Optics: measuring and testing – By shade or color – Tristimulus examination
Reexamination Certificate
2001-11-01
2003-12-30
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By shade or color
Tristimulus examination
C356S073000
Reexamination Certificate
active
06671050
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention concerns a color measuring method and device for printed matter, and in particular, concerns a color measuring method and device by which the differences in measurement results according to light source can be corrected. In many cases in the printing industry, the color of a specific part of each individual printed matter must be ascertained as an objective, numerical value. Various color measuring methods for printed matter have thus been known conventionally and various color measuring devices are actually being used.
In general, the color of an object is determined by the spectral reflectance of the object, the spectral intensity distribution of the light that illuminates the object, and the spectral sensitivity distribution of the visual system of the human being who is observing the object. A spectral reflectance that is measured for an object will thus be an important set of objective data that indicate the color of the object. However, the color of an object is also strongly affected by the spectral intensity distribution of the light that illuminates the object, in other words, by the spectral intensity distribution of the light source. Thus, for example, under a light source that contains a high amount of a red color component, the color of the object will be observed to be reddish. Therefore, the spectral intensity distribution of the light source is normally taken into consideration in measuring a spectral reflectance of an object.
To be more specific, the measurement of a spectral reflectance of a sample that is to be measured is carried out for example by the following method. First, a “perfect reflecting diffuser”, which is a plate-like object with a smooth surface that does not reflect light specularly and has a spectral reflectance of substantially close to 1.0 across the entire visible range, is prepared. Normally, a plate having barium sulfate or other white powder coated and solidified on the surface is used as the perfect reflecting diffuser. Next, a specific light source is made to illuminates the sample and the spectral intensity distribution of the reflected light that is obtained from the sample at this time is obtained. Next, light is irradiated onto the perfect reflecting diffuser using exactly the same light source and the spectral intensity distribution of the reflected light that is obtained from the perfect reflecting diffuser is obtained. As a device for measuring the spectral intensity distribution, a spectral radiance meter, etc., is generally used. Lastly, by dividing the spectral intensity distribution obtained for the sample by the spectral intensity distribution obtained for the perfect reflecting diffuser, the spectral reflectance of the sample that is not dependent on the light source can be obtained.
When the spectral reflectance of the sample has been obtained in the above manner, elements of the spectral sensitivity distribution of the human visual system are incorporated to provide data that objectively indicate the color that a human being senses in the observation process. The spectral reflectance data are data that indicate the reflectance in the visible wavelength range of 380 nm to 780 nm, and even if data are collected at each 5 nm of wavelength, the data will consist of numerical values indicating 81 values of reflectance and will thus be inconvenient to handle. Thus for color evaluation, a color is generally expressed using the tristimulus values (XYZ) of the XYZ colorimetric system stipulated by the Commission Internationale de l'Eclairage (CIE). The tristimulus values (XYZ) can be determined by a known method (refer for example to ISO/CIE 10527 CIE standard colorimetric observers, 1st Ed., 1991), which uses the spectral reflectance of the sample, the spectral sensitivity distribution of the human visual system, and the spectral intensity distribution of the light source that is used in observation, and are generally referred to as colorimetric values. As the spectral intensity distributions of typical light sources, the spectral intensity distributions, for example, of CIE standard illuminant D65, CIE illuminant D50, etc., which are stipulated in ISO/CIE 10526 CIE standard color illuminants, 1st Ed., 1991 can be used. Also with regard to the spectral sensitivity distribution of the human visual system, the color matching function for the XYZ colorimetric system stipulated in ISO/CIE 10527 CIE standard colorimetric observers, 1st Ed., 1991 can be used.
A general color measuring device for printed matter in the prior art has functions of measuring the color of a specific part of a printed matter based on the above-described principles of measuring color and determining objective data, such as the spectral reflectance, colorimetric values, etc. That is, a general color measuring device has a light source which illuminates light onto a printed matter that is to be measured, a spectral radiance meter which measures the spectral reflectance, and a computational processing unit which carries out various computations, and normally, a perfect reflecting diffuser is provided for carrying out corrections. When an operator performs the necessary computations in accordance to the above-described measurement procedure, the computational processing unit executes the necessary computations based on the collected data and thereby automatically obtains the spectral reflectance, colorimetric values, and other objective data.
With the above-described color measuring method, it should be possible to determine the spectral reflectance and colorimetric values of a sample that do not depend on the light source by carrying out the correction that makes uses of measurement results obtained using a perfect reflecting diffuser. However, in actuality, with a printed matter with which gradation is expressed by an area modulation method, such as offset printing, the measurement results obtained by a conventional color measuring method do not match the colors recognized by a human being in many cases. This is due to a fluorescent whitening (brightening) agent being contained in the printing paper. A fluorescent whitening agent is a colorless compound, which has the property of dyeing fibrous matter and emitting blue to purple fluorescent light corresponding to a wavelength near 420 nm and has the function of whitening the printed paper by emitting light that is complementary to the yellow color of fibers. To be more specific, stilbenzene dyes, etc. are used as fluorescent whitening agents in many printing papers. Though a sheet of paper that is comprised only of normal fibers looks yellowish, a sheet of paper to which a fluorescent whitening agent is added is increased in whiteness due to complementation by a blue to purple color (complementary color of yellow).
However, the intensity of fluorescence is normally strongly dependent on the wavelength of the light source, and a fluorescent whitening agent has the property of fluorescing strongly when illuminated with light of a high amount of ultraviolet components and weakening in fluorescence when illuminated with light of a low amount of ultraviolet components. Paper to which a fluorescent whitening agent is added thus seems bluish when observed under a fluorescent lamp, daytime sunlight, or other light source that contains a high amount of ultraviolet components and seems yellowish when observed under an illumination light source, such as a tungsten lamp. In a printed matter with which gradation is expressed by an area modulation method, such as offset printing, ink is transferred to a paper as a collection of microscopic units of adhesion (halftone dots in the case of offset printing). Gradation is expressed by this units of ink by controlling the area of the ink-adhered part per unit area, in other words the dot percent. The same applies likewise to color printed matter obtained by an inkjet printer. Thus with a halftone printed matter, the bare part of the paper to which ink is not adhered becomes an important element that affects color. For example, a h
Kudo Yoshiaki
Sugiyama Tohru
Dai Nippon Printing Co. Ltd.
Evans F. L.
Geisel Kara
Ladas & Parry
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