arXiv subjects

Timothy D. Wilkinson

Publications and source records attributed to Timothy D. Wilkinson.

25 records · Page 2Linked to original sources

arXiv2020

Lookup tables for phase randomisation in hardware generated holograms

The rise in virtual and mixed reality systems has prompted a resurgence of interest in two-dimensional and three-dimensional real-time computer generated holography. Phase randomisation is an integral part of holographic projection as it ensures independence in sub-frame techniques and reduces the edge enhancement seen in flat-phase images. Phase randomisation requires, however, the availability of a pseudo-random number generator as well as trigonometric functions such as cos and sin. On embedded devices such as field programmable gate arrays and digital signal processors this can be an unacceptable load and necessitate the use of proprietary intellectual property cores. Lookup tables are able to reduce the computational load but can run to many megabytes for even low-resolution systems. This paper introduces the use of lookup tables (LUTs) in the context of two common algorithms used for real-time holographic projection: Gerchberg-Saxton and One-Step Phase-Retrieval. A simulated study is carried out to investigate the use of relatively small lookup tables where random numbers are repeated in sequence. We find that the increase in error is low and tunable to under 5\% even for small look up tables. This result is also demonstrated experimentally. Finally, the implications of this study are discussed and conclusions drawn.

eess.SP↗

arXiv2020

Automated laser induced damage threshold testing applied to a ferroelectric spatial light modulator

Laser Induced Damage Thresholds (LIDTs) are a measure of the level of fluence an optical component may be expected to handle without observable damage. In this work we present an automated system for measurement of LIDTs for a wide range of components and include details of the data handling, image processing and code required. We then apply this system to LIDT measurements of a commercial HDP-1280-2 'BlueJay' ferroelectric display finding a LIDT of 9.2 W/cm2 Ø27μm, 5.5 W/cm2 Ø150μm and 3.2 W/cm2 Ø3.1mm with wavelength 1090+-5nm. Finally, the quality of the results obtained are discussed and conclusions drawn.

physics.ins-det↗

arXiv2019

Improving pixel differentiation in holographic images

Computer generated holography (CGH) has seen a resurgence in recent years due, in part, to the rise of virtual and mixed reality systems. The majority of approaches for CGH are based on a sampled Discrete Fourier Transform (DFT) and ignore the interstitial behaviour between sampling points in the replay field. In this paper we demonstrate that neighbouring replay field pixels can interfere significantly giving the visual impression of pixel movement and increased noise. We also demonstrate that increasing the separation between target pixels reduces the interference and improves pixel quality. This phenomena is demonstrated experimentally with close agreement between model and measured result. This work begins by introducing the concept of pixel differentiation before showing simulated models of pixel differentiation issues. Two mitigation approaches are introduced and an experimental system is then used to validate the simulation. Finally results are discussed and conclusions drawn.

physics.optics↗

arXiv2019

Variance and Error in One-Step Phase-Retrieval

Time multiplexed approaches for high frame-rate holographic displays have been around since the invention of One-Step Phase-Retrieval (OSPR) in the early 2000s. When discovered, formulations were created for variance reduction but other image quality metrics were ignored. This work sets out statistical models for the mean squared error (MSE) and structural similarity index (SSIM) behaviour of OSPR for a range of image types in order to better understand the effect of time multiplexing on visible images. This finds that while observed variances converges to zero as the number of frames per second increases, MSE converges to a non-zero value while SSIM converges quadratically to a non-unitary value.

eess.IV↗

arXiv2019

Characterising optical fibre transmission matrices using metasurface reflector stacks for lensless imaging without distal access

The ability to form images through hair-thin optical fibres promises to open up new applications from biomedical imaging to industrial inspection. Unfortunately, deployment has been limited because small changes in mechanical deformation (e.g. bending) and temperature can completely scramble optical information, distorting images. Since such changes are dynamic, correcting them requires measurement of the fibre transmission matrix (TM) in situ immediately before imaging. TM calibration typically requires access to both the proximal and distal facets of the fibre simultaneously, which is not feasible during most realistic usage scenarios without compromising the thin form factor with bulky distal optics. Here, we introduce a new approach to determine the TM of multi-mode fibre (MMF) or multi-core fibre (MCF) in a reflection-mode configuration without access to the distal facet. A thin stack of structured metasurface reflectors is used at the distal facet to introduce wavelength-dependent, spatially heterogeneous reflectance profiles. We derive a first-order fibre model that compensates these wavelength-dependent changes in the TM and show that, consequently, the reflected data at 3 wavelengths can be used to unambiguously reconstruct the full TM by an iterative optimisation algorithm. We then present a method for sample illumination and imaging following TM reconstruction. Unlike previous approaches, our method does not require the TM to be unitary making it applicable to physically realistic fibre systems. We demonstrate TM reconstruction and imaging first using simulated non-unitary fibres and noisy reflection matrices, then using much larger experimentally-measured TMs of a densely-packed MCF, and finally on an experimentally-measured multi-wavelength set of TMs recorded from a MMF. Our findings pave the way for online transmission matrix calibration in situ in hair-thin optical fibres

physics.optics↗

arXiv2019

Grayscale-to-color: Single-step fabrication of bespoke multispectral filter arrays

Conventional cameras, such as in smartphones, capture wideband red, green and blue (RGB) spectral components, replicating human vision. Multispectral imaging (MSI) captures spatial and spectral information beyond our vision but typically requires bulky optical components and is expensive. Snapshot multispectral image sensors have been proposed as a key enabler for a plethora of MSI applications, from diagnostic medical imaging to remote sensing. To achieve low-cost and compact designs, spatially variant multispectral filter arrays (MSFAs) based on thin-film optical components are deposited atop image sensors. Conventional MSFAs achieve spectral filtering through either multi-layer stacks or pigment, requiring: complex mixtures of materials; additional lithographic steps for each additional wavelength; and large thicknesses to achieve high transmission efficiency. By contrast, we show here for the first time a single-step grayscale lithographic process that enables fabrication of bespoke MSFAs based on the Fabry-Perot resonances of spatially variant metal-insulator-metal (MIM) cavities, where the exposure dose controls insulator (cavity) thickness. We demonstrate customizable MSFAs scalable up to N-wavelength bands spanning the visible and near-infrared with high transmission efficiency (~75%) and narrow linewidths (~50 nm). Using this technique, we achieve multispectral imaging of several spectrally distinct target using our bespoke MIM-MSFAs fitted to a monochrome CMOS image sensor. Our unique framework provides an attractive alternative to conventional MSFA manufacture, by reducing both fabrication complexity and cost of these intricate optical devices, while increasing customizability.

physics.app-ph↗

arXiv2018

Quantitative phase and polarisation endoscopy applied to detection of early oesophageal tumourigenesis

Phase and polarisation of coherent light are highly perturbed by interaction with microstructural changes in pre-malignant tissue, holding promise for label-free early cancer detection in endoscopically accessible tissues such as the gastrointestinal tract. Flexible optical fibres used in conventional diagnostic endoscopy scramble phase and polarisation, restricting clinicians instead to low-contrast amplitude-only imaging. Here, we unscramble phase and polarisation images by exploiting the near-diagonal multi-core fibre (MCF) transmission matrix to create a novel parallelised fibre characterisation architecture, scalable to arbitrary MCFs without additional experimental overhead. Our flexible MCF holographic endoscope produces full-field en-face images of amplitude, quantitative phase and resolved polarimetric properties using a low-cost laser diode and camera. We demonstrate that recovered phase enables computational re-focusing at working distances up to 1mm over a field-of-view up to 750$\times$750 $μm^2$. Furthermore, we demonstrate that the spatial distribution of phase and polarisation information enables label-free visualisation of early tumours in oesophageal mouse issue that are not identifiable using conventional amplitude-only information, a milestone towards future application for early cancer detection in endoscopy.

physics.med-ph↗