Subtraction correlation fringes in Digital Speckle Pattern Interferometry are described as a smoothly varying intensity distribution multiplied with high-frequency noise that is due to the random distribution of speckle intensity and speckle phase across the image plane. Although intensity modulation and speckle phase can be eliminated by phase stepping or other phase retrieval methods, noise is not eliminated completely due to the limited dynamic range of image processing, given by the saturation level of the camera, the digitisation depth and the electronic noise. Digital image processing techniques have been developed to reduce the speckle noise from the fringe pattern, but in most cases the fringes are not restored completely. In this paper the speckle noise is reduced before image digitisation by reducing the range of either the modulation intensity values or the speckle phase values, or both, in the interference pattern. This is done adaptively by using programmable twisted nematic LCDs. Since the manipulation is performed for each pixel independently in the reference beam, the imaging quality is retained and no blurring occurs. We quantitatively discuss the improvement and limitation of the method. The experimental verification is performed using an amplitude-only or phase-only spatial light modulator in a conventional ESPI setup.
We show improvement in the signal-to-noise ratio of speckle correlation fringe patterns by providing a conjugate phase to the scattered object beam. This conjugate phase negates the speckle phase and will leave the speckle intensity as the only statistically varying quantity. Since one of the random variables is reduced the signal-to-noise ratio improves. The variation of intensity in the fringe profile is reduced after the adaptive phase correction. This improvement is vividly seen after a simple morphological filtering. The experiment is performed using a twisted nematic liquid crystal display SLM in transmission.
A complex pupil filter using electrically address twisted nematic LCD spatial light modulators (SLMs) to obtain superresolution along the radial (transverse) direction is presented. The fact that the commercial LCDs can be modeled to obtain amplitude and phase-only SLMs has increased the scope of improving superresolving power dynamically by a suitable combination of both types in an optical system. In general, amplitude-only or phase-only filters result in unwanted increase of side lobe intensity which is disadvantageous for many applications such as microscopy and data storage. The main objective of the complex filter design is to increase the central to side lobe intensity ratio without affecting the size of the central spot. The complex filter performance is optimized for this purpose. The comparison of this filter with that of the conventional pupil filters of both types is shown by simulations. The complex filter is obtained by concatenating amplitude-only and phase-only SLMs in a 4F processor. The SLMs are addressed at video frame rate by a graphic card from the computer. The optimization procedure along with the experimental procedure are described and the results are discussed in this paper.