Browsing by Author "Hallam, Jonathan M."

Now showing 1 - 7 of 7
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    2D spatially-resolved depth-section microfluidic flow velocimetry using dual beam OCT
    (MDPI, 2020-03-27) Hallam, Jonathan M.; Rigas, Evangelos; Charrett, Thomas O. H.; Tatam, Ralph P.
    A dual beam optical coherence tomography (OCT) instrument has been developed for flow measurement that offers advantages over microscope derived imaging techniques. It requires only a single optical access port, allows simultaneous imaging of the microfluidic channel, does not require fluorescent seed particles, and can provide a millimetre-deep depth-section velocity profile (as opposed to horizontal-section). The dual beam instrument performs rapid re-sampling of particle positions, allowing measurement of faster flows. In this paper, we develop the methods and processes necessary to make 2D quantitative measurements of the flow-velocity using dual beam OCT and present exemplar results in a microfluidic chip. A 2D reference measurement of the Poiseuille flow in a microfluidic channel is presented over a spanwise depth range of 700 μm and streamwise length of 1600 μm with a spatial resolution of 10 μm, at velocities up to 50 mm/s. A measurement of a more complex flow field is also demonstrated in a sloped microfluidic section.
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    Dual-channel OCT for velocity measurement in microfluidic channels.
    (OSA, 2018-09-28) Rigas, Evangelos; Hallam, Jonathan M.; Ford, Helen D.; Charrett, Thomas O. H.; Tatam, Ralph P.
    A dual-beam Optical Coherence Tomography system has been developed, using a bespoke dual optical fibre, to simultaneously image microfluidic channel structures and measure high velocity flows (presently 250μm/s) from a single optical access point.
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    In-process range-resolved interferometric (RRI) 3D layer height measurements for wire+ arc additive manufacturing (WAAM)
    (IOP, 2022-01-10) Hallam, Jonathan M.; Kissinger, Thomas; Charrett, Thomas O. H.; Tatam, Ralph P.
    In this work a range resolved interferometry (RRI) instrument for absolute distance measurements is integrated into a wire + arc additive manufacturing (WAAM) system to provide in-process monitoring of layer height, and prospects for volume and profile monitoring are discussed. In this experiment titanium feedstock was used to create a 150 mm long, 13.5 mm high wall comprised of 11 WAAM deposited layers. The RRI in-process measurements are in very good agreement ($\lt$0.05 mm) with both mid-process, on-machine micrometre measurements taken by hand after each WAAM deposition, and post-process laser scanning measurements of the completed wall. The high depth of field allows direct referencing of the layer height measurements to the build plate making the measurement independent of the motion system and build plate bending, considerably lowering uncertainties. This, together with the capability for cost-effective in-process measurements in harsh environments, should make the proposed approach very interesting for routine use in WAAM systems.
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    Influence of aberrations on confocal-based remote refractive index measurements
    (Optical Society of America, 2019-08-13) Ford, Helen D.; Francis, Daniel; Hallam, Jonathan M.; Tatam, Ralph P.
    Confocal scanning combined with low-coherence interferometry is used to provide remote refractive index and thickness measurements of transparent materials. The influence of lens aberrations in the confocal measurement is assessed through ray-trace modeling of the axial point-spread functions generated using optical configurations comprised of paired aspherics and paired achromats. Off-axis parabolic mirrors are suggested as an alternative to lenses and are shown to exhibit much more symmetric profiles provided the system numerical aperture is not too high. The modeled results compare favorably with experimental data generated using an optical instrument comprised of a broadband source and line-scan spectrometer. Refractive index and thickness measurements are made with each configuration with most mirror pairings offering better than twice the repeatability and accuracy of either lens pairing.
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    Low-coherence and broadband confocal refractometry: reducing the measurement time
    (IOP Publishing, 2023-09-04) Francis, Daniel; Hallam, Jonathan M.; Tatam, Ralph P.
    This paper describes the steps taken to improve the measurement speed of a combined low-coherence and confocal refractive index measurement system. The instrument measures the refractive index and thickness of transparent plates using a fibre-based low-coherence interferometer with a line-scan spectrometer. The spectrometer allows on-line dispersion measurement which is necessary to derive the sample thickness t as well as both the phase and group refractive indices np, ng. The measurements were performed on a sample with six surfaces consisting of three glass windows mounted in a lens tube. Experimental results show that a measurement time of 4.4 s for the multi-layered object, which has a total thickness of approximately 10.5 mm, can be achieved whilst maintaining an accuracy of better than 0.1% for np, ng, and t. This represents an approximately hundredfold improvement over previously published measurements.
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    Metre-per-second microfluidic flow velocimetry with dual beam optical coherence tomography
    (Optical Society of America, 2019-08-19) Rigas, Evangelos; Hallam, Jonathan M.; Charrett, Thomas O. H.; Ford, H. D.; Tatam, Ralph P.
    A novel dual beam Optical Coherence Tomography (OCT) instrument has been developed for high velocity flow measurement, principally in microfluidics applications. The scanned dual beam approach creates a pair of image-frames separated by a small spatiotemporal offset. Metre-per-second flow measurement is achieved by rapid re-imaging by the second beam allowing for particle tracking between each image-frame of the pair. Flow at 1.06 m/s using a single optical access port has been measured, more than two orders of magnitude larger than previously reported OCT systems, at centimetre depth and with millimetre scale depth of field within a microfluidic chip, whilst simultaneously imaging the microfluidic channel structure.
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    The use of parabolic mirrors in combined low-coherence and confocal refractive index measurement
    (SPIE, 2019-06-21) Francis, Daniel; Ford, Helen D.; Hallam, Jonathan M.; Tatam, Ralph P.
    Low-coherence interferometry is combined with confocal scanning to provide remote refractive index and thickness measurements of transparent materials. The influence of lens aberrations in the confocal measurement is assessed through investigation of the axial point-spread functions (APSFs) generated using optical configurations comprised of paired aspherics and paired achromats. Off-axis parabolic mirrors are suggested as an alternative to lenses and are shown to exhibit much more symmetric APSFs provided the system numerical aperture is not too high. Refractive index and thickness measurements are made with each configuration with most mirror pairings offering better than twice the repeatability and accuracy of either lens pairing.