An integrating sphere is an apparatus which is primarily designed to collect electromagnetic radiation from a source or sample and to measure its total optical power. The sphere consists of a spherical cavity with a highly reflective, but diffuse, interior surface and small apertures for the entry and exit of radiation. Regardless of the trajectory of any incident waves, the interior surface will scatter all radiation evenly throughout the sphere, eliminating spatial non-uniformities (via spatial integration) but preserving the original power levels. This allows for an accurate gauge of the total flux emanating from the source. Most applications center around the measurement of source radiation – usually in the optical band – when high precision is needed. The sphere can also be used as a source itself. Integrating sphere introduction and more information>>
Radiometry
When combined with a highly responsive photodector, integrating spheres are used to accurately measure radiant flux either directly from a source or from some illuminated sample. Since radiometry in general is not confined to a particular wavelength band, the total spectral power of the source is measured. Thus, when the sphere is used as a radiometer, incoming radiation is filtered to exclude any undesirable or extraneous wavelengths per the given application. When coupled with an interferometer or diffraction grating to separate wavelengths, power can be measured on a spectral basis.
Photometry
When used as a photometer, the integrating sphere is configured for measuring in the optical spectrum, including infrared, visible (to humans) light, and ultraviolet bands. It is ideal for precise comparison of the luminous intensity of a direct light source, since the effects of outside sources and geometric spreading are eliminated. Furthermore, the attenuation of intense, collimated sources such as lasers is a direct function of the sphere geometry, allowing for a calculated measurement of the original beam intensity.
Material Scattering and Diffusion
When a diffuse sample material of interest is placed at the sphere input, an accurate measurement of its transmittance properties can be made by illuminating the sample with an outside source of known intensity. It is typically desired to view these results spectrally, as the spectral intensity is illustrative of certain material properties. Alternatively, a sample can be placed inside the sphere – opposite an unobstructed, calibrated source at the input – in order to measure its scattering properties. Like the transmittance results, scattering data are generally most useful when viewed as a function of wavelength.
Aside from its common use as a precision radiance flux measurement device for external sources and samples, the integrating sphere is also used as a uniform source of diffuse light for the calibration of other sources or optical instruments. Additionally, it can be used to determine the sum of the intensities of ambient light incident on its input aperture from all angles. Multipurpose commercial spheres feature various configurable light sources that can be used to suit each desired application