Sign & Digital Graphics

September '18

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26 • September 2018 • S I G N & D I G I T A L G R A P H I C S ELECTRIC SIGNAGE What is CRI? Color rendering index (CRI) is a quan- titative measure of how well a light source renders colors versus an ideal blackbody radiator. Without getting too technical, it is calculated by taking eight CIE (International Commission on Illumination) standard color data points and comparing them to a standard. A general CRI (Ra) is calculated by taking the arithmetic mean of these points. The smaller the average differ- ence in chromaticities versus the ideal standard, the higher the CRI. A CRI of 100 represents the maximum value. A table of typical CRI values of various light sources is shown in Figure 2. Let me make an important point. Higher CRI does not mean "better." CRI is important when you are back-illumi- nating graphics. Typically, a CRI value greater than 80 is more than accept- able. However, one should consider the graphic as well. Sometimes color appear- ance (typically determined by CCT) is more important than color fidelity (CRI). For instance, say you are trying to highlight a burger and fries, it may be more important to highlight warmer tones, even at the expense of a higher CRI. You may be surprised how well an 80 CRI product performs and may be disappointed with a 90+ CRI product depending on the graphic. As a general rule, when looking at LED backlighting for graphics, CCTs in the 4000-5000K range and above 80 CRI are the most versatile and will do a good job of light- ing both warmer and cooler colors in the same sign at high fidelity. An example would be the American flag, where a higher CRI will render all three colors accurately. CCTs and Cabinet Signs/ Channel Letters So what about cabinet lighting and channel letters? Here the decision is in some ways simpler. First, CRI values are of less importance. The only gen- eral caveat is that warmer text and print (red, orange, brown, etc.) render better "CCTs in the 4000-5000K range and above 80 CRI are the most versatile and will do a good job of light- ing both warmer and cooler colors in the same sign at high fidelity." Figure 2: Comparing CRI across various lamps.

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