensor. A filter, which limits the light measured by the detector to that portion of the spectrum that is absorbed by the color layer, is placed in front of the detector, which in turn, determines the apparent OD.
With no light passing, within the band of the filter, passing through the colorant, the light measured by the detector is substantially only the light that is diffusely reflected from the surface of the colorant. The value S, which can be expected to be the same over the entire spectrum, is calculated from the inverse logarithm to the base
10
of the OD measured through the filter. Even though the scatter is measured only over a limited wavelength band, the value achieved may be assumed to be constant over the entire visible region, since the same scatter mechanism is operative over the entire visible spectrum.
With knowledge of the value of S for prints on the particular substrate, the measured spectrum (or the spectrum computed for a computer image) can be corrected to determine which portion of the desired apparent spectrum must be supplied by light that passes twice through the color layer. Since S will be supplied by the scatter from the surface, the amount to be supplied by the light that passes through the printed colors can be calculated. This correction will apply to whether the color is being reproduced with a series of halftone process color separations or with a single specially formulated colorant.
An aspect of some preferred embodiments of the invention is related to more accurate measurement of color OD of printed substrates.
An aspect of some preferred embodiments of the invention is related to the more accurate det
Kritchman Eli
Shuster Michael
Steinhauer Jeffrey
Evans F. L.
Fenster & Company
Hewlett-Packard Indigo B.V.
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