Variable focal length radiometer for particle sizing

Various approaches exist to obtain information regarding aerosol particle size distributions. The Sauter mean diameter (SMD) measurement, which provides the mean size of the particles, has the advantage that it may be carried out with relatively inexpensive techniques. A description is given of a technique for inferring SMDs of aerosols using forward scattering of light (Fraunhofer scattering). The technique is based on the assumption of a Gaussian distribution of particle sizes. The introduction of an optical element with a variable focal length (a zoom lens) makes it possible to obtain a range of effective instrumental focal lengths. This approach broadens the range of particle sizes which may be determined with the aid of a single instrument.