LED technology: changing the future of lighting
Understanding the fundamentals of solid state lighting
The light-emitting diode (LED), one of the most energy-efficient and rapidly-developing lighting technologies, has the potential to fundamentally change the future of lighting in the United States.
That’s according to the Department of Energy (DOE), which also predicts that by 2027 widespread use of LEDs, also called solid state lighting, could save the equivalent annual electrical output of 44 large electric power plants (1,000 megawatts each), and a total savings of more than $30 billion at today’s electricity prices.
“The potential of LED technology to produce high-quality white light with unprecedented energy efficiency is the impetus for the intense level of research and development currently supported by the U.S. Department of Energy,” the federal agency said.
Origins of LED
In 1962, General Electric researcher Nick Holonyak, Jr., invented the first practical LEDs, which were used as red indicator lamps in electronic devices.
While standard, incandescent bulbs produced light using a glass enclosure containing a filament, which would eventually burn out, the new LED technology used tiny light bulbs illuminated by the movement of electrons in a semiconductor chip, called electroluminescense.
Industry was interested in the new technology because of its advantage over standard light bulbs: uses less power, has longer lifetime, produces little heat, and emits colored light.
Over the next two decades, the evolution of the technology and improvements in manufacturing efficiency led to the development of more colors, wider acceptance in the marketplace, and the growth of more applications such as calculators, digital watches and lab testing equipment.
By the 1980s, new technologies were boosting LED light output and efficiency leading to new applications as indicator lights in DVD players, microwave ovens, and other domestic appliances. Soon after, LEDs were being used in more rugged applications such as traffic lights and automobiles.
Today, the high efficiency and directional lighting of LEDs makes them ideal for many industrial uses – offices, warehouses, signs and displays, restaurants and public spaces, just to name a few.
On the consumer side, Christmas tree lights are among the most popular and most affordable LED consumer products on the market. They save 90% or more in utility costs, operate at cooler temperatures, and have an operational life span of roughly 20,000 hours (enough to last for 40 holiday seasons).
Challenges remain
The availability of LED products in the market continues to grow, with new generations of devices becoming available about every four to six months, according to the Department of Energy.
The quality and energy efficiency of LED products also varies widely, and many lighting manufacturers face a learning curve in integrating LED technology into lighting products.
“Manufacturers vary in their ability to do this effectively,” the Department of Energy (DOE) said.
The LED difference
LEDs differ from other lighting sources such as incandescent bulbs and compact fluorescent lamps (CFLs) in various ways.
LEDs don’t suddenly burn out like incandescent or CFL lamps, but instead gradually fade in brightness over time, a process known as lumen depreciation. The useful lifetime of a LED product is based on the number of operating hours until the LED is emitting 70% of its initial light output.
A typical incandescent lamp lasts about 1,000 hours; a comparable CFL lasts up to 10,000 hours, and some linear fluorescent lamp-ballast system can last more than 40,000 hours. But quality white LEDs in well-designed fixtures are expected to have a rated useful life of 50,000 hours or more.
Not unlike incandescent bulbs, which release 90% of their energy as heat, and CFLs, which release about 80% of their energy as heat, LEDs also emit a significant amount of heat. So, why are they cool to the touch?
Incandescent bulbs are hot to the touch because infrared radiation heats the glass enclosure. The intense heat in LED devices is not from infrared radiation but instead produced by the semiconductor chip.
Since higher temperatures in LEDs will result in lower light output, referred to as lumen depreciation, the heat needs to be removed using a heat sink, which keeps the device cool by dissipating the heat.
“Most high power LEDs convert only about 15 percent of the input power into light with the rest being lost as heat,” according to Materials for Advanced Packaging published in 2008 by Daniel Lu and C. P. Wong.
White-light LEDs
Unlike incandescent and fluorescent lamps, LEDs are not inherently white light sources. They monochromatic LEDs emit colors (except white) depending on the materials it’s made of, which explains their efficiency in colored light applications such as traffic lights and exit signs.
To use LEDs as a general light source it needs white light. There are two ways to do this:
- Phosphor conversion: a phosphor, which exhibits the phenomenon of luminescence, is used on or near the LED to emit full spectrum white light
- RGB systems: mixes light from multiple monochromatic LEDs (red, green, and blue) to produce white light.
Courtesy: Department of Energy
Quality of LED lighting
A couple of key terms to keep in mind when considering aspects of lighting quality are color appearance and the Color Rendering Index (CRI).
Color appearance, referred to as Correlated Color Temperature (CCT), measures color of the light source using Kelvin (K) temperature, which indicates the warmth or coolness of a lamp’s color appearance.
The lower the Kelvin temperature (2700–3000 K), the warmer the color of the light, while the higher the temperature (3600–5500 K), the cooler, and more bluish, the light appears.
For most indoor lighting applications and living spaces, warm white (2700K to 3600K) is preferred. Cool light (3600K to 5500K) is more appropriate for visual tasks because it produces higher contrast than warm light.
Until recently, most white LEDs had very high CCTs, often above 5000 Kelvin. High CCT light sources appear “cool” or bluish-white. While very high CCT LEDs are still common, products with neutral and warm-white LEDs are now sold on the market. They are less efficient than cool white LEDs, but have improved significantly, and the efficacy gap between cool and warm LEDs is narrowing, U.S. energy regulators say.
Types of White LEDs
| Lighting Type | Efficacy (lumens/watt) |
Lifetime (hours) |
Color Rendition Index (CRI) | Color Temperature (K) | Indoors/ Outdoors |
| Cool White LEDs | 60–92 | 25,000–50,000 | 70–90 (fair to good) | 5000 (cold) | Indoors/ outdoors |
| Warm White LEDs | 27–54 | 25,000–50,000 | 70–90 (fair to good) | 3300 (neutral) | Indoors/ outdoors |
Color Rendering Index, or CRI, is a widely accepted measure of how well a light source renders colors, compared to incandescent and daylight sources. The CRI scale of 0 and 100, with 100 representing perfect color rendering based on illumination by a 100-watt incandescent light bulb.
Since most objects are not a single color, but a combination of many colors, light sources deficient in certain colors may actually change the apparent color of an object.
A light source with a CRI of 80 or higher is considered acceptable for most indoor residential applications.
While CRI is generally considered by industry to be a more important lighting quality than CCT, it’s been found to be an inaccurate, unreliable predictor of color preference of LED lighting products as it can result in negative values for some LED lamps.
To address the shortcomings of the CRI for LED, or solid-state, light sources, a Color Quality Scale (CQS) was developed by the National Institute of Standards and Technology, an agency of the U.S. Department of Commerce. It is now being considered by the International Commission on Illumination (CIE) to replace the CRI.
Endorsing the new CQS measurement, Dr. James Brodrick, lighting program manager for the DOE’s Building Technologies Program, said, “Regardless of the type of light source, the CQS represents the color rendering qualities of white light more accurately than the CRI and is a far better predictor for colors that have a high red content, such as skin color and wood finishes – which is one of the CRI’s major weaknesses.”
They are also mono-directional, similar to a flashlight, which makes them ideal for applications such as recessed downlights and task lighting.
Other unique characteristics of LED lighting include:
- compact size
- long life and ease of maintenance
- resistance to breakage and vibration
- good performance in cold temperatures
- lack of infrared or ultraviolet emissions
- instant-on performance
- ability to provide dimming and color control
Typical LED Efficacy Compared to Conventional Lighting Technologies in 2010
| Product Type | Typical Luminous Efficacy (in lm/W) |
|---|---|
| LED cool white package | 130 |
| LED warm white package | 93 |
| LED A19 lamp (warm white) | 64 |
| LED PAR38 lamp (warm white) | 52.5 |
| High intensity discharge (high watt) Lamp and ballast |
120; 111 |
| Linear fluorescent Lamp and ballast |
118; 108 |
| High intensity discharge (low watt) Lamp and ballast |
104; 97 |
| Compact fluorescent lamp | 63 |
| Halogen | 20 |
| Incandescent | 15 |
Sources:
For more information on the basics of LED Lighting:
- LEDs Magazine
- Department of Energy LED Basics
- What is Color Rendering Index CRI
- Is CQS an Improvement on CRI?