KEYWORDS: Particles, Laser Doppler velocimetry, Nanoparticles, Atmospheric particles, Signal processing, Doppler effect, Calibration, Photon counting, Single photon, Signal to noise ratio
Laser Doppler anemometry offers a non-intrusive in-situ flow measurement method both for scientific and industrial
environments, especially in extreme conditions. Compared to the commercial LDA systems Nano-LDA device was
developed for simultaneous flow measurement, particle counting and sizing down to the nanometer size range. High
detection sensitivity was reached by applying single photon avalanche diodes with photon correlation technique and
special techniques for particle counting (burst selecting) and individual burst signal processing.
In this paper verifying measurements are performed with Palas 2.0 iP aerosol generator and differential mobility analyzer
down to 75nm paraffin particles, which is in accord with the lower size limit of the generator. In case of individual
particle velocity estimations the low SNR signal requires special prepare of the autocorrelation function such as
unfolding, zero padding and windowing. A detailed discussion is shown for the role of the different techniques in
velocity estimation.
The amplitude technique in the particle sizing requires a calibration process to determine the intensity loss of the system.
The model-based algorithm supports the calibration by the complete simulation of the measurement process and light
scattering. By this way a single calibration measurement for one kind of monodisperse particles can be enough. The
model-based algorithm is tested by measurements with monodisperse particles of different sizes set by the DMA.
A particle sizing algorithm is developed for nanoparticle sizing with laser Doppler anemometry/velocimetry systems. A model-based signal processing method is used to estimate the particle size from the autocorrelation curve corresponding to a single particle transit. Two kinds of figures of merit functions are combined to improve the sensitivity. The optimal setup parameters (refractive index, wavelength, and observation direction) and trajectory error are investigated in independent simulation studies. At 514-nm illumination wavelength, the most sensitive size region is found below 300 nm down to the sizing limit (20 photons on average from a single particle transit). The required laser power is searched for by the lower sizing limit based on the calibration measurement. The size estimation of a polystyrene sphere particle of 50 nm diameter requires at least 123-kW/cm2 laser power density at 350 nm, while 587 kW/cm2 at 514 nm is used in the studied system.
Signal processing method was developed for simultaneous particle counting, sizing and flow velocity measurements with
photon correlation laser Doppler anemometers. The high sensitivity of avalanche photodiodes in photon counting mode
assures the ability to catch the individual particles in the submicron/nanometer size range. A detailed discussion is given
about the optimal set up parameters (wavelength, detector position and refractive index) and calibration measurements
are shown to determine system parameters in a particular arrangement and simulations to estimate the lower size limit of
the particle characterization. Estimations are given for the required laser power to detect 20 photon counts in average for
a single particle transit as a lower limit of the particle sizing procedure. In the calibration measurement the most sensitive
size region was below 300nm down to the sizing limit (20 photons in average) at 514nm illumination wavelength. As a
numerical example we conclude that the size estimation of a polystyrene sphere particle of 50nm diameter requires at
least 123 kW/cm2 laser power density at 350nm while 587 kW/cm2 at 514nm in the studied system.
KEYWORDS: Particles, Signal to noise ratio, Photon counting, Laser Doppler velocimetry, Signal processing, Filtering (signal processing), Sensors, Data processing, Scattering, Algorithm development
The photon correlation Laser Doppler Anemometers were developed to measure the flow velocity also in the nanometer
particle range. An LDA signal processing method has been developed for dividing the raw data line of photon
correlation LDA into shorter parts corresponding to single particle transit (burst). The commonly used Lee filter was
applied with some modification and an intelligent burst finding algorithm was developed. By this way the LDA system
was adapted for single particle counting. The complete simulation algorithm gives an opportunity for discussing the
burst selecting and so the particle counting efficiency as a function of the SNR. Size estimation from the burst size was
discussed and compared to the model-based signal processing technique. The minimum detectable particle size was
estimated.
KEYWORDS: Particles, Signal processing, Laser Doppler velocimetry, Sensors, Error analysis, Monte Carlo methods, Signal detection, Nanoparticles, Doppler effect, Optimization (mathematics)
The lower limit of size measurement with PDA is about 150nm particle diameter, because of the small scattering efficiencies under this size. A model-based signal processing method is developed to determine the particle size from the autocorrelation function constructed in photon counting LDA systems by simulating the whole scattering and detecting process. An optimization algorithm is proposed to fit the computed autocorrelation function to the measured one. The errors of this signal processing method are discussed.
A planar phase Doppler system and a photon correlation LDA system was simulated regarding to particle size measurement below 150nm size range. Light scattering on Gaussian beams was simulated by Lorenz-Mie theory (LMT) and superimposed on the detectors' surface. The planar PDA system was found unable to size measurement in this range. Particle size measurement from photon correlated LDA signal is discussed particularly. Adaptability of method developed for visibility measuring was investigated. We established that the total number of photon counts from passing particles is a good estimator of particle size in this size range, because scattered intensity does not oscillate with the size changes. The two particle sizing methods were compared. The photon counting method was found simpler and more sensitive. Dependence on particle trajectory is investigated and found it is comparable for the two methods.
A photon correlation Laser Doppler Anemometer system for the simultaneous measurement of sub-micrometer particle size and velocity is introduced. From the results of Monte Carlo simulations it is shown that the FFT of the auto-correlation function of the detected scattered intensity contains the information on particle size. Experimental results on polystiren latex particles of 100 nm size are discussed.
KEYWORDS: Aerosols, Particles, Scattering, Mie scattering, Atmospheric particles, Light scattering, Monte Carlo methods, Laser scattering, Laser range finders, Refractive index
Due to the statistical nature of scattered light there is an apparent propagation delay uncertainty when detecting scattered impulses originated from different scattering volumes. Because of the randomness of particle number, size and refractive index - scattered intensity is a random variable itself. For this reason the scattered impulse builds up randomly. If light propagation is assessed by its scattered trace, the effect leads to an apparent propagation delay uncertainty. The phenomenon is similar to the so called jitter in electrotechnics. This delay is a function of aerosol concentration and size distribution. The scope of this work is to measure and calculate the extent of this uncertainty for use in subsequent measurement units.
A photon correlation LDA system for the simultaneous measurement of sub-micrometer particle size and velocity is introduced. Following the results of Monte Carlo simulations it is shown that the FFT of the auto-correlation function of the detected scattered intensity contains the information on particle size. Experimental results on polystiren latex particles of 100 nanometer size are discussed.
We propose the measurement of particle size, which is based on the visibility measurement of the pre-detection signal corresponding to particle transit of the sensing volume in a photon correlation LDA arrangement. It is shown that a good estimate of the visibility is the ratio of the contents of two specific channels of the Fourier transform of the autocorrelation function. We show that in a wide range of experimental conditions this ratio is a monotonous function of particle size. This circumstance leads to the possibility of defining calibration curves for practical devices.
An existence of pairs of rigorously correlated photons (biphotons) from nonlinear parametric down conversion has made it possible to develop a technique for measuring the absolute value of quantum efficiency of the optical detectors, both in photon-counting and in analog regimes, without using any standard light sources. A new technique for the measurement of quantum efficiency using single photomultiplier was evaluated. It is based on the measurement of the ratio between the single- and double- electron peaks in its pulse-height distribution.
KEYWORDS: Particles, Signal detection, Laser development, Control systems, Calibration, Liquids, Optimization (mathematics), High dynamic range imaging, Electronic filtering, Spherical lenses
A new liquid-borne particle counter is described that utilizes an optimized optical setup to determine the size distribution and concentration by two different methods and to estimate the shape-factor of the suspended particles. The aim of this optimization was to achieve the appropriate optical conditions for such type of measurement, viz. determining the geometry of the illumination and detection, and calculating the testing volume and dependence of the parameters of detected signal on the particle size for pulse height and pulse duration methods performed at the same illumination and detection conditions. The main benefit of the developed device is the high dynamic range of the size determination of the micron-size liquid-borne particles and the possibility of estimating their shape-factor. The proposed measurement method was incorporated in a PC- controlled LQB-1-200-L-T liquid-borne laser particle counter developed on the basis of our previous APC-03-2 and APC-03- 2A air-borne particle counters and a LQB-1-200 liquid-borne particle counter.
Several proposals are known for the simultaneous measurement of particle velocities and size. In this study we propose to measure the ratio of the contents of two specific channels of the Fourier transform of the autocorrelation function of the pre-detection signal corresponding to single particle transit of the sensing volume. We show that in a wide range of experimental conditions this ratio is a monotonous function of particle size. This circumstance leads to the possibility of defining calibration curves for practical devices.
A new liquid-borne particle counter is described that utilizes an optimized optical set-up to determine the size distribution and concentration by two different methods and to estimate the shape-factor of the suspended particles. The aim of this optimization was to achieve the appropriate optical conditions for such type of measurement, viz. determining the geometry of the illumination and detection, and calculating the testing volume and dependence of the parameters of detected signal on the particle size for pulse height and pulse duration methods performed at the same illumination and detection conditions. The possibility of estimating the shape-factor of micron-size particles is discussed. The proposed measurement method was incorporated in a PC-controlled LQB-1-200-L-T liquid-borne laser particle counter developed on the basis of previous APC- 03-2 and APC-03-2A air-borne particle counters and a LQB-1-200 liquid-borne particle counter.
KEYWORDS: Particles, Signal detection, Liquids, High dynamic range imaging, Calibration, Laser development, Sensors, Semiconductor lasers, Light scattering, Control systems
A new liquid-borne particle counters is described that utilizes an optimized optical set-up to determine the size distribution and concentration by two different methods and to estimate the shape-factor of the suspended particles. The aim of this optimization was to achieve the appropriate optical conditions for such type of measurement, viz. determining the geometry of the illumination and detection, and calculating the testing volume and dependence of the parameters of detected signal on the particle size for pulse height and pulse duration methods performed at the same illumination and detection conditions. The main benefit of the developed device is the high dynamic range of the size determination of the micron-size liquid-borne particles and the possibility of estimating their shape-factor. The proposed measurement method was incorporated in a PC- controlled LQB-1-200-L-T liquid-borne laser particle counter developed on the basis of previous APC-03-2 and APC-03-2A air-borne particle counters and a LQB-1-200 liquid-borne particle counter.
Several methods (particle counting and sizing, (beta) -ray absorption monitoring, gravimetric measurements, microscopy) were all applied to study the airborne dust particle pollution in Budapest, at various locations and during different seasons of the year. In addition, the airborne dust particle concentration was determined at several altitudes and several locations as a function of particle size. The results obtained by different measurements demonstrate that in certain cases the measured pollution exceeded the values permitted by Hungarian health standards.
KEYWORDS: Signal detection, Sensors, Interferometers, Heterodyning, Signal processing, Data acquisition, Motion analysis, Analog electronics, Resolution enhancement technologies, Motion measurement
The development of a data acquisition system for a high resolution motion analyzer is discussed. The displacement measurement is based on a Michelson-type heterodyne interlerometer. The gained detector signals of the interferometer are processed by the direct phase comparison method. This method makes it possible to achieve a resolution equivalent of 1/512 of the applied optical wavelength, without the intricate signal conditioning needed by the commonly used phase-locked loop frequency multiplication or analog signal multiplication methods. The main technical data of the system are 1.25-nm resolution, 20-m measuring distance, and 60-kHz sample rate. The accuracy is better than ± 10 nm ± 0.1 ppm within a velocity range of ± 1.8 m/s and an acceleration range of ± 9900 m/s2.
A light-scattering airborne particle counter with a new optical design having the optical sampling chamber outside the laser resonator is presented. This device can be used to measure the size distribution of particles from a diameter of 0.3 μm in a wide concentration range with high reliability and stability. Some results of the applications of the device in highly contaminated environments are reviewed.
Time-interval statistics of the trigger impulses, corresponding to photo-electron impulses of certain rate are studied for laser anemometry use. Expressions are given for the frequency distribution of time intervals between trigger impulses. The distribution function of the channel content as probability variable is determined. The results of numerical calculations are presented for the case of two models of illumination patterns of the sample. It is proved in both cases that on the frequency distribution curve there occurs maxima corresponding to the particle velocity. By using the image of a diffraction grating proposed here, the present signal processing can directly measure the velocity distribution of particles with higher precision. The full statistical description presented provides the possibility of the comparison with other signal processing methods.
A new quick-look method of sample analysis and a new equipment to carry out it, was developed by the authors. It's intended to use for special tasks occurring in the geology and in environmental protection. The layers in the ground are directly shot by a high energy pulse of a Nd:YAG laser and the picture of the excited plasma is observed trough a fibre optical system . The atomic emission spectra detected by the surface unit are stored in data files of special format. The spectrum files are analysed by the SAMPO 90 programme on an NB386 computer just at the site. The first methodological experiments were performed in the laser laboratories of Pees University. The new equipment consists of a sonde (measuring head) connected to the surface unit trough a fibre optical system. The surface unit contains the supply unit, the polycromator and a RAM. The measuring and the data acquisition is controlled by a 386 Note Book Computer. The whole system seems to be a portable laboratory to determine the components of rock samples or the contamination in the soil.
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