Integrating Spheres

An integrating sphere (also called Ulbricht sphere after Friedrich Ulbricht) is an optical device for various purposes such as

• measuring the optical flux from a laser diode, light-emitting diode (LED) or bulb
• measuring the transmittivity or reflectivity of optical components, even when the partly scatter the light
• illuminating some device very uniformly

Figure 1: Integrating sphere for measuring optical powers independent of the spatial beam properties.

The device (see Fig. 1) is a hollow sphere with a diffusely reflecting internal surface, typically two or more small openings (ports) for introducing light or attaching a photodetector, and often some so-called baffles, which are light barriers used to prevent direct illumination of a detector by a light source. The arrangement causes many diffuse reflections of the introduced light before it reaches a detector, so that the light flux becomes very uniform at the detector, and nearly independent of the spatial and polarization properties of the introduced light: the detected optical power depends only on the total introduced power. In that way, the total output power of alaser diode can be measured, even if the beam divergence is fairly large.

Apart from such measurement purposes, an integrating sphere can be used to illuminate a device very uniformly. This can be important e.g. for testing the homogeneity of digital imaging equipment (e.g. CCD arrays).

Ideally, the coating on the inner side of the integrating sphere has a very high reflectivity over the required wavelength range, and the reflection is very diffuse. If the optical losses in the sphere and through the small ports are low, the multiple reflections can lead to a fairly high optical intensityinside the sphere and consequently to a high optical efficiency, even if the sphere is much larger than the light source and the detector.