An integrating sphere (IS) acts like a diffuser, preserving power but neglecting spatial information

The integrating sphere can be manufactured at sizes from 1 mm up to 3 m in diameter. For diffuse reflectance, transmittance, and scattering measurements in spectroscopy (as in turbidity), an integrating sphere is the best choice. A tiny 16 mm integrating sphere is incorporated into a detector for radiant power measurement (bottom; courtesy of Gigahertz-Optic). This 65 in. spectral lamp measurement system is used in standards laboratories around the world as a calibration source for lamps and LEDs (top; courtesy of Sphere optics).

Spheres are also used to measure total luminous flux and total spectral radiance. The first major advantage of a sphere over power meters is attenuation. An integrating sphere (IS) acts like a diffuser, preserving power but neglecting spatial information. In addition to capturing all the light from any light source, an IS can be calibrated to NIST-traceable standards to measure spectral flux. “In sphere-based power measurement, 1 µW is a typical threshold for good signal-to-noise ratio, versus 1 nW or less for conventional power meters.” Darrell says, “The integrating sphere can be used in power measurements where a conventional meter might be damaged by the power level of the source. It is a uniform attenuator.” This can also be a negative as spheres are significantly less sensitive than conventional power meters.

Second, it helps to understand when integrating spheres are needed and how they work. When should you use an IS rather than a power meter or spectrometer? A sphere has a couple of major advantages over conventional power meters, Chris Darrell, vice president, sales and applications engineering, Sphere optics (Concord, NH), explains. “The first is uniform response independent of spatial and angular information. Spheres do not care about the angular profile of the source and spatial distribution—only input power.” This is useful for measurements of diodes or fibers with angular divergence that impinges on the quality of the power measurement. By definition, a sphere will wipe out this information. Darrell says, on the other hand, if you want beam profiles and angular information, then use a goniometer or beam profiler.

Art Springsteen, president of Avian Technologies (Sunapee, NH) says, “Typically integrating spheres are used when light, whether it’s transmitted, reflected, or emitted, is scattered and one wants to catch the most possible light.” Light entering the sphere undergoes multiple scattering reflections, so that rays that are incident on any point on the inner surface are equally distributed, minimizing the effects of the original direction of the rays.