Measurement of Color

Color is a very important parameter for many manufactured products including paint, cosmetics, foods, beverages, pharmaceutical liquids and tablets, textiles, and so on. If the calcium supplements in a bottle are snpposed to be pink, aU the tablets in the bottle should be the same color. All of the bottles in a lot shonld be the same color. If they are not, consumers may think there is something wrong with the tablets. Both the generation and sensation of color are complex and depend 

Several important measures of wine quality ean be evaluated by mathematical combination of absorbance values at multiple wavelengths (Bain). Wine color intensity, a measure of how dark the wine is, is ealeulated from the sum of the absorbanees at 420, 520, and 620 nm. The wine hue is a measure of the appearance of the wine and is ealeulated from the ratio of absorbance at 420 nm to absorbance at 520 nm. The Thermo Fisher Seientifie software on the Evolution Array UV/VIS spee-trophotometer ean ealeulate the intensity, hue, and the CIE L a b values, as well as the eolor dif-ferenee values (the delta values) eompared to a standard. Red wines exhibit an absorbanee between 400 and 650 nm, eentered at about 500 nm, due to the presence of anthocyanin. No such peak appears in the spectra of white wines. The CIE eolor measurements are earried out in transmittance mode. Table 5.12 shows results for the color and color difference measurements. 

Source Modified from Bain, G., Wine color analysis using the Evolution array UV-visible spectrophotometer, Thermo Fisher Scientific Application Note 51852. Application note and data Thermo Fisher Scientific (www.thennofisher.com). Used with permission. 

You can see from the L values that the white wines are lighter than the reds in fact, one of the white wines has an L value 100. The L = 100 value is for diffuse white samples that give specular reflection may be higher than 100. You can also see that the a values for the red wines are positive, as would be expected, while the white wines are negative (more greenish than red in color). 

There are three major faaors that affect the accuracy and precision of quantitative absorption measuranents the instrument, the skill of the analyst, and the method variables. Instruments vary in the quality of their optical, mechanical, and electrical systems and also in their data processing. Each instrument has fixed limitations these mnst be understood by the analyst and optimized when possible. Wavelength calibration must be checked routinely using recognized wavelength standards. Holmium oxide standards are commonly used for this purpose. Stray light, transmittance, resolution, and other instrument parameters should be checked regularly. The analyst must optimize slit 

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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. 

Color. Many water samples have a yellow to brownish-yeUow color which is caused by natural substances, eg, leaves, bark, humus, and peat material. Turbidity in a sample can make the measurement of color uncertain and is usually removed by centrifiigation prior to analysis. The color is usually measured by comparison of the sample with known concentrations of colored solutions. A platinum—cobalt solution is used as the standard, and the unit of color is that produced by 1 mg/L platinum as chloroplatinate ion. The standard is prepared from potassium chloroplatinate (K PtCl ) and cobalt chloride (C0CI26H2O). The sample may also be compared to suitably caUbrated special glass color disks. 

Techniques for efficiently and economically measuring the other important characteristics, ie, length, strength, and position of weakness in the fiber, are now in commercial use. Existing color-measuring equipment can be used to measure the color (whiteness or yellowness) of washed wool, but accurate measuring of colored-fiber content remains a problem (5,8). 

In this work hybrid method is suggested to determine cationic surfactants in water. It is based on preconcentration of cationic surfactants in the some of ion associates with acidic dyes on the paper filter and measurement of color intensity by solid-phase specdophotomenic method or visual comparison. 

Intermediate methods include the earliest procedure based on Stein s equation [33] and one based on Samuels equation [34]. Among the direct methods is an IR spectroscopic method based on the measurement of the dichroic ratio (R), of amorphous absorption bands. In the investigations [35], the amorphous bands 898 cm" and 1368 cm", for which the angles of transition moment are a898 = 39 and aneg = 80 , respectively, were used. Other methods are spectroscopy of polarized fluorescent radiation [35,36], measurement of color di-... 

The indirect methods discussed thus far have dealt with measurement of color only as it can be correlated with physical characteristics of materials and the effect of these materials on radiant energy. As has been pointed out, the reflectance spectro-photometric curve describes a property of the material. A change in the reflectance spectrophotometric properties may not always result in a change in visual color. The reason is that color of the object is not an unchangeable characteristic of the object itself, dependent only upon these reflectance properties, but is also dependent upon the quality of the illuminating light and the sensitivity of the observer s eye. Thus the measurement and description of visual color are psychophysical problems... 

The methods described make possible the objective measurement of color of foods and a designation in standardized psychophysical terms. However, the psychological significance of food colors is not directly apparent from results expressed... 

Because the quality and health aspects of foods cannot be measured by a single index, it necessarily follows that the subject of control methods in the canned food industry is very broad, and includes chemical, physical, organoleptic, and bacteriological tests, only the first of which is discussed here. The measurement of color, odor, optical clarity, texture, viscosity, and chemical composition has been used to evaluate canned foods, but in many cases the methods that are applicable to one product are either not applicable to another, or can be used only after considerable modification. 

In the preceding section, we presented principles of spectroscopy over the entire electromagnetic spectrum. The most important spectroscopic methods are those in the visible spectral region where food colorants can be perceived by the human eye. Human perception and the physical analysis of food colorants operate differently. The human perception with which we shall deal in Section 1.5 is difficult to normalize. However, the intention to standardize human color perception based on the abilities of most individuals led to a variety of protocols that regulate in detail how, with physical methods, human color perception can be simulated. In any case, a sophisticated instrumental set up is required. We present certain details related to optical spectroscopy here. For practical purposes, one must discriminate between measurements in the absorbance mode and those in the reflection mode. The latter mode is more important for direct measurement of colorants in food samples. To characterize pure or extracted food colorants the absorption mode should be used. 

In a recent application, the appearance testing of tablets through measurement of color changes has been automated through the use of fiber optic probes... 

The effect of particle size, and hence dispersion, on the coloring properties of aluminum lake dyes has been studied through quantitative measurement of color in compressed formulations [47], It was found that reduction in the particle size for the input lake material resulted in an increase in color strength, and that particles of submicron size contributed greatly to the observed effects. Analysis of the formulations using the parameters of the 1931 CIE system could only lead to a qualitative estimation of the effects, but use of the 1976 CIEL m v system provided a superior evaluation of the trends. With the latter system, the effects of dispersion on hue, chroma, lightness, and total color differences were quantitatively related to human visual perception. 

Because coloristic assessments are essentially judgments of color effects, coloristic practice long rested solely on the colorist s trained eye. Today, the measurement of color is a mature field of science, and colorists employ theories of the optical behavior of pigmented layers. 

Transparency, gloss, color, refractive index, and reflectance are the properties normally associated with aesthetics of plastic materials. In some areas, changes in optical properties, increases in haze after abrasion testing (285), color differences after weathering, and birefringence analysis of residual stress within a transparent part (286) are all used to measure the effects of applied stresses. Measurements of color, gloss, refractive index, and haze apply to many products beyond plastics and use similar techniques. Reference should be made to this general topic for detailed information (see Color). 

SPECTROCHEMICAL ANALYSIS (Visible). Chemical systems that exhibit a selective light absorptive capacity are colored. Hence, the terms colorimetric analysis and colorimetry often are used to designate the measurement of such systems when the objective is to determine the concentration of die constituent responsible for the color. The use of the term colorimetry in this respect is not to be confused with the use of the same term in physics where the term refers strictly to the measurement of color. See also Colorimetry. 

A strict definition of color includes (1) the object appearance that depends on light, object, and observer, and (2) the visual perception described with color names. Color is a primary attribute of appearance and it can be quantified. The measurement of color is known as colorimetry. The colorimetric principles associated with the response of the normal eye are important when reviewing color analysis. The eye-brain combination is sensitive, flexible. 

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