Chromaticity, coordinates

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. 

Ghromaticity Diagrams. The CIE 1931 chromaticity diagram uses the chromaticity coordinates ... 

The advantage of chromaticity coordinates is that we now have a set of nonnalized values which we can use to compare colors having different intensity values (and thus different energy values as weU). Furthermore, we need only specify x and y since x + y + z = 1.00. This allows us to specify monochromatic radiation in terms of our chromaticity coordinates. Actually, we can plot a three-dimensional value on a flat (2-dimensional) surface. 

If we have a certain color, a change in intensity has a major effect on what we see (in both reflectance and emittance). For example, if we have a blue, at low intensity we see a bluish-black, while at high intensity we see a bluish-white. Yet, the hue has not changed, only the intensity. This effect is particularly significant in reflectance since we can have a "light-blue" and a "dark-blue", without a change in chromaticity coordinates. 

The R,G,B values of the Berger formula measured by tristimulus colorimeters are linearly related to the X,Y,Z tristimulus values of the CIE system. The Stensby formula incorporates the L,a,b tristimulus values of the Hunter system. In the CIE 1982 formula, x and yn are the chromaticity coordinates of the D65 (2° or 10° observer) light source. 

Only two of the three chromaticity coordinates need to be actually specified, since... 

The recently synthesized PPV 68, in which the oxadiazole group is separated from the PPV backbone by an oxygen atom, is a very soluble material with optical band gap of 2.36 eV and yellowish-orange emission color (chromaticity coordinates by the Commission Internationale de l Eclairage, CIE a 0.50, y = 0.47 591 nm) [124]. An extremely high-... 

For the blue pixel, the standard CIE 1931 color chromaticity coordinates are (0.14-0.16, 0.11-0.15). Since a relatively large band gap is required for blue emitters, the appropriate blue host materials with even larger band gap are needed to optimize the energy transfer requirements. The main challenge in designing the blue emitter or its host is the device stability. 

Chromaticity coordinates are two numbers (denoted jc and y) which together represent in the CIE system the colour quality. 

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. 

They are represented as coordinates in a color plane. The chromaticity coordinates x and y are used to specify the saturation and hue of any color in the CIE chromaticity diagram. See Figure 4 a for illumination D 65. The CIE spectral tristimulus value y (2) corresponds to the lightness sensitivity curve of the human eye. Therefore, a third color variable is specified in addition to x and y, namely the CIE tristimulus value Y, which is a measure of lightness. 

The normalization projects all colors onto the plane at X + Y + Z = 1. Therefore, the third coordinate is redundant and we have to only specify the coordinates x and y. The third coordinate is then given by = 1 — x — y. Since the weights as defined by the standard observer are all positive, the coordinates x, y. and will also be positive. Owing to the constraint x + y + = 1, they will all be smaller than one. Instead of specifying a color by the triplet X, Y, and Z, we can also specify the color by the chromaticity coordinates x and y as well as the absolute luminance Y. The absolute coordinates X and Z are then given as (Poynton 2003)... 

The matrix T which transforms the linear RGB coordinates of the display device to XYZ space can be computed as follows (Watt 2000) In case of a color monitor, let i, y, with i e r, g, hj be the chromaticity coordinates of the phosphors red, green, and blue. Note that z, = L — jc —. We can rewrite the matrix T as... 

QxQy coordinates are complementary xy chromaticity coordinates as shown in Fig. 3. These are analogues of the xy coordinates which are calculated from tristimulus XYZ values in order to plot color points in a two-dimensional diagram [11]. In this case, QxQy values are calculated by the same procedure adopted in the case of xy coordinates, but the transmission spectra T(X) are replaced with the absorption spectra A(X). The equations for QxQy are given as follows ... 

Fig. 9 QxQy chromaticity coordinate of the optode sensors with and without the screening dye as a function of Li+ concentration...

Y/(X + Y + Z), which are equal to x and y, respectively, are called chromaticity coordinates z can also be calculated, but no more information is obtained. The tristimulus values, X, Y, and Z, can be calculated from spectrophotometric data and then be used for calculating the chro-... 

---