As light emitting diodes (LEDs) cannot be manufactured with consistent optical properties—its brightness and colour can vary from component to component, even in the same production batch—LEDs have to be tested during production and in their final application, for example, in the luminaries. Comprehensive optical characterisation is also essential during R&D for LEDs and for LED based products. Throughout the development process from R&D, through production and packaging, to luminaire design, there are many industry standards that need to be met. Therefore, a number of test and measurement (T&M) instruments are required, which also need to keep pace with the evolving lighting standards and technological upgradations.
By Richa Chakravarty
Tuesday, December 13, 2011: Different tests required
While testing LEDs, one needs to measure electrical parameters (voltage, current, etc), thermal parameters, and light output. Electrical, thermal and optical (ETO) characteristics of LED tests are performed on LED chips, LED drivers, and on the finished product. The optical test is important to check parameters like luminous flux and luminaire efficacy. For optimising the performance of the LEDs and luminaire, a variety of testing equipment are required as per the specifications of a product.
The typical setup used for LED measurements consists of a wafer prober, a source measure unit (SMU), and probes that apply an electrical current to the device under test (DUT) and measure its electrical parameters. An LED test system also includes light collection optics, such as an integrating sphere or optical fibre, and a spectrometer to measure the spectral output and optical power of the DUT.
Instruments used for testing electrical parameters
From measuring operating voltage, input current, frequency and power factor, there are various tests conducted on LED luminaires. Among the electrical tests conducted are the luminous-intensity-voltage (L-I-V) tests. All electrical tests like the HV test, electromagnetic interference test, electromagnetic compatibility test and surge test, relate to the electrical functioning of the product, and are performed on the LED drivers.
SMU, in-circuit testers, power supplies, digital multimeters (DMM), and power analysers are some of the instruments that help in testing electrical parameters. While DMMs are used for basic measurements like voltage, current, resistance, etc, power supplies are used for injecting power or stimulating the device. While oscilloscopes help in measuring waveforms and frequency, a power analyser is used to measure the efficiency of the drive circuit, including the LED driver IC.
An SMU enables a wide range of accurate and fast current versus voltage (IV) measurements. It combines the capability of the current source, voltage source, current meter and voltage meter along with the capability to switch easily between these various functions, into a single instrument. This gives it the ability to evaluate the IV characteristics of devices across all four measurement quadrants without the need for any additional equipment. “For quick IV (current versus voltage) characterisation, SMUs that have the capability to output and measure both voltage and current, are used. Besides this, SMUs also possess a compliance feature that allows a limit to be placed on the voltage or current output, in order to prevent device damage,” informs Sadaf Arif Siddiqui, senior technical
marketing engineer, Agilent Technologies India Pvt Ltd. Agilent offers the B2900A series single or dual channel SMU units that have a wide range of IV measurement capabilities for a variety of two-terminal and three-terminal devices. SMUs in this series have a resident graphical use interface (GUI) that makes it easy to perform a variety of tests and save measurement data without having to write any code.
Digital multimeters also help in the accurate measurement of current and voltage settings for different parameters in luminaries. Along with the DMM SM5011A, Scientific Mes-Technik offers highly efficient power supplies from Delta Elektronika, Netherlands. “While selecting the test instruments, one has to ensure consistent performance and accuracies are maintained for the measuring instruments. In designing and testing LED drivers, the high rise pulse can easily be analysed on 100 MHz digital oscilloscopes. Similarly, HM8115 is the power meter with a good resolution of 1 mW, which is used for measuring accurate power for luminaries during final testing and approval,” explains Anirudha Kela, managing director, Scientific Mes-Technik Pvt Ltd.
Instruments used for testing thermal parameters
For high power LEDs, thermal parameters are the most critical and important, as these are very sensitive to junction temperature. The most widely acceptable test standard, IESNA LM, recommends LED manufacturers to report the flux of an LED at a particular temperature. At present, LED and LED lamps and lanterns
cannot achieve ideal service life due to the low performance of LED. The LED junction temperature in the working status is not only the direct key to its life and efficacy, but is also influenced by the thermal resistance under the same service conditions. The feasibility of the heat conduction system design of the LED also directly affects the life of lamps and lanterns. Therefore, the thermal performance test for the LED has direct significance on LED production, application and research. LED thermal and electrical performance analysers and thermal and spectroradiometeric systems for LEDs are good instruments to do thermal performance tests.
LED thermal and electrical performance analysers can measure junction temperature, the K factor, forward voltage, forward current, automatically record the changing curve of the parameters at a given time, track the instantaneous changes in junction temperature, and do the high resolution measurement of the relationship between junction temperature and voltage, with the time.
Thermal and spectroradiometric systems can not only test the thermal performance (LED thermal resistance, etc) of single LEDs, but also conduct integration testing on optics, colour, electricity and heat, and record the changing curve of the parameters with time, set programmable thermal sink temperatures and forward currents of LEDs to simulate different working conditions of LEDs.
Instruments used for testing optical parameters
Optical T&M of LEDs involves the measurement of the light output, colour or spectral characteristics of the device under test as a function of drive current, viewing angle, or other conditions. Simple tests yield a single data point; more complex tests yield a function relating an optical value to another parameter.
An optical test measures parameters like luminous flux (lm) for a single LED and luminaire efficacy. Luminaire efficacy is measured in lumens/watts, that is, total light output divided by total electrical input power. “Out of the many parameters of the LED, total luminous flux is the fundamental quantity. Total flux means the total flux integrated in all the directions (that is, 4π steradian solid angle). In some applications, instead of total flux, partial measurement of the flux is done, ignoring flux in unintended directions, for example, backwards,” informs Anil G Chowta, director, Falcon Electro-Tek Pvt Ltd. Falcon has introduced the FO-LH200 to cater to the need of LED parameter measurement including luminous intensity and luminous flux. It is designed to measure the absolute spectral power of the light under test, and also measures spectral radiant flux, peak wavelength, dominant wavelength, centre wavelength, excitation purity, chromaticity coordinates, correlated colour temperature and the colour rendering index.
Every LED chip undergoes a photometry test for its optical parameters before it is used in any design. When a luminaire, like a module is made of a series of LEDs, it is also put through the same test. Simple photometric measurements may be performed with a photometer or a spectroradiometer. Tests in this category include
luminous flux (lumens) and luminous intensity (candela). Simple radiometric measurements may be performed with a radiometer or a spectroradiometer. Tests in this category include radiant flux (watts) and radiant intensity (W/sr). Colour measurements for LEDs require the use of a spectroradiometer. Tests in this category include chromaticity, correlated colour temperature (CCT), colour rendering index (CRI), and dominant wavelength and purity.
For the measurement of total luminous flux, goniophotometers or integrating sphere systems are used and for the measurement of partial LED flux. Antrix Associates, exclusive distributor of Withlight (Korea based company), offers a complete range of LED testing instruments ranging from spectrometer, goniometer, integrated sphere system, LED thermal test, etc.
“LED luminance efficiency is measured through lux meters. We offer the 51002 lux meter that helps in measuring the lux levels. Our solution offers a high speed data acquisition system for lamp lifecycle tests and LED chip forward voltage measurements that help high speed square wave current to measure forward voltage for estimation of LED operating temperature,” says Kumaraswamy ML, deputy general manager, TMI and communications product Group, Yokogawa India Ltd.
Luminous intensity is another parameter of an LED that is a measure of the directional output of the LED. The procedure involved is to make a measurement of the
flux incident on a detector at a measured distance from the LED and to calculate the solid angle by dividing the area of the detector by the distance squared. This is measured with the help of an integrating sphere system adapter, which allows measurement of luminous flux and radiant power. LED test fixture is used for pushing the LED into the opening of the sphere in such a way that all the light radiation is captured into the integrating sphere. The interior of the integrating sphere has a highly reflective and diffusive white coating, which is especially flat for this purpose. Falcon has designed unique adapter CIE127-2007 that provides measurements of luminous intensity and all spectral parameters.
Another important optical test is for colour measurement and consistency. It is important to understand the colour dominant wavelength, chromaticity and correlated colour temperature. Colour quantity measurements are most uncertain, and may result in unexpected results.
Spectroradiometers are used for measuring spectral power distribution in the visible region of wavelengths. High speed, CCD based spectroradiometers can be customised to any LED measurement application. Lightweight and portable, these devices provide high precision, are fast and give accurate research grade measurements to meet demanding product applications. Optomech Engineers Pvt Ltd, Orlab Instruments Pvt Ltd, and VS Instrument Corp are some of the India players that manufacture UV-Vis and double beam spectroradiometer. However, leading companies in this domain includes China based Everfine Co Ltd and German company Instruments Systems.
Instruments used for reliabilty tests
Apart from electrical and optical tests, it is equally important that LEDs are measured on life span parameters. LEDs are predicted to work for 100,000 hours, while LED products last 70,000-80,000 working hours. As it is practically impossible to perform the life tests of an LED to see whether or not it would last for 10 years, accelerated tests are carried on the products.
Characteristics of the product are monitored for extreme conditions. “Certain reliability tests such as endurance tests using power cycling meters, environmental tests (change of temperature, dry heat, cold, damp heat, steady state, damp heat cyclic and salt mist tests), ingress protection, wind force tests, etc, are used to guage the product’s life,” shares Sambaiah V, general manager, operations, MIC Electronics Ltd. To measure reliability and endurance test, MIC offers power cycling meter that measures a minimum of 2000 cycles. These test are done in the labs which involves mathematical and statistical estimation and predictions that are based on voltage fluctuations and thermal properties. Power cycling meter, and humidity chambers measure environmental tests. ESPEC Corp, a leading US based company specialises in manufacturing pharmaceutical testing, automotive, solar and battery testing devices. It offers wide range of specialty chambers, thermal shock and environmental test chambers.
An ETO characterisation system measures the spectral light output of LEDs while precisely controlling electrical and thermal parameters. With today’s automated test systems, one can 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.
T&M equipment used for testing LEDs and LED products
- Digital multimeters (DMM)
- In-circuit testers
- Integrating spheres for luminous flux measurement
- LED profilers
- LED stations
- Optical spectrum analysers
- Power analysers
- Source measure units (SMU)
- Measuring adapter for luminous intensity and averaged LED intensity
- LED test sockets
- LED luminous intensity meters
- LED thermal and electrical
- performance analysers
- Aging and optical attenuation testers
- Multiplex luminous flux
- maintenance meters
Procuring testing instruments is one of the biggest challenges faced by buyers. Major testing instruments related to optical tests, which include luminous flux and luminaire efficacy, luminous intensity and colour consistency, are still not available with domestic players. In the absence of instruments like goniophotometers, spectro-radiometers, etc, with the domestic players, buyers have to look to global players for help in meeting their testing requirements. However, instruments for measuring electrical and thermal parameters are available with domestic players.
While making a purchase decision, priority should be given to companies that have a long and good track record of releasing new instruments with cutting edge technologies, apart from being affordable. Customers should also look for local servicing as well as calibration support. Shares Praveen Kumar Gupta, director, Avante Global Services Pvt Ltd, “While selecting LED test equipment, buyers should ensure that the device serves their purpose. They should select the instrument based on long term goals. The equipment should be capable of covering testing needs even after technological advancements take place from time to time.”
Some other criteria buyers should look for are:
- Instruments (for example, oscilloscopes) should have high sampling and high memory for data storage
- Data acquisition systems must have high speed with accuracy
- Source units should have high resolution and high speed
- Power analysers must have low power factor accuracy, and should be able to sense low current in mA range. They should follow IEC and KEMA regulations for lamp and LED testing
- Optical spectrum analysers should have very high resolution bandwidth as well as high dynamic range
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