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Papers by Daniel Claus

Digital Holography and Three-Dimensional Imaging, 2016

We present a depth resolved hyperspectral digital holography setup and its application for the re... more We present a depth resolved hyperspectral digital holography setup and its application for the reconstruction of amplitude and phase at different depth sections for different wavelength. For example the technique can be used for the discrimination of cells.

Unconventional Optical Imaging, 2018

A model describing the signal generation in chromatic confocal imaging is presented here. It can ... more A model describing the signal generation in chromatic confocal imaging is presented here. It can be used to understand the signal development process accounting for wave-optical phenomena using scalar wave theory. The influence of the optics in terms of aberrations, the specimen in terms of roughness and further parameters on the signal generation process will be investigated. Moreover, the possibility to adapt the model to investigate other spectral imaging systems, such as chromatic confocal spectral interferometry will also be shown.

The incoherent imaging properties of ptychography are discussed in this paper. Usually a coherent... more The incoherent imaging properties of ptychography are discussed in this paper. Usually a coherent light source is employed in ptychography for the recording of the diffraction patterns. However, in combination with a curved illumination it is possible to obtain an image quality of the reconstructed images that is equal to the one known from incoherent imaging. The underlying principle and results to demonstrate these findings are presented in this paper. Moreover, it will be shown that consequently not only the coherent speckle noise but likewise the resolution can be increased.

Engineering Research Express, 2021

This article discusses the fabrication and performance of a multi-channel fibre switch, consistin... more This article discusses the fabrication and performance of a multi-channel fibre switch, consisting of 19 single-mode fibres, with enhanced coupling efficiency due to micro-optics, directly printed via two-photon-polymerization on the end-face of each fibre. The use of high-resolution two-photon-polymerization not only allows the enhancement of the coupling efficiency with respect to the coupling device in use but likewise offers great freedom in the arrangement of the used fibres. This letter gives a thorough explanation of the fabrication method as well as the optical simulations for the lenses on the fibre assembly.

Surface Topography: Metrology and Properties, 2020

This paper addresses the effect and influence of wave optical aberrations and surface roughness o... more This paper addresses the effect and influence of wave optical aberrations and surface roughness on the chromatic confocal signal and resulting measurement errors. Two possible approaches exist for implementing chromatic confocal imaging based on either refraction or diffraction. Both concepts are compared and an expression for the expected chromatic longitudinal aberrations when using a diffractive optical element is derived. Since most chromatic confocal sensors are point sensors, the discussion on wave-optical aberrations is focused on spherical aberrations. Against common belief, the effect of spherical aberrations cannot be eliminated in the calibration process using for instance a piezo mounted mirror. It will be shown in the following that even a diffraction limited system with peak to valley spherical aberration smaller than 0.25 wavelength suffers from measurement errors. Experimental results will be shown to highlight this important issue. In order to develop a deeper under...

Sensors, 2019

In this paper, we have applied a recently developed complex-domain hyperspectral denoiser for the... more In this paper, we have applied a recently developed complex-domain hyperspectral denoiser for the object recognition task, which is performed by the correlation analysis of investigated objects’ spectra with the fingerprint spectra from the same object. Extensive experiments carried out on noisy data from digital hyperspectral holography demonstrate a significant enhancement of the recognition accuracy of signals masked by noise, when the advanced noise suppression is applied.

Optics and Lasers in Engineering, 2020

A new denoising algorithm for hyperspectral complex domain data has been developed and studied. T... more A new denoising algorithm for hyperspectral complex domain data has been developed and studied. This algorithm is based on the complex domain block-matching 3D filter including the 3D Wiener

Computer Optics, 2018

The topography of surface relief gratings was studied by digital holographic microscopy. The appl... more The topography of surface relief gratings was studied by digital holographic microscopy. The applicability of the method for quantitative measurements of surface microstructure at nanoscale was demonstrated. The method for wavefront reconstruction of surface relief from a digital hologram recorded in off-axis configuration was also applied. The main feature is noise filtration due to the presence of noise in the recorded intensity distribution and the use of all orders of the hologram. Reconstruction results proved a better effectiveness of our approach for topography studying of relief grating patterned on a ChG As2S3 – Se nanomultilayers in comparison with standard Fourier Transform and Atom Force Microscope methods.

$Research paper thumbnail of Diffraction-limited superresolution ptychography in the Rayleigh–Sommerfeld regime$

Journal of the Optical Society of America A, 2018

This is a repository copy of Diffraction-limited superresolution ptychography in the Rayleigh-Som... more This is a repository copy of Diffraction-limited superresolution ptychography in the Rayleigh-Sommerfeld regime.

Seminars in Orthodontics, 2018

Structure  Use of digital models in orthodontics  Indirect approaches to generate digital full-... more Structure  Use of digital models in orthodontics  Indirect approaches to generate digital full-arch models  Technical principles applied for direct 3D intraoral scanning  Accuracy of intraorally scanned full-arch models

Applied Sciences, 2018

Quantitative phase imaging can reveal morphological features without having to stain the biologic... more Quantitative phase imaging can reveal morphological features without having to stain the biological sample. This property has important implications for intraoperative applications since the time spent during histopathology can be reduced from a few minutes to a few seconds. However, most common quantitative phase imaging techniques are based on the interferometric principle, which makes them more prone to disturbing environmental influences, such as temperature drift and air turbulence. In the last decade, with the advance of computing power, many different iterative quantitative phase imaging techniques, which only require the recording of the diffracted wavefield, and therefore offer increased robustness towards environmental disturbances, have been proposed. These are particularly well-suited for the application outside the well-controlled lab environment such as an operating theatre. The optical performance of our developed iterative phase retrieval method based on variable wav...

Journal of Medical Imaging, 2018

Early detection of cancer can significantly increase the survival chances of patients. Palpation ... more Early detection of cancer can significantly increase the survival chances of patients. Palpation is a traditional method in order to detect cancer; however, in minimally invasive surgery the surgeon is deprived of the sense of touch. We demonstrate how shearing elastography can recover elastic parameters and furthermore can be used to localize stiffness imhomogenities even if hidden underneath the surface. Furthermore, the influence of size and depth of the stiffness imhomogenities on the detection accuracy and localization is investigated.

Journal of Biomedical Optics, 2017

This paper relates to the improvement of conventional surgical stereo microscopy via the applicat... more This paper relates to the improvement of conventional surgical stereo microscopy via the application of digital recording devices and adaptive optics. The research is aimed at improving the working conditions of the surgeon during the operation, such that free head movement is possible. The depth clues known from conventional stereo microscopy in interaction with the human eye's functionality, such as convergence, disparity, angular elevation, parallax, and accommodation, are implemented in a digital recording system via adaptive optomechanical components. Two laterally moving pupil apertures have been used mimicking the digital implementation of the eye's vergence and head motion. The natural eye's accommodation is mimicked via the application of a tunable lens. Additionally, another system has been built, which enables tracking the surgeon's eye pupil through a digital displaying stereoscopic microscope to supply the necessary information for steering the recording system. The optomechanical design and experimental results for both systems, digital recording stereoscopic microscope and pupil tracking system, are shown.

International Conference on Applications of Optics and Photonics, 2011

ABSTRACT In the last two decades digital holography emerged as one of the most promising techniqu... more ABSTRACT In the last two decades digital holography emerged as one of the most promising techniques for obtaining the complex object-wave (amplitude and phase). However, due to the coherent nature of the light source used in digital holography, the reconstructed hologram is subjected to speckle noise. Moreover, the resolution and size of the sensor employed in digital holography are smaller compared to the formerly used holographic plates in optical holography. This results in a reduced resolution for the reconstructed hologram. This paper discusses two resolution improvement methods, which are both based on the same recording process. However, the recorded data is processed differently to obtain a resolution improved reconstruction. The two methods are compared in terms of corresponding optical resolution, phase accuracy and processing time.

Computers in Biology and Medicine, 2016

Modern microsurgery is a long and complex task requiring the surgeon to handle multiple microscop... more Modern microsurgery is a long and complex task requiring the surgeon to handle multiple microscope controls while performing the surgery. Eye tracking provides an additional mean of interaction for the surgeon that could be used to alleviate this situation, diminishing surgeon fatigue and surgery time, thus decreasing risks of infection and human error. In this paper, we introduce a novel algorithm for pupil detection tailored for eye images acquired through an unmodified microscope ocular. The proposed approach, the Hough transform, and six state-of-the-art pupil detection algorithms were evaluated on over 4000 hand-labeled images acquired from a digital operating microscope with a non-intrusive monitoring system for the surgeon eyes integrated. Our results show that the proposed method reaches detection rates up to 71% for an error of ≈ 3% w.r.t the input image diagonal; none of the state-of-the-art pupil detection algorithms performed satisfactorily. The algorithm and hand-labeled data set can be downloaded at:(on acceptance)

Journal of Medical Imaging, 2017

This article [J. Med. Imag. 4(1), 014505 (2017)] was originally published with the captions for F... more This article [J. Med. Imag. 4(1), 014505 (2017)] was originally published with the captions for Figs. 8 and 9 transposed. The captions and figures below are correctly matched. This article was corrected online on 23 May 2017. Fig. 8 Stress distributions obtained using the hyperelastic Arruda-Boyce model for Abaqus (a) σ xx at the front surface, (b, c, d) 3-D stress distribution along (b) the x-direction σ xx , (c) y-direction σ yy , (d) and z-direction σ zz with indenter positioned on top of inhomogeneity, (e) 3-D stress distribution along z-direction σ zz with indenter positioned in 10 mm distance to inhomogeneity.

This thesis discusses the creation, acquisition and processing of digital holograms. Several tech... more This thesis discusses the creation, acquisition and processing of digital holograms. Several techniques to improve the optical resolution have been investigated and developed. The optical resolution of numerically reconstructed digital holograms is restricted by both the sampling frequency and the overall sensor-size of the digital camera chip used. This thesis explores the limitations on the optical resolution of the holograms obtained. A typical sensor-size and sampling frequency for digital holograms is 10 mm and 100 lp/mm, respectively, whereas holographic plates used for optical holography can be more than a meter in size and have a sampling frequency of 3000 lp/mm. In order to take full advantage of the benefits digital holography offers, such as fast image acquisition and direct phase accessibility, the problem of reduced resolution needs to be overcome. Three resolution improvement methods have been developed in the scope of this PhD thesis. Prior to implementing the resolution improvement methods, different holographic setups have been analyzed, using the Space-bandwidth product (SBP) to calculate the information distribution both in the recording and reconstruction process. The first resolution improvement method is based on the synthetic aperture method. In this manner an increased sensor area can be obtained resulting in a larger numerical aperture (NA). A larger NA enables a more detailed reconstruction. The problem encountered in doing this is that an increased optical iii resolution results in a smaller depth of field. This has been overcome in this thesis by applying the extended depth of field method. As a result a high resolution in focus reconstruction of all longitudinal object regions is obtained. Moreover, the extended depth of field method allows a topological mapping of the object. The second resolution improvement method is based on sampling the interference pattern with sub-pixel accuracy. This was carried out on a CMOS-sensor and implemented by moving the light sensitive pixel-area into the dead zone in a 4x4 grid to cover whole the pixel-area. As a result the sensor's sampling frequency is doubled. The increased sampling frequency permits a reduction of the recording distance which results in an increased optical resolution of the reconstructed hologram. The third and novel approach described in this thesis has been to increase the optical resolution stored in a digital hologram by the combination of the synthetic aperture and the sub-pixel sampling methodBy analogy with the Fresnel-. The resolution improvement methods have been demonstrated both for lens-less digital holography and digital holographic microscopy.

Novel Biophotonics Techniques and Applications III, 2015

During minimally invasive surgery the visual (3 dimensional) and mechanical (haptic) feedback is ... more During minimally invasive surgery the visual (3 dimensional) and mechanical (haptic) feedback is restricted or even non-existing, which imposes a serious loss of important information for decision making. Information about the mechanical properties of the biological tissue helps the surgeon to localize tissue abnormalities (benign vs. malign tissue). The work described here is directed towards assisting the surgeon during minimally invasive surgery, which in particular relates to the segmentation and navigation based on the recovery of mechanical properties. Besides the development of noninvasive elastographic measurement techniques, a reliable constitutive FE-model of the organ (describing its mechanical properties) is generated resulting in a further improvement of the segmentation and localization process. At first silicon phantoms, with and without foreign bodies have been generated for the purpose of testing the transfer of information (delivery and processing of data). The stress-strain curve was recorded and embedded in the FE-model (Arruda-Boyce). Two dimensional (2D) displacement maps have experimentally been obtained from the phantom, which were in good agreement with the FE simulation.

The microscopic properties of the lunar regolith such as the shape, the surface texture and the s... more The microscopic properties of the lunar regolith such as the shape, the surface texture and the size distribution are required for an understanding of both past surface processes and potential hazards for future human exploration [1]. To reveal the particle morphology at the sub micrometer scale, scanning-probe microscopy (SPM), first used on the 2008 Phoenix mission [1], is a proven approach; however, there are two main challenges for the measurement of lunar particles. Firstly, the SPM tip is liable to move particles during scanning, even when using the lower contact forces of the dynamic-mode imaging. Hence the particles need to be stabilised during imaging. Secondly, typically the AFM tip extends about 10 μm from its cantilever, so larger particles protruding more than this height above their substrates cannot be scanned completely. To immobilize particles and eliminate large particles during SPM scanning, micromachined Si substrates, which have been successfully applied in the ...

$Research paper thumbnail of Pixel size adjustment in coherent diffractive imaging within the Rayleigh–Sommerfeld regime$

Applied Optics, 2015

The reconstruction of the smallest resolvable object detail in digital holography and coherent di... more The reconstruction of the smallest resolvable object detail in digital holography and coherent diffractive imaging when the detector is mounted close to the object of interest is restricted by the sensor&amp;#39;s pixel size. Very high resolution information is intrinsically encoded in the data because the effective numerical aperture (NA) of the detector (its solid angular size as subtended at the object plane) is very high. The correct physical propagation model to use in the reconstruction process for this setup should be based on the Rayleigh-Sommerfeld diffraction integral, which is commonly implemented via a convolution operation. However, the convolution operation has the drawback that the pixel size of the propagation calculation is preserved between the object and the detector, and so the maximum resolution of the reconstruction is limited by the detector pixel size, not its effective NA. Here we show that this problem can be overcome via the introduction of a numerical spherical lens with adjustable magnification. This approach enables the reconstruction of object details smaller than the detector pixel size or of objects that extend beyond the size of the detector. It will have applications in all forms of near-field lensless microscopy.