Based on the actual needs of LED various applications, LED test needs to include many contents such as electrical properties, optical characteristics, switch characteristics, color characteristics, thermal characteristics and reliability.
LED is unipolar PN junction diode that is composed of inorganic semiconductor materials, it is one kind of semiconductor PN junction diodes; its voltage-current relationship is called as voltage-current characteristic. LED electrical properties parameters include forward current, forward voltage, back current and back voltage; LED must work under proper current and voltage. The LED electrical characteristics test can get LED maximum allowable forward voltage, forward current, back voltage and current. In addition, it can test LED best work power.
LED electrical properties usually use corresponding constant current and constant voltage power supply to do test.
Similar with other light source, LED optical characteristics test includes luminous flux and luminous efficiency, radiation flux, radiation efficiency, light intensity and light intensity distribution and spectral parameters.
There are two ways to test flux, integrating sphere method and goniophotometer method. Goniophotometer is the most accuracy way to test flux, but it takes a long time, so generally we adopt integrating sphere method to test luminous flux. Current integrating sphere method for LED luminous flux test has two test structures, one is to place LED in the middle of integrating sphere and another one is to place on the wall.
After luminous flux test, we can get LED luminous efficiency with digital power meter. The test method of radiation flux and radiation efficiency is similar with luminous flux and luminous efficiency test.
2. Luminous intensity and luminous intensity distribution characteristics
The light intensity of point light source is uniform distribution in all directions of space, the test result will not change when you use different receiving aperture detector at different distance, but the light intensity distribution of LED is inconsistent, which makes the test result change with the test distance and the aperture of detector. Therefore, CIE-127 puts forward 2 kinds recommended test conditions that make every LED do light intensity test and assessment under the same condition, CIE-127 condition has been quoted by LED manufacture and test organization.