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.
• 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.
• 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.”
The complete product 3-D goniometry ensures that the design of luminaries is as per the requirement and it does not have any dark spot or error. There are some latest, compact, low-cost goniometer measurement assemblies optimised for fast, accurate angular measurements. These are compatible with spectro-radiometers, monochrometers and photometers, making them ideally suitable for performing colour and/or intensity measurements in labs or production lines.
• LM-79-08 for electrical and photometric measurements of solidstate lighting products
• LM-80-08 for measuring the lumen maintenance of LED light sources
• RP-16-05 for adding SSL product terms
• TM-16-05 tech memorandum of SSL
CIE (Commission Internationale de L’Eclairage)
• 127-2007 (for single low-power LEDs only)
• CIE-127 (2007) A&B (for intensity measurement of discrete LEDs)
• ASSIST (RPI Lighting Research Center)
Recommendation for testing and evaluating luminaires used in directional lighting
• ANSI (American National Standards Institute)
• C78.377-2008 for chromaticity of SSL sources (indoor only)
• DOE (US Department of Energy)
• ENERGY STAR program requirements for SSL luminaires
Spatial distribution is a measurement of intensity as a function of viewing angle. This measurement is traditionally performed by holding the test lamp stationary and swinging a small-aperture detector in an arc about the device under test. A complete hemispherical characterisation of the device can be assembled by rotating the plane of measurement with respect to the device, and repeating this test for a number of meridional angular scans.
For mapping the angular characterisation, you need to manually rotate the LED. However, new assemblies have an automated axial range that eliminates the need for manual rotation. New equipment also feature high-speed USB interface to capture and store data points on a computer.
LEDs are predicted to work for 100,000 hours, while LED products last 70,000-80,000 working hours. To actually test it, will you keep the luminaire on for 24 hours a day for eight-ten years? Well, not possible practically. Most of the test results and conclusions are approximated as the function of some other parameter. “Lifetime of LED products is determined as a measurement that is function of temperature,” explains Mahajan. “Thermal evaluation or checking optical parameters as a function of temperature is critical to predict the life of the product.”
“As it is practically impossible to perform life-test of an LED for ten years, accelerated tests are carried on the products. Characteristics of the product are monitored for extreme conditions. So if the product is to work at 50°C, we take it to the oven and test it for 150°C. The graph of LED reaction helps in determining the approximate life. Thus the life span of the product is concluded,” says Arora.
Agreeing to this, Chui says, “Tests and measurements solutions for time-consuming parameters like lumen maintenance (LM) and lifespan (LS) of LED products eventually conclude with mathematical or statistical predictions or estimations. To end up with more credible T&M results from LM and LS, more elaborate mathematical models or statistical analyses should be invoked to perform more exact and precise predictions or estimations.”
While explaining how the tests are actually conducted, Arora says, “If the LED is recommended by the manufacturer for 700mA current, the actual tests are performed at 50 per cent of the rated value, i.e., 350 mA or 450 mA. Practical conditions such as voltage fluctuations and surges contribute to the overall life of the LED. Based on this, if the LED manufacturer talks of 100,000 hours, to be on very conservative side, take 50 per cent of the rated life.”
You can have best design and best luminosity, but if you don’t have proper heat management, it will affect the life-span of the product.
Colour measurement and consistency
“One of the most important parameters of LED products along with the total luminous flux is colour quantities. It is important to understand the colour-dominant wavelength, chromaticity and correlated colour temperature,” explains Mahajan. Integrating spheres along with spectroradiometer are used to measure colour quantities.
Colour quantity measurements are most uncertain, and may result in large unexpected results. National Institute of Standards and Technology (NIST) has established a reference spectroradiometer, tailored for LED colour measurement and spectral colour measurement. Spectroradiometers are used to measure spectral power distribution in the visible region of wavelengths. Charge-coupled-device (CCD) spectrometers are sensitive to a wide range of wavelengths, from infrared and optical to ultraviolet and X-rays. Some of the test stations have fast-scanning 16-bit, 2048-element CCD spectrometer configured to cover wavelength in the range of 280 nm to 800 nm.
“Normally, red-green-blue (RGB) and intensity tests are performed on LEDs. The LED analyser allows you to perform fast and automatic tests of both the colour and intensity (brightness) of the LED. In this, the light from LEDs is collected by plastic optical fibres and carried to the LED analyser board for measurement and analysis,” shares Jim Hill, director of sales and marketing, Intrinsic Quality.
Eye safety tests for LEDs
As quoted on the website of Orb Optronix, “A system determined to be safe may fall into a different classification with addition of an optic as simple as lens.” Development of powerful LEDs performing to their best in narrow spectres of intense light has exposed the consumer market to another health hazard. A lot of research is going on to study the eye-safety testing and evaluation of LEDs. Companies like Orb Optronix have applied for ANSI- and IESNA-recommended practices to take care of such a situation.
Designing for safety is a concern to both the LED manufacturers as well as product designers. But in optics, LED manufacturers do not have control over how their LED chip will be used. In optics, the total result cannot be determined by combining individual results, which makes testing role of LED-based product manufacturers more critical than LED component manufacturers.
Consumer safety is the key responsibility in product testing. It is necessary to carefully evaluate every hazardous condition and measure spectral radiance.
Expectations and goals for future
The only way to distinguish between good and bad products is the quality, for which standards and tests are very important. “Testing and qualifications are the market differentiator when so many low-cost, low-quality products enter the market,” feels Mahajan. Introduction of cheaper LED lamps by unorganised players has hampered the penetration of good-quality certified LED products. LED lighting is expected to have a great future, but the penetration largely depends on the affordability.
There needs to be some kind of specifications and standardisation as early as possible. BEE, Central Power Research Institute and other organisations in India are working towards it. At the LED Lighting Congress, Kumar voiced another challenge, “Even though we have standards, we still don’t have a national-level testing laboratory. Within this year, the work for the national test facility for LEDs will begin.” Kumar also announced an impending global tender for a million LED lamps expected this year for which BEE will announce the specifications.
“When it comes to LED products, the focus is on achieving maximum light output, i.e., maximum number of lumens with minimal electrical consumption. The move is towards getting better luminous efficacy, better thermal management, better life time and colour consistency,” says Mahajan. Standardisation and measurement will play an important role in this.
The author is a senior technology journalist at EFY