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Luminous Efficacy and Color Quality Advances in LED Systems

Light-emitting diodes (LEDs) are a viable alternative to incumbent and emerging lighting technologies in many application areas, including outdoor and general lighting solutions. To lead against other technologies, LEDs must improve to out-compete the alternatives in terms of luminous efficacy and color quality.

Luminous efficacy and proper color rendition is a function of the LED, the thermal management and driver and power-supply efficiency of the luminaire. As a result, lighting solution designers need to consider the complete system.

Luminous efficacy of a source

Luminous efficacy is a measure of how efficiently a light source produces visible light – or the ratio of luminous flux to power. Depending on context, the power can be either the radiant flux of the source's output, or it can be the total electric power consumed by the source. Most often, luminous efficacy of a source is measured in terms of lumens per watt (lm/W), which is increasingly used by standards bodies and regulatory agencies.

For example, the U.S. Department of Energy (DOE) has set hard goals for LED luminous efficacy, seeking to have cost-effective, market-ready warm white LEDs producing 160 lm/W by 2025.

While there is every indication that these goals are achievable — for example Cree's XLamp XM-L LEDs are one of the most efficient on the market right now — LEDs are only one part of LED-based solutions.

Luminous efficacy of the system

Luminaire efficacy should take into consideration the LED's luminous efficacy of a source in light of the relative efficiency of other system components.

For example, "LEDs also require supplementary electronics, usually called drivers," wrote the DOE on its solid-state lighting website. "The driver converts line power to the appropriate voltage (typically between 2 and 4 Vdc for high-brightness LEDs) and current (generally 200 to 1,000 mA), and may also include dimming and/or color-correction controls.

"Currently available LED drivers are typically about 85 percent efficient. So LED efficacy should be discounted by 15 percent to account for the driver." Other system components, including the power supply or the fixture’s overall thermal profile, also will have an effect on the total luminaire efficacy.

At the moment, LEDs are surpassing compact fluorescent lighting in terms of luminaire efficacy. Of course, efficacy by itself is not enough to a make a good and competitive lighting solution. Low-pressure sodium lamps, for example, can outperform LEDs and most other light sources in terms of luminous efficacy, but they have a nearly monochromatic light that poorly renders colors.

Color rendering

For most lighting applications, the light source must accurately reproduce the colors of the objects it illuminates. An example might be down lighting in a kitchen, where one would not want the luminaire to make naturally bright fruit or vegetables seem dull or, perhaps, even spoiled.

Recently, the U.S. National Institute of Standards and Technology, which is part of the Department of Commerce, proposed the Color Quality Scale. This qualitative measurement seeks to improve upon the aging Color Rendering Index (CRI) for comparing the color-rendering capabilities of fluorescent lights, which was introduced more than 40 years ago by the International Commission on Illumination.

However, whether using the newer Color Quality Scale or the CRI, color rendering should be a significant consideration in lighting solution design.