Optics: measuring and testing – For size of particles – By particle light scattering
Patent
1985-01-16
1987-10-20
Rosenberger, R. A.
Optics: measuring and testing
For size of particles
By particle light scattering
356343, G01N 1502
Patent
active
047010517
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a laser-doppler-apparatus for determining the size of moving spherical particles in a fluid flow, comprising a laser beam generator for generating two spatially separated laser beams, focusing means for focusing the laser beams so that in a focusing plane, the focused beams intersect each other under a given angle in an intersecting point in a measuring volume passed by a part of the fluid flow, photoelectric detector means for detecting the scattered radiation caused by a moving particle in the fluid flow in spatially different positions outside the measuring volume and generating electrical signals in response thereto, and signal processing means for determining the particle size on the basis of said electrical signals.
In optical measuring methods for determining the size of moving particles in a fluid flow by use, for instance, of laser radiation, the measurement may take place without physical interference with the measuring medium in a small volume of space, the measuring volume, which is passed by the fluid flow or a part thereof and is defined by intersection between two incoming laser beams, on one hand, and by location within the viewing field of an optical detector, on the other hand. Moreover, the measurement may be performed in real time at a high data collection rate and with a possibility of getting simultaneous information about the size and velocity of moving micro elements in the fluid flow.
Such micro elements may have the form of solid particles, droplets in gases or gas bubbles in liquids, and in the following all three kinds are considered to be encompassed by the designation "particles".
The methods hitherto in use for optical determination of particle size have mainly been based on measurement of the intensity of the scattered radiation from a particle passing the measuring volume. However, the spatial function of the scattered radiation from small particles is a complicated function of the direction of the scattered radiation relative to the direction of the incoming radiation, the direction of polarization of the radiation and the size, shape and material of the particle.
In order to reduce this complexity, it has been suggested, inter alia, to measure the intensity of the scattered radiation concurrently in two or more directions or measure the overall intensity within a spatial angle in a direction normal to the incoming light.
Furthermore, measuring methods are known for optical article size measurement which are based on detection of different properties of so-called laser-doppler signals. Thus, for measuring the velocity of particles in a flowing medium, it is well known to measure the doppler shift of the frequency of scattered laser radiation from the moving particle. In general, this principle for measurement of flow velocity is based on measurement of the difference between the doppler shifts of scattered radiation from two different laser beams intersecting each other under a given angle in the measuring volume, the two beams of scattered radiation being caused to impinge simultaneously and in overlapping relationship on the surface of a photo detector. Thereby, the difference frequency between the two doppler-shifted radiation frequencies is formed, and this difference frequency is direcly proportional to the velocity component of the particle normal to the bisector of the two incoming beams in the plane of the incoming beams.
In addition to the information about the velocity of a moving particle associated with the frequency, a doppler signal also comprises information about other properties of the particle, such as its size, and, therefore, different methods have also been suggested for deriving information about the size of moving particles from doppler signals. Thus, a relatively simple prior art method is based on measurement of the amplitude of a low-pass-filtered doppler signal.
From an article of W. M. Farmer, "Measurement of Particle Size, Number Density and Velocity Using a Laser Interferometer", in Applied Optics, Vol. 11, November 19
REFERENCES:
patent: 4179218 (1979-12-01), Erdmann et al.
patent: 4540283 (1985-09-01), Bachalo
"Measurement of Particle Size, Number Density, and Velocity Using a Laser terferometer"Farmer, 11/1972, vol. 11, #11, Applied Optics.
Buchhave Preben
Knuhtsen John
Olldag Peder E. S.
Cooper Crystal
Dantec Electronik, Medicinsk Og Videnskabeligt Maleudstyr A/S
Rosenberger R. A.
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