Because LED has the characteristics that it has small volume, directional emitting light, high brightness, PN junction, there are many new problems in access of quality and measurement methods. Different applications determined the requirements of the LED performance. From the aspect of the optical properties, LED used to display, they are mainly the brightness, distribution perspective, color etc.parameters. For general lighting LED, it pays more attention to the flux, the beam spatial distribution, color, color characteristics etc. parameters; for biological applications LED, it pays more attention to biological effective radiated power, effective irradiance etc. parameters. In addition, the light emitting diode is not only as a light source, but also as a semiconductor power device, so it must be from the optical, electrical and thermal etc. aspects to evaluate the quality of it comprehensively.
From the view of the current structure and the industrial development of LED products, lighting LED products mainly need to consider several aspects of optical performance, electrical properties, thermal properties, radiation safety and life parameters.
Optical properties. Performance requirements for the optical properties of LED are mainly related to the spectra, brightness and color etc. According to the new industry standards “ Semiconductor Lighting-emitting Diode Test Method”, the main parameters are emission peak wavelength, spectral radiation bandwidth, axial luminous intensity, beam half intensity angle, luminous flux, radiation flux, luminous efficiency, chromaticity coordinates, color temperature, color purity and dominant wavelength, color rendering index(These parameters can be tested by spectroradiometer) and other parameters. Display LED, is mainly the visual intuitive effect, so the correlated color temperature and color index is not required, but for lighting white LED, these two parameters are particularly important, They are important indicators of the atmosphere and effects of lighting, but color purity and dominant wavelength are generally nor required.
Electrical properties. PN junction electric characteristics of LED, determines the LED in lighting applications as the distinct from the electrical characteristics of the traditional light source, namely the nonlinear one –way conductive properties, low voltage driving and electrostatic sensitivity characteristics. At present, the main measurement parameters include the forward driving current, forward voltage, reverse leakage current, reverse breakdown voltage and electrostatic sensitivity etc.
Thermal properties. To enhance the luminous efficiency and power of lighting LED is one of the key problems in current LED industry development, at the same time, PN junction temperature and shell heat issue is particularly important, typically expressed in the thermal resistance, shell temperature, junction temperature and other parameters.
Radiation safety. At present, the International Electrotechnical Commission IEC makes the LED products equivalent to the semiconductor laser requirements of radiation safety testing and demonstration. Because LED is narrow beam , high brightness light emitting device, considering the radiation may damage the retina, therefore, for different applications of LED, the international standards stipulates the limit value requirements of the effective radiation and testing methods, in the European Union and the United States at present, the radiation safety of lighting LED products is required to execute as a compulsory safety requirement.
Reliability and life. The reliability indicator is to measure the work ability of LED in different environments. It is very important in LCD back light and large screen display. Life is to evaluate the quality indicator of LED products available cycle, usually expressed by the effective life and end life. In lighting applications, effective life is the last time that the flux of LED attenuates to the specified percentage of initial value at the rated power condition.