Nitesh Pandey

Resolution limits in practical digital holographic systems

We examine some fundamental theoretical limits on the abil- ity of practical digital holography DH systems to resolve detail in an image. Unlike conventional diffraction-limited imaging systems, where a projected image of the limiting aperture is used to define the system performance, there are at least three major effects that determine the performance of a DH system: i The spacing between adjacent pixels on the CCD, ii an averaging effect introduced by the finite size of these pixels, and iii the finite extent of the camera face itself. Using a theo- retical model, we define a single expression that accounts for all these physical effects. With this model, we explore several different DH record- ing techniques: off-axis and inline, considering both the dc terms, as well as the real and twin images that are features of the holographic record- ing process. Our analysis shows that the imaging operation is shift vari- ant and we demonstrate this using a simple example. We examine how our theoretical model can be used to optimize CCD design for lensless DH capture. We present a series of experimental results to confirm the validity of our theoretical model, demonstrating recovery of super- Nyquist frequencies for the first time.

Extended viewing angle holographic display system with tilted SLMs in a circular configuration

This paper presents an extended viewing angle holographic display for reconstruction of real world objects in which the capture and display systems are decoupled. This is achieved by employing multiple tilted spatial light modulators (SLMs) arranged in a circular configuration. In order to prove the proper reconstruction and visual perception of holographic images the Wigner distribution function is employed. We describe both the capture system using a single static camera with a rotating object and a holographic display utilizing six tilted SLMs. The experimental results based on the reconstruction of computer generated and real world scenes are presented. The coherent noise removal procedure is described and implemented. The experiments prove the possibility to view images reconstructed in the display binocularly and with good quality.

Digital Holographic Capture and Optoelectronic Reconstruction for 3D Displays

The application of digital holography as a viable solution to 3D capture and display technology is examined. A review of the current state of the field is presented in which some of the major challenges involved in a digital holographic solution are highlighted. These challenges include (i) the removal of the DC and conjugate image terms, which are features of the holographic recording process, (ii) the reduction of speckle noise, a characteristic of a coherent imaging process, (iii) increasing the angular range of perspective of digital holograms (iv) and replaying captured and/or processed digital holograms using spatial light modulators. Each of these challenges are examined theoretically and several solutions are put forward. Experimental results are presented that demonstrate the validity of the theoretical solutions.

Quantization noise and its reduction in lensless Fourier digital holography

Digital holography is an imaging technique that enables recovery of topographic 3D information about an object under investigation. In digital holography, an interference pattern is recorded on a digital camera. Therefore, quantization of the recorded hologram is an integral part of the imaging process. We study the influence of quantization error in the recorded holograms on the fidelity of both the intensity and phase of the reconstructed image. We limit our analysis to the case of lensless Fourier off-axis digital holograms. We derive a theoretical model to predict the effect of quantization noise and we validate this model using experimental results. Based on this, we also show how the resultant noise in the reconstructed image, as well as the speckle that is inherent in digital holography, can be conveniently suppressed by standard speckle reduction techniques. We show that high-quality images can be obtained from binary holograms when speckle reduction is performed.

Twin removal in digital holography using diffuse illumination.

A method to numerically remove the twin image for inline digital holography, using multiple digital holograms, is discussed. Each individual hologram is recorded by using a statistically independent speckle field to illuminate the object. If the holograms are recorded in this manner and then numerically reconstructed, the twin image appears as a different speckle pattern in each of the reconstructions. By performing speckle-reduction techniques the presence of the twin image can be greatly reduced. A theoretical model is developed, and experimental results are presented that validate this approach. We show experimentally that the dc object intensity term can also be removed by using this technique.

Zooming algorithms for Digital Holography

Digital Holography is an imaging modality made up of two parts: (1) using a digital camera to record the interference pattern between a field scattered from an object and a known reference field so that the complex object wavefield can be obtained and (2) replaying or reconstructing the hologram on a computer by simulating the propagation of the object wavefield back to the object plane. Thus an image is obtained. The most commonly used algorithms for the reconstruction algorithm are the so-called direct method and the spectral method of calculating the Fresnel Transform, which describes free space propagation in the paraxial approximation. These algorithms differ in the output range that they display. For the direct method, the output image size is proportional to the distance making it more appropriate for large objects at large distances. The spectral method has an output image size equal to the size of the CCD. We show how to adapt this latter algorithm in a simple way to allow it to generate any output range and in any location making it far more versatile for zooming in on specific regions of our reconstructed image.

Speed up of Fresnel transforms for digital holography using pre- computed chirp and GPU processing.

We show how the reconstruction of digital holograms can be speeded up on ordinary computers by precomputing the chirp factor in the Fresnel transform for a given detector array size. The speedup in time is shown for various hologram sizes. We also run the same algorithm on a Nvidia GPU. The speedup and the error introduced due to quantizing to different levels is investigated. Additionally a variance based

Fixed-point numercial-reconstruction for digital holographic microscopy.

In this Letter, we study the reconstruction of digital holograms of microscopic objects using a fixed-point representation of the numercial-reconstruction process. For different bit levels in our fixed-point reconstruction algorithm, we investigate the errors introduced to both the reconstructed image intensity and the unwrapped quantitative phase information. Experimental results based on a microscopic lens array are provided.

Compensation of reference beam sphericity in a multi-perspective digital holography based record-display setup.

The presence of a residual sphericity in a reference beam causes magnification in the reconstructed image in digital holography. We discuss a method to estimate the relative sphericities in a multi-perspective multi-camera digital hologram recording unit. The digital holograms can then be compensated with numerical quadratic phase factors so that the object appears with the same magnification in all the reconstructions.

Quantization Noise: An Additional Constraint for the Extended Sampling Theorem

Recovery of spatial frequencies above the Nyquist limit is of interest in digital holography. We examine how finite pixel size and quantization error introduced by a CCD camera effect the recovery of these frequencies.