Theory of Using Aluminum and An Integrating Sphere in Photometry

I. Definition of an integrating sphere:
An integrating sphere is an empty, hollow cavity covered with a diffuse, white reflective coating, also known as a photopic ball or flux ball. There is one or more holes on the wall for entering light and for placing the light receiver. The inner wall of the integrating sphere should be a good spherical surface; usually the deviation from the ideal sphere is not more than 0.2% of the inside diameter. The inner wall of the sphere is coated with ideal diffusion material, which is a material with diffuse reflectivity close to 1. BaSO4 is usually used. Mix it evenly with adhesive and then spray it on the inner wall. The spectral reflectance of BaSO4 coating in the visible spectrum can reach 99%.

LISUN 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 accruate than the traditional integrating sphere.

 

The light entering the integrating sphere is repeatedly reflected by the inner wall coating, forming a uniform lighting on the inner wall. In order to obtain higher measurement accuracy, the aperture of the integrating sphere should be as small as possible. The aperture ratio is defined as the ratio of the spherical area of the opening of the integrating sphere to the total internal wall area. There are two types of integrating sphere structures in general. One is the 2pi structure, and the other is the 4pi structure. The new type loading table integrating sphere IS-*MA introduced by Lushan can be used to test four types of different types of SSL, including all directional or only one directional emitting (ignoring direction).

The IS-*M**P photometric integrating sphere (lateral opening) can also be used for two-way testing. 4pi test is simply to test the lamp in the middle of the integrating sphere, and 2pi is to test the lamp in the side opening. 4pi tests are suitable for all types of SSL including all directional or only one directional emitting (ignoring direction), 2pi tests are suitable for forward emitters (ignoring direction with large heat sinks and no directional emission), and for LEDs without backward emission.

II. The functions of integrating sphere are as following

1. Light Receiver
The light entering the integrating sphere passes through a small hole on the integrating sphere and sets one or two light detectors such as selenium photovoltaic cells or photomultiplier tubes on the inner wall. The light current output by the light detector is proportional to the illuminance on the inner wall of the integrating sphere, that is also called illuminance integrating sphere, proportional to the light flux entering the integrating sphere. In this way, you can know the change of the light flux entering the integrating sphere according to the change of the output light current.

2. Uniformly lit surface
Several lamps (usually four or six) are symmetrically and uniformly set on the inner wall of the integrating sphere, opposite the exit hole. The light emitted from the lamp is repeatedly reflected by the inner wall and forms a uniform and bright lit ball surface. It can be used as the uniform brightness and large field (2w>140 degree) surface (the entrance pupil of the optical system is basically consistent with the exit hole) of the measured optical system. This integrating sphere is used to measure the fading coefficient and illuminance uniformity of photographic objectives.

3. Spherical Parallel Light Pipe
Two holes are opened at both ends of the horizontal axis line of the integrating sphere. One hole is used to install a collimating lens, and the focal length of the collimating lens is equal to the diameter of the inner wall of the sphere. A few lamps are symmetrically set on the shell near the objective mirror, and it is required that the light they emit cannot be directly illuminated on the objective mirror. Another hole is plugged with a center-opening plug, and a horn-shaped shutter with black absorber layer on the inner wall is plugged outside the plug, so that all the light entering the plug hole is completely absorbed in the shutter. Thus, the plug and shutter together constitute an absorber. In this way, the sphere simulates a completely black target in a bright sky for the collimator.

When the hole plug and the shutter are removed and replaced by a white plug, the sphere simulates a bright sky with uniform brightness. An integrating sphere with the collimating lens, lamp and black and white plugs is called a spherical parallel light pipe, which is used to measure the scattered light coefficient of the collimator. During the measurement, the illuminance of the “black body target” and the “white plug” is measured by the photodetector respectively, that is, the corresponding indication values measured by the photodetector. After calculation, the scattered light coefficient of the collimator can be obtained.

Lisun Instruments Limited was found by LISUN GROUP in 2003. LISUN quality system has been strictly certified by ISO9001:2015. As a CIE Membership, LISUN products are designed based on CIE, IEC and other international or national standards. All products passed CE certificate and authenticated by the third party lab.

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Tags：LPCE-2 (LMS-9000)