The latest developments in light-emitting diode (LED) technology – part 1
Solid-state lighting (SSL) is creating a revolution in lighting. Two light sources that constitute SSL (inorganic light-emitting diodes or LEDs and sister technology organic light-emitting diodes, otherwise known as OLEDs) have the potential to replace many light sources that either heat a tungsten filament to incandescence or that use filaments within a glass envelope to create an ionized arc stream, which are the fluorescent and HID lamps.
The development of LEDs, which provide a concentrated beam of light, is about 10 yr ahead of OLEDs, which are based on inorganic (non-carbon) materials and provide a diffuse area light source. An OLED consists of a layer of semiconducting polymer sandwiched between two conductive layers that act as electrodes. When a current is passed between these electrodes, the polymer gives off light. The light is created by electrons released from one electrode layer falling into positively charged holes that have been generated by the polymer’s interaction with the other layer.
Although high-brightness LEDs (HBLEDs) currently suffer from a 30% yield in manufacture and other shortcomings in their operation, their advantages include: directional light, compact size, breakage resistance, controllability, instant on, no infrared or ultraviolet energy radiation, relatively long life and efficiency, absence of radiated heat, and no mercury content.
Up to now, luminaires have consisted of a replaceable light source held in position by a socket(s) within the metal fixture enclosure. The first large market for LED sales is replacing the soon-to-be-banned incandescent A and PAR lamps. Thus, manufacturers offer HBLED bulbs as an energy-efficient substitute in the millions of sockets presently using inefficient sources — a market that will be there for many years to come.
The U.S. Department of Energy (DOE) is advancing HBLED acceptance through industry alliances, workshops, standards development, its SSL CALiPER testing program, the ENERGY STAR certification program, and the Next Generation Lighting Industry Alliance (NGLIA). Through these efforts, the DOE is fostering the design and manufacture of new lighting systems that optimize all of the characteristics of the LED.
The HBLED sources being developed today vary widely in their construction and features. HBLED makers are continuously figuring out ways to coax more lumens from the tiny die and increase color stability. They are also assembling an array of half a dozen or more LED dies into an array, panel, bar, or strip to meet or exceed the light output and efficiency of traditional products with more uniform light distribution.
Because we are at the headwaters of HBLED development, false claims and inaccurate information regarding performance have been marring their entry into the lighting industry. Understandably, the only way to avoid confusion and to validate manufacturers marketing claims is to apply standards. Thus, operational characteristics unique to LEDs, such as lumen maintenance, thermal management within a fixture, and color consistency, are addressed in three standards issued in 2008.
Full life testing is not practical because these sources are expected to have lifetimes exceeding 50,000 hr, which would be 5.6 yr. For that reason, a report called TM-21, “Lumen Depreciation Lifetime Estimation Method for LED Light Sources,” describes how to extrapolate short-term test data to predict the lumen output extended over tens of thousands of hours. The lumen output is measured every 1,000 hr to at least 6,000 hr. The reduction in output at these six points is plotted on a chart and extrapolated for 36,000 hr. If the LED is tested for 10,000 hr, the extrapolation can extend to 60,000 hr. When lumen output drops, 30% is considered the end of useful life for an LED source. In addition, LM-80 recommends testing the chromaticity of the source, because an excessive shift in color temperature is another measure of end-of-life.
Light is radiant energy from the sun that covers a portion of the electromagnetic spectrum called the visible light spectrum, which extends upward in wavelength from violet (380 nm) to red (620 nm to 760 nm) — an even balance of these wavelengths composes pure white visible light.
In the past, because of the limitations of phosphor technology, fluorescent lamp makers urged users to accept the “white” color of these lamps by introducing subjective terms, such as “warm white” or “cool white,” to categorize the “warmth” or “coolness” of the light and to create a comparison to the incandescent source. LED light sources have the same challenges — because of the limitations of the LED phosphors, the difficulty in achieving an exact wavelength in a single PCB-type LED die during manufacture, and because an LED die produces a saturated color. Thus, two terms used in the lighting industry — color temperature and color rendering index — are important in the new standards adopted for the HBLED industry. Let’s explore these two terms in order to better understand their significance.
Color temperature expresses the relative color appearance of a white light source, whether it looks more yellow/gold (warm) or more blue (cool) in terms of the range of hues of white. Consider that daylight contains all of the colors of the visible spectrum, but we judge daylight to be “warm” at sunrise and sunset and to be “cool” at noon on an overcast day.
Understanding color temperature starts with understanding the black body radiation and the Kelvin temperature scale. A theoretical black body is an object that has no color and is black because it absorbs all radiation incident on its surface and emits no radiation at 0 Kelvin. On the Kelvin temperature scale, 0 Kelvin corresponds to -273°C, or absolute zero, where all molecular motion ceases. This theoretical black body radiator, a block of black metal made of carbon and tungsten through which electric current is passed, starts out emitting no radiation. However, as this black body is heated, it emits radiation in the visible spectrum — with the surface changing in color from red to orange to yellow and finally to a blue-white color, going from 1,000K to 6,500K.
Thus, the color temperature of a light source is the temperature at which the heated black body matches the color (appearance) of the light source in question. This means that if we were to compare the lamp’s color to a black body at 6,500K (an overcast sky), the two would appear to be identical to a human observer. Another term for color temperature is chromaticity.
A color temperature designation is truly accurate only for an incandescent lamp because it produces a continuous spectrum, at a color temperature of about 2,700K, as mentioned above. All of the other sources — only gaseous discharge lamps until the development of HBLEDs — that produce a somewhat discontinuous spectrum are referred to as having a correlated color temperature (CCT). ANSI C78.377-2008 establishes eight acceptable CCTs for LED products, ranging from 2,700K (similar to incandescent) to 6,500K (similar to daylight fluorescent).
This change in color across the visible spectrum using Kelvin temperature is plotted on what is called the International Commission on Illumination (CIE) chromaticity diagram of which the plot is called the black body locus. Essentially, using a graph with an X and Y axis, the CIE diagram has the full range of spectral colors represented by their wavelengths and distributed around the edge of the “triangle” or the “color space,” as shown in Fig. 2 (click here to see Fig. 2). The perimeter of the triangle encompasses spectrally pure colors (seen in nature only in rainbows and prisms) ranging from red to blue. Moving toward the center of the triangle dilutes the color until it ultimately becomes white. By specifying the X and Y coordinates related to the triangle, any color can be located on the diagram. The color path of the black body locus, as mentioned above, is located in the general white area of the chromaticity diagram. Therefore, a light source can be characterized by comparing it to the temperature of the black body radiator, which is closest to it on the CIE chromaticity diagram.
Color rendering index (CRI) — a key reference in selecting a light source — has been used to compare fluorescent and HID lamps for more than 40 yr. It expresses the degree to which a range of eight pastel color samples, of low and medium saturation, will appear “familiar” or “natural” under a specific light source, compared to a reference light source of a similar color temperature, using a scale of 0 to 100. The average differences measured are subtracted from 100 to get the CRI. If the lamp under test has a correlated color temperature of less than 5,000K, the black body radiator is usually the reference (an incandescent lamp).
The CIE uses the incandescent source as the reference, because a tungsten filament is similar to a black body radiator. Thus, an incandescent source has a very high CRI — close to 100. At present, the CRI is also used to determine the color-rendering capabilities of HBLEDs, and this rating is used for specification, compliance, and certification purposes in the Energy Star program.
However, in a technical report 177:2007, “Color Rendering of White LED Light Sources,” the CIE group concludes that the existing CRI is not useful in determining the color-rendering capabilities of white LED light sources. Research indicates that even LEDs with low CRI can produce visually appealing light. As a result, the National Institute of Standards and Technology (NIST) developed a new measurement standard in which changes in hue and saturation will not be weighted equally.
The new standard, called a color quality scale (CQS), uses a different and larger set of colors and some other modifications, promising to more accurately reflect the observed color appearance of white light sources, including those that use multiple color LEDs to generate white light. The new standard will be applied to all existing white light sources, both the traditional lamps and all solid-state sources. This determination has significant implications for the outdoor lighting industry, which attempts to hold LEDs to the same CRI standard as other light sources.
Now that we’ve covered the basics of how LED technology works as well as the latest standards development on this front, we will address manufacturing details, getting the most out of your lumens, and electronic control of this technology in Part 2 of this article next month.