Digital hologram sequences have great potential for the recording of 3D scenes of moving macroscopic objects as
their numerical reconstruction can yield a range of perspective views of the scene. Digital holograms inherently
have large information content and lossless coding of holographic data is rather inefficient due to the speckled
nature of the interference fringes they contain.
Lossy coding of still holograms and hologram sequences has shown promising results. By definition, lossy
compression introduces errors in the reconstruction. In all of the previous studies, numerical metrics were used
to measure the compression error and through it, the coding quality. Digital hologram reconstructions are highly
speckled and the speckle pattern is very sensitive to data changes. Hence, numerical quality metrics can be
misleading. For example, for low compression ratios, a numerically significant coding error can have visually
negligible effects. Yet, in several cases, it is of high interest to know how much lossy compression can be achieved,
while maintaining the reconstruction quality at visually lossless levels.
Using an experimental threshold estimation method, the staircase algorithm, we determined the highest
compression ratio that was not perceptible to human observers for objects compressed with Dirac and
MPEG-4 compression methods. This level of compression can be regarded as the point below which compression
is perceptually lossless although physically the compression is lossy. It was found that up to 4 to 7.5 fold
compression can be obtained with the above methods without any perceptible change in the appearance of video
Recording and real time reconstruction of digital hologram sequences have recently become feasible. The amount
of information that such hologram sequences contain results in voluminous data files, rendering their storage
and transmission impractical. As a result, compression of digital hologram sequences is of utmost importance
for practical applications of hologram sequences. In the absence of a specific hologram sequence compression
technique, a first concern is how a high-performance conventional video compression technique would perform.
Such a technique would not be optimized for hologram sequences but would provide a threshold that all hologram
sequence compression techniques should reach.
In this paper, the use of MPEG-4 part 2 video coding algorithm for the compression of hologram sequences is
investigated. Although the algorithm was originally developed for the compression of ordinary video, we apply it
on digital hologram sequences and investigate its performance. For this, appropriate digital hologram sequences
are used to asses how the coding algorithm affects their information content. In addition, we investigate whether
MPEG-4 interframe coding, which aims to achieve compression by exploiting similarities across adjacent frames
of the sequence, offers any advantage compared to intraframe coding, where each frame is coded independently.
Results show that the MPEG-4 coding algorithm can successfully compress hologram sequences to compression
rates of ~ 20 : 1 while retaining the reconstruction quality of the hologram sequences.
Crystallization is a widely used chemical process that finds applications in pharmaceutical industries. In an industrial
crystallization process, it is not only important to produce pure crystals but also to control the shape and size of the
crystals, as they affect the efficiency of downstream processes and the dissolution property of the drug. The effectiveness
of control algorithms depend on the availability of on-line,
real-time information about these critical properties. In this
paper, we investigate the use of lens-less in-line digital holographic microscopy for size and shape measurements for
crystallization processes. For this purpose, we use non-crystalline spherical microparticles and carbon fibers with known
sizes present in a liquid suspension as test systems. We propose an algorithm to extract size and shape information for a
population of microparticles from the experimentally recorded digital holograms. The measurements obtained from the
proposed method show good agreement with the corresponding known size and shape of the particles.
Digital holography has been reported as an effective tool for particle analysis. Other image-based techniques have small
depth of focus allowing only 2D analysis at microscopic level. On the other hand, digital holography offers the ability to
study volume samples from a single recording as reconstructions at different depths can be obtained. This paper focuses
on the processing of the digital hologram that follows its recording in order to obtain particle size. We present a stepwise
processing procedure with discussion on aspects such as reconstruction, background correction, segmentation,
focusing, magnification and particles' feature extraction. Solutions to common obstacles faced during particle analysis
which include ways to obtain fixed size reconstructions, automatically determine the threshold value, calculate
magnification, and locate particles' depth position using effective focusing metrics are highlighted. Real holograms of
microparticles are used to illustrate and explain the different stages of the procedure. Experimental results show that the
proposed algorithm can effectively extract particle size information from recorded digital holograms.
Digital holographic interferometry allows accurate measurements on a microscopic level. As the number and size
of the recorded digital holograms increase so does their data volume. As a result the volume of holographic data can
substantially constrain applications where storage or transmittance of such data is required. Compression of holographic
data in order to reduce their storage requirements has been studied. The speckled nature of the interferograms makes
their compression nontrivial; however image compression algorithms such as JPEG, JPEG2000 and Set Partitioning In
Hierarchical Trees (SPIHT) have been shown to perform adequately. So far the compression effects of the holographic
interferograms using such coding methods have mainly been studied in terms of errors at the reconstruction intensity.
On the other hand, metrology applications usually rely on the holograms' reconstructed phase. In this paper we
investigate hologram compression and how it affects the reconstructed phase. Holographic interferometry experiments
are carried out to investigate measurement error due to interferograms compression using image compression methods.
The results indicate that compression can be achieved while the measurement error due to compression is retained low.
Numerous phase-shifting interferometry digital hologram compression techniques have been proposed and investigated in the literature. We review some of the most important of them and compare their performance under the same conditions. Comparison is performed based on reconstruction quality. As holograms contain three dimensional information we investigate the compression effects on different distance reconstructions and reconstructions corresponding to different viewing angles. In this way we evaluate the compression performance of the methods not only on a single reconstruction, as it was done so far, but for aspects of the whole range of the holographic three dimensional information. We use holograms of multiple object scenes or objects with sufficient details on different depths so that both parallax and depth effects can be demonstrated.
The improved reconstruction quality that phase-shifting interferometry digital holography offers compared to
conventional digital holography comes with an extra computation cost, increased recording complexity and additional
data storage demands. The compression techniques that have been proposed to date aim at compressing the complex
wavefront at the plane of the recording camera. We propose data compression at an earlier stage of the phase-shifting
procedure that may lead to enhanced compression rates. More specifically we propose the compression of the
interference patterns which are input to the phase-shifting algorithm. These interferograms are gray scale images
making possible the use of standard image compression techniques for their compression. We investigate the use of
baseline JPEG, JPEG-2000 and Set Partitioning in Hierarchical Trees for their compression. In order to verify the use of
the proposed methods we apply them to real holographic data and compare their performance with other coding
techniques that have been proposed in the literature. The study reveals that the proposed methods not only outperforms
existing techniques in terms of compression performance but also they offer additional advantages including increased
flexibility into the choice of the compression parameters as well as increased compatibility.