Integrating sphere

An integrating sphere is a hollow optical instrument whose diffuse reflective interior collects radiation and averages its spatial distribution across the sphere's inner surface.

Diffuse reflection: Reflection that scatters incident light over many directions rather than preserving a single reflected direction. Repeated diffuse reflections spread incoming radiation across the sphere's interior.
Total luminous flux: The total visible-light output of a source, weighted by the human eye's photopic sensitivity. An integrating sphere measures lamp output without requiring directional intensity scans.
Radiant flux: The rate at which electromagnetic radiation transfers energy, measured in watts. The sphere redistributes this total optical power without ideally changing it.
Goniophotometer: An instrument that measures optical intensity as a function of emission angle. It resolves angular distribution, while a sphere generally collapses that information into total output.
Lambertian surface: A surface whose apparent brightness is independent of viewing direction and whose emitted intensity follows Lambert's cosine law. The sphere's coating approximates this behavior to produce uniform radiance.
Spectral reflectance: The fraction of incident light reflected by a material at each wavelength. A sphere collects reflected light over many directions for material reflectance measurements.
Radiance: Radiant power per projected area per solid angle, describing directional optical output. The sphere's diffuse field is commonly characterized by its interior radiance.
Monochromator: An optical device that isolates a narrow range of wavelengths from a broader spectrum. A monochromator selects wavelengths; the sphere instead collects light over many directions.
Sphere multiplier: A factor describing how repeated reflections increase the irradiance inside an integrating sphere relative to a single reflection. It quantifies how coating reflectance and port area determine the sphere's amplification.
Spectrophotometry: The measurement of how much a sample absorbs, transmits, or reflects light across wavelengths. Sphere attachments enable measurements of scattering samples that defeat narrow-beam setups.

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