Decorrelation in an interferometric set-up appears due to movements of the speckle pattern. In the case of rigid body
movements the effect of decorrelation severely limits the performance of speckle interferometers. If the movement is
larger than the speckle size the wanted phase information of the deformation is lost.
Phase modulating spatial light modulators (SLMs) provide a new method to non-mechanically deflect and shape light.
By using the SLM for scanning the field-of-view and focusing at different distances it is possible to measure intensity
speckle patterns in a three-dimensional volume. These intensity images can then be cross correlated to give a three-dimensional
correlation coefficient of the speckle pattern.
If an SLM is utilized in an interferometric set-up it is possible to compensate for unwanted movements during an
experiment. The measured correlation coefficient will then provide information regarding how large movements that are
allowed with maintained performance of the interferometer. It is shown that for large movements the SLM can be used to
retrieve phase maps.
The use of complex amplitude correlation to determine the deformation field for in-plane motions in digital speckle pattern interferometry (DSPI) is investigated. The result is compared to experiments where only DSPI-algorithms, as well as where combined DSPI-intensity correlation are used. Experiments with and without large rigid body motions are studied. An advantage of using complex amplitude correlation instead of intensity correlation is that the phase change describing the deformation is retrieved directly from the correlation peak and there is no need to compensate for the large movement and then use interferometric algorithms to obtain the phase information. A discovered drawback of this method is that the correlation values drops very quick if there is a phase gradient larger than π across the sub image used for the cross correlation. This means that in order to use the complex amplitude correlation the size of the subimages must be proportional to the magnitude of the present deformation gradient. Or, a third parameter in the cross-correlation algorithm that compensates for the phase variation is needed.
Leendertz dual beam symmetric illumination - normal observation arrangement is widely employed for real time evaluation of in-plane displacement components as well as surface shape. Instead of observing along the optical axis, we have examined the Leendertz arrangement by observing the scattered light along the direction of illumination beams, and imaged as two separate images onto a photo sensor of a CCD camera. In this paper, we will show that the interference between the speckle fields generated from one of the illuminating beams with the specular reflection speckle fields from the second illuminating beam, is responsible for fringe formation along the directions of observation. The interferometer is a combination of two channels; each of which senses independently and simultaneously the information pertaining to either the in-plane displacement component of a deformation vector, or surface relief variation of a three dimensional object. A summary of possible information that can be extracted from the present arrangement is also highlighted. Experimental results using a four-frame phase shifting technique are illustrated.
KEYWORDS: Digital signal processing, Speckle, Digital photography, Photography, Holography, Interferometry, Shearography, Speckle interferometry, Motion measurement, Digital holography
A general problem in optical metrology is to measure a deformation field when this field is added to a translation or a rotational motion. Methods like Speckle Photography (SP) do handle large rigid body motions but the result might be of too poor accuracy to resolve the deformation field. Interferometric methods on the other hand might measure the deformation field but the bulk motion makes the fringes disappear. By combining digital speckle photography, with Speckle Interferometry, or with shearography, such measuring situations can be mastered.
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