When comparing LED lighting performance to conventional lighting, buyers will want to consider energy efficiency, operating life and lumen depreciation, light output/distribution, color quality, color shift, dimmability, and expected lifetime.
The final energy efficiency of any lighting system depends on more than the efficacy of the light source itself. In the case of LED lighting, driver efficiency, luminaire optical losses, and luminaire thermal factors also determine luminaire efficacy, expressed as lumens (amount of light output) per watt of electricity used (lm/W). For more information, see Energy Efficiency of LEDs.
Life and lumen depreciation
All light sources diminish in output over their operating life. In comparing the light output and energy efficiency of different types of light sources and fixtures, it is important to consider mean or end-of-life efficacy, and not initial efficacy. Lumen depreciation is an essential consideration in evaluating the design life of LED lamps, which typically do not fail like some conventional light sources. Instead, design life ends at the point where light output is projected to drop below useful levels. The most common lifetime specification is L70, which represents the estimated operating hours before light output falls to 70 percent of initial levels. For more information, see Lifetime and Reliability.
It is impossible to directly measure the lifetime of an SSL product that's expected to last for 50,000 hours or more, which works out to nearly six years of continuous, 24/7 use. SSL technology is changing so rapidly that any product is sure to be superseded by several generations of newer models before that much time has passed. This means that product lifetime has to be extrapolated rather than directly tested. Furthermore, an LED luminaire is a complex system in which many other components also come into play, such as the driver, electrical connections, fixture housing, and optics. Problems with these other elements may sometimes lead to catastrophic failure, or may instead accelerate lumen depreciation, and should be taken into consideration by manufacturers when describing product life. Researchers are working to gain a better understanding of all failure mechanisms that come into play with an LED luminaire. Meanwhile, a DOE-industry working group has published a report, LED Luminaire Lifetime: Recommendations for Testing and Reporting, that details the issues and offers specific guidance for accurate reporting of lifetime.
Light output and distribution
LEDs are inherently directional sources and offer better optical control than traditional energy-efficient sources like fluorescent and metal halide lamps. With fewer total lumens, LED luminaires can produce high light levels on nearby surfaces (e.g., task lighting) or low light levels on more distant surfaces (e.g., pole-mounted parking lot luminaires). To maximize energy efficiency and lighting quality, it is important to evaluate each luminaire against the specific light level and uniformity requirements of each application. For more information, see Establishing LED Equivalency.
The two most common measurements that provide useful guidance on LED color quality are correlated color temperature (CCT), which measures relative color appearance of white light from lower (warm/yellow) to higher (cool/blue) values on the Kelvin (K) scale; and color rendering index (CRI), which provides a comparison of how colors are rendered by LEDs relative to a reference light source (either incandescent or daylight). While these two color quality measures have been used for many years in describing conventional lighting, they only get you in the general ballpark for selecting and matching lamp colors. Two LED light sources with identical CCTs can render colors very differently due to differences in the spectra. And CRI value can be poor at predicting the quality of the appearance of saturated red objects, and doesn't correspond well to human perception of color quality. As a supplement to CRI, a lamp's R9 value describes how closely it renders a saturated red color sample, relative to the reference illuminant. And another value, Duv, quantifies the distance between the chromaticity of a given light source and a blackbody radiator of equal CCT. A number of new color-rendering metrics have been proposed in recent years, but none have been widely adopted as of yet. For more information, see LED Color Characteristics.
For some applications, such as retail lighting, an excessive shift in color over time could mean the end of a luminaire's useful life. Standards are under development for color shift; in the interim, products should be segmented into one of three categories—lamp replacement, standard-grade luminaire, or specification-grade luminaire—with color shift treated differently for each category. For more information, see LED Color Stability.
LEDs are in theory fully dimmable, but they are not compatible with all dimmer controls designed for incandescent lamps. As LED lighting becomes more common for household applications, fully integrated LED dimming controls may become a reality in new construction. In the meantime, LED products will need to be designed to use dimmers that were made for incandescent products. Some LED light fixture manufacturers publish lists of specific dimmer products tested and approved for use with their fixtures.
See the discussion of Standards for more details on industry efforts to improve how SSL product performance is measured and reported.