A High-Sensitivity Low-Cost Optical Particle Counter Design

Abstract

We report the design of a small optical particle counter with high sensitivity and low construction cost for atmospheric aerosol measurements. Particle sensing is based on the detection of the forward scattering of laser light. The separation of the laser beam and scattered light is achieved with a novel yet simple optical system. A laboratory prototype system with a 405-nm laser and photomultiplier tube detector has successfully detected polystyrene latex particles as small as 125 nm in diameter with unit efficiency. Theoretical calculations suggest that a lower detectable size limit of 100 nm can be achieved with reduction of background scattered light. The new counter will be useful in a variety of ground-based as well as small balloon-borne applications such as vertical profiling and in situ measurement of particles from explosive volcanic eruptions.

A light-weight, high-sensitivity particle spectrometer for PM2.5 aerosol measurements

“…The amplitude of the high frequency component is comparable to the offset. This noise is similar to that observed in the previous work (Gao et al 2013a) and is often indistinguishable from pulses caused by small particles. The majority of stray light comes from Rayleigh scattering by air molecules, evidenced by a marked decrease (>2£) in noise when the enclosure is evacuated to lower than 100 hPa.…”

“…It consists of a diode laser, a collimating lens, and two cylindrical lenses. The laser and associated beam-forming optics are the same as those reported in Gao et al (2013a). A 405-nm laser diode (Sony SLD3234VF 100 mW or similar) is press-fit into a brass housing.…”

“…Note that the primary source of noise in the original prototype (Gao et al 2013a) was also likely due to Rayleigh scattering, and that the cause of the high frequency noise has not been determined. Low levels of light from an LED and an incandescence bulb caused similar high frequency noises, suggesting that it is a feature of the PMT and/or detection electronics.…”

“…The most significant change from our original design (Gao et al 2013a) is in the geometry for the collection of scattered light. The new design (which uses a spherical mirror to collect light scattered sideways instead of a focusing lens to collect forward scattered light) is more sensitive for particles below 200 nm in diameter, and less sensitive to larger particles.…”

“…Various designs have been developed with a range of performance and complexity, some of which lend themselves to straightforward miniaturization. In this vein, we have previously reported a low-cost, high-sensitivity OPC prototype (Gao et al 2013a). Now, we have further proved the design feasibility in the laboratory, improved certain elements (primarily in the light-collection optics used for detection) and constructed and deployed several working instruments.…”