The integrating sphere is a cavity sphere whose inner wall is coated with white diffuse reflective material, also known as photometric sphere, luminous flux sphere, etc. One or several window holes are opened on the spherical wall, which are used as light inlet holes and receiving holes for placing light receiving devices. The inner wall of the integrating sphere should be a good spherical surface, and it is generally required that its deviation from the ideal spherical surface should not be greater than 0.2% of the inner diameter. The inner wall of the sphere is coated with an ideal diffuse reflection material, that is, a material with a diffuse reflection coefficient close to 1. The commonly used material is magnesium oxide or barium sulfate, which is mixed with colloidal binder and sprayed on the inner wall. The spectral reflectance of the magnesium oxide coating in the visible spectrum range is above 99%. In this way, the light entering the integrating sphere is repeatedly reflected by the inner wall coating to form uniform illumination on the inner wall. In order to obtain higher measurement accuracy, the aperture ratio of the integrating sphere should be as small as possible. The opening ratio is defined as the ratio of the area of the sphere at the opening of the integrating sphere to the area of the entire inner wall of the sphere.