Improve the accuracy of LED luminous flux testing in integrating spheres

According to the particularity of LED luminous flux measurement, the design of the integrating sphere for LED measurement adopts unique optimization, combined with high reflectivity diffuse reflection materials, which greatly improves the stability and accuracy of the system. Experimental results show that the stability and consistency of the system are much higher than other common LED test systems. It is a system truly suitable for measuring LED optical parameters.

Introduction:

Unlike traditional light sources, the luminous flux measurement of LED light sources poses a great challenge to the accuracy of the equipment in the process of using an integrating sphere to measure luminous flux. On the one hand, compared with traditional light sources, LEDs generally have stronger directionality and do 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 direct light from 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 fluctuations. In ordinary test systems, there are differences in LEDs with different lighting angles. The same LED placed in different directions has the same emission at different positions. Even though the rated luminous flux is the same; the actual measured values ​​are different. According to the customer's verification results, the LED placement direction of the ordinary LED test system always affects the luminous flux measurement results by more than 50% (the difference between the maximum signal and the minimum signal of the same LED measured in different directions)

When measuring different luminous angles of different LEDs, due to the difference in distribution of the internal surface of the integrating sphere, the distribution of direct reflection has different effects on the detector, which directly affects the difference in measurement accuracy (as shown in Figure 1)

Improve the accuracy of LED luminous flux testing in the integrating sphere
Figure 1: Different lighting angles have different effects on LED measurement

On the other hand, LED test systems usually use tungsten halogen 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.

Analysis:

The internal reflection characteristics of the integrating sphere are one of the key factors that affect the directionality of LEDs on measurement accuracy. In ordinary LED test systems, the reflectivity and Lambertian properties of the surface coating of the integrating sphere are not ideal. One reason is low reflectivity, and the other is poor diffuse reflection properties. The result of the low reflectivity of the integrating sphere surface is that the direct light of the LED gradually attenuates after several reflections. However, in the entire light mixing process, both direct illumination light and reflected light account for a large proportion, which is dominant. In some cases, low-reflectivity materials can create a strong shadowing effect on the back of the baffle probe. However, it is the light and shadow effect of straight line reflection that leads to inaccurate measurements.

In addition, low diffuse reflectance will seriously affect signal attenuation. Since light is reflected multiple times in the integrating sphere during the light measurement process, each reflection will produce a certain attenuation, but the impact of reflectance on light intensity is enhanced after multiple reflections. For example, if the reflected light is reflected 15 times by the integrating sphere, if there is a 5% difference in reflectivity, the signal attenuation may be more than doubled. In fact, the reflectivity of the integrating sphere varies much more than this.

Current LED test systems do not yet use standard LEDs as standard light sources. During the measurement process, we still choose to use a standard tungsten halogen lamp as the standard light source. Since the external structure of the standard lamp and the measurement LED are 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:

The LPCE-2 spectrometer & integrating sphere LED test system is an LED test system developed by Shanghai Lishan Electronics. It fully complies with the relevant requirements of LM-79 and CIE, and effectively solves the various defects of traditional LED test systems. Compared with the traditional large-scale assembly production technology of integrating spheres, Lisun Electronics uses one-time molding technology to produce integrating spheres, whose shape completely conforms to the 4π or 2π spherical structure. Lisun Electronics integrating sphere also uses high reflection and diffusivity coating, so that the lamp is designed in the opening position to align with the detector position. This improvement allows for good consistency in test results even when using highly directional LEDs or using positional mode under extreme conditions.

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

In view of the accuracy problem of the above LED test results, the LPCE-2 test system was used to conduct corresponding tests. The test conditions are as follows: use 5 high-brightness green LEDs with a power of about 0.35W and a lighting angle of about 30°. The LPCE-2 test system is used for 9 measurement positions, representing possible LED position patterns, as shown in Figure 3.

Conclusion:

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 detector's opening, the peak value of the luminous flux test result is still less than 5%. This is a very good test result. During the actual test process, the repeatability error of LED luminous flux measurement was 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 reliable guarantee. This standard system not only greatly supports the R&D and production of LEDs, but is also an ideal choice for optical performance measurement in the LED industry.