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The integrating sphere consists of a hollow sphere, the inner surface of which is coated with a non-selective matte finish with a high reflectance, creating diffuse reflection. The illuminance of any point on the sphere that is shielded from the direct rays of the light source under study is proportional to the luminous flux of this source (or, more generally, to the radiant flux).
One of the key parameters of the sphere is its diameter, which must significantly exceed the dimensions of the light sources being measured; consequently, spheres with diameters of up to 16 ft (5 meters) are constructed for measuring luminous fluxes. Sometimes, the radiation under study is introduced into the sphere through an aperture that is small compared to its diameter.
First developed by R. Ulbricht in 1894, the photometric sphere was used to measure the radiant power of newly developed electric light bulbs. Today, integrating spheres are widely used for light and color measurements—specifically for measuring luminous flux, color temperature, color coordinates, and color rendering (CRI) of lamps and luminaires, as well as the reflectance and transmittance of materials. They also serve as sources of uniform radiation with exceptionally even luminance distribution across the entire exit port, making them ideal for testing and calibrating imaging systems, photometers, CCD cameras, detector arrays, and remote sensing instruments.
Structurally, all integrating spheres consist of a metal shell with a reflective coating. However, manufacturers offer a wide range of configurations and optional features. Depending on their size, the spheres may include up to two internal lamp holders and/or one or two optical ports on the surface, as well as interfaces for connecting power supplies and measuring instruments for samples placed inside the sphere.