To test LED electrical characteristics ( Electrical Safety Test ), optical characteristics,switching characteristics,color characteristics, thermal characteristics and reliability.
Semiconductor light emitting diode (LED) has been widely used in indicator light, signal lamp, instrument display, mobile phone backlighting, car light source and so on, especially the development of white LED technology, LED has been more and more widely applied in lighting area. But in the past, there is no comprehensive national standards and industrial standards for LED measurement. And the production practice can only use the relative parameters as the basis, there is a big disagreement among the different manufacturers, users, research institutions, which has caused a serious impact on the development of the domestic LED industry. Therefore, the national standard of the test methods of semiconductor light emitting diode (LED) arises at the historic moment.
LED Measuring Method
Based on the actual demand of all the fields of LED application, LED test needs to include many aspects such as the electrical characteristics, optical characteristics, switching characteristics, color characteristics, LED thermal characteristics, reliability and so on.
1. Electrical Characteristics
LED is the unipolarity PN junction diode made of the semiconductor inorganic materials ( LED Junction Temperature Test ). It is one of the semiconductor PN junction diode, the relationship between the current and voltage is called volt-ampere characters. It can be seen from figure 1 that LED electrical characteristics parameters include the positive current, positive voltage, reverse current and reverse voltage. LED must be driven by the appropriate current voltage to work properly. Through the LED electrical characteristics test, we can get the maximum allowed forward voltage, positive current and reverse voltage and current of LED, we can also test the best working power of LED. The electrical characteristics test of LED generally uses the volometer to test under the corresponding constant current and constant voltage.
2. Optical Characteristics
Similar to other light sources, LED optical characteristics test mainly included flux ( Flux Meter ) and luminous efficiency, LED radiation fluxes and radiation efficiency, light intensity and light intensity distribution characteristics and spectrum parameters etc.
(1) Luminous Flux and Luminous Efficiency
There are two methods that can be used to test the luminous flux: integrating sphere method and goniophotometer method. The goniophotometer method is the most accurate method to test the luminous flux, because its time consuming is longer, we usually use the integrating sphere method to test the luminous flux. There are two test structures for the existing integrating sphere method to measure the luminous flux of LED. The first method is to place the measured LED in center, another method is to place it on the wall of the ball
In addition, because when using the integrating sphere method to test the luminous flux, the light source’s absorption of the light will affect the result of the test. Therefore, the auxiliary light is often introduced. After testing the luminous flux, with the electrical parameters, it can measure the luminous efficiency of LED. The test method of the radiation fluxes and radiation efficiency is similar to the test of the luminous flux and luminous efficiency.
(2) Light Intensity and Light Intensity Distribution Characteristics
Illuminant light intensity in each direction of the space has uniform distribution. The test results received from the detectors of different receiving apertures in different distance are the same. However, because the different light distribution of LED, the test result changes with the measuring distance and the detector aperture.Therefore, CIE-127 puts forward two recommended test conditions to carry out LED light intensity test and evaluation in the same conditions, at present the CIE-127 conditions have been recommended by the LED manufacturers and inspection agencies.
(3) Spectrum Parameters
The spectral characteristics parameters of LED mainly include peak emission wavelength, spectral radiate bandwidth, spectral power distribution and so on. The spectrum of monochro monochromatic LEDs is the single wave, properties is represented by the peak wavelength and bandwidth, and the spectrum of the white LED is made up of a variety of monochromatic spectrum synthesis. All the spectral characteristics of LED can be represented by the spectrum power distribution, and the calculation of the spectrum power distribution of LED can also obtaine the chromaticity parameters.
Spectrum power distribution test needs to be conducted through the light splitting to distinguish all colored light from the mixed light. We generally use the prism and grating to realize the spectral photometry.