An integrating sphere is a hollow sphere with a highly diffusing, high-reflectance inner coating (Spectralon/PTFE, barium sulfate, or diffuse white paint). Light entering the sphere is scattered many times and becomes nearly uniform , so detectors at any port measure an average/“integrated” quantity. That makes the sphere ideal for measuring total flux, reflectance/transmittance (both diffuse + specular), quantum yields, and for providing uniform illumination.
