There are two principles upon which an integrating sphere’s operation is based: the internal diffraction of light and the diffusion of the light beams to the various photosensitive detectors.
Integrating spheres have several functions, but their main purpose is to measure the output from divergent light sources. As the name implies, it is spherical in shape (although several models are have a cuboid outer housing) and it is hollow. This construction is based, in part on the structure of base spheres. The sphere is internally coated with a highly reflective surface and contains a variety of specialized equipment, including baffles and detector ports. Upon entering the sphere, approximately 99% of this light is diffracted throughout the sphere multiple times as if it were in a clear prism; this allows each individual beam to reach the same intensity. When one of the photo detectors is struck by a light beam, the detector measures the beam’s intensity in terms of proportional power or a sum of all ambient and divergent light inside the integrating sphere. The resulting radiance exiting the sphere extends for a full hemisphere while the irradiance striking the internal sphere wall is incident from a full hemisphere.
Integrating spheres must have a uniform reflective surface with a diffuse reflectivity that is at or near 100%. Proper calibration and configuration of the sphere and any related equipment is of the utmost importance in order to gain accurate, reliable readings. Each wavelength (visible light, ultraviolet, infrared) requires a different internal coating to be used. In each case, the internal coating needs to be free of fluorescence or the photo detectors inside the sphere will give faulty measurements due to the absorption of short-wavelength light by fluorescent materials.
The integrating sphere has several uses in photonics, but its most historic use is the measurement of luminous flux. Innovations resulting from these studies have led to the invention and manufacturing of common products such as LED and LCD backlights. Because of its properties as a uniform radiance source, the sphere can be used for imaging and nonimaging illumination. The sphere’s geometry allows it to capture and measure the power of laser diode beams. By increasing the number of ports and adjusting the sphere size, the sphere can take measurements from various incident angles. Currently, integrating spheres have three main applications: integrating sphere light detectors (LSD), integrating sphere light sources (ISS), and integrating spheres for photometric material properties (ISMP). The internal configuration of structures and diffraction components varies between each sphere. Of these three, LSD’s are the most prominent application. Their primary function is the measuring of reflectance and transmittance resulting from light diffraction. Prior to the introduction of the integrating sphere, it was nearly impossible to carry out field radiometry without getting erroneous results. With their advent, however, integrating spheres, ISMP’s to be specific, have successfully proven their worth as reliable radiometry instruments. Similar to spectroscopy, the sphere’s readings can aid in classifying various materials based on their chemical composition.
