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When comparing LED lighting products to their conventional counterparts, buyers and specifiers have a number of different parameters to consider. One, of course, is energy efficiency, which is one of LED lighting’s best-known potential advantages, and which can translate into considerable cost savings over time. However, the energy efficiency of any lighting system depends on more than just the efficacy of the light source itself. In the case of LED lighting, driver efficiency, optical losses, and thermal factors also come into play. Additional parameters and considerations are detailed below.
Correlated color temperature (CCT) measures the relative color appearance of white light, from lower (warm/yellow) to higher (cool/blue) values on the Kelvin (K) scale; while color rendering index (CRI) provides a comparison of how colors are rendered by a light source relative to either incandescent or daylight. Although these two metrics have long been used, they only get you in the general ballpark for selecting and matching lamp colors. Due to differences in their spectral power distributions, two LED light sources with identical CCTs can appear quite different; while CRI can be a poor predictor of real-world color rendering – especially with saturated red objects – and doesn't correlate well with human color-quality perception. Two other metrics – R9 and Duv – are often used to supplement CCT and CRI, and a new method for evaluating light-source color, TM-30-15, has been developed by the Illuminating Engineering Society (IES). For more information, see the Fact Sheets LED Color Characteristics, True Colors, and Evaluating Color Rendition Using IES-TM-30-15.
Some LED light sources can last much longer than their conventional counterparts – 50,000 hours or longer – but it’s important to consider mean or end-of-life efficacy rather than initial efficacy. All light sources diminish in output over the course of their operating life. But whereas many conventional sources burn out altogether, LED sources typically don’t, instead showing gradual lumen depreciation that can make it hard to determine the end of their useful life. The most common threshold point for determining the end of an LED source’s useful life is when its output has fallen to 70% of initial levels – a point designated as L70. For more information, see the Fact Sheet Lifetime and Reliability and the report LED Luminaire Lifetime: Recommendations for Testing and Reporting.
For some applications, such as retail lighting, an excessive shift in color over time could mean the end of a luminaire’s useful life, even if there has been little or no lumen depreciation. Standards covering color shift are in development, but in the meantime, for color-critical applications, specifiers should carefully review LM-80 data for LED packages and in-situ temperature measurement data for lamps or luminaires. These data will at least allow the specifier to estimate chromaticity shift over a defined operating period, usually 6,000 hours, under stated temperature conditions. For more information, see the Fact Sheet LED Color Stability and the GATEWAY Report Color Maintenance of LEDs in Laboratory and Field Applications.
LIGHT OUTPUT AND DISTRIBUTION
It’s important to also consider light output and distribution when comparing different lighting products. Because they’re directional light sources, LEDs offer better optical control than conventional sources, which means that with fewer total lumens, they can produce high light levels on nearby surfaces (e.g., with task lighting) and low light levels on more-distant surfaces (e.g., with pole-mounted parking-lot luminaires). To maximize energy efficiency and lighting quality, it’s important to evaluate each luminaire against the specific light level and uniformity requirements of the application.
In theory, LEDs are fully dimmable, but they aren’t compatible with many installed dimmer controls, most of which were designed for incandescent lamps. As LED lighting becomes more widely used in residential applications, integrated LED dimming controls may become a reality in new construction. In the meantime, LED products will need to use dimmers that were made for incandescent products. Some manufacturers publish lists of specific dimmer products tested and approved for use with their LED fixtures. Incompatibility can cause a host of performance problems, including flicker. For more information, see the GATEWAY Report Dimming LEDs with Phase-Cut Dimmers: The Specifier’s Process for Maximizing Success.