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Lisun LED Test Solution and Measuring Method Part2: Switching, Color, Thermal and Reliability Characteristics

3 Switching Characteristics

LED switching characteristics means the variation characteristics of the optics, electrics and colors. Through the LED switching characteristics test, we can obtain the change rule of the instant working state of the passes and power failure. We can not only learn the LED loss, but we can use it to guide the design of LED driver module.

4 Color Characteristics

LED color ( Colorimeter ) characteristics mainly include chromaticity coordinate, dominant wavelength, color purity, color temperature and color rendering, etc. LED color characteristic is particularly important to white LED. The existing LED test method of the color characteristics are spectrophotometry and integration. Spectrophotometry is to use the monochromatic to measure LED spectrum power distribution, and then use the chroma weigh function integration to get the corresponding chromaticity parameters; Integration is to use certain color filter together with the photoelectric detector to directly measure the chromaticity parameters; The accuracy of the spectrophotometry is substantially higher than the integration.

5. Thermal Characteristics

The LED thermal characteristics mainly refers to the thermal resistance and junction temperature. Thermal resistance refers to the ratio of the temperature difference and the power dissipation along the heat channel. Junction temperature refers to the LED PN junction temperature. LED thermal resistance and junction temperature are the important factors that affect LED photo electricity performance.

Generally there are two existing ways for LED junction temperature measurement: one way is to use the infrared measuring temperature microscope or miniature thermocouple to measure LED chip’s surface temperature, which is seen as the LED junction temperature, but the accuracy is not enough; Another way is to use the reverse ratio change between the positive bias and the junction temperature under the given bias current source to determine the LED junction temperature, this method is more complex.

6. Reliability

The reliability of LED includes the electrostatic sensitivity characteristics, life, environmental characteristics, etc.

Electrostatic sensitivity characteristics refer the electrostatic discharge voltage that LED bears. For some LEDs, because the resistance rate is high, and the positive and negative electrode distance is short, if the static charge of both ends accumulate to a certain value, the electrostatic voltage will breakdown PN junction, in serious case, it can breakdown the PN junction and cause LED to lose efficacy. We must therefore test the electrostatic sensitivity characteristics of LED; get the electrostatic discharge fault LED critical voltage. At present, we generally adopts human model, machine model, charging device model to simulate the electrostatic discharge phenomenon in real life.

In order to observe the change rules of the optical property when LED is continuously used for a long term. We need to conduct the sampling LED testing method to get LED life parameters through the long-term observation and statistics.

For the test of LED environmental characteristics ( Environmental Chamber ), simulation of all kinds of natural hit LED encountered in the application, commonly includes: high and low temperature impact test, humidity cycle test ( Temperature and Humidity Chamber ), moisture test, salt spray test, sand and dust test test, irradiation test, vibration and impact test, drop test, centrifugal acceleration test, etc.

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