![]() ![]() Volume 8 Issue 1
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October 2004
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What is full-spectrum index?Full-spectrum index (FSI) is a mathematical measure of how much a light source's spectrum deviates from an equal-energy spectrum (NLPIP 2003). An equal-energy spectrum is an imaginary spectrum that provides the same radiant power at all wavelengths, thus representing a "full" spectrum. Therefore, for humans to see object colors, a light source must generate light from more than one region of the visible spectrum. Subtle differences in the perceived colors of objects arise from slight differences in the spectral reflectance of those objects. If a light source does not provide radiant power at those wavelengths where the spectral reflectances of those objects differ slightly, the objects will appear to have the same color. Therefore, a lamp that emits radiant power at all visible wavelengths would be expected to have good color rendering properties. For more information about color rendering, see "How are the color rendering properties of light sources defined?". Among electric light sources, light from Xenon lamps most closely resembles a full spectrum. The same can be said for the 5500 K phase of daylight. Both of these sources have been experimentally shown to be excellent at revealing subtle differences in color that can not be seen under other types of lamp spectra (Deng et al. 2004). Light sources with deficiencies in some parts of the spectrum will have poorer FSI values and will be less effective at rendering subtle differences in object colors. Appendix B provides a calculation method for assessing FSI. The NLPIP report Lighting Answers: Full-Spectrum Light Sources contains details on FSI, along with FSI values for many commercial light sources.
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