Color measurements Hunter

Hunter, Richard S. 2001. The Basics of Color Perception and Measurement. Hunter Associates Laboratory, Inc., Ed., January, 2006. http //www.hunterlab.com/pdf/ color.pdf. 

The CIE Lab color scale was used to measure Hunter L (Lightness), Hunter +a (redness), Hunter -a (greenness). Hunter +b (yellowness), and Hunter -b" " (blueness) values. Color values of gels were measured using a Chroma Meter (Model CR-310, Minolta Corporation, Ramsey, NJ). The instrument was standardized for surimi gel measurement by using a Minolta calibration plate (No. 18133009 = 94.5, 0.3160, 0.3330) and a... 

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. 

Measurement of Whiteness. The Ciba-Geigy Plastic White Scale is effective in the visual assessment of white effects (79), but the availabihty of this scale is limited. Most evaluations are carried out (ca 1993) by instmmental measurements, utilising the GIF chromaticity coordinates or the Hunter Uniform Color System (see Color). Spectrophotometers and colorimeters designed to measure fluorescent samples must have reversed optics, ie, the sample is illuminated by a polychromatic source and the reflected light passes through the analy2er to the detector. 

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. 

Color Difference Evaluation. Shade evaluation is comparable in importance to relative strength evaluation for dyes. This is of interest to both dye manufacturer and dye user for purposes of quaUty control. Objective evaluation of color differences is desirable because of the well-known variabihty of observers. A considerable number of color difference formulas that intend to transform the visually nonuniform International Commission on Illumination (CIE) tristimulus color space into a visually uniform space have been proposed over the years. Although many of them have proven to be of considerable practical value (Hunter Lab formula, Friele-MacAdam-Chickering (FMC) formula, Adams-Nickerson formula, etc), none has been found to be satisfactorily accurate for small color difference evaluation. Correlation coefficients for the correlation between average visually determined color difference values and those based on measurement and calculation with a formula are typically of a magnitude of approximately 0.7 or below. In the interest of uniformity of international usage, the CIE has proposed two color difference formulas (CIELAB and CIELUV) one of which (CIELAB) is particularly suitable for appHcation on textiles (see Color). 

Photoelectric-Colorimetric Method. Although the recording spectrophotometer is, for food work at least, a research tool, another instrument, the Hunter multipurpose reflectometer (4), is available and may prove to be applicable to industrial quality control. (The newer Hunter color and color difference meter which eliminates considerable calculation will probably be even more directly applicable. Another make of reflection meter has recently been made available commercially that uses filters similar to those developed by Hunter and can be used to obtain a similar type of data.) This instrument is not a spectrophotometer, for it does not primarily measure the variation of any property of samples with respect to wave length, but certain colorimetric indexes are calculated from separate readings with amber, blue, and green filters, designated A, B, and G, respectively. The most useful indexes in food color work obtainable with this type of instrument have been G, which gives a... 

Tests were carried out as designated in ASTM Method D-925-83 related to staining of surfaces by contact, migration, or by diffusion. Hunter color values were measured on the L-scale. On this scale 100 is white and 0 is black. 

For most other purposes, both before and after processing, color is measured using a colorimeter such as a Minolta Chroma Meter (Minolta Corp., Ramsey, NJ). The instrument is calibrated against a standard-white (Minolta) reference plate. Hunter L (whiteness), a (greenness) and b (yellowness) values were determined for each sample and made directly from the potato surface (Nourian et al., 2003). 

In the work with cottonseed flours, we used the Hunterlab color meter D25D2A and expressed these measurements as Hunter L, a, b color values. These are coordinates of the three-dimensional opponent-color space shown in Figure 2. The L value measures lightness, or the amount of light reflected or transmitted by the object. The a and b values are the chromaticity coordinates from which information about hue and saturation can be obtained. The a value measures redness when plus and greenness when minus. The b value measures yellowness when plus and blueness when minus. 

Figure F5.1.10 The Hunter L,a,b color space was designed for measuring color differences. Figure courtesy of GretagMacbeth. This black and white facsimile of the figure is intended only as a placeholder for full-color version of figure go to http //www.currentprotocols.com/colorfigures...

Many industrial color applications work with a color spectrophotometer to measure CIELAB color differences. CIELAB works well in specifying the color of an object, but color differences calculated with the formula have limited value for pass/fail decisions. These component color differences (dL, da, db ) are calculated by simply subtracting the L a b values for the standard from the L a b values for the trial. The dL, da, db values should indicate the direction of color difference from the standard (e.g., lighter or darker). They are commonly used to establish the allowable color limits from a standard. However, in applying these delta differences as limits for color acceptability, they should be adjusted for different colors. CIELAB, like Hunter L,a,b, is not a visually uniform color space. The limitations of CIELAB are not well understood throughout industry and the numbers continue to be applied without being adjusted for different colors and products. 

Many single number color scales have been developed for foods by converting X, Y, Z or L, a, b color specifications to a number indicating grade or quality on the desired scale. Such a scale was developed by Hunter to measure the citrus redness CR, and the citrus yellowness CY of the juice. The formulae are derived as follows ... 

Yellowness Index. The yellowness of the 50-mil plaques used for stabilization studies was measured directly from a Hunter D-25 color difference meter equipped with a 90° sphere attachment. The samples were placed in the reflectance port of the sphere using a standard white ceramic as backing. The instrument is designed to give direct yellowness index values as described in ASTM D-1925. 

Color The color of the film can affect the consumer acceptance of coated produce (60). Five different color systems XYZ, Yxy, L a b, L C H° and Hunter Lab are used to express absolute chromaticity. Color system XYZ is measured as tristimulus values of color and forms the basis for most calculations. In the color system Yxy, Y is a lightness factor expressed as a percentage based on a perfect reflection of 100%. x and y are chromaticity coordinates defined by the following equations ... 

Note 2 Color of the samples and color fade was measured using two criteria, that is, using a densitometer (the percent reduction in reflected optical density), and L, a, and b units of the Hunter color space, along with color difference, AE. In general, better results were obtained when optical densities were measured. 

A section of 10 g of bacterial cellulose was cut into 1 cm sections to measure color appearance. The values of L, a, and b were measured by a Hunter Lab Color Quest (Memmert, Germany) colorimeter with the CIELAB color system. These values were then used to calculate chroma (C ) and hue angle (/lab) values. L indicates lightness, with a scale ranging from 0 (black) to 100 (white). Positives and negatives in a represent red and green, whereas positives and negatives in b represent yellow and blue, respectively. 

Hunter Lab. Instruments for Color and Appearance Measurement. Reston, VA (1988). 

Solution color refers to the yellowness of the cellulose acetate solution. The most meaningful measurement is the yellowness index (YI) for an 18% solution of cellulose acetate in 9/1 methylene chloride methanol (by weight). Measurement is made by the Hunter colorimeter described above. Hemicelluloses in the pulp affect the color of the solution of cellulose acetate, but there are other factors that relate to solution color including brightness of the wood pulp and cellulose acetate processing conditions. 

NIST total color difference n. The total color difference, AE, calculated by means of the NIST color difference equation, sometimes referred to as a Judd or a Judd unit. The term is frequently erroneously used to refer to the color difference calculated by other equations normalized to agree in magnitude on the average to an NBS unit, or to the AE measured on tristimulus colorimeters with electrical circuits designed to give an approximation of the NIST color difference equation, frequently one of the Hunter E a, b equations. Such incorrect use of the term is to be discouraged. Description of the exact equation and method of measurement used will avoid confusion (NIST, www.nist.gov). 

The principles and standard procedures used in the determination of color can be found in ASTM E308. Equipment is conunercially available for measuring optical properties of polymers for these purposes. Major suppliers include Gardner (gloss meter for specular gloss, haze meter for haze and luminous transmittance, and clarity meter for specular transmission) and Hunter Lab (spectrocolorimeter). 

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