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Old 04-30-2012, 06:08 PM
steveparrott steveparrott is offline
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Join Date: Jun 2005
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In response to this question of using color filters vs. phosphor coatings, a pHD LED development engineer (tops in the field) had this answer:
"Color filters are subtractive. That is they can only remove light generated by the LED. This is an extremely inefficient, and in my view, counter-productive way to achieve a desired CCT (Correlated Color Temperature). It is the light equivalent of cutting off your left arm to lose weight! It is very misleading to suggest that adding color filters accurately adjusts CCT.

It should also be noted that the use of color filters greatly diminishes CRI (Color Rendering Index). CRI refers to how well colors of objects are reproduced compared to standards. The filters enhance the primary colors: Red, Green, Blue, Yellow. But they do this by sacrificing a substantial portion of the other colors by absorbing them! They also sacrifice the efficiency of the LED. According to the definition of CRI all colors of the visual spectrum are taken into account, so the TRUE CRI would be very low since the other colors have been removed; e.g., a Yellow filter throws away some or all of the LED light generated in the Red, Green and Blue part of the spectrum.

It would be vastly superior to use white LEDs of the particular color temperature to generate the desired light. For example, using such (red) filters might make a red flower "pop", but the blue ones would look very dull indeed! The overall scene would sacrifice one color in favor of another, all the while tossing away any semblance of energy efficiency."
The engineer went on to explain (in very complex terms) that the use of color filters actually negates CCT determination. According to the CIE (group that defines CCT), you can only determine CCT when the light source lies near the plankian (black body) locus. All white LED's are (by definition) located near this locus. Light sources that have color tints (such as produced through color filters) are distant from the locus - they're no longer white sources. They may visually approximate a certain CCT, but they will not render colors in the same way as will white sources at that CCT.
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