Standards and tests are essential to evaluate the quality and longevity of LEDs and luminaires. So test and measurement solutions have to keep pace with the evolving LED lighting standards
SHWETA DHADIWAL BAID
Though the market for light-emitting diodes (LEDs) is poised to grow and replace compact fluorescent lamps (CFLs) and high-intensity discharge (HID) lamps, LED lighting has its own share of challenges. At recently held Executive Congress on LED Lighting in Delhi, Saurabh Kumar, secretary, Bureau of Energy Efficiency (BEE), said that the lack of technical specifications for LEDs is the biggest constraining factor in India.
There are certain international bodies working on standards for LED luminaires. “Commission Internationale de L’Eclairage (CIE), Illuminating Engineering Society (IES) and American National Standards Institute (ANSI) standards are universally accepted to test and characterise optical and electrical parameters of LEDs,” shares Atulya Sahay, director-Photonics Marketing Company. “For consumers, Energy Star is voluntary energy-efficiency labeling that is awarded to a product that meets strict energy guidelines set by US Department of Energy (DOE) and US Environmental Protection Agency (EPA).”
“Government’s concrete policies to advocate green products will encourage manufacturers to produce and consumers to purchase eco-friendly products,” adds Carol Chiu, marketing manager, GlacialTech.
Standards are continuously evolving to improve the quality and longevity of the products. “Test and measurement solutions are required to function according to the new standards. The most recent standards IES LM79 and IES LM80 were not in existence six years ago, so the older testing solutions cannot be used to perform the new tests. T&M solutions are evolving to catch up with the evolving standards,” explains Vikrant Mahajan, applications engineer, Labsphere.
There is a huge variation from one manufacturer to the other in terms of specifications of a product. Mahajan shares, “T&M providers work with the manufacturers to find what they want to test and build. Normally, 70 per cent of the requirements can be satisfied by commercial off-the-shelf equipment, but the rest 30 per cent is permutation and combination through customised solutions.”
The three main tests performed on LEDs and luminaires are electrical, thermal and optical tests. All these tests are done to get the specifications of the luminaire comprising the LED chip, LED driver and packaging. Nimish Buddhadev, business head-India, Tayo Industrial Group, says, “The biggest challenge is to test its life, consistency and then the driver quality.”
Electrical, thermal and optical (ETO) characteristics of LEDs help product designers to estimate the performance of their LED-based modules and products. The tests are performed on LED chips, LED drivers, the package and then on the finished product.
[stextbox id=”info” caption=”Parameters for LED characterisation”]• Total light output
• Spectral radiant flux
• Spectral luminous flux
• Colour (chromaticity)
• CCT (correlated colour temperature)
• Angular distribution
• Junction temperature
• Forward voltage
• Eye safety (ANSI RP-27, IEC 62475, CIES009)
“Every LED chip undergoes photometry test for its optical parameters before it is used in any design. There are automated machines that capture all the features of the LEDs with computer software. When a luminaire, say, a module or a product, is made out of a series of LEDs, it is also put through the same test,” shares Gaurav Arora, marketing manager, Goldwyn.
Agreeing to this, Deepak Loomba, managing director and CEO, DeCore, says, “Many people often mistake the optical output characteristics of an LED to be the final product’s, which is not so. An LED when converted into a product after being enclosed in cases looses light, thereby reducing the effective lumen. So final optical testing like in case of any other lighting device should be done after LEDs are packaged as a final LED application.”
LED driver is an important electronic component as it interfaces with the actual power supply. “All the electrical tests like HV test, electromagnetic interference test, electromagnetic compatibility test and surge test related to electrical functioning of the product are performed on the LED drivers.” After the LED driver, it’s the filtered and clean power which is fed to the LED chip. Hence quality and selection of LED drivers matter a lot.
“In electrical tests, luminous-intensity-voltage (L-I-V) tests are conducted, while for optical tests the most important parameters are luminous flux (lm) for single LED or luminaire efficacy in lumen/watt, i.e., total light output divided by total electrical input power,” shares Sahay.
[stextbox id=”info” caption=”T&M Equipment for LEDs and LED products”]• Radiometer
• Integrating sphere
• LED profilers
Luminous efficacy is the most important parameter for LED manufacturers to dominate other lighting devices such as incandescent lamps and CFLs. Sahay explains, “For high-power LEDs, thermal is a very critical and most important parameter, as these are very sensitive to junction temperature. The most widely acceptable test standard IESNA LM-79 recommends LED manufacturers to report the flux of an LED at a particular temperature.”
“For accurate high-power LED testing, heat-sink material properties are a must as all the light and efficiency characteristics of an LED might change in case the heat-sink on which it is mounted changes,” shares Loomba.
An ETO characterisation system measures the spectral light output of LEDs while precisely controlling electrical and thermal parameters. With today’s automated test systems, you are able to select the input test parameter ranges and start the test. Advanced data acquisition and data capture capabilities provide all the necessary data to make fundamental design decisions regarding LED selection, thermal management, luminous efficacy, etc. Some of the ETO systems are capable of displaying the relative data in the form of graphs.
Goniometry for directivity parameters
LED goniometric test is used in far-field pattern and angular characterisation of an LED. Buddhadev says, “Single-chip LED goniometry performed on the chip itself in two-dimension (2-D) helps to know the angle of maximum lumens, distribution and lux level at certain distance, beam angle, etc. The goniometric test in 3-D helps in characterising the distribution of LED lumens in both X and Y axes with respect to the viewing angle. This helps to design reflector and LED lenses. Designing the reflector and lens without knowledge of angular parameters means just shoot-and-shot in blind night.”