How to improve the accuracy of the LED luminous flux test in the integrating sphere

Different from the traditional light source, the luminous flux measurement of the LED light source poses a great challenge to the accuracy of the equipment in the process of measuring the luminous flux using the integrating sphere. On the one hand, compared with traditional light sources, LEDs generally have stronger directivity and will not emit light uniformly throughout the space. This feature makes the direct light distribution of the LED on the surface of the integrating sphere uneven. This uneven distribution will cause the direct light of different LEDs to have different reflection characteristics of the detector. Since the position of the detector and the position of the baffle are fixed, the direct performance of various reflection distributions is signal fluctuation. In a common test system, LEDs with different light-emitting angles are different, and the same LED has the same emission at different positions in different placement directions. Even if the rated luminous flux is the same; the actual measured value is different. According to the customer's verification results, the LED placement direction of the ordinary LED test system always affects the luminous flux measurement result by more than 50% (the difference between the maximum signal and the minimum signal of the same LED measured in different directions)

When measuring the different light-emitting angles of different LEDs, the distribution of direct reflection has different effects on the detector due to the difference in the distribution of the inner surface of the integrating sphere, which directly affects the difference in measurement accuracy

On the other hand, LED test systems usually use halogen tungsten lamps as standard light sources. Compared with LEDs, the standard lamps used are very different in appearance, lighting distribution characteristics and spectral characteristics. Therefore, the difference between the two should be corrected by the absorption coefficient.

The internal reflection characteristic of the integrating sphere is one of the key factors that make the LED directivity affect the measurement accuracy. In the ordinary LED test system, the reflectivity and Lambertian characteristics of the surface coating of the integrating sphere are not ideal. One reason is low reflectivity, and the other reason is poor diffuse reflection characteristics. The result of the low reflectivity of the integrating sphere surface is that the direct light of the LED is the result that the direct light of the LED gradually attenuates after several reflections. However, in the whole process of light mixing, direct illumination light and reflected light both occupy a large proportion, which is dominant. In some cases, low-reflectivity materials will have a strong shadow effect on the back of the baffle probe. However, this is the light and shadow effect of the straight line reflection that causes the measurement to be inaccurate.

In addition, lower diffuse reflectance will seriously affect signal attenuation. In the light measurement process, light is reflected multiple times in the integrating sphere, and each reflection will produce a certain attenuation, but the influence of reflectance on light intensity is strengthened after multiple reflections. For example, if the reflected light is reflected 15 times in the integrating sphere, if there is a 5% difference between the reflectances, the signal attenuation may more than double. In fact, the difference in reflectivity of the integrating sphere far exceeds this point.

The current LED test system has not been used as a standard LED as a standard light source. In the measurement process, we still choose to use standard tungsten halogen lamps as the standard light source. Because the external structure of the standard lamp and the measured LED is very different, including the light absorption effect of the LED lamp holder and the difference between the standard lamp installation position and the LED installation position, all of these are important factors that affect the accuracy of the test results.

solution:

LPCE-2 spectrometer & integrating sphere LED test system is a set of LED test system developed by Shanghai Lishan Electronics, which fully meets the requirements of LM-79 and CIE, and effectively solves various defects of traditional LED test system.

LPCE-2 uses standard tungsten halogen lamps as standard lamps combined with optional auxiliary lamps to measure the impact of the difference between LED lamp holders and standard lamp holders on the test results. This standard lamp has been strictly calibrated by Lisun Electronic Calibration Laboratory; the test results can be traced back to NIM.

In view of the accuracy of the above-mentioned LED test results, the LPCE-2 test system is used for corresponding tests. The test conditions are as follows: 5 high-brightness green LEDs are used, the power is about 0.35W, and the illumination angle is about 30°. The LPCE-2 test system is used for 9 measurement positions, which respectively indicate the possible LED position modes.

The relationship between the measured luminous flux and the LED position pattern is shown in Figure 4 and Figure 5. It can be seen from the test results that even in the most extreme case, when the LED is placed before and after the opening of the detector, the peak value of the luminous flux test result is still less than 5%. This is a very good test result. In the actual test process, the repeated error of the LED luminous flux measurement is far less than 0.1%. It can be seen that the test results of the LPCE-2 test system are reliable and stable, and can provide a reliable guarantee. This standard system not only greatly supports the development and production of LEDs, but also an ideal choice for optical performance measurement in the LED industry.