Measuring Instruments and Data Processing for the Determination of Particle Size Distribution
Swithenbank et al. (1976) have developed an instrument based on Fraunhofer’s diffraction which is shown schematically in Fig. 4.1. It consists of a small 1 mW He-Ne laser fitted with a spatial filter and a collimating lens which provides a parallel beam of monochromatic coherent light. The particles are placed in this beam and the scattered light is collected by a lens and brought to a focus on a special detector placed in the focal plane. The detector consists of thirty semicircular, photosensitive rings. The signal is transmitted to a PDP8-A minicomputer via a multiplexor -and analogue/digital converter unit and the whole system is controlled by a teleprinter, which is also the output device. The detector/lens system used gives approximately a hundred-to-one particle size ratio; a 105 mm lens gives a particle size range from 2 μm to 197 μm, a 300 mm lens a range from 5.7 μm to 563 μm, and a 630 mm lens a range from 12 μm to 1182 μm. For measuring in high ambient lighting where parasitic effects may be significant, the photodetector can be protected by a red filter (for an He-Ne laser). Although it would be possible to use an interference filter to improve the signal/noise ratio, careful calibration would be needed at the higher angles since the pass band of the interference filter depends on the angle of incidence of the light. When the instrument is used for droplet and airborne particle size measurement the laser transmitter and beam expander are mounted at one end of a rigid aluminium channel. At the opposite end of the channel is the multi-element detector system. The particles to be measured are caused to pass through one laser beam between the transmitter and receiver.
KeywordsParticle Size Distribution Light Beam Focal Plane Interference Filter Photographic Plate
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