An integrating sphere is used to measure a divergent light source. As shown in the illustration, an integrating sphere has its inner surface coated with a surface that highly reflects (typically 99%) in a scattering, nonspecular way. Thus when a divergent beam hits the walls of the integrating sphere, the light is reflected and scattered many times until the light hitting any place on the walls of the sphere has the same intensity.
A detector placed in the sphere thus gets the same intensity as anywhere else and the power the detector detects is proportional to the total incident power independent of the beam divergence. (The detector is so arranged that it only sees scattered light and not the incident beam.) An ideal integrating sphere has a surface with reflective properties are Lambertian. This means that light incident on the surface is scattered uniformly in all directions in the 2pi steradians solid angle above the surface. The surface used in Ophir integrating sphere heads closely approximates a Lambertian surface.
The 3A-IS series has two 50mm integrating spheres in series with a photodiode detector. The two series spheres scramble up the light very well, thus giving output very independent of incident beam divergence angle. The two spheres in series also ensure that the light hitting the detector is greatly reduced in intensity thus allowing use up to 3 Watts even though photodiodes saturate at about 1mW. There are two integrating sphere models, the 3A-IS with a silicon photodiode for 400 – 1100nm and the 3A-IS-IRG with an InGaAs detector for 800 – 1700nm.