Standardized color space

An important point about colorimetry is that all the standard color spaces use relative radiometric units and not relative photon counts. This is important because a spectrophotometer usually measures counts that are proportional to the number of received photons at a defined wavelength. A correction fimction that performs at the same time the correction of the detector sensitivity at each wavelength as well as the conversion to radiometric units is then needed... 

To prepare an image for display or other interpretation, several steps are typically taken. First, the red, green, and blue camera responses (RGB) are mapped to a standard color space, the most common example being sRGB [3]. Displays with different primaries can be calibrated to provide accurate color under the assumption that the capture response closely approximates the sRGB standard. To transform device RGB to sRGB, a 3 x 3 color correction matrix is used as shown in the equation below. 

Experienced color matchers can achieve a good color match by trial and error without using any instmmentation. In some cases, however, this technique can be a lengthy process, and should the desired match be outside the color space defined by the available color standards, the technician might spend too much time just to determine that the match is not possible. To get the most cost-effective match using a low metamerism in the shortest possible time, the use of a computet color matching system is preferable. 

Hunter L,a,b and Other Color Spaces. The CIELAB and CIELUV color spaces were the outgrowth of a large and complex group of interrelated early systems and have replaced essentially all of them except for the 1942 Hunter Y,a,b group of color spaces (3). This was the earliest practical opponent-based system which is still widely used. In this system, for illuminant C and the 2° standard observer ... 

In the CIELAB and CIELUV color spaces, the difference between a batch sample and a reference standard designated with a subscript s, can be designated by its components, eg, AAL = L — L. The three-dimensional total color differences are given by EucHdian geometry as the 1976 CIE lYa b and 1976 CIE lYu Y color difference formulas ... 

There are instances in which one manufacturer may designate a bluish red dye as Red 4B and another manufacturer uses Violet 2R for the same dye. To resolve such a problem the manufacturers pattern leaflets should be consulted. These show actual dyed pieces of cloth so the colors of the dyes in question can be compared directly in the actual appHcation. Alternatively, colors can be specified in terms of color space coordinates. The Cielab system is becoming the standard in this system the color of a dye is defined by three numbers, the L, a, and b coordinates (see Color). 

When Munsell devised his color space, he did so on the basis of minimum observable color perception steps. But the problem with the Munsell System was one of reproducibility, which the CIE Standard Observer cured. In formulating a color match, one wants to be able to predict the correct concentration of colorants required, whose scattering and absorption properties are known, i.e.- the lightness, so as to match the sample submitted, starting with their spectrophotometric curves. In practice, this is not so simple, since two colors must have identical spectrophotometric curves to be exactly equal. It turns out that the human eye will identify the two colors to be equal if their spectrophotometric reflectances are reasonably close. Two colors may appear to be equal under Daylight illumination, but quite different under incandescent lamp illumination. These colors are known zus "metamers" and the phenomenon "metamerism". 

CIE color system, 7 304. See also Commission Internationale de l clairage (CIE) chromacity diagrams, 7 313-315 standard illuminants, 7 315-316 CIEDE200 color difference equation, 7 322 CIELAB, 7 304 CIELAB Color Space, 7 320 CIELAB color values, of flax fiber, 11 614t Cielab dye nomenclature system, 9 244 CIELAB Metric Color Spaces, 7 320 CIELAB system, 19 428, 451 CIELUV Color Space, 7 319-320 CIE Standard Observer, 7 311-312 Cigarette filters... 

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 ... 

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. 

The encoding process used for standard definition television (SDTV) is shown in Figure 5.12. First, the nonlinear quantity luma is computed from the nonlinear RGB values. Then the color differences between the red channel and luma as well as the blue channel and luma are computed. These two differences are called chroma. The R — Y channel is scaled by 0.564 and is denoted by PR. The B — Y channel is scaled by 0.713 and is denoted by Pb- This is called the Y PbPr color space. The transform shown in Figure 5.12 can be... 

Let us now have a different look on the use of local space average color for color constancy using color shifts. Consider the standard RGB color space. A unit cube is spanned by the... 

Apart from producing a constant output irrespective of the illuminant, the entire color space should be used. We can compute the standard deviation of the pixels of a single image in RGB chromaticity space and average the standard deviation over all images of the image set. This will measure how well the available color space is used. The color... 

Stokes M, Anderson M, Chandrasekar S and Motta R 1996 A Standard Default Color Space for the Internet - sRGB. Technical report, Version 1.10. 

The Farnsworth-Munsell 100-Hue test [40] offers a complete set of limit standards combined with standardized sources that are easily, administered to an individual to assess and quantify their color acuity. These tests evaluate the ability to discern between graduated subtle shade color differences (i.e., green to red, purple to red, blue to violet, and green to blue), which provide an overall assessment of an individual s ability to discern color nuance within different color spaces. This method is commonly used today to evaluate different individuals abilities to perceive color differences and, when given on a regular basis, serves as a color reference standard to evaluate color-matching personnel. 

There are other minor nonequivalencies of ISO test procedures compared to ASTM standards, but for color difference the ISO Test Procedure No. 105 is unique. Those who use Colour Measurement Committee (CMC) procedures—particularly CMC 2 1 Lightness to Color ratio—claim that it facilitates a uniform description for acceptability decisions that is better than any other system in existence. These equations permit the use of a single number tolerance, DEcmc, in a nearly uniform color space. The CMC formula is a modification to the perceptibility CIELAB formula. It is fully described elsewhere in this book, but it deserves some brief notice here because, after all, it is an ISO procedure. The CMC developed the basic British Standard No. 6923, Calculation of Small Color Differences. Soon afterward, in 1989, the American Association of Textile Chemists and Colorists (AATCC) adopted AATCC Test Method 173, CMC Calculation of Small Color Differences for Acceptability. Ford Motor Company indicated a preference for using CMC 2 1 ratio color difference for plastics weathering data for plastics interior trim materials. 

Subsequent to determining standard and lot tristimulus XYZ values, one simply plugs those calculated values into the CIE Lab color equations. Why the CIE system Because CMC uses the DLab values, generated in CIE color space, to determine DC, DH, and eventually CMC DE. 

The color measurement of fried foods is traditionally done either instrumentally or visually through trained panelists. Human evaluation is quite subjective, tedious, and subject to error of perception. Hence, it is least recommended for the color determination of fried foods. The instrumentation procedure involves the use of color spaces and numerical values to create, visualize, and represent color in 2D and 3D spaces (Trusell et al., 2005). Food color is usnally measured using the L a b color indices. This is the international standard color measurement technique, adopted by the Commission Internationale d Eclairage (CIE) in 1976. L is a measure of luminance or lightness on a scale of 0-100, a is a measure of greenness to redness, and b is a measure of blueness to yellowness. This technique provides measurement similar to how human eye perceive colors (Hunt, 1991). 

L a b color space A method of depicting color that is uniform and recommended by the Comission Internationale d Eclairage for standard color description. L - psychometric lightness (0 = black 100 = white) a psychometric red ( +ve values), green (-ve values) b psychometric yellow (+ve values), blue (-ve values). 

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