Study on the accuracy of LED optical parameter test based on integrating sphere <br>Yu Limin Greg Mckee
(Lanfei Optics Labspherere Inc.)
Abstract: According to the particularity of LED luminous flux measurement, unique optimization is carried out in the design of integrating sphere for LED measurement. At the same time, diffuse reflection material with high reflectivity is adopted, which greatly improves the stability and accuracy of the system. The experimental results show that the stability and consistency of the system is much higher than other common LED test systems. It is a system that is truly suitable for measuring LED optical parameters.
Keywords: LED measurement integrating sphere
In the process of measuring the luminous flux using the integrating sphere, unlike the ordinary light source, the luminous flux measurement of the LED light source poses a great challenge to the equipment in terms of the test accuracy. On the one hand, LEDs generally have a stronger directionality than ordinary light sources, and generally do not emit light uniformly throughout the entire space. This characteristic makes the distribution of the direct light of the LED on the surface of the integrating sphere unevenly distributed, and the uneven distribution directly causes the direct reflection light of different LEDs to have different reflection characteristics with respect to the detector. Because the position of the detector port and the setting of the baffle are fixed, the different reflection distributions directly appear as signal fluctuations. In the ordinary measurement system, different forward divergence angle LEDs, different placement directions of the same LED, different positions in the same direction, etc., even if the luminous flux is consistent, the measured values â€‹â€‹show great difference. According to the customer's verification results, the influence of the LED placement direction of the ordinary LED measurement system on the luminous flux measurement result often exceeds 50% (the difference between the maximum signal and the minimum signal measured by the same LED in different directions).
When measuring different illumination angles of different LEDs, the influence of the direct reflection distribution on the detector is different due to the difference in the distribution of the inner surface of the integrating sphere, which directly affects the difference in the accuracy of the two measurements. As shown in Figure 1.
On the other hand, the LED measuring system usually uses a tungsten halogen lamp as a standard light source, and the standard lamp itself has a large difference with the LED in terms of its actual shape, distribution characteristics of light emission, and spectral characteristics. Therefore, the difference between the two must be corrected as necessary.
An important reason for the influence of the directivity of the LED on the measurement accuracy is the reflection characteristic of the inner surface of the integrating sphere. In conventional LED measurement systems, the reflectivity and Lambertian properties of the integrating sphere surface coating are not ideal. One is that the reflectance is low, and the other is that the diffuse reflection characteristics are not good. One result of the low reflectivity sphere surface is that the direct illumination of the LED is gradually attenuated after a few reflections, and the direct and direct reflections are large during the entire light mixing process. The proportion has played a leading role. Under certain conditions, low reflectivity materials can produce strong shadowing effects at the rear detector of the baffle under certain conditions. It is the straight reflected light and shadow effect that leads to inaccurate measurements.
In addition, the lower diffuse reflectance is very damaging to the signal. Since the light is reflected multiple times in the integrating sphere during the light measurement process, each reflection has a certain attenuation, and the influence of the high and low reflectivity on the light intensity is enhanced after multiple reflections. For example, if the reflected light is reflected 15 times in the ball, if the reflectance of the two differs by 5%, the attenuation of the signal may more than double. In fact, the difference in reflectivity in the integrating sphere is far more than that.
Current LED test systems are not yet available as standard LEDs for standard light sources, and a calibrated, stably driven tungsten halogen lamp is used as the standard source during the measurement process. Since the outline structure of the standard lamp and the LED to be tested are very different, the LED bracket has an absorption effect on the light, and the difference between the standard lamp mounting position and the LED mounting position is an important factor affecting the accuracy of the measurement result.
Solution and test results:
In the current LED test system, in order to overcome the above problems and improve the accuracy, the specified LED is usually tested in a special installation direction, or the system is designed such that the light source to be tested directly faces the detection port or the back port mode. However, one angle cannot solve all the problems, and the effects of LEDs with different illumination angles and reflectivity still need to be solved.
When designing the LED measurement system, Labsphere has fully optimized the factors affecting the measurement accuracy according to the actual use, so that the sensitivity of the system to the LED direction is minimized. That is, no special angles and directions need to be specified during the measurement. Even under extreme conditions, the use of extremely directional LEDs, using extreme conditions of placement, the results of the measurement still maintain good consistency.
Labsphere's LED measurement system is designed in strict accordance with the CIE standard. The integrated sphere inner surface coating uses the patented Spectralon@ or Spectraflect@ as the reflective layer with extremely high reflection efficiency and good Lambertian reflection characteristics. The reflectance is greater than 99% and 98% in the visible range, respectively. Good reflective materials ensure better mixing characteristics and uniform light distribution, resulting in direct illumination of direct reflection characteristics and a smaller shadowing effect of the baffle. In addition, the design of the baffle and the design of the detection port have been optimized, using a diffuser device to minimize the sensitivity of the detection port to direct reflected light. Special considerations have been made on the joints of the inner surface of the sphere.
The LED system uses a calibrated tungsten halogen lamp as a standard lamp, and an auxiliary lamp scheme is used to compensate for the influence of the difference between the LED bracket to be tested and the standard lamp bracket on the measurement result. The standard lamp was rigorously calibrated in Labsphere's calibration laboratory in the United States and the results were traceable to NIST. The standard lamp operates at a constant color temperature of 3000K, maintains a constant light output flux, and provides NIST traceable spectral radiant flux data. The standard and auxiliary lamps use a fixed-current fixed-power drive instead of an Adjustable Power Supply to minimize power dissipation. Long-term stability is better than 0.02%, ensuring the reliability of light output results.
Under the system conditions, the targeted test of the accuracy of the LED measurement results described above was carried out. The test conditions are as follows: high-brightness green? 5 LED, power is about 0.35W, and the illumination angle is about 30?. The lumen values â€‹â€‹and error results of the test are shown in Table 1 and Figure 2.
The LEDs use nine measurement orientations, representing the possible LED placement. It contains the extremes of minimizing and maximizing the impact on the detector. From the measurement results, even in the most extreme cases, where the LED faces the detector opening and the LED is facing away from the opening, the peak-to-peak value of the luminous flux measurement is still less than 5%. This is a very good test result. In the actual application process, LEDs will not be placed in such extreme situations. In this test, a simple test stand is used. In the case of positioning error, the luminous flux error of the same position measurement is less than 0.1%. The repeatability error of the luminous flux measurement of the LED during the actual test is much less than 0.1%. It can be seen that Labsphere's LED optical performance system measurement results are reliable and stable, which will guarantee the performance of the product. The use of such a standard system can greatly support the research, development and production of LEDs, and is an ideal choice for optical performance measurement in the LED industry.
About the author: Xiao Dong (Mr. Roger Xiao), Labsphere (Labsphere Inc.) Asia Pacific Marketing Director
(This article is from the 2008 China Optoelectronic Industry High-level Forum Proceedings)
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