Prototype particle size analyzer incorporating variable focal length optics

Abstract

A prototype particle size analyzer incorporating an innovative variable focal length optical system has been constructed. The device greatly minimizes some particle sizing errors inherent in current analyzers. It also has the advantage of being capable of measuring a wide range of particle diameters inexpensively and accurately. An existing five lens optical train was modified to allow each lens to be adjusted in three planes and then mounted on a rigid twin rail base to alleviate alignment problems. A precision linear positioning motor was installed to drive a zoom lens that adjusts the system focal length from 1850mm to 5550mm. A photosensor section that measures the intensities of forward scattered light at two different angles was built and evaluated. An analog computer interface has been built into the system to provide the device with automatic and manual control. During normal automatic operation the computer drives the zoom lens until an optimum intensity ratio is sensed. The system focal length is then determined which yields the corresponding particle size. Successful operation during initial testing has been achieved. Future refinements are suggested.