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Rare Earth-Doped Phosphors for White LEDs Chapter 278 7 (Brill, 1998; Fairman et al., 1977) but, together with the first, are necessary to determine the color of light. The tristimulus values X, Y, and Z of a light source are defined as, ððð X 1⁄4 k PðlÞ x"ðlÞdl, Y 1⁄4 k PðlÞ y"ðlÞdl, Z 1⁄4 k PðlÞ z"ðlÞdl where k is a proportionality constant and P(l), named as spectral power distri- bution (SPD), is the radiant power emitted by a source at each wavelength or band of wavelengths over the entire spectrum (eg, in units of W/nm). The SPD provides a complete description of the spectral properties of a light source. The Y value is equal to one of the photometric quantities (such as luminous flux) of the light source if k 1⁄4 683 lm/W is used (see later, luminous efficacy of radiation, LER). Tristimulus values were derived such that X1⁄4Y1⁄4Z is equal-energy white. The chromaticity coordinates (x,y) are calcu- lated as x1⁄4X/(X+Y+Z) and y1⁄4Y/(X+Y+Z). Any color of light can be repre- sented on the CIE 1931 (x,y) chromaticity diagram, which is shown in Fig. 4B. The chromaticity diagram is a guitar pick shaped 2D plot of all pos- sible hue and saturation values of a light source, independent of luminance. The perimeter of the guitar pick is the locus of chromaticity coordinates for the spectral colors (ie, monochromatic light, wavelengths in nm). 2.3.1 Correlated Color Temperature The black arc plotted in the interior of the chromaticity diagram gives the chro- maticity coordinates of emission from a blackbody at temperatures between 1000 K and infinity. It is called the Planckian locus and is a convenient way of representing a white light source. Positions along the Planckian locus can be specified by the temperature of the blackbody, called the color temperature, thereby reducing the hundreds of numbers in an SPD, or the two numbers of chromaticity coordinate, to a single number. A nonblackbody source is often described by the correlated color temperature (CCT), which is the temperature of a blackbody radiator that most closely resembles the color from a source of equal brightness. Lines of a specified (constant) CCT would cross the Planckian locus at coordinates of that blackbody temperature, as indicated in Fig. 4B. A powerful feature of the chromaticity diagram is that light produced by mixing output from two sources having different chromaticity coordinates will fall on the line connecting the coordinates of the sources alone; the position along the line is determined by the weighted average of the brightness of each source. Similarly, the chromaticity coordinate of light produced by mixing three light sources will fall within the triangle formed by the chromaticity coordinates of the three sources as the vertices. It is important to note that any color falling outside this triangle cannot be produced by mixing the three sources. How can we determine whether the spectrum of a light source is suitable for general lighting? The two major issues of the efficiency of the radiation and the color quality of the light are important.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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