Prof. Marc S. Walton Profile

Prof. Marc S. Walton

at Northwestern Univ

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Publications (4)

Proceedings Article | 22 July 2019 Presentation

Investigation of reflectance-based pigment classification in layered media (Conference Presentation)

Lionel Fiske, Oliver Cossairt, Aggelos Katsaggelos, Marc Walton

Proceedings Volume 11058, 110580E (2019) https://doi.org/10.1117/12.2525382

KEYWORDS: Reflectivity, Associative arrays, X-ray fluorescence spectroscopy, Chemical elements, Multilayers

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Proceedings Article | 12 July 2019 Presentation + Paper

Maximizing the microscope: instrument design and data processing strategies for hyperspectral imaging of cross-sectional cultural heritage samples

Billie Males, Lindsay Oakley, Oliver Cossairt, Marc Walton

Proceedings Volume 11058, 110580C (2019) https://doi.org/10.1117/12.2526098

KEYWORDS: Microscopes, Particles, Hyperspectral imaging, Cultural heritage, Data processing, Spectral resolution, Luminescence, Scattering, Reflectivity, Light sources

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The use of non-invasive hyperspectral imaging techniques has become standard practice in the materials analysis and study of precious cultural heritage objects such as drawings, paintings, murals and more. However, the non-linear mixing of spectral signatures from complex and heterogenous objects with multiple colorants present below the resolution limits of the camera can complicate material identification. Consequently, ground truth measurements are still usually obtained from microscopic samples removed and embedded to expose stratigraphy and obtain sub-surface information about the artist’s material choices and technique. This work considers a microscopic spectral imaging technique capable of mapping molecular information in such micro samples at high spatial and spectral resolution while avoiding some of the challenges of complimentary techniques, such as swamping fluorescence in Raman spectroscopy or long integration times using FT-IR spectroscopy. Construction of a dark field hyperspectral microscope for cultural heritage samples is described using a tunable light source to illuminate the sample monochromatically from the visible to near infrared wavelengths, with the diffusely reflected light collected from the specimen with a long working distance, 20x objective. The illumination and detection arms were decoupled to better focus the power of the tunable light source across the tunable range through Köhler illumination optics. By mounting the optical train on a rotating arm, we can achieve multiple angles of illumination and optimize lighting conditions. The sample is also rotated in order to reconstruct an even distribution of light across the field of view. This multi-axis movement capability also provides exciting opportunities to leverage more than simple spectral information from an image series such as surface topography and differential phase contrast information. The developed microscope was used create a library of spectral signatures for comparison to painting cross sections, and the ability of the microscope to identify and examine individual pigment particles was tested.

Proceedings Article | 12 July 2019 Presentation + Paper

Time-domain optical coherence tomography can measure artworks with high penetration and high resolution

Bingjie Xu, Kuan He, Pengxiao Hao, Jian Gao, Florian Willomitzer, Aggelos Katsaggelos, John Tumblin, Oliver Cossairt, Marc Walton

Proceedings Volume 11058, 110580M (2019) https://doi.org/10.1117/12.2525649

KEYWORDS: Optical coherence tomography, Mirrors, Sensors, Signal to noise ratio, Imaging systems, Data acquisition, Image resolution, Cultural heritage, Reflectivity, Control systems

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Accurate measurements of the geometric shape and the internal structure of cultural artifacts are of great importance for the analysis and understanding of artworks such as paintings. Often their complex layers, delicate materials, high value and uniqueness preclude all but the sparsest sample-based measurements (microtomy or embedding of small chips of paint). In the last decade, optical coherence tomography (OCT) has enabled dense point-wise measurements of layered surfaces to create 3D images with axial resolutions at micron scales. Commercial OCT systems at biologically-useful wavelengths (900 nm to 1.3 μm) can reveal some painting layers, strong scattering and absorption at these wavelengths severely limits the penetration depth. While Fourierdomain methods increase measurement speed and eliminate moving parts, they also reduce signal-to-noise ratios and increase equipment costs. In this paper, we present an improved lower-cost time-domain OCT (TD-OCT) system for deeper, high-resolution 3D imaging of painting layers. Assembled entirely from recently-available commercially-made parts, its 2x2 fused fiber-optic coupler forms an interferometer without a delicate, manuallyaligned beam-splitter, its low-cost broadband Q-switched super-continuum laser source supplies 20 KHz 0.4-2.4 μm coherent pulses that penetrate deeply into the sample matrix, and its single low-cost InGaAs amplified photodetector replaces the sensitive spectroscopic camera required by Fourier domain OCT (FD-OCT) systems. Our fiber and filter choices operate at 2.0±0.2 μm wavelengths, as these may later help us characterize scattering and absorption characteristics, and yield axial resolution of about 4.85 μm, surprisingly close to the theoretical maximum of 4.41 μm. We show that despite the moving parts that make TD-OCT measurements more timeconsuming, replacing the spectroscopic camera required by FD-OCT with a single-pixel detector offers strong advantages. This detector measures interference power at all wavelengths simultaneously, but at a single depth, enabling the system to reach its axial resolution limits by simply using more time to acquire more samples per Ascan. We characterize the system performance using material samples that match real works of art. Our system provides an economical and practical way to improve 3D imaging performance for cultural heritage applications in terms of penetration, resolution, and dynamic range.

Proceedings Article | 1 May 2017 Presentation + Paper

Fluorescence lifetime estimation using a dynamic vision sensor

Fengqiang Li, Nathan Matsuda, Marc Walton, Oliver Cossairt

Proceedings Volume 10222, 102220N (2017) https://doi.org/10.1117/12.2262153

KEYWORDS: Luminescence, Sensors, Computational imaging, Modulation, Imaging systems, Calibration, Semiconductor lasers, X-rays, Time metrology, Motion measurement, X-ray fluorescence spectroscopy

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