LEDs: The Future Is Here
By Brent Ehrlich
For years we’ve been hearing that light emitting diodes, or LEDs, are the greatest thing to happen to lighting since the incandescent bulb replaced gas lights. Until recently, LEDs have been a promise left unfulfilled—an expensive specialty product that was impractical for most applications—but that is changing.
Though some LED lighting still suffers from many of the problems that plagued early fluorescents, including poor color rendering, inconsistent dimming, questionable quality, and high first costs, the technology has been advancing rapidly. In some applications, LED lighting now provides better energy performance, has a longer lifespan, and requires less maintenance than most of its incandescent, halogen, and fluorescent competitors.
“For general lighting applications, just this year LEDs have started to get to a point where there are some viable options,” says Glenn Heinmiller, principal at Lam Partners Architectural Lighting Design. LEDs may have finally crossed over from boutique products to serious lighting options.
Changing the Way We Look at Light
We’ve been used to incandescent bulbs that produce light by heating a metal filament until it glows, and to light sources that excite a gas using an electric arc, such as fluorescent and metal halide fixtures—but LEDs are different. LEDs are solid-state lighting (SSL), using semiconductors to produce light as current is run through them. They share more in common with a computer chip than they do with the incandescent light bulb.
LEDs have a number of advantages over other lighting technologies. LEDs are small, powerful, and versatile and can be used effectively in everything from task lighting to street lights, including applications where using other conventional lighting technology is impossible or impractical—such as in flexible ropes or where changing colors are desired for displays. Yet they are tough enough to be used in applications with high vibration that would damage conventional fixtures; they also work well in cold temperatures, turn on instantly, can be controlled digitally, and, unlike most fluorescents, can be cycled on and off repeatedly with no ill effects. LEDs' best features, though, are their long service life and their energy efficiency—which is improving by the day.
Lighting efficacy is measured in lumens of light produced per watt of energy consumed, or lumens per watt (lpw). The efficacy of LEDs is currently about as good as most fluorescent technologies—around 50 lpw—and LEDs are far better than incandescent bulbs, which max out at around 20 lpw. The efficacy of LEDs has been improving, with some LED replacement bulbs reaching 90 lpw. This efficacy will continue to pay off for years since a well-made LED luminaire should last 30,000 to 50,000 hours. For comparison, incandescent bulbs burn out after 1,000 to 3,000 hours, and compact fluorescent lamps (CFLs) after about 9,000 hours. When LEDs do fail, they don’t fail suddenly. They slowly fade away, officially “burning out” when they hit 70% of their initial light output in a process known as lumen depreciation.
Major electronics companies not known for lighting, such as Samsung, Toshiba, and LG, have now entered the LED lighting market. With all of this competition and industry input, LED innovation and performance are improving daily, the volume of products is increasing, and the cost is dropping—but new problems are also arising.
Putting Together the LED
Complete LED luminaires (including LED replacement lamps, or bulbs) incorporate the LED chip or chips, a heat management system, and electronics. The LED chip produces light when electricity flows across its semiconductor material from the positive side (p-side or anode) to the negative side (n-side or cathode) across the “p-n junction,” releasing energy as a photon of light, along with some heat. Using different semiconductor materials or changing the engineering produces different colors of light. The most common semiconductor material, indium gallium nitride (InGaN), generally creates blue and violet LEDs, while others, like indium gallium aluminum phosphite (InGaAlP), are used to create red, orange, and yellow LEDs.
LED chips are very small (typically about 1⁄16 in.2, or 40 mm2) and are mounted onto a frame; the frame is usually covered by an epoxy case, which doubles as a lens. The frame incorporates a reflector and is designed to dissipate heat away from the LED chip, while the lens maximizes light output and may also contain phosphors used to change the color of the light. LEDs come in different sizes and configurations, and some contain multiple chips, but in general they are usually less than a quarter of an inch wide.
All LED luminaires include a heat sink. Very efficient LEDs convert about 20% of their energy into light and release 80% as heat, and this heat builds up and reduces efficiency and light output. When LEDs are grouped together and other electronic components are added, the temperature in the luminaire can increase dramatically. Manufacturers manage this heat using heat sinks made from large pieces of metal, typically aluminum, shaped into fins to maximize surface area. These fins are visible on most LED replacement light bulbs or on the backs of downlights.
Dimmers and other electronics
Unlike traditional light bulbs, LEDs run on direct current (DC) and usually require a driver, or power supply, that converts the alternating current (AC) from the utility to DC and protects the electronics from voltage fluctuations. Controls for dimming and building automation can also be integrated into the luminaire. In an LED replacement lamp (a type of luminaire that is essentially an LED light bulb), the driver and electronics that enable dimming are contained in the base of the lamp, but in other LED luminaires, the driver can be either part of the luminaire or external.
There are two main types of drivers: constant-current drivers are made for downlights and other fixtures that use one light per driver, and constant-voltage drivers are used with multiple lights connected in parallel. Drivers are critical to the efficacy and overall performance of an LED luminaire and need to be selected so they are compatible with the LED voltage and with other components. They also need to be durable to match the LED’s long lifespan. Drivers need to be incorporated into a lighting system properly, since long runs of wiring can introduce a “hum” or flicker in the LEDs, and too many fixtures on one driver can overload the system. To ensure compatibility, LED manufacturers often supply lists of drivers that work with their products.
The optics can also affect the efficiency of the luminaire. Conventional light sources like incandescents are omnidirectional, spreading light in all directions. LEDs, on the other hand, produce light from a flat surface and send it outward in one direction, so more light is directed at the target and less light is trapped in the luminaire. An LED’s primary optics cover the chip and help improve initial light output, but the light still has to get to its target so secondary optics typically adjust the beam angle or create a more diffuse light.
Ironically, the directional nature of LEDs hurts the performance of the first-generation of LED A19 replacement lamps (those shaped like a conventional incandescent bulb with a screw-mount). These lamps directed the light toward the ceiling and not down at the desk or table, which is a problem if you are trying to use the light to read or write. New LED replacement lamps use a combination of design and secondary optics to recreate a consistent, omnidirectional light that mimics that of a conventional incandescent lamp. This feature has now become a requirement for any Energy Star-rated A19 replacements.
We can compare the efficacy of conventional lighting, which is measured under a constant 77°F (25°C) temperature using relative photometry, because a light bulb and fixture with known light outputs are used as a reference; when a new bulb is introduced, the difference in light output can be easily calculated. This produces a source efficacy (in lpw) for the bulb. LED luminaires cannot be measured this way because the LED cannot be separated from the heat sink and other components, so performance has to be measured based on the entire luminaire using absolute photometry, which produces a luminaire efficacy.
Adding to the complexity, LED chip manufacturers often publish efficacies based on the chip alone, with data taken in one short snapshot at 77°F (25°C). These numbers can exceed 200 lpw, but losses from the heat sink, driver, and optics would otherwise add up to reduce the LED chip efficacy by about half to arrive at the final luminaire efficacy. Staying ahead of the performance curve can be tricky, but Glenn Heinmiller insists, “You have to evaluate everything. That’s what our clients are paying us to do. We have to be constantly on top of advances in LED technology. We can’t depend on the analysis we did a year ago, because the numbers are different.”
Avoiding the LED Blues
Although the first generation of LEDs had a cold, blue-white light, the light quality from LEDs has improved dramatically in the past couple years and is now close to matching or even surpassing that of conventional light sources.
Unlike incandescent lights, which produce the full spectrum of light, LED chips are engineered to produce only one wavelength of light. A fuller spectrum of white light is created by either mixing red, green, and blue—with the final result called RGB LEDs—or by using phosphors that generate white light when hit by photons from blue LEDs. The phosphors can be applied directly to the LED or placed on a layer separate from the LED; the latter is known as a remote phosphor. Most LEDs now use phosphor technology, and remote phosphors are becoming increasingly popular and may maintain their original color longer. These remote phosphors are visible as the yellow coatings found on many LEDs.
Limitations of measuring LED color
The standard metrics for LED color, or chromaticity, are the color rendering index (CRI) and the correlated color temperature (CCT). CRI is a measure of how accurately a light source renders the colors of an object as compared to a reference source using a scale from 0 to 100, with 100 being the most “accurate.” An incandescent bulb has a CRI of 100, and LEDs are now often above 80, with some reaching as high as 98.
CCT is measured in Kelvins and is similar to the colors given off by an ideal metal heated up to the point of glowing. In general terms, color temperatures below 3,200K are considered “warm” because they contain more reddish or yellowish tones, and those above it are considered “cool” because they contain blue tones.
Unfortunately, CCT and CRI do not necessarily provide the best metrics for judging color accuracy, for a host of reasons ranging from the limited number of color samples used in CRI testing to inaccuracies that come up at the far ends of the scale. Because of these limitations, the International Commission on Illumination does not recommend using CRI as a metric for white-light LEDs, and a new standard called the Color Quality Scale, or CQS, is in development. Until that standard is adopted, the U.S. Department of Energy (DOE) suggests CRI is useful when comparing different light sources (i.e., electric and daylighting) and when CCTs are the same; but DOE recommends not using CRI as the sole basis for choosing an LED product.
LM-79: The One Standard You Need to Know
There are guidelines for LED electronics put out by the National Electrical Manufacturers Association (NEMA) and the American National Standards Institute (ANSI), but there are no standards that will ensure that when you purchase an LED product it will perform in a certain way. The only one that can be used for those trying to judge the quality of an LED product is LM-79. “When I am evaluating product, the first thing I ask is, ‘Do you have an LM-79 report?’” said Naomi Miller, senior lighting engineer at Pacific Northwest National Laboratory, which manages DOE’s lighting programs.
LM-79-08 Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products describes standardized test methods for measuring the efficacy and color quality of an entire luminaire. Individual LED chips or separate fixtures are not covered. LM-79 is the basis for DOE’s Commercially Available LED Product Evaluation and Reporting (CALiPER) program, which independently tests LED luminaires through qualified laboratories and creates detailed test reports, summary reports for each year of testing, and benchmark reports for various product applications. LM-79 is also used as the basis for U.S. Environmental Protection Agency (EPA) Energy Star Qualified LED Lamp Program and for the Lighting Facts label.
Product labels and databases
There are several databases available that list LED luminaires tested to LM-79 standards. Selecting products from these lists helps ensure the quality of the LED luminaire, but there are limitations. Testing and verification take time, and with the rapid pace of change in the LED industry, those specifying LEDs will want to check with manufacturers to verify that information is up to date.
Sponsored by DOE, the Lighting Facts program () has developed performance labels for LEDs that look similar to a nutrition label on food packaging, providing a quick summary of performance data based on LM-79 test reports. Lighting Facts also lists participating manufacturers, retailers and distributors, lighting professionals, and energy-efficiency programs.
Energy Star (www.energystar.gov) rates LED replacement bulbs using the same metric it uses for CFLs. These bulbs have to be three times as efficient as comparable incandescents and meet standards for lumen maintenance, rated lifetime, color consistency, CRI, and other factors. They also have to shine light in all directions to illuminate a desk as well as the ceiling. Commercial LED luminaires that meet the Energy Star requirements are tested to LM-79 and have a CRI of at least 80. Performance criteria differ depending on product type, but the LED luminaire manufacturer has to provide a list of compatible dimmers and disclose any known incompatibilities with other controls, occupancy sensors, or other electronics. Utility rebate programs often require Energy Star ratings.
Designlights Consortium () is comprised of utilities and energy-efficiency programs that have developed a Qualified Product Database of LED luminaires. The organization has qualifying standards for 19 product categories based on light output, lumen density, luminaire efficacy, CCT, CRI, lumen maintenance, and warranty. The organization uses Energy Star as the basis for many of the standards and develops its own for categories with no standards. Participating utilities and energy-efficiency programs can use the organization’s product database as the basis for refunds, but they suggest using Energy Star-qualified products when applicable and encourage use of the Lighting Facts label.
Problems with LEDs
LED luminaires are complicated electronic systems, and getting them to work with other controls in the real world is full of variables and potential pitfalls. Here are some problems to look out for.
Getting flicker out
In the days of magnetic ballasts, light from fluorescent fixtures would sometimes flicker rapidly. Electronic ballasts fixed the problem, but flicker may have returned as a problem for LEDs. According to Naomi Miller, “Most LED products don’t flicker, but flicker is created in some LED products because they turn completely on and off once or twice per AC cycle, so with the U.S.’s 60 hertz electrical distribution, these products will have full output and then no output for half the cycle.”
Good-quality drivers smooth out the light output and don’t have this problem, but others suffer from visible flicker produced by this on-and-off cycling. Miller says flicker might not be noticeable at first, only appearing as a strobe effect when your eyes move in relation to the LED light source (or vice versa). “Flicker is noticeable by about 10% of the population,” Miller said, “but whether it is visible or not, it can result in headaches, eye strain, more repetitive behaviors in autistic children, and may slow down your ability to read and process information.“ In some applications—schools, health care, or industry—it might be more of a concern.
The data on the health impacts of flicker are based on testing from the days of early fluorescent fixtures, and adoption of LED lighting is too new to know for certain what health effects, if any, may be caused by flicker from LED lighting.
There is no reliable metric for predicting flicker, so Miller suggests that before installing 100 LEDs in a project, install five, test them, and get feedback from occupants. Once they are installed, she says, you can test for flicker by using a flicker wheel or by just quickly waving your finger back and forth in front of the light. If you see a strobe effect, you have flicker. In that case, make sure a compatible driver or dimmer (see below) is being used, and try changing the dimmer to a switch. Also, contact the dealer or manufacturer: manufacturers are trying to solve these problems, and customer feedback can be helpful.
Dimming an LED can also cause flicker—as well as other problems. “A dimmer cuts part of the sinusoidal AC waveform delivered to the light,” Miller explained (AC current fluctuates from positive to negative in a sine wave 60 times per second), “and in an LED that can mean the light is only on for a fraction of the AC cycle and you only get half the light.” Incandescent light dims smoothly because the heated element retains heat and provides light during the “off” portion of the cycle (this is called persistence). An LED has no persistence, so it is instant on/instant off and hence the chopped waveform of the dimmer can cause the LED to flicker.
Dimmable LEDs are common, but there is no standard for what “dimmable” means. An LED may dim but then buzz, change colors, or shut off prematurely, and there is a limit to how many LEDs can be placed on one dimmer. “If you are specifying LED products and doing dimming, do your homework and make sure the dimmer and driver are compatible,” Miller advises.
Getting the LED blues
In 2010 the International Dark-Sky Association published a study of outdoor lighting such as LEDs with color temperatures of 6,000K that have blue tones and concluded, “There are substantially more deleterious effects to humans, wildlife, and astronomical resources associated with blue-rich light.” Some of the claims involve light pollution from blue-white LED fixtures that affects visibility, but the authors also suggest that the blue light might disrupt circadian rhythms in humans and wildlife.
The authors of the study admit there are no firm data on the relationship between melatonin disruption and blue LEDs, and DOE disagrees with the study’s conclusions. DOE is particularly concerned because a 6,000K LED is 30% more efficacious than a 3,000K (whose color is preferred by most people), so the lost energy resulting from a switch to a 3,000K LED would be equal to the annual amount consumed by 3.7 million households. Efficiency traditionally trumps color quality in nighttime lighting. However, LED manufacturers are improving the efficiency of warm white LEDs daily.
Another possible source of pollution could be LED disposal. A study published in Environmental Science and Technology shows that LEDs that were crushed and subjected to conditions that mimic those in a landfill leached metals that exceeded California’s hazardous waste standards (see “LEDs Exceed California Hazardous Waste Standard,” EBN Feb. 2011). Many LED chips comply with the EU’s Restriction of Hazardous Substance (RoHS) directive, which helps reduce the presence of potential pollutants, but LEDs should be disposed of responsibly in the same manner as computers and other electronics.
Can We Keep Up?
Two years ago, I attended a presentation by Jim Benya, principal at Benya Lighting Design, where he declared that LEDs were a specialty product and didn’t make sense for most lighting applications. So when I spoke with him for this article I was surprised to hear that he now embraces them for many applications. What changed his mind? Within the past year LED performance has improved dramatically, he says, and costs have dropped to the point where they can even challenge many fluorescent lighting systems.
Benya is now incorporating LEDs into more projects as the technology develops. He uses only LEDs for downlights and for residential and hospitality landscape lighting. Heinmiller is using LEDs in retail track lighting, street lighting, and even some downlights, but both Benya and Heinmiller are quick to point out that there are a lot of variables that go into choosing an LED, and in some cases their higher cost makes them a less viable choice (see the sidebar, “Where Do LEDs Pay?” for deeper analysis).
As the tables in the sidebar show, an inexpensive, efficient 2x4 fluorescent is still a better choice than an LED equivalent, but LEDs are an excellent choice for track lighting compared to halogen, with simple payback as fast as one year. The commercial recessed LED downlight may not make as much economic sense as the CFL, but the LED is slightly more efficacious, so if energy savings is your primary goal and payback is not so important, then LED downlights make sense. Heinmiller points out that high labor costs for relamping or putting an expensive dimming ballast on the CFL might bring the costs closer to the LED price tag. For the residential-grade downlight, LED is a “no-brainer” as long as you are comfortable with the color and dimming capability. He said the color is not necessarily bad, but it’s not the same as what many people are used to.
Choosing an LED Product
Because there are no standards that ensure an LED product will perform in a certain way, selecting and using an LED luminaire can still be a huge challenge. While product databases like Lighting Facts and Energy Star might help narrow the search, there is usually no guarantee the product will perform as advertised in your project.
While Naomi Miller relies heavily on LM-79 reports, for those with less lighting experience she suggests reviewing a Lighting Facts label, which contains a simplified version of the data. Most importantly, she insists that “you really need to see it before you specify it: do mock- ups so you know how it is going to perform.“
“We don’t have any way of knowing if an LED is any good,” Jim Benya said. “You can’t even trust three-quarters of the products.” So he recommends going with a trusted manufacturer. “It takes a fully integrated system with thermal and electrical management and real serious testing. If the company you are working with has done these right, you can relax more and count on the product.”
Benya acknowledges that there are applications for which LEDs are currently not cost-effective, such as T5 replacements, but he also has this word of advice for those concerned with payback: “Paying for itself is a fantasy. The money you use to buy LED lighting systems is capital, and the money you use to amortize it is operating funds. The funds don’t cross from one to the other. With LEDs you have to lay out more capital.” He says that anyone who selects an LED based on the lowest cost alone is “an idiot.”
Glenn Heinmiller acknowledges the importance of testing products but says, “The DOE programs are really important because they encourage the development of high-quality products, but I don’t think designers are looking at DOE testing data so much. Even with CALiPER, you don’t know what the specific product is, and the technology is changing so fast the data is out of date by the time you see it.” He continued, “We want to see independent lab test data on the specific product that we are considering.” This rapid pace of industry change makes selecting products particularly challenging. “We are getting samples of products, testing them out, and talking with manufacturers, but the manufacturers often don’t even have the answers.” In one case he had looked at an outdoor lighting product and when he got around to reviewing it a few months later, the technology had changed so much that the manufacturer had to redesign the fixture.
“It is safer to go with an established manufacturer. That is not to say some small startup isn’t making a fantastic product,” Heinmiller says, but “buyer beware.”
What Lies Ahead?
One of the more interesting products to come out in the last year is an edge lighting technology. It uses LEDs placed on the edge of a frame along with optics that serve as the lens to provide an evenly distributed 80 CRI light across the surface of the luminaire. The optics can be fine-tuned so the light can be directed as needed, and the lens can either be opaque (so it looks similar to a conventional 2x2) or clear. The LEDs are not visible along the edge, so with the clear lens the luminaire can be integrated into the ceiling so it is barely noticeable.
Organic LEDs (OLED) have the potential to open up real design possibilities and could even change the way we light our buildings. Like LEDs, OLEDs are solid-state lighting, but they come in a flat, flexible film form that, theoretically, can be made nearly any size. OLED technology can be found in some smart phones and in lighting. OLED’s flexible film provides a diffuse light that can be bent into shapes, used to cover objects, or even incorporated into the building itself. OLEDs are currently very expensive, the efficacy isn’t great, and their size is limited, but their potential is enormous.
The rapid advancement of LED technology is already having a profound impact on how we light our buildings. Third-party case studies of LED installations compiled by DOE, Energy Star, and other programs demonstrate the potential energy savings from using current LED technology, with verified energy savings over comparable conventional product installations exceeding 50%. Moving forward, energy savings will only improve as more efficient, more powerful LED chips are used. These more efficient chips will produce more light so fewer luminaires will be required, leading to additional savings. And these advances are happening right now. The average efficacy of products tested by CALiPER has improved from 21 to 57 lpw since 2007, and DOE predicts the maximum efficiency for LEDs will reach 250 lpw by 2020, with LED luminaire efficiency expected to reach 219 lpw. DOE might have to adjust for the pace of innovation, however, since LED chipmaker Cree has already produced and tested a proof-of-concept LED chip that reaches 231 lpw.
LEDs are no longer a lighting fad. With careful selection and when used in the right application, they are powerful tools for bringing down energy use in our buildings. So if you still think LEDs aren’t viable lighting options or if you have been waiting for the technology to mature, Heinmiller has a word of caution for lighting designers: “If you don’t keep up with LED technology, you are going to be left in the dust.”