The Role of Integrating Sphere
Integrating Sphere plays the role in below three aspects.
1. Optical Receiver
The measured light enters the sphere through the hole of integrating sphere. We set up one or two photo detectors on the inner wall such as selenium photovoltaic cells or photomultiplier tubes. The output light detector of the photo detector is proportional to the illuminance of the integrating sphere inner wall, which is proportional to the luminous that enters the integrating sphere. In this way, we could obtain the flux change of integrating sphere according to the change of output photocurrent.
2. Uniformly illuminated surface
Several bulbs (usually four or six) are evenly arranged symmetrically on the inner wall of the integrating sphere. The light emitted by the bulb diffuses on the inner wall several times to form a uniform bright luminous sphere, which can be used as the uniform brightness, large field of view (2w>140 degrees) object surface (Optical system entrance pupil and the exit hole is basically coincident) of the measured optical system. The integrating sphere is used to measure the vignetting coefficient of the photographic objective lens and the uniformity measurement of the illuminance of image plane.
3. Spherical parallel light tube
Open two holes at the horizontal axis at both ends of the sphere. One hole is fitted with a collimating objective lens, and the focal length of the collimating objective lens is equal to the inner diameter of the sphere. Symmetrically set a few bubs with horizontal axis in the vicinity of the objective lens on the side of the sphere, the light emitted by the bulbs can’t hit directly the objective lens. The other hole is fitted with a stopper with a central opening, and a horned extinction tube with a black absorbing layer on the inner wall is inserted so that the light entering the extinction tube through the stopper hole is completely absorbed.
Thus, the stopper with hole and the extinction tube form a bold body, so that the sphere will simulate a black object in the bright sky for the collimator. Remove the stopper with hole and the extinction tube, put the white stopper and the sphere will simulate a uniform brightness sky. The integrating sphere with collimating objective lens, bulb and black and white stopper is called the spherical parallel light tube, which is used to measure the stray light coefficient of the telephoto coefficient. During the measurement, the illuminance of the blackbody target image and the “white stopper” image are measured by the photo detector. We could obtain the stray light coefficient of the measured telescope after calculation the corresponding indication value measured by the photo detector. If the image is not total black when the telescope images the black target in the bright sky, then there is some stray light to the image plane except the telescope images the target.