| Phys198 | April 21, 1997 |
The two common methods of speckle metrology are speckle photography and speckle correlation interferometry. Speckle photography was discussed earlier. This lecture will concentrate on speckle interferometry.
Comparison of the two methods
Speckle photography has already been discussed and represents applications of speckle measurement methods in which no coherent reference wave is mixed with the object wave, but the speckles are simply regarded as markers, and the spatial separation between speckle sets produces fringes in the output which can be analyzed by Young's fringes or Fourier filtering.
Speckle correlation interferometry, on the other hand, represents measurement systems in which a coherent reference wave is mixed with the object wave, and the phase difference between them produces changes in the observed fringe output.
In terms of the measurement range each is best suited for, the two methods are complementary. Speckle photography requires that the displacement being measured be larger than the size of the speckles themselves, while speckle interferometry requires the opposite, i.e., speckle interferometry requires that the displacement be less than the speckle size. Speckle photography is used for in-plane measurements only, while speckle interferometry can be used either for in-plane or out-of-plane measurements. Another difference between the two methods is that in speckle photography high sensitivity is achieved by using small speckles (speckle size can be controlled by varying the f# of the lens imaging system), while sensitivity in speckle interferometry is basically determined by the geometry of the setup, with large speckles frequently used to improve the performance of CCD camera sensors. Finally, speckle photography gives the complete magnitude of displacement along the direction of motion, while speckle interferometry only gives the displacement component along a selected direction.
Speckle Interferometry
Several systems are discussed in Sections 20.2-20.4 of the textbook. The figures given here are all taken from the text. Three setups each are shown for obtaining in-plane and out-of-plane displacement data, respectively. In all cases, the direction of the sensitivity vector must be determined, just as in the case of holographic interferometry. Note that in getting in-plane data, only projected motion along a single direction is obtained. As in the case of holographic interferometry, two independent measurements must be made to determine total in-plane motion.
1. Out-of-plane measurement (z-axis) with normal reference beam (high sensitivity) :
Phase shift is given by
Get fringes when
2. Out-of-plane measurement (z-axis) with two non-parallel illumination beams from same side of normal (less sensitivity) :
Phase shift is given by
with fringes occurring when
3. In-plane
measurement (e.g., x-axis) with symmetric illumination beams from
opposite sides of normal :
Phase shift is given by
with fringes occurring when
Additional setups for
obtaining in-plane displacement data are shown at the right.
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