Analyzing the Results of LED Integrating Sphere Tests

The research and development of LED light source has become the focus of the world. Governments in various countries are constantly improving the standards of LED products, and have higher requirements for the testing of LED products. Quality supervision, third-party testing and professional certification agencies and enterprises have new demands for testing equipment. For the measurement of optical characteristics of LED and its finished lamps, there is still no unified test method. At present, most of them use LED Integrating Sphere or Distribution Photometer for the measurement of optical characteristics of LED.

The testing system of distribution photometer has high accuracy, but is very expensive, complex in operation, and needs 6-8 hours for measurement, which cannot meet the actual production and testing requirements of small and medium-sized enterprises. The accuracy of the integrating sphere test system is lower, but the price is low, easy to operate, and the measurement speed is fast. It only takes about 30 minutes for a single measurement, but due to the lack of a dedicated LED standard light source, the measurement results have low credibility. Therefore, we used the integrating sphere test system to measure the optical and chromatic characteristics of LED/COB ceiling lamps by putting them in different positions and orientations, and then used the high-precision distribution photometer to measure the relevant optical and chromatic parameters of the same LED/COB ceiling lamps. To study and analyze the test method and error compensation of LED finished lamps with integrating sphere.

I. Relationship between Optical Characteristics of LED and Position
Traditional packaged LED is generally circular or cylindrical, with a luminous angle of 5° -90°. Moreover, the light intensity distribution of LED light source is uneven, that is, the light intensity is the largest in the normal direction. As the angle between the light emitting direction and the normal direction increases, its light intensity is an approximate Lambert source, and its light emitting area is also relatively small, i.e. the light intensity of LED is the maximum when the light is perpendicular to the LED light emitting surface, that is, the angle with the axis is zero degrees. As the angle increases, the brightness will decrease.

II. Principle of LED Integrating Sphere Test
LED Integrating Sphere Test system uses the relative method to measure the luminous flux, spectral distribution and chromaticity of LED light source. The standard light source of the same type as the tested LED is used to calibrate the integrating sphere photometer and spectral radiance meter to obtain better measurement uncertainty. This test uses a narrow beam angle D65 standard light source specially calibrated for LED. In the LED integrating sphere test method, objects such as light sources, baffles, brackets, etc. placed in the integrating sphere will change the light distribution in the sphere, forming self-absorption effect. When the lamp is in different positions or angles, the light distribution in the sphere is also different. During the test, the light source is generally placed at the center of the LED integrating sphere, and the photometer and spectral radiance meter are arranged on the detection window of the LED integrating sphere respectively to measure the luminous flux and average spectral power distribution of the light source, then calculate the spectral correction factor according to the measurement results and correct the measurement value of the photometer, calculate the spectral correction factor.

III. Principle of Distribution Photometer Measurement
At enough light-emitting planes, the illuminance detected by the distribution photometer at each point on the virtual spherical surface with the measuring distance between the light detecting detector and the center of the light emission of the tested LED source as the radius is measured with a small angle interval. The plane angle interval is generally 5°, and the angle interval within the plane is generally 1°. When the size of the tested LED module is large or the beam angle is narrow, smaller plane interval and angle step should be adopted to ensure the completeness of illuminance sampling.

IV. Test Phenomenon

(1) Luminous flux. When the COB ceiling light is placed forward, although it is blocked by the baffle, some light still penetrates directly to the detector, so that the maximum measured value is 235.62lm; When the COB ceiling light is facing backward, the light is scattered onto the detector, and the minimum measured value is 219.38lm, with the extreme value difference of 16.24lm.
(2) Color temperature. Color temperature is related to color coordinates. As the color temperature rises, the color becomes colder. The maximum color temperature measured when the LED lamp faces forward is 6661.4K, the measured value when facing upward is 6268.2K, and the measured value when facing downward is 6219.2K. there is no more than 1% difference between other situations and the measured value downward.
(3) Color rendering indexRa. Spectrum determines color rendering. When the spectrum of each band is continuous and abundant, the color rendering is good; When some spectrum bands are missing or less, the color rendering is poor. The maximum color rendering index measured when the COB ceiling light is facing forward is 85.18, the minimum color rendering index measured when facing backward is 83.94, with a difference of 1.24 between the two.
(4) Luminous efficacy. The distribution of measured value of luminous efficacy under different angles is basically the same as that of luminous flux. When the COB ceiling light was facing forward, the maximum luminous efficacy measured was 82.28lm/W; When facing the baffle, the minimum luminous efficacy measured was 75.66lm/W. The extreme value difference was 6.62lm/W.
(5) The main wavelength. The main wavelength reflects the difference between the spectrum of the tested light source and the standard corpus spectrum. The minimum main wavelength measured when the COB ceiling light is facing forward is 475.4nm. The measured results of other situations are not far from the measured results when facing downward, which are 479.68nm.

V.  Summary
The test results show that for LED light source, its spatial luminous intensity and chromaticity parameters are different due to its own non-uniformity of light emission. The comparison and analysis of the measurement results of 3WCOB LED ceiling lamp measured by LED integrating sphere test system and distribution photometer test system shows that there is a certain difference, but the difference is not big. Taking into account the characteristics of fast measurement speed, easy operation and low price of integrating sphere measurement, it is suggested to use LED integrating sphere test system for the measurement of finished lamps, and put it in the center of the integrating sphere downward for corresponding compensation for different measurement parameters.

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