Samuel Lawman

High precision dynamic multi-interface profilometry with optical coherence tomography

Optical coherence tomography (OCT) has mostly been used for high-speed volume imaging but its profilometry potentials have not been fully exploited. This paper demonstrates high precision (as good as ~50 nm) multi-interface profilometry using a Fourier domain OCT system without special antivibration devices. The precision is up to 2 orders of magnitude better than the depth resolution of the OCT. Detailed analysis of the precision achieved for different surfaces is presented. The multi-interface profiles are obtained as a by-product of the tomography data. OCT has the advantage in speed and sensitivity at detecting rough and internal interfaces versus conventional optical profilometry. An application of the technique to the dynamic monitoring of varnish drying on paintlike substrates is demonstrated, which provides a better understanding of the formation of surface roughness. The technique has potential benefits in the fields of art conservation, coatings technology, and soft matter physics.

High resolution corneal and single pulse imaging with line field spectral domain optical coherence tomography

We report the development of a Spectral Domain Line Field Optical Coherence Tomography (LF-OCT) system, using a broad bandwidth and spatial coherent Super-Continuum (SC) source. With conventional quasi-Continuous Wave (CW) setup we achieve axial resolutions up to 2.1 μm in air and 3D volume imaging speeds up to 213 kA-Scan/s. Furthermore, we report the use of a single SC pulse, of 2 ns duration, to temporally gate an OCT B-Scan image of 70 A-Scans. This is the equivalent of 35 GA-Scans/s. We apply the CW setup for high resolution imaging of the fine structures of a human cornea sample ex-vivo. The single pulse setup is applied to imaging of a coated pharmaceutical tablet. The fixed pattern noise due to spectral noise is removed by subtracting the median magnitude A-Scan. We also demonstrate that the Fourier phase can be used to remove aberration caused artefacts.

Nondestructive analysis of automotive paints with spectral domain optical coherence tomography.

We have demonstrated for the first time, to our knowledge, the use of optical coherence tomography (OCT) as an analytical tool for nondestructively characterizing the individual paint layer thickness of multiple layered automotive paints. A graph-based segmentation method was used for automatic analysis of the thickness distribution for the top layers of solid color paints. The thicknesses measured with OCT were in good agreement with the optical microscope and ultrasonic techniques that are the current standard in the automobile industry. Because of its high axial resolution (5.5 μm), the OCT technique was shown to be able to resolve the thickness of individual paint layers down to 11 μm. With its high lateral resolution (12.4 μm), the OCT system was also able to measure the cross-sectional area of the aluminum flakes in a metallic automotive paint. The range of values measured was 300-1850  μm2. In summary, the proposed OCT is a noncontact, high-resolution technique that has the potential for inclusion as part of the quality assurance process in automobile coating.

Non-destructive analysis of flake properties in automotive paints with full-field optical coherence tomography and 3D segmentation

Automotive coating systems are designed to protect vehicle bodies from corrosion and enhance their aesthetic value. The number, size and orientation of small metallic flakes in the base coat of the paint has a significant effect on the appearance of automotive bodies. It is important for quality assurance (QA) to be able to measure the properties of these small flakes, which are approximately 10μm in radius, yet current QA techniques are limited to measuring layer thickness. We design and develop a time-domain (TD) full-field (FF) optical coherence tomography (OCT) system to scan automotive panels volumetrically, non-destructively and without contact. We develop and integrate a segmentation method to automatically distinguish flakes and allow measurement of their properties. We test our integrated system on nine sections of five panels and demonstrate that this integrated approach can characterise small flakes in automotive coating systems in 3D, calculating the number, size and orientation accurately and consistently. This has the potential to significantly impact QA testing in the automotive industry.

Imaging of Corneal Neovascularization: Optical Coherence Tomography Angiography and Fluorescence Angiography

Purpose The purpose of this study was to compare optical coherence tomography angiography (OCTA) and indocyanine green angiography (ICGA) for the assessment of corneal neovascularization (CoNV). Methods Patients with CoNV extending at least 3 mm into the cornea were included. All patients underwent corneal imaging at the same visit. Images were recorded using the AngioVue OCTA system (Optovue, Inc.) with the long corneal adaptor module (CAM-L). ICGA images were recorded with fluorescent filters using the Heidelberg system (HRA2 Scanning Laser Ophthalmoscope; Heidelberg Engineering). Images were graded for quality by two independent observers. Vessel parameters: area, number, diameter, branch and end points, and tortuosity, were compared between devices. Bland-Altman plots were used to assess differences between parameters. Results Fifteen patients with CoNV predominantly associated with microbial keratitis were included. Mean subjective image quality score was better for ICGA (3.3 ± 0.9) than for OCTA (2.1 ± 1.2, P = 0.002), with almost perfect interobserver agreement for ICGA images (κ = 0.83) and substantial agreement for OCTA images (κ = 0.69). Agreement of grading of all investigated vessel parameters between ICGA and OCT images was slight to moderate, with significant differences found for vessel diameter (-8.98 μm, P = 0.01, 95% limits of agreement [LOA]: -15.89 to -2.07), number of branch (25.93, P = 0.09, 95% LOA: -4.31 to 56.17), and terminal points (49, P = 0.05, 95% LOA: 0.78 to 97.22). Conclusion Compared with ICGA, current OCTA systems are less precise in capturing small vessels in CoNV complexes, and validation studies are needed for OCTA segmentation software. OCTA, however, complements ICGA by providing evidence of red blood cell flow, which together with depth information, may be helpful when planning treatment of CoNV.

Fourier domain optical coherence tomography for high-precision profilometry

A Fourier domain (FD) optical coherence tomography (OCT) system is shown to be capable of profilometry with two orders of magnitude better accuracy than the axial imaging resolution of the system. High precision OCT profilometry not only achieves similar accuracy as commercial white light interferometry based profilometers but is also capable of profilometry on complex subsurface structures with multiple interfaces of low reflectance. An accuracy of 55nm was achieved with a ThorLabs SROCT on a lab bench without special anti-vibration devices. This technique has the potential for a range of applications, such as high precision refractive index measurements and simultaneous dynamic monitoring of the interface structure of a drying varnish and the substrate.

Supercontinuum ultra-high resolution line-field OCT; experimental spectrograph comparison and comparison with current clinical OCT systems by the imaging of a human cornea

Ultra high axial resolution (UHR) was demonstrated early in the development of optical coherence tomography (OCT), but has not yet reached clinical practice. We present the combination of supercontinuum light source and line field (LF-) OCT as a technical and economical route to get UHR-OCT into clinic and other OCT application areas. We directly compare images of a human donor cornea taken with low and high resolution current generation clinical OCT systems with UHR-LF-OCT. These images highlight the massive information increase of UHR-OCT. Application to pharmaceutical pellets, and the functionality and imaging performance of different imaging spectrograph choices for LF- OCT are also demonstrated.

Applications of optical coherence tomography in the non-contact assessment of automotive paints

The multiple layer paint systems on modern cars serve two end purposes, they firstly protect against corrosion and secondly give the desired visual appearance. To ensure consistent corrosion protection and appearance, suitable Quality Assurance (QA) measures on the final product are required. Various (layer thickness and consistency, layer composition, flake statistics, surface profile and layer dryness) parameters are of importance, each with specific techniques that can measure one or some of them but no technique that can measure all or most of them. Optical Coherence Tomography (OCT) is a 3D imaging technique with micrometre resolution. Since 2016, OCT measurements of layer thickness and consistency, layer composition fingerprint and flake statistics have been reported. In this paper we demonstrate two more novel applications of OCT to automotive paints. Firstly, we use OCT to quantify unwanted surface texture, which leads to an “orange peel” visual defect. This was done by measuring the surface profiles of automotive paints, with an unoptimised precision of 37 nm over lateral range of 7 mm, to quantify texture of less than 500 nm. Secondly, we demonstrate that OCT can measure how dry a coating layer is by measuring how fast it is still shrinking quasiinstantaneously, using Fourier phase sensitivity.

Fast blur detection and parametric deconvolution of retinal fundus images

Blur is a significant problem in medical imaging which can hinder diagnosis and prevent further automated or manual processing. The problem of restoring an image from blur degradation remains a challenging task in image processing. Semi-blind deblurring is a useful technique which may be developed to restore the underlying sharp image given some assumed or known information about the cause of degradation. Existing models assume that the blur is of a particular type, such as Gaussian, and do not allow for the approximation of images corrupted by other blur types which are not easily incorporated into deblurring frameworks. We present an automated approach to image deconvolution which assumes that the cause of blur belongs to a set of common types. We develop a hierarchical approach with convolutional neural networks (CNNs) to distinguish between blur types, achieving an accuracy of 0.96 across a test set of 900 images, and to determine the blur strength, achieving accuracy of 0.77 across 1500 test images. Given this, we are able to reconstruct the underlying image to mean ISNR of 7.53.

Deformation velocity imaging using optical coherence tomography and its applications to the cornea

Optical coherence tomography (OCT) can monitor human donor corneas non-invasively during the de-swelling process following storage for corneal transplantation, but currently only resultant thickness as a function of time is extracted. To visualize and quantify the mechanism of de-swelling, we present a method exploiting the nanometer sensitivity of the Fourier phase in OCT data to image deformation velocities. The technique was demonstrated by non-invasively showing during de-swelling that osmotic flow through an intact epithelium is negligible and removing the endothelium approximately doubled the initial flow at that interface. The increased functional data further enabled the validation of a mathematical model of the cornea. Included is an efficient method of measuring high temporal resolution (1 minute demonstrated) corneal thickness, using automated collection and semi-automated graph search segmentation. These methods expand OCT capabilities to measure volume change processes for tissues and materials.