Colorimetry, tristimulus

Cahbration with standard reflectance and transmittance samples should be routinely used for optimum results in spectrophotometry and colorimetry. Cahbration of the wavelength (32) and photometric (33) scales is also advisable. The cahbration of a white reflectance standard in terms of the perfect reflecting diffuse, T, has been discussed (34), as have diagnostic tiles for tristimulus colorimetry (35). A collaborative reference program is available on instmment performance (36). 

Osorio, C. et al., Application of tristimulus colorimetry to obtain natural additives from fruits. 1. Color evaluation during osmotic dehydration, in Proceedings of 4th International Congress on Pigments in Food, Hohenheim, Germany, Carle, R. et al., Eds., Shaker Verlag, Aachen, 2006, Yll. 

The appearance of tablets and powders during accelerated stability testing can be quantified using tristimulus colorimetry [29]. In this work, various formulations were stored under stress conditions, and the tristimulus parameters... 

In the field of colorimetry, the most important method of characterizing a color is to state the CIE tristimulus values X, Y, Z, which are calculated with the formulas... 

Colorimetry [1.17] - [1.19]. The principles of colorimetry are based on the fact that all color stimuli can be simulated by additively mixing only three selected color stimuli (trichromatic principle). A color stimulus can, however, also be produced by mixing the spectral colors. Thus, it has a spectral distribution, which in the case of nonluminous, perceived colors is called the spectral reflectance q (2). After defining three reference stimuli, the trichromatic principle allows a three-dimensional color space to be built up in which the color coordinates (tristimulus values) can be interpreted as components of a vector (CIE system for standards, see Table 1, Colorimetry CIE = Commission Internationale de l Eclairage). For uncolored illumination the three CIE tristimulus values depend on the spectral reflectance as follows ... 

The first is to imitate the analysis made by the eye in terms of responses to three stimuli. This technique, known as "tristimulus colorimetry," sets out to measure X, Y, and Z directly. 

Colour can be useful when describing different batches of drug substance, since it can sometimes be used as an indicator of solvent presence or, more importantly, of degradation. In addition, subtle differences in colour may be due to variations in the particle size distribution. Usually colour is subjective and is based on individual perception however, more quantitative measurements can be obtained by using, e.g., tristimulus colorimetry (Nyqvist et al. 1980 Ve-muri et al. 1985 Nyqvist and Wadsten 1986 Stock 1993). 

The original CIE system of colorimetry was designed for colour specification and has only limited use as a colour-appearance model. It can be easily shown that the CIE tristimulus values that can be used effectively to specify a surface colour do not correlate well with the appearance of that surface. This chapter introduces the distinction between colour specification and colour appearance and then reviews how simple adaptive processes that pool information at different spatial positions in an image can be used as a basis for colour-appearance models. A typical colour-appearance model, CIECAM97S, is then described. Finally, some further uses of colour-appearance models are discussed. 

There are a bewildering variety of methods and instruments available to the food technologist in the field of colorrr measurement. When one is approaching the subject for the first time or when attempting to devise a method for a material outside the normal experience, the wealth of possibilities available sometimes makes the choice difficrrlt. It is the purpose of this chapter to attempt to identify a systematic approach in order to ease the task. The approach is concerned primarily with the use of tristimulus colorimetry and reflectance spectrophotometry. These two techniques will be discussed in detail along with colour scales and formulae commonly in use. The key points which may influence measured results will also be described followed by advice on the selection, preparation and presentation of samples. Finally, detailed references to work carried out in the field of food colour measirrement and where to access further information will be provided. 

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. 

ASTM D 5386-93b. Standard test method for color of hquids using tristimulus colorimetry. 

Prasad KMMK, Raheem S, Vijayalekshmi P, and Sastri CK (1996) Basic aspects and appHcations of tristimulus colorimetry. Talanta 43 1187-1206. 

Integration (colorimetry) n. The process of summing the products of the three values the illuminant, the observer response characteristics and the reflectance or transmittance of an object at specified wavelengths in order to obtain the tristimulus values. 

In a book devoted to advances in a field, we cannot write a treatise on color, even if it lay within our competence to do so. We must therefore presuppose that the reader will acquaint himself with the concepts discussed in the above-mentioned monographs. Additional background is furnished by the following National Bureau of Standards publications Spectrophotometry (Circular 484), Photoelectric Tristimulus Colorimetry with Three Filters (Circular C429), Colorimetry (Circular 478). We shall refer to these and to research articles in the Journal of the Optical Society of America, when necessary, and proceed to discuss their application to foods, emphasizing those problems with which we have had direct, if limited, experience. 

Hunter, R. S. 1942. Photoelectric tristimulus colorimetry with three filters. Nat l Bur. Standards Circ. C429. 

Nickerson, D. J. 1935. Disk colorimetry including a comparison of methods for computing tristimulus values for certain disks. J. Opt. Soc. Amer. 26, 253. 

The modem tristimulus colorimeter or spectrophotometer uses the principles of colorimetry to provide the analyst with a description of the colored sample which can be expressed in terms consistent with one of the accepted color systems (e.g., CBEL a b ). These instmments can be used in either a reflectance or transmittance mode consequently, both opaque and transparent samples can be accommodated. A number of models are available fi-om various manufacturers with a variety of capabilities, including stationary, portable, and even on-line units. 

Test method for color and color-difference meeisurement by tristimulus (filter) colorimetry, ASTM Standard, Book of Standards, Vol. 06.01 ASTM E1347-06, ASTM International, West Conshohocken, PA, 2009. 

FIGURE 3 Tristimulus values of the equal-energy spectrum of the 1931 CIE system of colorimetry. [From Billmeyer, F. W, Jr, and Saltzmann, M. (1981). Principles of Color Technology, 2nd ed. Copyright 1981 John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.]... 

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