An integrating sphere is an optical accessory used with spectrophotometers and spectroradiometers to collect and average the total light scattered, reflected or transmitted by a sample. The instrument consists of a hollow sphere with an interior coating of a near-perfect diffuse reflector, typically barium sulphate or polytetrafluoroethylene, with one or more ports for sample placement, illumination and detection. Light entering or generated within the sphere is reflected many times off the matte interior so that any small region of the sphere wall samples a representative average of the total light, regardless of the angular distribution of the original beam.

In gemmology the integrating sphere is encountered in the laboratory rather than at the bench. Spectrophotometric measurements of total absorption, transmission or reflectance for opaque, translucent and strongly scattering materials such as turquoise, jade, opal and treated stones rely on integrating-sphere accessories to handle the diffuse component that a simple in-line measurement would miss. Reference standards including diffuse reflectance plates are calibrated against integrating-sphere measurements, and the device is also central to fluorescence quantum-yield determination in gem materials when the analyst needs the total emitted light rather than a directional sample.

For the working dealer the term is unlikely to come up directly, but it underlies the published spectroscopic data on which laboratory identification reports rely.