Color measurements Munsell

If results of color measurements are expressed in Munsell notation, a reader can use Munsell color charts as an aid in visualizing approximate ranges of color differences involved. Such a means has been suggested (15) for expressing color of light-colored juices. The necessary experimental data were obtained with a reflection meter similar to the reflectometer described. 

Some of the oldest color-determining instruments are the additive visual colorimeters. They establish the basis for all color measurements which fundamentally assign a unique numerical value to a particular color. When the spinning disc technique is applied to the Munsell color... 

Table 1. The tri-stimulus values of Munsell color standard plates obtained from two color measuring approaches.

Methods are described for determining the extent to which original natural color is preserved in processing and subsequent storage of foods. Color differences may be evaluated indirectly in terms of some physical characteristic of the sample or extracted fraction thereof that is largely responsible for the color characteristics. For evaluation more directly in terms of what the observer actually sees, color differences are measured by reflectance spectrophotometry and photoelectric colorimetry and expressed as differences in psychophysical indexes such as luminous reflectance and chromaticity. The reflectance spectro-photometric method provides time-constant records in research investigation on foods, while photoelectric colorimeters and reflectometers may prove useful in industrial color applications. Psychophysical notation may be converted by standard methods to the colorimetrically more descriptive terms of Munsell hue, value, and chroma. Here color charts are useful for a direct evaluation of results. 

Conversion tables and charts now available make it possible to express I.C.I. data in forms in which a specified color and the significance of measured color differences can be more easily visualized. For example, I.C.I. values calculated from objective instrumental readings can be converted into the Munsell notation which evaluates the three psychological color attributes—hue, lightness (Munsell value), saturation (Munsell chroma)—on scales of approximately equal visual steps. In addition, the Munsell color charts offer one of the most convenient sources of material standards for direct color comparisons. 

Once this fact was rccdized, it was understood that an average of what each person saw would have to be made if a standard system weis to be formed and promulgated. This led to the concept of the "Standard Observer". Thus, the research required to define and measure color took a completely different path from the original methods such as the Munsell Color Tree. 

Initially, the gray area of the Mondrian was chosen and the energy given off by the patch on the Mondrian was measured. The subject was asked to match the corresponding colors for all of the areas of the color Mondrian from the book of colors. Next, a red area of the Mondrian was selected. The illumination of the color Mondrian was adjusted such that the energy given off by the red area was equivalent to the gray area that had been previously matched. The illumination of the Munsell book of colors always remained the... 

We have used two techniques in an attempt to compare the "color" of the various adsorbents. First, they have been compared with Munsell soil color charts (17). Second, their diffuse reflectance spectra have been measured (vs. white filter paper as reference). These results are summarized in Tables III and IV, respectively. 

Color Charts. The colors of pigments can be routinely measured by visual comparison with color charts like the one issued by the Munsell Color Company. This method may achieve a high precision because of the eye s sensitivity to small (relative) color changes. Of paramount importance for reliable application is a uniform light source. The major drawback of the method is that the resolution of color assignments is limited by the resolution of the color chips (see 3.2.3). Interpolation between chips is possible in principle, but difficult in practice as it has to be done in a three-dimensional space. 

Figure 5 Change of chromaticity with cell depth for four wines. From transreflectance measurements using a near 07diffuse instrument geometry, illuminant C, 2° observer. Cell 1, depth 0.25 mm cell 2, depth 1 mm cell 3, depth 2 mm cell 4, depth 5 mm cell 5, depth 10 mm. Lines of Munsell constant Hue and Chroma have been superimposed. (Reproduced with permission from Hutchings JB (1999) Food Color and Appearance, 2nd edn. Gaithersburg, MD Aspen Publishers.)...

There are, necessarily, limitations to the above. Normally, the specular component of the reflected light is eliminated, and since the color matches are metameric, viewing conditions must be specified. Also, the greatest usefulness lies in the measurement of surface colors. The object should be opaque and nonfluorescent. The nature of the surface of tomato pur4e is far from ideal. Although many difficulties arise in borderline cases, and the result is dependent on the skill and color vision of the observer, the method has served a useful purpose. If however we examine the wording of the PMA specification, a sample shall contain as much or more red than that produced by spinning the specified Munsell discs. . . , it... 

To determine color tolerances, we need ideally an assessment of color differences on a tridimensional color scale. Judd discusses the various attempts to measure the color differences. These include Nickerson s index of fading 7, based on Munsell spacing, the changes in hue, chroma, and value AH, AC, AF) being appropriately weighted for perceptibility differences, and his own contribution, AE, the N.B.S. unit. This has been redefined in terms of Hunter s (a,13) chromatic space. The simplest expression of this quantity is given by Scofield (1943) ... 

A CMVS was designed and evaluated on colors perceived by human. The machine vision measured tri-stimulus values in R,G,B color coordinate were transformed to L a b color space for better representation of human perceived colors. The CMVS measurements were evaluated using Munsell standard color chips, a colorimeter, and a sensory panel. Excellent correlations were found between the CMVS and the colorimeter, which is designed to quantify human perceived colors. The CMVS s ability in quantifying human perceived colors was evidenced by accurate measurement of Munsell standard color chips and the excellent correlations with the sensory panel perceived colors (R>0.90). From the experiments, it was possible to resolve up to 0.7 CIE unit color difference of the tested colors. 

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