Improve The Accuracy Of Integrated Sphere For Led Test

According to the specialness of LED flux measurement, the design of integrated sphere for LED measurement adopts unique optimization, combining high reflectivity diffuse reflector material, which greatly improved system stability and accuracy.

The experimental results show that the stability and consistency of the system are far superior to those of other common LED testing systems. It is a truly suitable system for measuring the optical parameters of LEDs.

LISUN LPCE-2 Integrated Sphere Spectroradiometer LED Testing System is for single LEDs and LED lighting products light measurement. LED’s quality should be tested by checking its photometric, colorimetric and electrical parameters. According to CIE 177, CIE84,  CIE-13.3, IES LM-79-19, Optical-Engineering-49-3-033602, COMMISSION DELEGATED REGULATION (EU) 2019/2015, IESNA LM-63-2, IES-LM-80 and ANSI-C78.377, it recommends to using an array spectroradiometer with an integrating sphere to test SSL products.

The LPCE-2 system is applied with LMS-9000C High Precision CCD Spectroradiometer or LMS-9500C Scientific Grade CCD Spectroradiometer, and A molding integrating sphere with holder base. This sphere is more round and the test result is more accruacy than the traditional integrating sphere.

1. Introduction:

Different from traditional light sources, the measurement of LED light flux poses great challenges to the accuracy of the equipment when using integrated spheres for measurement. On the one hand, compared with traditional light sources, LEDs usually have stronger directionality and are no longer evenly illuminated in the whole space. This characteristic makes the direct light distribution on the surface of the integrated sphere of LED uneven. This uneven distribution will lead to different detection characteristics of the reflector for different LEDs.

Due to the fixed position of the reflector and baffle, direct performance of various reflectance distributions is signal fluctuation. In general test systems, there are differences in the LED of different emission angles, and the same LED has different placements with different positions and the same emission. Even with the same rated light flux; actual measurement values are different. According to customer verification results, the placement direction of LED in general LED test systems always affects the light flux measuring result more than 50% (the maximum signal and the minimum signal difference of the same LED measured in different directions).

Figure 1: Different lighting angles have different effects on LED measurements

When measuring different LEDs with different emission angles, due to the distribution difference inside the integrated sphere, the distribution of direct reflection has different impacts on the detector, which directly affects the difference of measurement accuracy (as shown in Figure 1).

On the other hand, LED test systems often employ tungsten lamps as the standard light source, which have considerable difference in appearance, lighting distribution characteristics and spectral characteristics compared to LED. Therefore, the difference should be corrected by absorption coefficient.

2. Analysis:
The internal reflection properties of the integrating sphere is one key factor affecting the measurement accuracy of LED directionality. In ordinary LED testing systems, the reflection rate and Lambertian characteristics of the coating on the integrating sphere’s surface are not ideal. One cause of this is its low reflectance, the other is its poor diffuse reflection characteristics. The result of a low-reflectance integrated sphere’s surface is that LED direct light gradually attenuates after a few times of reflection. However, in the process of light mixing, direct illumination and reflected light take up a large proportion, which is mainly dominant. In some cases, low reflection materials will produce a strong shadow effect on the back of the baffle probe. However, it is the light shadow effect caused by the linear reflection that leads to inaccurate measurement.

In addition, a lower diffuse reflectance will seriously affect the attenuation of signals. Because in the process of optical measurement, light is reflected multiple times in the integrated sphere, and each reflection will cause a certain degree of attenuation, but the influence of reflectivity on light intensity will be strengthened after multiple reflections.

For example, if the reflectance between the reflections in the integrating sphere is reflected 15 times, and there is a 5% difference in reflectivity, the signal attenuation may be more than doubled. In fact, the differences in the reflectance of the integrated sphere are far more than this.

At present, LED test systems have not yet been used as the standard light source for standard LED. In the measurement process, we still choose to use standard tungsten lamps as standard light sources. Due to the great difference between the external structure of standard lamps and measured LED, including the influence of LED light absorbance and the difference between the installation positions of standard lamps and LED, all these are important factors affecting the accuracy of test results.

3. Solution:
LPCE-2 Spectrophotometer & Integrating Sphere LED Testing System is developed by Shanghai Lishan Electronics, which completely meets the requirements of LM-79 and CIE, effectively solving the various defects of traditional LED testing systems. Compared with the large scale assembly and production technology of traditional integrated sphere, Lishan Electronics adopts one-time molding technology to produce integrating spheres, and its shape completely conforms to the spherical structure of 4π or 2π. LISUN Electronics Integrated Sphere also uses high reflection and diffuse coefficient coating, making the lamp is open-ended position design pointing to the detector position. Even if directional LEDs are used or position mode is used under extreme conditions, this improvement keeps the test results consistent.

LPCE-2 uses a standard tungsten lamp as the standard lamp and an optional auxiliary lamp to measure the difference between the LED seat and the standard seat and its impact on the test result. The standard lamp has been strictly calibrated by LISUN Electronics Calibration Laboratory; the test results are traceable to NIM.

In response to the accuracy of the LED test results, LPCE-2 test system was used for the corresponding test. The test conditions are as follows: 5 high brightness green LEDs, power is about 0.35W, illumination angle is about 30°. The LPCE-2 test system is used in 9 measurement positions, respectively indicating the possible LED position mode, as shown in Figure 2.

Figure 2: Different LED position patterns

4. Conclusion:

Figure 3: Luminous flux corresponding to different LED test positions

The relationship between the tested flux and the LED position mode is shown in Figure 3. From the test results, it can be seen that even under the most extreme conditions, when the LED is placed in front of and behind the detector opening, the peak of the flux test results remains below 5%. This is an exceptionally good test result. During the actual testing, the repeatability error of LED 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 reliable assurance. This standard system not only greatly supports the research and development and production of LED, but also is an ideal choice for optical performance measurement in the LED industry.

Tags：LPCE-2 (LMS-9000)