“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 material, so that the stability and accuracy of the system have been greatly improved. Experimental results show that the stability and consistency of the system are much higher than other common LED test systems. It is truly a system suitable for the measurement of LED optical parameters.
Abstract: 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 material, 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 truly a system suitable for the measurement of LED optical parameters.
Introduction: Unlike traditional light sources, the luminous flux measurement of the LED light source poses a great challenge to the accuracy of the equipment in the process of measuring luminous flux using an 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 different light-emitting angles of different LEDs, the distribution of direct reflections 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 (as shown in Figure 1)
Figure 1: Different illumination angles have different effects on LED measurement
Improve the accuracy of the LED luminous flux test in the integrating sphere
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 characteristics of the integrating sphere are 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 gradually attenuated after several reflections. However, in the whole process of light mixing, direct irradiated light and reflected light both account for 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 which causes the measurement to be inaccurate.
In addition, the lower diffuse reflectance will seriously affect the attenuation of the signal. 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 the 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 be more than doubled. 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 a standard tungsten halogen lamp 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.
The LPCE-2 spectrometer & integrating sphere LED test system is a set of LED test system developed by Shanghai Lisun Electronics, which fully meets the requirements of LM-79 and CIE, and effectively solves the various defects of the traditional LED test system.
Compared with the traditional large-scale assembly and production technology of integrating spheres, Lisun Electronics adopts one-time molding technology to produce integrating spheres, and its shape fully conforms to the spherical structure of 4π or 2π. Lisun Electronic Integrating Sphere also adopts high reflection and diffusion rate coating, so that the opening position of the lamp is designed to align with the position of the detector. Even if you use a highly directional LED or use position mode under extreme conditions, this improvement keeps the test results in good consistency.
LPCE-2 uses a standard tungsten halogen lamp as a 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 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 possible LED position modes, as shown in Figure 3.
Figure 2: Different LED position modes
The relationship between the measured luminous flux and the LED position mode 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 repeat 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, which can provide a reliable guarantee. This standard system not only greatly supports the development and production of LEDs, but is also an ideal choice for optical performance measurement in the LED industry.
Figure 3: Luminous flux corresponding to different LED test positions
Figure 5: The relationship between LED test position and luminous flux
Shanghai Lishan Quality System has passed ISO9001:2015 strict certification. As a member of CIE, LISUN products are designed based on CIE, IEC and other international or national standards. All products have passed CE certification and third-party laboratory certification.
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Link to this article：How to improve the accuracy of the LED luminous flux test in the integrating sphere