LED product quality analysis and judgment criteria

In order to develop lighting LED technology , developed countries attach great importance to the research of LED test methods and standards. For example, the National Institute of Standards Testing (NIST) organizes internationally renowned test experts to conduct LED test research, focusing on test methods such as LED luminescence characteristics, temperature characteristics and light fading characteristics, and attempts to establish a complete set of LED test methods and technical standards in LED testing. The aspect has already taken the lead in the world.

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

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

Optical performance: The optical performance of LEDs mainly involves performance requirements in terms of spectrum, luminosity and chromaticity. According to the newly established industry standard "Semiconductor LED Test Method", there are mainly illuminating peak wavelength, spectral radiance bandwidth, axial illuminating intensity, beam half-intensity angle, luminous flux, radiant flux, luminous efficiency, chromaticity coordinates, correlated color temperature, Color purity and parameters such as dominant wavelength and color rendering index. LEDs for display are mainly visual and visual effects, so there is no requirement for correlated color temperature and color rendering index. For white LEDs for illumination, the above two parameters are especially important. It is an important indicator of lighting atmosphere and effect, and color Purity and dominant wavelength are generally not required.

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

Thermal performance: LED lighting efficiency and power improvement are one of the key issues in the current LED industry development. At the same time, LED PN junction temperature and housing heat dissipation issues are particularly important, generally using thermal resistance, housing temperature, The junction temperature and other parameters are indicated.

Radiation safety: At present, the International Electrotechnical Commission IEC equates LED products with the requirements of semiconductor lasers for safety testing and demonstration of radiation. Because LED is a narrow beam, high-brightness light-emitting device, considering its radiation may be harmful to the human eye retina, the international standard specifies the limits and test methods for effective radiation for LEDs used in different occasions. Radiation safety for lighting LED products is currently implemented as a mandatory safety requirement in the European Union and the United States.

Reliability and longevity: Reliability metrics are a measure of the ability of LEDs to function properly in a variety of environments. Especially important in LCD backlights and large screen displays. Lifetime is a quality indicator that evaluates the useful life of an LED product and is usually expressed in terms of useful life or end of life. In lighting applications, the effective life is the time it takes for the LED to decay to a specified percentage of the initial value at rated power.