What is LED?
"A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electro luminescence. LEDs are small extended sources with extra optics added to the chip, which emit a complex intensity spatial distribution. The color of the emitted light depends on the composition and condition of the semi conducting material used, and can be infrared, visible or near-ultraviolet.
Rubin Braunstein of the Radio Corporation of America first reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955. Experimenters at Texas Instruments, Bob Biard and Gary Pittman, found in 1961 that gallium arsenide gave off infrared (invisible) light when electric current was applied. Biard and Pittman were able to establish the priority of their work and received the patent for the infrared light-emitting diode. Nick Holonyak Jr. of the General Electric Company developed the first practical visible-spectrum LED in 1962.
The Illuminating Engineering Society of North America Standards
LM79-08 Approved Method:
Electrical and Photometric Measurements of Solid-State Lighting Products
Test data allows the specifier or end user to evaluate the suitability of the SSL lighting system for its use in a particular application or to compare SSL lighting systems against one another. LM79 provides for the total luminous flux, electrical power, efficacy and chromaticity.
Why is this important with LED’s?
Relative photometry utilizes a lighting manufacturer’s standard product, photometric testing is accomplished using a reference lamp and a reference ballast or transformer if required. Therefore relative photometry does not provide electrical power information. It also does not take into account heat characteristics of the system regarding efficacy or chromaticity. In fact it does not provide chromaticity characteristics at all. All of these issues are very important when evaluating SSL systems.
Absolute photometry requires the lighting manufacture to submit the complete SSL lighting system for measurement so that the resulting data reflects the actual flux, colorimetric performance and the electrical power measurements of the lighting fixture packaged for its intended use.
The importance of absolute photometry and the IESNA LM79 testing standard is that an LED is an electronic device. While an LED has no IR or UV in its beam it does create heat. The LED directly converts electrons to photons unlike the traditional lamps that convert electrons into heat or exciting an internal gas. The point at which the LED converts the electrons into photons is the P/N junction. This point is measured for heat as the junction temperature or TJ. This heat must be conducted away as fast as possible. This is known as heat sinking. The better the system is at moving this heat quickly will allow the LED to operate cooler and will provide for a higher efficacy, better lumen maintenance and a more consistent color output over time.
Several things within the system can affect the TJ. The driver within the system will determine the forward current at which the LED’s are driven. This will determine the lumens produced by the LED’s with a diminishing return for higher drive current due to increased TJ. Of equal importance is how well the LED design handles heat dissipation to the electronic board, how well the electronic board dissipates heat to the substrate and how well the substrate dissipates heat to the heat sink and then, how well the fixture manufacturer dissipates this heat away from the fixture.
LM80-08 Approved Method:
Measuring Lumen Maintenance of LED Light Sources
LM80 covers lumen maintenance measurement for LED packages, arrays and modules. It does not cover any other aspects of LED performance and must be supplied to the lighting manufacturer by the LED manufacturer. This is why B-K Lighting and Teka Illumination will only use LED’s from manufacturers that have tested their LED’s to this standard.
IESNA LM80 Sets the standards for uniform test methods for LED manufacturers under controlled conditions for measuring LED lumen maintenance while controlling the LED’s case temperature, the forward voltage and forward current to the LED. It also requires the LED manufacturer to measure at a 55°C, 85°C and one other case temperature chosen by the manufacturer, typically at 110°C. It also requires the lumen maintenance data for at least 6,000 hours of constant DC mode operation. The preferred method is 10,000 hours.
How do we use this information?
LM80 does not speak to this issue, but LED manufacturers then extrapolate this data to provide lumen maintenance out to L70 or useful lumens life. At this time IESNA is working on TM21 that will standardize this extrapolation method for all LED manufacturers.
Traditional lamps have a lamp life expressed in a 50% mortality rate. In other words half of the lamps will have ceased to operate at X hours. While traditional lamps experience lumen deprecation over their life, this is usually taken into consideration during the calculation phase of the design. Since LED’s could theoretically operate forever the industry has established the L70 standard or where the lumen output of the LED system will have depreciated to 70% of initial lumens. This is the theoretical extrapolation that is done using the LM80 information and the lighting manufacturers tested TJ. These numbers are usually in the range of 30,000 to 50,000 hours and beyond.
What does this mean in real life?
The designer must ensure the SSL lighting system “being considered for specification” has been tested to the LM79 standards and that the SSL lighting system manufacturer can provide the LM80 data from the LED manufacturer. A typical halogen MR16 lamp has a lamp life of 4,000 to 5,000 hours with lumen maintenance of 70% at end of life. A typical pulse start metal halide lamp has a lamp life of 12,000 to 15,000 hours with lumen maintenance of 50% at end of life.