Tristimulus colorimeter

The science of color measurement has been explored by various authors (127,128). AATCC evaluation procedure no. 6 describes a method for instmmental measurement of color of a textile fabric. AATCC evaluation procedure no. 7 may be used to determine the color difference between two fabrics of a similar shade. Instmmentation may be either a spectrophotometer for measuring reflectance versus wavelength, or a colorimeter for measuring tristimulus values under specified illumination. If a spectrophotometer is used, however, the instmment must be equipped with tristimulus integrators capable of producing data in terms of CIE X, Y, and Z tristimulus values. 

Colorimeters. Also known as tristimulus colorimeters, these are instniments that do not measure spectral data but typically use four broad-band filters to approximate the jy, and the two peaks of the x color-matching functions of the standard observer curves of Figure 7. They may have lower accuracy and be less expensive, but they can serve adequately for most industrial color control functions. Examples of colorimeters are the BYK-Gardner Co. XL-835 the Hunter Lab D25 series the Minolta CA, CL, CS, CT, and CR series (the last of these is portable with an interface) and the portable X-Rite 918. 

Objective Evaluation of Color. In recent years a method has been devised and internationally adopted (International Commission on Illumination, I.C.I.) that makes possible objective specification of color in terms of equivalent stimuli. It provides a common language for description of the color of an object illuminated by a standard illuminant and viewed by a standard observer (H). Reflectance spectro-photometric curves, such as those described above, provide the necessary data. The results are expressed in one of two systems the tristimulus system in which the equivalent stimulus is a mixture of three standard primaries, or the heterogeneous-homogeneous system in which the equivalent stimulus is a mixture of light from a standard heterogeneous illuminant and a pure spectrum color (dominant wave-length-purity system). These systems provide a means of expressing the objective time-constant spectrophotometric results in numerical form, more suitable for tabulation and correlation studies. In the application to food work, the necessary experimental data have been obtained with spectrophotometers or certain photoelectric colorimeters. 

The spectral characteristics of the source, photocells, and the three filters are such that approximate I.C.I. tristimulus values may be calculated (5) and from these a specification in terms of luminous reflectance, dominant wave length, and purity can be obtained. Hardy has cautioned (3), however, that the usefulness of such an instrument as a tristimulus colorimeter depends upon the standardization and constancy of the spectral characteristics of the light source, cell, and filters. 

Cohen, E. and Saguy, L, A rapid method of determination of betanin and vulgaxanthin 1 in beet powder using a general purpose tristimulus colorimeter, Ztschr. Lebensm. Unters. Forsch., 175, 31, 1982. 

Instrumental color measurements eliminate subjectivity, are more precise, take less time, and are simpler to perform. However, to evaluate instrumental results properly, the physics of the measurement processes must be considered. Three types of color measurement instruments are used for food the monochromatic colorimeter, the tristimulus colorimeter, and the colorimetric spectrophotometer. 

The R,G,B values of the Berger formula measured by tristimulus colorimeters are linearly related to the X,Y,Z tristimulus values of the CIE system. The Stensby formula incorporates the L,a,b tristimulus values of the Hunter system. In the CIE 1982 formula, x and yn are the chromaticity coordinates of the D65 (2° or 10° observer) light source. 

The actual determination of color is made with a photoelectric tristimulus colorimeter fitted with a CIE illuminating source. Measurements are made relative to a standard (usually magnesium oxide or barium sulfate) taken through... 

Trisphenol epoxy novolacs, 10 371-372 Tristimulus colorimeters, 7 325 Trisubstituted alkene synthesis, 13 653 Trisubstituted silyl group, in silylation, 22 691... 

Methods of Determination. Lightness. The white pigment powder is compressed in a suitable powder press to give an even, matt surface. The CIE tristimulus value Y is measured with color measuring equipment. For standards, see Table 1 ( Lightness ). Apparatus spectrophotometer or tristimulus colorimeter, powder press, white standard. 

Black-Ground Method. The relative scattering power is determined from the tristimulus values Y of the pigmented medium applied in various film thicknesses to black substrates. Compared with the gray paste method, the black-ground method has the advantage that it is not restricted to any particular test medium. Apparatus spectrophotometer or tristimulus colorimeter. 

Evaluation of Color Changes (see Section 1.3.2). Color changes are differences in lightness, chroma, and hue such as exist between identical samples with different histories. Evaluation methods are particularly suitable for the determination and evaluation of color changes that occur following physical and chemical stresses on pigmented materials. The CIE tristimulus values X, Y, and Z of the samples or of different test locations on a sample are determined with a colorimeter. The CIELAB... 

Luminescence color is determined by visual comparison, by calculation of the color coordinates x and y from the emission spectrum, or by means of a tristimulus colorimeter under the excitation definitive for the application (UV radiation, cathode rays, etc.). 

Tristimulus colorimeters are used to measure color in certain food applications. They combine light source, filters, and photodetectors to reproduce the CIE Standard Observer response functions (Fig. F5.1.8). Colorimeters, having broad band-pass filters, do not measure the spectral data. Without spectral data they cannot offer the choice of either observer or... 

The measurement of color on tristimulus colorimeters will depend on the type of machine and its operating instructions. It should be noted that orange juice color readings are affected by incorporated air and by variations in temperature. The juice should be deaerated and brought to 80°F i 2° before readings are made. Color value readings will have to be correlated with visual evaluations to be meaningful. 

As pink or red—fleshed grapefruit ripen, the color of the juice loses the distinct coloration and gives a juice that is definitely amber with a very slight brownish cast. Mixtures of white-fleshed and pink-fleshed grapefruit have a dull appearance and may look grayish to dull amber. There are no visual color standards for grapefruit juice products. However, color characteristics may be measured on tristimulus colorimeters using the L, a, b, color notation. 

Color measurements on Parylene-C film were determined with a Minolta Chromameter 221, a colorimeter with output limited to CIE chromaticity or tristimulus values and CIELAB L, a and b color coordinates. Measurements on the films after various exposure times were recorded with the sample mounted over the white calibration plate. 

The uniformity was assessed by selecting representative samples from the dyed lot and determining the color (tristimulus values) on a standard colorimeter (DIANO/LSCE Automate System). The difference in color of each specimen from the average color of all specimens was determined using the FMC II color difference formula (5J. In this system one unit of color difference is defined as the minimum perceptible difference in color. Thus spots or unlevel dyeing could be readily identified by variations in color difference between specimens from the same dyeing. 

Initial inspection of the dyed carpet suggested that good color reproducibility and uniformity had been achieved. Ten samples were cut from each of the dyed samples and the color measured on the Diano Colorimeter. Each sample was measured three times to average variations due to instrument changes and sample texture differences. Tristimulus values (X,Y,Z) were calculated from the average of the three measurements. 

One of the simplest tristimulus colorimeters is the Lovibond Tintometer. The apparatus consists essentially of a viewing tube and light reflected from a standard white surface. One half of the field is illuminated by the light reflected by, or transmitted through, the specimen under examination. The other half is illuminated directly by a beam of standard light. Into the path of the latter beam coloured glass filters of the subtractive primaries are inserted and moved by mechanical means. 

Since X, Y, and Zare known by examination with a tristimulus colorimeter, the only unknown quantities are Ci, c, and C3, namely the concentrations of dyestuffs required to match the pattern. These equations, however, can only be solved by making a series of approximations, a procedure which was far too laborious until digital computers became available. It is easy for a computer to calculate the c values for a first trial dyeing from data fed into it relating to the dyes selected as most suitable. A trial dyeing is made with the predicted recipe and it is examined in a colorimeter. Its tristimulus values are fed into the computer, when the necessary corrections can be ascertained from the following equations ... 

Vemuri S, Taracatac C, Skluzacek R. Color stability of ascorbic acid tablets measured by a tristimulus colorimeter. Drug Dev Ind Pharm 1985 11 207-222. 

The tristimulus method has theoretical advantages, where the materials to be measured are fluorescent but, there are serious practical problems in assuming that a tristimulus colorimeter exactly matches human vision, that is, in eliminating color blindness from the instrument. 

Two commonly used types of color measurement equipment are a colorimeter and a spectrophotometer. A tristimulus colorimeter has three main components ... 

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