Instrumental color measurements, methods

Wirth M. Instrumental color measurement a method for judging the appearance of tablets. J Pharm Sci 1991 80(12) 1177—1179. 

With the best observing conditions, it is possible for the trained observer to compete with photoelectric colorimeters for detection of small color differences in samples which can be observed simultaneously. However, the human observer cannot ordinarily make accurate color comparisons over a period of time if memory of sample color is involved. This factor and others, such as variability among observers and color blindness, make it important to control or eliminate the subjective factor in color grading. In this respect, objective methods, which make use of instruments such as spectrophotometers or carefully calibrated colorimeters with conditions of observation carefully standardized, provide the most reliable means of obtaining precise color measurements. 

The science of color measurement has been explored by various authors (127,128). AATCC evaluation procedure no. 6 describes a method for instrumental 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. Instrumentation 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 instrument must be equipped with tristimulus integrators capable of producing data in terms of CIE X, Y, and Z tristimulus values. 

The development of the observer response functions is the foundation for color measured by an instrument. The Standard Observer established a recognized method for converting... 

The science and methods of colorimetry have been reviewed as a system of color analysis. The color measurement concludes successfully when the instrument data agree with the visual evaluation. The color data from the instrument provide a consistent, objective, and documented way to evaluate color. However, the final color analysis is the judgment made... 

The instruments used for color measurements are nowadays spectrophotometers determining the reflectance of a sample. Three-filter colorimeters, trying to mimic the spectral response of the human eye are now next to obsolete. For applications in the field of uni-pigments photometer illuminating/viewing geometries are standardized as methods A and B, and are designed to suit the individual application (see Section 1.4.2). For standards, see Table 1.1 ( Color Differences, Conditions/ Fvaluation... ). 

By the use of color standards for the indicator, the pH of a solution may be estimated to about 0.1 unit by the indicator method. A more satisfactory general method of determining the pH of a solution is by use of an instrument that measures the hydrogen-ion concentration, making use of a measurement of the... 

The American Public Health Association (APHA) also developed a method for evaluating the color of wastewater. Initially, this method was used as an indication of water purity and involved making comparisons of test samples with dilutions of a 500-ppm Pt-Co stock solution. In the APHA index system, distilled water is assigned a value of 0 (zero) and the stock Pt-Co solution has a value of 500. Details pertaining to the preparation of solutions and sample measurements are provided in ASTM D1209-93. In addition, ASTM D1209 describes how to correlate data from color measurement instruments with data from physical APHA and Pt-Co color standards. 

Finally, we obtained Chromaticity Coordinates which we could use to plot various hue values. If we have a Color Comparator, then we can measure any hue and compare it to any other. But, if we do not, we either build or buy one, or resort to an instrumental method. The Color Comparator uses a human observer to specify color. If we wish to use an instrumental method, then we must correct each component of the instrument to the chroma response of the Standard Observer. While the Color Comparator given above in 6.7.15. was perfectly satisfactory for setting up a system of chromaticity coordinates, it was difficult and awkward to use. What was really required was an instriunental method of color measurement. 

Instrumentation and the associated software for measuring and matching color has advanced rapidly in recent years as the instruments have become more accurate, smaller, portable, more user friendly, and more affordable. Comment is given by Klefmghaus [42] on recent developments in color measurements, and descriptions of instruments available can be found in Modern Plastics International [43, 44 ], The goal is to correlate with visual a,ssessmcnt. but as Mulholland reports [45] the color equations are not perfect. Abrams [46] points out the objectivity of instrumental methods and some of the problems of correlation vvith visual assessments and gives advice on the selection and use of samples for matching. 

In the manufacture of color master batches, the current industry practice is to perform eritieal color measurements off-line. Typically, a sample of the pelletized color concentrate is diluted with natural resin at a standard ratio and milled, extruded or injection molded into a physieal form suitable for visual and instrumental evaluation. These methods are slow and labor intensive. Furthermore, they do not lend themselves well to statistical process eontrol strategies because of the time lag between production and testing. Since relatively few samples can be examined, laboratory measurements may not give a true indication of the consistency of the concentrate product over the entire manufacturing process. 

Objective color measurement is based on tristimulus colorimetry. " This instrumental method measures tristimulus values of light transmitted by a sample and compares the results to the values transmitted by distilled water. The results can be recalculated to the platinum-cobalt scale referred to in the previous standards. The results can also be interpreted by normal methods of color measurement to yellowness index, color, color depth, etc. 

See alsa Chemometrics and Statistics Multivariate Calibration Techniques. Color Measurement. Extraction Solvent Extraction Principles. Flow Injection Analysis Detection Techniques. Food and Nutritional Analysis Water and Minerals. Kinetic Methods Principles and Instrumentation Catalytic Techniques. Optical Spectroscopy Detection Devices. Spectrophotometry Overview Derivative Techniques Biochemical Applications Pharmaceutical Applications. Spot Tests. Water Analysis Overview. 

With regard to color measurement, it is fair to observe that the Model 6500 was not designed as a colorimetric instrument. The optical geometry does not comply with specifications of the Commission International de L Eclairage (CIE), there is no method of setting the zero of the reflectance scale, the white standard is not specified, and, in the authors experience, it varies appreciably between instruments. Furthermore there is no provision in the software to calculate color values from the spectral data. To some extent these objections can be overcome by treating color values as though they were constituents but this yields a level of performance below the true capabilities of the instrument. This point is discussed further in the Calibration Mathematics Section. 

The use of color instrumentation for the evaluation of color has several advantages over the human eye. The instrumental method removes the subjective nature of the visual evaluation of color and allows for quantification of color and color difference. Instrumental methods are also more sensitive and the information more reliable and reproducible. Instrumental assessment of color and color difference has long been used in the printing, coating, paper, plastics, and textile industries. A detailed description of the instrumentation used is outside the scope of this chapter. The reader can find more information in the various publications on color and color measurement found in the Reference section, in particular, Ref. 7. 

The method relies on the instrumental measurement of the degree of yellowness under daylight illumination of close to colorless or close to white translucent or opaque plastics. The measurement is made on pellets and is based on the tristimulus values that are obtained with a colorimeter. Methods are provided to determine the color measurements, such as yellowness index, CIE X, Y, Z, and Hunter L, a, b, or CIE L, a, b. A series of specimens that is compared should have similar gloss, texture, etc. 

Shown in Fig. 6.8 is an early electronic Klett-Summerson colorimeter that was a step above the color comparator shown above. With this instrument the analyst could now abandon the unreliable human eye as detector and begin to rely on a variety of detection and signal recording methods. We have moved a long way from Duboscq and Klett, and yet these are the instruments that started us down the pathway of rapid, accurate, sensitive color measurement for a variety of purposes. 

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