Testing instruments: the lever for analyzing and judging LED product quality

1. Comprehensive consideration of performance parameters

Semiconductor light emitting diodes (LEDs) have many new problems in quality evaluation and detection methods due to their small size, directional light emission, high brightness, and PN junction electrical characteristics. Different applications determine the performance requirements for LED products. From the perspective of optical performance, LEDs used for display are mainly concerned with parameters such as brightness, viewing angle distribution, and color. LEDs used for general lighting pay more attention to parameters such as luminous flux, spatial distribution of light beams, color, and color rendering characteristics, while LEDs used in biological applications are more concerned with parameters such as biological effective radiation power and effective radiation illumination. In addition, light emitting diodes are both a light source and a power-type semiconductor device, so their quality must be comprehensively evaluated from many aspects such as optics, electricity, and heat.

From the perspective of the current structure and industrial development of LED products, lighting LED products mainly need to consider parameters such as optical performance, electrical performance, thermal performance, radiation safety and life span.

Optical performance. The optical performance of LED mainly involves performance requirements in terms of spectrum, photometry and chromaticity. According to the newly formulated industry standard "Test Method for Semiconductor Light Emitting Diodes", the main parameters include peak wavelength, spectral radiation bandwidth, axial luminous intensity, beam half-intensity angle, luminous flux, radiant flux, luminous efficiency, chromaticity coordinates, correlated color temperature, color purity and dominant wavelength, color rendering index and other parameters. LEDs used for display are mainly for visual intuitive effects, so there are no requirements for correlated color temperature and color rendering index. For white light LEDs used for lighting, the above two parameters are particularly important. They are important indicators of lighting atmosphere and effects, while color purity and dominant wavelength are generally not required.

Electrical properties. The electrical properties of the PN junction of LEDs determine the electrical properties of LEDs in lighting applications that are different from those of traditional light sources, namely, unidirectional nonlinear conductivity, low voltage drive, and sensitivity to static electricity. Currently, the main measurement parameters include forward drive current, forward voltage drop, reverse leakage current, reverse breakdown voltage, and electrostatic sensitivity.

Thermal performance. Improving the luminous efficiency and power of LEDs for lighting is one of the key issues in the current development of the LED industry. At the same time, the PN junction temperature of LEDs and the heat dissipation of the shell are particularly important, which are generally expressed by parameters such as thermal resistance, shell temperature, and junction temperature.

Radiation safety. Currently, the International Electrotechnical Commission (IEC) has made radiation safety testing and demonstrations for LED products equivalent to those for semiconductor lasers. Since LEDs are narrow-beam, high-brightness light-emitting devices, and considering the possible harm of their radiation to the human retina, international standards have stipulated effective radiation limit requirements and test methods for LEDs used in different occasions. Currently, in the EU and the US, radiation safety for lighting LED products is implemented as a mandatory safety requirement.

Reliability and lifespan. Reliability is a measure of the ability of LEDs to work properly in various environments. It is particularly important in LCD backlight sources and large-screen displays. Lifespan is a quality indicator for evaluating the usable cycle of LED products, usually expressed as effective lifespan or final lifespan. In lighting applications, effective lifespan refers to the time it takes for the luminous flux of an LED to decay to a specified percentage of the initial value under rated power conditions.

2. Pay attention to the formulation of measurement standards

The characteristics of LED, such as small light-emitting surface, narrow light beam and high brightness, determine the particularity of its detection. In order to deal with this problem, CIE established two technical committees, "TC2-45 LED Measurement" and "TC2-46 CIE/ISO LED Intensity Measurement Standard". The CIE TC2-34 group held a meeting at the Vienna headquarters in October 1997 to formulate and recommend the CIE 127-1997 LED measurement standard, which involves LED radiance, photometry and chromaticity measurement. However, due to the rapid development of LED technology in recent years, especially white light LED products for lighting, many problems have not been considered in the past. Therefore, at the CIE annual meeting held in Tokyo, Japan in 1999, representatives of developed countries proposed that CIE TC2-34 formulate standards for white light LED lighting equipment, and the Japanese delegation also submitted two draft standards for white light LEDs for general lighting.

In order to develop lighting LED technology, developed countries attach great importance to the research of LED testing methods and standards. For example, the National Institute of Standards and Technology (NIST) of the United States organized internationally renowned testing experts to carry out research on LED testing, focusing on the testing methods of LED luminous characteristics, temperature characteristics and light decay characteristics, and trying to establish a complete set of LED testing methods and technical standards. It has been at the forefront of the world in LED testing. Japan has established the "White Light LED Testing Research Committee" to specialize in the testing methods and technical standards of white light LEDs for lighting. In order to seize the commanding heights of LED research, developed countries in the world have invested a lot of manpower and material resources in LED standards and testing, and focus on the selection of LED characteristic parameters and testing methods in terms of standards.

In my country, there is no corresponding national standard for semiconductor light emitting diode test methods, so there are often great disputes among different manufacturers and users. In recent years, the Optoelectronic Devices Branch of China Optical Association has successively organized many academic seminars and exchanges on semiconductor light emitting diode test methods. Industry professionals have gradually formed a relatively unified understanding and formulated a unified industry standard SJ/T2355-2005 "Semiconductor Light Emitting Diode Test Method", which has played an important role in product exchanges and comparisons within the industry. This standard not only adopts the CIE127-1997 "Measurement of LEDs" method, but also combines the development needs of power white light LEDs for lighting, and adds color rendering characteristics, junction temperature and other parameters of the measurement method, which provides an extremely important basis for the development of lighting LED products.

In recent years, the development of multi-chip or multi-tube LED lamps has also been very rapid, and there is no corresponding testing standard specifically for LED lamps at home and abroad. However, as a lighting application product, the International Electrotechnical Commission IEC and the International Commission on Illumination CIE have relevant measurement standards. The central light intensity and beam angle of LED lamps for lighting can refer to the IEC61341-1994 standard. Similarly, the national standard GB/T 19658-2005 "Method for measuring the central light intensity and beam angle of reflector lamps" has been drafted by Zhejiang University Sanse Instrument Co., Ltd. and implemented in August 2005. For the measurement of spectral radiation and color, you can refer to CIE NO.63 document and national standard GB/T7922-2003.

As people pay more attention to photobiological safety, according to the requirements of the International Electrotechnical Commission IEC60825 standard, LED lamps or lighting products must be tested for radiation safety in accordance with the requirements similar to those of laser devices. Domestic companies generally do not pay enough attention to this. As more domestic LED products enter international markets such as the United States and the European Union, more complex radiation safety testing issues will be involved. For the rapidly developing wide-beam LED products, the requirements of the above IEC standards may be too harsh. For conventional lighting lamps and lamp systems, considering the possible harm to human skin and eye health, the International Electrotechnical Commission adopted the International Commission on Illumination's document CIE S009/E2002 "Photobiological Safety of Lamps and Lamp Systems" as the official IEC standard in 2002. In order to respond to international changes, my country also formulated corresponding standards in 2004. The standard was drafted by the National Electric Light Source Testing Center (Beijing) and Zhejiang University Sanse Instrument Co., Ltd. and will be officially released in the near future.

3. Strive to develop testing instruments

Internationally, countries such as the United States, Germany and Japan started early in the field of LED testing instruments and have formed certain characteristics. In China, LED testing instruments have developed very rapidly in recent years, and have gradually developed testing instruments for all links of the upstream, midstream and downstream of the LED chip, luminous material, LED tube, LED lamp and lamp industry, including testing instruments for laboratory research and development, automatic testing and sorting equipment on the production line, and instruments for product quality inspection. Measurement indicators include: spectrum, photometry, chromaticity, radiation, electrical parameters, thermal resistance, reliability and life.

In terms of laboratory research and development and quality testing instruments, they can basically meet the general needs of domestic LED product development. Domestic instruments often have a relatively large price advantage, only a fraction of the price of imported instruments, which has played a certain positive role in improving the initial product quality of many domestic small and medium-sized LED companies. However, due to the technical level of some instrument manufacturing and the gap in understanding of LED measurement standards, there are often obvious differences in instrument measurement results between manufacturers. A typical situation, such as the measurement of axial strong light of light-emitting diodes, generally has poor comparability of measurement results. For the same LED tube, the measurement results between two manufacturers may have an error of more than 10% or even several tens of percent, which far exceeds the general photometric measurement accuracy range.

At present, special attention should be paid to the following issues in LED photoelectric measurement:

First, the standard of measurement.

The light radiation of a light emitting diode is actually a directional imaging beam, so the luminous intensity cannot be measured and calculated according to the photometric measurement rules in general textbooks. In other words, in general, the luminous intensity cannot be simply calculated using the illumination on the detection surface and the inverse square law of distance. The CIE 127-1997 "Measurement of Light Emitting Diodes" publication defines the intensity test of LEDs as the concept of average intensity and stipulates a unified test structure, including the size of the detector receiving surface and the requirements for the measurement distance. This lays the foundation for accurate testing and comparison of LEDs. Although the CIE document is not an international standard, it has been widely recognized and adopted internationally. my country's LED industry standards are completely consistent with the methods of the CIE document.

Second, photometry measures the spectral response of the sensor.

At present, the photometric sensor used in LED measuring instruments is composed of silicon photodiodes and corresponding visual spectral response correction filters. In order to make the spectral response function of the detector consistent with the CIE standard observer spectral luminous efficiency function V (λ), it is generally necessary to consist of multiple filters. Due to the limitations of materials and processes, there are certain differences in the spectral matching of sensors of some instruments. When the standard source used for factory calibration of the instrument (usually a 2856K tungsten filament lamp) and the measured LED photometric value will produce obvious deviations, and it is often more obvious for some monochromatic LEDs. Therefore, a high-precision photometric detector with a good spectral response curve at each wavelength should be used, or the spectral radiation method should be used for measurement, and the weighted integration by the computer can be obtained to obtain accurate measurement results. Otherwise, the instrument must be calibrated or corrected with LED standard tubes to obtain more consistent results.

Third, the directionality of the measurement.

The light emitted by a light emitting diode is highly directional, and the positioning of the measurement direction will significantly affect the accuracy of the measurement results. In particular, in the measurement of the axial light intensity of LEDs, some instruments do not limit the direction of the LED measurement, which makes it difficult to ensure measurement accuracy.

In the test power supply drive of LED, the influence of the increase of the junction temperature of LED itself on the electrical parameters and light emission cannot be ignored. Therefore, the ambient temperature and the temperature balance of the device during measurement are very important measurement conditions.

Judging from the existing LED testing instruments at home and abroad, they are basically aimed at the testing requirements of light-emitting diodes. With the development of power LEDs, the unification of testing methods and the requirements of instruments are attracting more and more attention. Many existing testing instruments will bring many new problems to the measurement of lighting LEDs. Therefore, in the process of purchasing and using LED measuring instruments, it is necessary to determine according to the product type, characteristics and relevant domestic and international standards.

With the rapid development of the LED industry, the industry should formulate unified testing methods and standards as soon as possible in response to the requirements of different stages of LED products, and form testing instruments that meet actual needs and have China's independent intellectual property rights, so as to facilitate the mutual cooperation and coordinated development of the upstream, midstream and downstream industrial chains, and promote the improvement of the competitiveness of the lighting LED market. It is worth noting that the testing instrument is the lever for product quality analysis and judgment, and the accuracy and reliability of the instrument should be the most important indicators. China's manufacturing enterprises should develop more testing instruments with international advanced levels to meet the needs of the continuous development of the lighting LED industry.