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Spotlight on Optics


  • February 2014

Optics InfoBase > Spotlight on Optics > Miniaturized sensor for particles in air using Fresnel ring lenses and an enhanced intensity ratio technique

Miniaturized sensor for particles in air using Fresnel ring lenses and an enhanced intensity ratio technique

Published in Applied Optics, Vol. 53 Issue 4, pp.625-633 (2014)
by Robert Schrobenhauser, Rainer Strzoda, Alexander Hartmann, Maximilian Fleischer, and Markus-Christian Amann

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Spotlight summary: The implementation of optical sensors for the analysis of aerosols is an important thrust in instrumentation engineering, given the widespread occurrence of technological processes that lead to aerosol creation and the discovery of the influence of aerosol particles on human health. The current trend is to employ laser particle counters, since they allow the analysis of the concentration and properties of individual particles for a wide range of the particle sizes.

The authors of this article have developed an aerosol sensor consisting of a focused laser beam, a inlet nozzle system that sends aerosol particles across the laser beam waist, and a system for collecting onto two photodetectors the radiation scattered by the particles within the angular ranges 15.6° -- 29.4° and 29.4° -- 37°, respectively. Two concentric Fresnel lenses are used for collecting the scattered radiation, allowing to reduce the weight and cost of the device and to increase the signal-to-noise ratio. The nozzle directs the aerosol particles into the laser beam waist with deviations of no more than 1 mm. The errors arising from these deviations are incorporated into the data-processing method, which helps mitigating their effect in the resulting measurement.

The identification of the scattering amplitudes and time of flight consists of four steps. In the first step, the cross-covariance of both scattering signals is calculated, and this is used to filter out signals with low signal-to-noise ratio. In the second step, the resulting signal is analyzed for symmetric Gaussian scattering events, and the amplitudes and time of flight are determined. This reduces the problem of coincident particles. In the third step, the scattering amplitude of each event is determined in both scattering signals. Finally, in the fourth step, the scattering amplitudes of both angle intervals and amplitude ratios are determined.

The authors apply amplitude ratios for the identification of particle size. To reduce the error resulting from uncertainty in refractive index, they use the sum of signals from both photodetectors in combination with scattering modeling based on the Mie theory.

A characteristic feature of this article is its detailed analysis of the processes leading to deviations from the ideal performance of the sensor. The analysis of such processes provided by the authors will undoubtedly draw the attention of other researchers in the area. One can expect follow-up research in the field of aerosol fluent aerodynamics for the focusing of particles on the axis of a laser, as well as in the processing of signals from several photodetectors for particle characterization.

--Vladimir Berdnik

Technical Division: Light–Matter Interactions
ToC Category: Scattering
OCIS Codes: (120.4570) Instrumentation, measurement, and metrology : Optical design of instruments
(220.4830) Optical design and fabrication : Systems design
(290.5850) Scattering : Scattering, particles
(290.2558) Scattering : Forward scattering

Posted on February 04, 2014

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