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Integrating Sphere

An Integrating sphere (also known as an Ulbricht sphere) is an optical component consisting of a hollow spherical cavity with its interior covered with a diffuse white reflective coating, with small holes for entrance and exit ports. Its relevant property is a uniform scattering or diffusing effect. Light rays incident on any point on the inner surface are, by multiple scattering reflections, distributed equally to all other points. The effects of the original direction of light are minimized. An integrating sphere may be thought of as a diffuser which preserves power but destroys spatial information. It is typically used with some light source and a detector for optical power measurement.

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The theory of a light-collecting cubical box was described by W. E. Sumpner in 1892. The practical implementation of the integrating sphere was due to work by R. Ulbricht (1849-1923) published in 1900. It has become a standard instrument in photometry and radiometry. It has the advantage over a goniophotometer for measuring the light produced by a source that total power can be obtained in a single measurement.

Applications

Light scattered by the interior of the integrating sphere is evenly distributed over all angles. The integrating sphere is used in optical measurements. The total power (flux) of a light source can be measured without inaccuracy caused by the directional characteristics of the source.

Reflection and absorption of samples can be studied. The sphere creates a reference radiation source that can be used to provide a photometric standard.

Integrating spheres are used for a variety of optical, photometric or radiometric measurements. They are used to measure the total light radiated in all directions from a lamp. An integrating sphere can be used to measure the diffuse reflectance of surfaces, providing an average over all angles of illumination and observation. An integrating sphere can be used to create a light source with apparent intensity uniform over all positions within its circular aperture and independent of direction except for the cosine function inherent to ideally diffuse radiating surfaces (Lambertian surfaces).

Since all the light incident on the input port is collected, a detector connected to an integrating sphere can accurately measure the sum of the entire ambient light incident on a small circular aperture. The total power of a laser beam can be measured, free from the effects of beam shape, incident direction, and incident position.

Materials

The optical properties of the lining of the sphere greatly affect its accuracy. Different coatings must be used at visible, infrared and ultraviolet wavelengths. High-powered illumination sources may heat or damage the coating, so an integrating sphere will be rated for a maximum level of incident power. Various coating materials are used. Early experimenters used a deposit of magnesium oxide. Barium sulfatehas a usefully flat reflectance over the visible spectrum. Finely-deposited gold is used for infrared measurements. Various proprietary PTFE compounds are also used for visible light measurements.

The theory of the integrating sphere assumes a uniform inside surface. Ports for detectors and sources must be small, less than about 5% of the surface area of the sphere, for the theoretical assumptions to be valid. Unused ports in commercially built spheres may have matching plugs made up, with the interior surface of the plug coated with the same material as the rest of the sphere. A baffle may be inserted in the sphere so that the direct path of light from a source to a detector is blocked, since this light will have non-uniform distribution.

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Integrating Sphere

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