An interferometer for detecting the velocity of small particles in a fluid includes relatively static means comprising a source of coherent radiation and a beam splitter means mounted in a fixed relationship for directing beams of coherent radiation from the source along two predetermined paths comprising a test path which will pass through the fluid under test and a reference path, and a scattering means mounted in the reference path for scattering the radiation in that path into a plurality of angles; and rotatable means, comprising a structure pivotably connected to said static means and rotatable about an axis which passes through the intersection of the reference path with the scattering means and through the intersection of the test path with the fluid under test, and optical components mounted on the said structure comprising recombination means for interferingly recombining radiation scattered into identical angles from both paths and thereby forming interference fringes which indicate the local velocity of the fluid under test.
A laser Doppler velocimeter is described which is capable of effectively suring two different velocity components of a fluid simultaneously. Such a velocimeter includes a pair of coherent beams of laser light which are focused to an intersection point through which flow particles within the fluid whose velocity is to be measured. Both beams are plane polarized with the plane of polarization of one being rotated normally with respect to the other, with the result that the scattered radiation is separable into two different beams respectively corresponding to the two incident beams. Such scattered radiation is Doppler shifted by the moving particles and is collected for conventionally providing a measurement of the velocity of any particle flowing through the intersection point on a path which is generally transverse thereto. Moreover, the wavelength of the light scattered by the particles from one of the beams is compared to the wavelength of such beam prior to it being Doppler shifted by the moving particles. This comparison provides a measurement of another component of the particle velocity, which measurement can be combined with the first measurement to provide a resultant velocity in a two-dimensional reference frame.
A flow field seeded with small particles is illuminated by collimated, monochromatic laser light sheet 16. Doppler shifted scattering from particle motion is imaged by an optical system 22. An optical frequency-to-intensity converter 24 is located at the image plane such that the transmitted image contains a simultaneous two-dimensional measurement of flow velocity along a direction determined by a laser beam and observer (converter) direction. These images can be observed directly or through a TV-2-D array camera and monitor or processed through a computer system 28.
An apparatus to evaluate objectively the concentration and motility of particles suspended in a sample of liquid, and particularly to an apparatus to solve objectively the problem of evaluating these important characteristics in a very simple manner and in a short time by exploiting the Doppler effect. The sample to be examined is subjected to one of the two sub-beams resulting from the suitable division of a coherent monochromatic light beam emitted by a polarized LASER, while the other sub-beam is considered as a reference light sub-beam. Because of the very little difference between the frequencies of the original and scattered light a beat is originated in the rejoined two sub-beams which is dependent on the characteristics of the examined sample. Electromechanical means are provided to position automatically and sequentially a set of test-pieces to be examined according to a program and a data processor is provided to process the output signal of the rejoined sub-beams, so that the desired evaluations are displayed on a monitor or printed as alpha-numerical results and/or graphic diagrams. All of the mechanical, electromechanical, optical and data processor members are enclosed within a single box-container as component members of the apparatus.
A method for measuring a direction of a laminar air flow uses a particle stream tracing an air flow and having a grain size substantially uniform along the particle stream. The direction of the particle stream is detected by a first and a second rectangular laser beams crossing each other in the vicinity of the particle stream at right angles and rotatable around an axis passing through the cross point of the laser beams. The second laser beam is further rotatable around the axis thereof. When the particle stream crosses the laser beams along the longer sides of the rectangular radiant area of the laser beam, the amount of a scattered light generated from the laser beam is large. Hence, the direction of the particle stream is detected by the rotational angle of the second laser beam. The direction of an air flow can be measured with a good accuracy because the measurement does not disturb the original air flow.
A laser velocimeter (12) has a laser beam generator (18) for generating first and second laser beams (20, 22) having a waist (40) of reduced cross-section area. A first focusing lens (28) has a first focal length F1 for focusing the first and second laser beams (20, 22) so that their waists focused by the lens (28) intersect at a focal point (26) at which a measurement is to be made. A second focusing lens (30) is positioned between the laser beam generator (18) and the first focusing lens (28). The second focusing lens (30) has a second focal length F2. The second focusing lens (30) is positioned from the first focusing lens (28) at a distance corresponding to a sum of the first and the second focal lengths F1 and F2. A third focusing lens (36) having a third focal length F3 is positioned between the laser beam generator (18) and the second lens (30). The third lens (36) is positioned from the second lens (30) a distance corresponding to the sum of the second and third focal lengths F3 and F2. The third focusing lens (36) is positioned in front of the beam waists (40) formed by the laser beam generator (18) a distance corresponding to the third focal length F3. The focusing lenses (28, 30 and 36) coact to focus the laser beams (20, 22) so that their waists intersect at a first focus spot of size W1. The second and third lenses (30, 36) are interchangeable in position by pivoting about a point between them to focus the laser beams (20, 22) so tat their waists intersect at a second focus spot size W2 different than the first focus spot size W1. Photodetector (31) is connected to computer (41), which processes signals derived from scattered light by the photodetector (31) to give measurements of the scattered light.
