Unlike incandescent lamps, LEDs are not inherently white light sources. Instead, LEDs emit nearly monochromatic light, making them highly efficient for colored light applications such as traffic lights and exit signs. However, to be used as a general light source, white light is needed. White light can be achieved with LEDs in three ways:
- Phosphor conversion, in which a phosphor is used on or near the LED to convert the colored light to white light
- RGB systems, in which light from multiple monochromatic LEDs (e.g., red, green, and blue) is mixed, resulting in white light
- A hybrid method, which uses both phosphor-converted (PC) and monochromatic LEDs.
The potential of LED technology to produce high-quality white light with unprecedented energy efficiency is the primary motivation for the intense level of research and development currently supported by the U.S. Department of Energy.
Future of LED
There are many white LED products available on the market, and the number continues to grow, with new generations of devices constantly emerging. While many of these products perform quite well, their quality and energy efficiency can vary widely. There are standards, test procedures, and resources such as LED Lighting Facts®, ENERGY STAR®, and the DesignLights Consortium’s™ Qualified Products List that can enable buyers to make informed decisions when evaluating LED lighting.
LED lighting technology has improved dramatically over the past 10 years. Improvements in technology have enabled LEDs to achieve among the highest efficiencies of available white-light sources. Improvements in manufacturing have enabled LED products to achieve a low enough cost that there has been measurable LED adoption in all general illumination applications.
Despite this progress, further improvements are both possible and desirable. The technology can be improved in efficiency and in other features, such as color quality, light distribution, form factor, and building integration. The manufacturing technology for LED lighting can also be improved to reduce cost and increase market penetration, resulting in the greatest possible energy savings for the nation.
LEDs offer the potential for cutting general lighting energy use nearly in half by 2030, saving energy dollars and carbon emissions in the process. Their unique characteristics—including compact size, long life and ease of maintenance, resistance to breakage and vibration, good performance in cold temperatures, lack of infrared or ultraviolet emissions, and instant-on performance—are beneficial in many lighting applications. The ability to be dimmed and to provide color control are other benefits of LED lights.
One of the defining features of LEDs is that they emit light in a specific direction, which reduces the need for reflectors and diffusers that can lower efficiency. In contrast, fluorescent and “bulb”-shaped incandescent lamps emit light in all directions, with the result that much of the light they produce is lost within the fixture or escapes in a direction not useful for the intended application. With many fixture types, including recessed downlights, troffers, and undercabinet fixtures, it is not uncommon for only 50 to 60% of the total light produced to be emitted.
In addition, LED sources are inherently dimmable and instantaneously controllable, and they can be readily integrated with sensor and control systems, thus enabling further energy savings through the use of occupancy sensing, daylight harvesting, and local control of light levels. What’s more, the high-speed modulation capability of semiconductor light sources has introduced new opportunities and features, such as indoor positioning capabilities, and LED technology also offers the prospect of full color control over the light spectrum. What this all adds up to is the potential to improve the performance and value of lighting in totally new ways.
LED lighting has the potential to be more energy efficient than any other known lighting technology. But, two aspects of energy efficiency are important to consider: the efficiency of the LED device itself (source efficacy) and how well the device and fixture work together in providing the necessary lighting (luminaire efficacy). How much electricity is consumed depends not only on the LED device, but also on the lighting fixture design. Because they are sensitive to thermal and electrical conditions, LEDs must be carefully integrated into lighting fixtures. The efficiency of a poorly designed fixture that uses even the best LEDs will be only a fraction of what it would be if the fixture were well-designed, and the design can also affect lumen maintenance.
Energy performance of white LED products continues to improve rapidly. DOE’s long-term R&D goal calls for cost-effective, warm-white LED packages producing 255 lumens per watt. This chart shows typical luminous efficacies for traditional and LED sources, including ballast losses as applicable.
LED Efficacy Compared to Conventional Lighting Technologies
|Product Type||Luminous Efficacy (in lm/W)|
|LED A19 lamp (dimmable, warm white)||78|
|LED PAR38 lamp (warm white)||70|
|LED T8 tube (neutral white)||107|
|LED 6″ downlight (warm white)||64|
|LED troffer 2’x4′ (warm white)||94|
|LED high/low-bay fixture (warm white)||102|
|LED street light||96|
|High intensity discharge system (high watt)||115|
|Linear fluorescent system||108|
|High intensity discharge system (low watt)||104|
|Compact fluorescent lamp A19 replacement||70|
|Compact fluorescent lamp A19 replacement (dimmable)||70|
Key aspects of high-quality light are the color appearance of the light itself, which is described by its correlated color temperature (CCT), and how the light affects the color appearance of objects, which is commonly called color rendition. Color rendition can be quantified using the color rendering index (CRI), or with one of several other recently developed metrics. LED light sources have demonstrated that they can achieve a wide range of color quality, depending on the demands of the lighting application. However, in order to achieve high levels of color quality, there are typically cost and efficiency tradeoffs. In general, a minimum CRI of 80 is recommended for interior lighting, and LED products can readily achieve this performance. CRIs of 90 or higher indicate excellent color fidelity; LEDs can also meet this threshold. CRI is far from a perfect metric and is especially poor at predicting the fidelity of saturated reds, for which the supplemental value R9 is often used. New metrics, such as the fidelity index (Rf) and the gamut index (Rg), which are described in IES TM-30-15, can provide a more comprehensive evaluation of color rendering. Learn more about TM-30-15 and LED color characteristics.
LED luminaire useful life is often described by the number of operating hours until the LED luminaire is emitting 70 percent of its initial light output. Good-quality white LED lighting products are expected to have a useful life of 30,000 to 50,000 hours or even longer. A typical incandescent lamp lasts about 1,000 hours; a comparable CFL, 8,000 to 10,000 hours; and the best linear fluorescent lamps, more than 30,000 hours. Learn more about LED lifetime and reliability.
A primary cause of lumen depreciation is heat generated at the LED junction. Unlike other light sources, LEDs don’t emit heat as infrared radiation, so it must be removed by conduction or convection. Thermal management is arguably the most important aspect of successful LED system design.
Costs of LED lighting products vary widely. Good-quality LED products may carry a significant cost premium compared to standard lighting technologies. However, costs are declining rapidly. LED package prices declined to approximately $1/klm by 2016, resulting in dramatically reduced LED lamp and luminaire prices. In general, LED lighting products are still more expensive than their conventional counterparts, but when the costs of energy and maintenance are included in the total cost of ownership, LED-based products can have a distinct advantage. Learn more about cost-effectiveness trends in the latest SSL R&D Plan.