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8 Handbook on the Physics and Chemistry of Rare Earths 2.3.2 Luminous Efficacy of Radiation Luminous efficacy (lm/W), where the term “efficacy” is usually used instead of “efficiency” for radiation sources, is a parameter describing how bright the radiation is perceived by the average human eye. It scales with the eye sensi- tivity curve. The highest human eye sensitivity is located around at 555 nm, green, with the highest possible LER (1⁄4683 lm/W on photopic vision). There- fore, 683 lm/W is the highest possible efficiency that can ever be obtained from a light source: When 100% of the electrical power is converted to light at a wavelength of 555 nm, the efficacy of the light source is 683 lm/W. Any other spectrum will yield a lower LER, as the human eye is less sensitive for other wavelengths. In order to obtain white light, emission in the red and blue is necessary, so the LER of white light is significantly lower than 683 lm/W, being on the order of 350 lm/W. In general, one has to find a compromise between high LER and good color quality of the light source. 2.3.3 Color Rendering Index CRI is a measure for color quality. The CRI gages how colors of certain stan- dard reference objects appear when illuminated by a test light source, com- pared to the colors observed under illumination by a reference light source of the same CCT. If there is no difference between the colors rendered by the reference and test light sources, the test source would have a CRI of 100. Obviously, the choice of this reference is very important, since it defines what the “true” colors of objects are. In the definition of the CRI, a several reference sources are used depending on the type of test source. First, the spectrum of the test source is compared to that of a blackbody radiator. When the CCT of the test source is below 5000 K, the reference source used for the calculation of the CRI is a black body radiator of the same CCT. Above 5000 K, a standard daylight spectrum of the same CCT, derived from the D65 standard illuminant and defined by the CIE, is used. Light from a standard incandescent light bulb has a CRI near 100. CRI is calculated from the SPD of the test light source. To evaluate the CRI of real light sources, the SPD must be measured, although mathematical models exist to simulate the SPDs of various white LEDs (Ohno, 2004). The value often quoted as “CRI” on commercially available lighting pro- ducts is properly called the Ra (formally CIE Ra) value, ie, CRI is a term for general use, while Ra is an international standard term which is called “general color rendering index.” Fourteen different color standard samples are used to evaluate Ra, of which the first eight are used for calculating Ra. Here, the special CRI, Ri, is calculated for each of the eight samples using the following equation: Ri 1⁄4 100 4:6DEi where DEi is the color coordinate difference between the test color and the eight standard samples, on the CIE 1964 W*U*V* uniform color spacePDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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