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LED Quality Evaluation

Semiconductor light emitting diode (LED) has many new problems in the aspect of quality evaluation and LED test method because of its characteristics such as small volume, directional light emission, high brightness, PN electrical characteristics and so on. Different applications determine the performance requirements of LED products. From the aspect of the optical proper including the light attenuation, the parameters of LED used for displaying mainly are the brightness, the perspective distribution and the color. LED used for general lighting will pay more attention to the parameters such as the flux which can be tested by flux meter, the spatial distribution of the beam, the color and the color rendering and so on. However, LED for biological applications is more concerned about the parameters such as the biological effective radiated power, the effective radiation intensity of illumination. In addition, light emitting diode is not only a kind of light source but also a kind of power-type semiconductor device, so the quality must be comprehensively evaluated from many other aspects such as the optics, the electrics, the thermology and so on.

From the point of view of the current structure of LED products and industry development, LED products for lighting mainly consider the parameters such as the optical properties, electrical properties, thermal properties, radiation( spectroradiometer ) safety and life.

The optical performance of LED mostly concerns the spectrum, the luminosity, the color, and other aspects of the performance requirements including the optical attenuation. According to the new set of the industry standard “Semiconductor Emitting Diode Testing Method”, the main parameters are emitting peak wavelength, spectral radiant glow bandwidth, axial luminous intensity, half strength beam, flux, radiation fluxes, luminous efficiency, color product coordinates, relevant color temperature , color purity, main wavelength, and the color index. When doing the LED test, luminous intensity measurement(luminance meter), LED aging test and color temperature test(color temperature meter) is rather important. LED used for displaying mainly is the intuitive visual effect, therefore the relevant color and the color index do not make the request. However, the above two parameters is very important to the white LED used for illumination. It is an important index for the lighting effect and atmosphere. The color purity and main wavelength generally have no requirements.

The LED’s PN electrical characteristics determines the electrical characteristics in LED lighting applications which is different from traditional light source, that is, one-way nonlinear electric property, low voltage driver and sensitive of electrostatic, etc. At present the main measuring parameters includes the positive drive current, and positive pressure drop, reverse leakage current, reverse breakdown voltage and electrostatic sensitivity, etc.

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