LED measurement system: integrating sphere measurement method

What is a integrating sphere:
The integrating sphere is also known as a hollow sphere with a highly reflective inner surface. It is mainly a high-efficiency device that collects the scattering or emission of light by samples placed inside or outside the ball and close to a certain window, or the light emitted by the light source itself. It can be used to accurately measure the optical reflection and transmission performance of materials, as well as the radiance, brightness, or chromaticity of light sources.

The working principle of the LED measurement system:
The light is collected by the integrating sphere through the sampling port, and after multiple reflections inside the integrating sphere, the light will be very evenly presented at various positions inside the integrating sphere. When using the light inside the integrating sphere for measurement, it can make our measurement results more reliable. The advantage of using an integrating sphere structure is that it can reduce and eliminate measurement errors caused by differences in light shape, divergence angle, and response at different positions on the detector.

The Function of the LED measurement system
1. Optical receiver
The measured light enters the sphere through the small hole on the integrating sphere, and one or two photodetectors, such as selenium photocell or Photomultiplier tube, are arranged on the inner wall. The photocurrent output by the photodetector is directly proportional to the illumination on the inner wall of the integrating sphere, that is, directly proportional to the luminous flux entering the integrating sphere. In this way, the change in the light flux entering the integrating sphere can be determined based on the change in output photocurrent.

2. Evenly illuminated surface
Symmetrically and evenly install several light bulbs (usually four or six) on the inner wall of the integrating sphere and the light outlet hole. The light emitted by the bulb is diffused multiple times through the inner wall to form a uniform and bright luminous sphere, which can be used as the object surface of the tested optical system with uniform brightness and a large field of view (2w>140 degrees) (the optical system’s inlet and outlet holes are basically coincident). This integrating sphere is used for measuring the vignetting coefficient and image plane illumination uniformity of photographic objectives.

3. Spherical collimator
Open two holes at both ends of the horizontal axis of the integrating sphere. Install a collimating objective through a hole, with the focal length of the collimating objective equal to the diameter of the inner wall of the sphere. Install several bulbs symmetrically with the horizontal axis on the shell near the objective lens, ensuring that the light emitted by them cannot directly reach the objective lens. Install a plug with a central opening on the other hole, and then insert a horn shaped extinction tube with a black absorption layer on the inner wall outside the plug, so that the light entering the extinction tube through the plug hole is completely absorbed.

Therefore, the perforated plug and the extinction tube together form a blackbody, so that when aimed at the straight objective, the sphere will simulate a completely black target in a bright sky. Remove the hole plug and extinction tube, replace them with a white plug, and the sphere will simulate a sky with uniform brightness. The integrating sphere with a collimating objective, a light bulb, and black and white stoppers is called a spherical collimator, which is used to measure the stray light coefficient of the telescope.

During measurement, the illuminance of the blackbody target image and the “white plug” image are measured using a photodetector, which is the corresponding indicator value measured by the photodetector. After calculation, the stray light coefficient of the measured telescope can be obtained. Because if the imaging of a blackbody target in a bright sky by a telescope is not completely black, it indicates that in addition to imaging the target, the telescope also has stray light shining on the image surface.

Why use a LED measurement system?
Generally speaking, when used carefully, optical diffusers can reduce minor errors caused by uneven distribution of incident light sources on the detector or slight deviation of the beam during measurement, thus improving the accuracy of measurement. However, in more precise measurements, you must use an integrating sphere as an optical diffuser to minimize the aforementioned errors.

When using a LED measurement system to measure the luminous flux (Lumen), it can make the measurement results more reliable. The integrating sphere can reduce and eliminate measurement errors caused by the shape of light, divergence angle, and differences in response at different positions on the detector.

The integrating sphere can also be paired with a spectrometer, connecting the light output hole of the integrating sphere to the incident grating of the spectrometer to ensure that the angle of the light source to be measured entering the spectrometer is the same, greatly improving the reproducibility of measurement.

The specific measurement and usage methods of the integrating sphere measurement mainly include the following four aspects:
1. Preparation: Prepare an integration sphere of appropriate size, a standard light source with a luminous flux close to that of the lamp light source, and maintain an ambient temperature of around 25 degrees Celsius, without wind blowing into the integration sphere;

2. Inspection: Install the standard light source at the center of the integrating sphere, connect the current source and power meter, and then light up the standard light source. Adjust to continuous testing on the operation interface of the integrating sphere software until the luminous flux reaches stability, and record its luminous flux value;

3. Calibration: The calibration operation is similar to inspection, except that zero calibration is required before lighting up the standard light source. After the light flux reaches stability, input the standard color temperature and standard light flux of the standard light source in the operation interface of the integrating sphere software, and then click to start calibration. The integrating sphere tester will automatically complete calibration;

4. Test sample: Install the sample into the integrating sphere, light up the sample, close the integrating sphere, and start testing. After the light flux stabilizes, record the values.

LPCE-2 Integrating 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.

High Precision Spectroradiometer Integrating Sphere System LPCE-2(LMS-9000)

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