May 12 2014
An Ulbricht sphere, also known as an integrating sphere, is an optical device used for measuring the reflectivity/transmittivity of optical components, and optical flux, from an LED, or bulb, and a laser diode.
In 1900, R. Ulbricht proposed the use of the integrating sphere with diffusely reflecting walls for light integration. The device works on the property of diffusing effect and uniform scattering. The accuracy of the device is mainly affected by the optical properties of the reflecting wall coatings.
Early experimenters used magnesium oxide deposits as coating materials. Various polytetrafluoroethylene compounds have been widely used as a coating material for measurements in the visible spectrum.
Barium sulfate and finely deposited gold are used for infrared measurements.
Working Principle
An Ulbricht sphere is a hollow, spherical device with its inner region coated with a white reflective coating. It has two or more small entrance and exit ports for the introduction of a light beam, or for attaching a photodetector or baffles.
Baffles are light barriers, which prevent direct illumination of the detector by a light source.
This arrangement allows a number of diffuse reflections of the incident light, before the light strikes upon the detector. As a result, the light flux becomes uniform and independent of the polarization and spatial properties of the incident light. The detected optical power is based on the power of the total amount of incident light.
Applications
Light scattered by the interior of the Ulbricht sphere is uniformly distributed over all angles. For this reason, the device can be used for a number of applications, which include:
- Studying the reflection and absorption of samples
- Measurement of diffuse reflectance of surfaces
- Testing the homogeneity of imaging devices
- Photometric or radiometric measurements
References