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ELECTRIC SIGNAGE W hite LEDs are the most common form of sign backlighting and are being used in applications from channel letters to cabinets and graphic advertis- ing boxes. However, not all white LEDs are created equal. There are a range of hues from a yellow-red to a blue-white coloration. Depending on the applica- tion, these color variations can have a significant impact on the effectiveness, perceived "brightness" and quality of your sign. In this article, we will provide an overview of color temperature and color rendering and how to choose the correct "color" of white to optimize your sign application. First, we need to define some key variables. Correlated color temperature ( CCT) is a description of the warmth or cool- ness of a light source. When a piece of metal is heated, the color appears red in appearance and gradually changes color to orange, yellow, white, blue-white, and finally a deeper blue color. The tem- perature of this metal is a thermal mea- surement in degrees Kelvin or absolute temperature. While LED lighting does not perfectly match the output of this piece of metal, we use this convention to describe the appearance of light as it relates to this type of black body radia- tor. The sun is an ideal blackbody radia- tor and above atmosphere has a CCT of about 5900K. Depending upon the time of day and conditions, the CCT we see varies between 5000K at horizon daylight to 6500K during overcast conditions. In general lighting, warmer CCTs which mimic candlelight are used to create relaxation and enhance skin tones, while cooler CCTs are used to enhance concen- tration in office environments (Figure 1). J. Bryan Vincent is an expert in the field of solid state lighting and electronic materi- als. He has dedicated the past 10 years to developing LED solu- tions specific to the sign industry and is a partner at Principal LED. He has a bachelor's degree in chemistry and a Ph.D. in chemistry/materials science. 74 • March 2017 • S I G N & D I G I T A L G R A P H I C S L E D I N S I G H T S Key Variables of Color Understanding color rendering and color temperature in illuminated signs B Y J . B R Y A N V I N C E N T , P H . D . Figure 1: Typical correlated color temperatures of various light sources.