Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Reexamination Certificate
2001-07-12
2003-03-18
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
C356S028000, C359S003000
Reexamination Certificate
active
06535276
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a method of measuring the three dimensional position of particles, in particular the velocity of particles in motion in a fluid medium.
2. Description of the Related Art
Several techniques to measure the three dimensional position of particles in a fluid, in particular the velocity of particles in a fluid, have been developed. The computations of the local velocities are obtained by analyzing the local motions of the particles.
The first family of techniques concern the methods based on Laser Doppler Velocimetry (LDV) wherein the fluid volume under test is illuminated by two coherent laser beams. The interference between the two beams creates a pattern with parallel fringes with a spatial frequency that is depending on the angle between the two propagation directions. When a particle is crossing the volume, it travels across the sequence of the light fringes and diffuses light proportionally to the local light intensity. Therefore, the diffused light is modulated by a frequency that is determined by the speed of the particle and the fringe spacing. The diffused light is detected and its frequency is computed in such a way that the speed of the particle is estimated. LDV technique gives an accurate measurement but measures the global velocity in one direction in a local region.
The second family concerns the techniques developed for Particle Intensity Velocimetry (PIV) wherein the volume under test is illuminated with a tin light sheet. This one is perpendicularly observed by a video camera that images the particles illuminated by the light sheet. The velocimetry is computed by analyzing the particle motion in sequences of images. The PIV is a two dimensional method and no information about motions parallel to the optical axis of the camera lens can be measured.
The third family concerns the so-called Photogrammetric methods wherein the illuminated volume under test is observed by several video cameras (
2
-
4
) with different viewing directions. The cameras are triggered in such a way that the images are recorded at the same time by the different cameras. The digitized images are processed and each particle is located in the different images. The relative positions of the particles in each image allow computing the three dimensional position of each particle. The three dimensional velocity maps are obtained by analyzing sequences of images. The main drawbacks of this system are:
The limited depth of view available with the classical imaging lenses. This becomes crucial when the volume of interest is small (typically less than 1 cm
3
).
The angles of viewing directions lead often to hidden parts of the volume of interest.
The fourth family is based on Digital holographic methods. In particular, a digital holographic method has been disclosed by Skarman, Wozniac and Becker, “Simultaneous 3D-PIV and temperature measurement using a new CCD based holographic interferometer”, Flow Meas. Instruction., Vol. 7, N°1, pp 1-6, 1996. This technique uses a digital refocus of the particle images. The optical set up is an interferometer. A laser beam (object beam) is transmitted through the volume under test and is imaged by a lens on the input face of a video camera. A second coherent beam is also incident on the input face of the camera. The two beams are interfering in such a way that the amplitude and the phase of the object beam can be computed by digital methods. As only one plane of the volume under test can be imaged, there is an important loss of information. However, the digital refocus allows to reconstruct the images of the particles.
The proposed method presents several drawbacks. First, due to a transmission illumination, only large particles can be considered. In the case of small particles, the disturbances of the optical field that they create are too weak to be measured with accuracy. Second, the computation of the optical phase is performed by the so-called phase stepping method. It requests to record several video frames with small changes of the optical paths introduced in the reference beam. This small changes of the optical paths takes time as the recording of the several video frames. During processing, the particles have to be sufficiently immobile in such a way that the method may only be used for low velocities. The three dimensional velocities are computed by analyzing sequences of images.
The last family concerns holographic methods using holographic recording media which can be thermoplastic films, silver halide films, etc. During the recording step, a hologram sequence of the moving particles is recorded. After processing, (thermal processing for thermoplastic, wet processing for the silver halide films), the holograms are reconstructed by illumination with a laser beam. The hologram is able to record the three dimensional information that can be measured with an imaging system like a video camera placed on a translation stage. Therefore the particle position measurements along the optical axis request mechanical motion of the imaging system.
This system presents several limitations. First, the holographic materials are always of weak sensitivity requesting a long exposure time or a high power laser. Secondly, the system needs a mechanical motion that is a source of positioning errors. Finally, the number of holograms that can be recorded in a sequence is limited. In practice, it is difficult to have more than 250 holograms.
SUMMARY OF THE INVENTION
The present invention aims to provide an improved digital holographic method for measuring the position of particles in a fluid medium, in particular for measuring the velocity of the particles. The method is able to refocus the image of the particles (seeds) moving in the fluid medium without the necessity of having several images being captured to get the complete three dimensional information, i.e., the amplitude and the phase.
More precisely, the present invention relates to a method which can be applied even to the measurement of the velocity of small and very small particles moving in fluid. With appropriate adjustments of the optical systems, the size range of the particles is typically from 500 &mgr;m to 0.1 &mgr;m.
Finally, the present invention concerns a method, which is able to measure the velocity of the fluid even if the speed of the particles is rather important. An estimation of the speed range of the particles is from 0 m/s to 600 w/s, where w is the size of the particles.
The present invention also relates to a method, which provides for improved speed processing, compared to most of the techniques of the state of the art.
Another aspect of the present invention is to provide an apparatus for performing said method.
Other advantages are described in the following detailed description.
The present invention is related to a method of measuring the three dimensional position of particles, possibly in motion, in a fluid medium contained in a sample by recording a digital hologram of said particles on an image sensor and by reconstructing the image of said particles from said hologram, the recording comprising:
providing a source beam with a coherent source;
generating at least two beams from said source beam, namely a reference beam and at least one object beam, said reference beam and said object beam being mutually coherent;
illuminating said sample by condensing said object beam onto said sample in order to obtain a scattered object beam for each particle;
transforming said scattered object beam into a spherical converging object beam toward said image sensor for each particle;
forming a diverging spherical beam from said reference beam;
superposing said spherical converging object beam for each particle and said diverging spherical beam on said image sensor, thereby obtaining on said image sensor an hologram of said particles by interfering said beams.
Preferably, said object beam is obtained by reflection of the source beam and said reference beam is obtained by transmission of the source beam.
In the case of
Buczinski Stephen C.
Knobbe Martens Olson & Bear LLP
Universite Libre de Bruxelles
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