Browsing by Author "James, David"

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    Behaviour of characteristic modes on patch antennas in multilayered media
    (ARMMS: Automated Radio Frequency and Microwave Measurement Society, 2022-04-26) Adamson, Michael F.; Morrow, Ivor L.; James, David
    A design methodology is described supported by Characteristic Mode Analysis (CMA) to enhance the radiation gain from a patch antenna using multiple dielectric superstrates. Analytic expressions for the radiation field patterns and impedance match are derived for the patch eigenmodes. An equivalent commercial simulation model is used to determine the associated characteristic modes. Eigenmode and CMA behaviour are studied as low cost planar dielectric sheets are stacked above the antenna to affect an increase in the radiated field gain. Numerical results are compared with laboratory measurements made on several prototype dielectric radome antenna configurations and demonstrate an impedance bandwidth of 3.6% and radiated field pattern gain of 11.5dBi for a double layer superstrate antenna.
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    Comparative noise performance of a coded aperture spectral imager
    (SPIE, 2016-10) Piper, James; Yuen, Peter W. T.; Godfree, Peter; Ding, Mengjia; Soori, Umair; Selvagumar, Senthurran; James, David
    Novel types of spectral sensors using coded apertures may offer various advantages over conventional designs, especially the possibility of compressive measurements that could exceed the expected spatial, temporal or spectral resolution of the system. However, the nature of the measurement process imposes certain limitations, especially on the noise performance of the sensor. This paper considers a particular type of coded-aperture spectral imager and uses analytical and numerical modelling to compare its expected noise performance with conventional hyperspectral sensors. It is shown that conventional sensors may have an advantage in conditions where signal levels are high, such as bright light or slow scanning, but that coded-aperture sensors may be advantageous in low-signal conditions
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    Design of a tunable snapshot multispectral imaging system through ray tracing simulation
    (MDPI, 2019-01-05) Ding, Mengjia; Yuen, Peter W. T.; Piper, Jonathan; Godfree, Peter; Chatterjee, Ayan; Zahidi, Usman; Selvagumar, Senthurran; James, David; Richardson, Mark A.
    Research on snapshot multispectral imaging has been popular in the remote sensing community due to the high demands of video-rate remote sensing system for various applications. Existing snapshot multispectral imaging techniques are mainly of a fixed wavelength type, which limits their practical usefulness. This paper describes a tunable multispectral snapshot system by using a dual prism assembly as the dispersion element of the coded aperture snapshot spectral imagers (CASSI). Spectral tuning is achieved by adjusting the air gap displacement of the dual prism assembly. Typical spectral shifts of about 1 nm at 400 nm and 12 nm at 700 nm wavelength have been achieved in the present design when the air-gap of the dual prism is changed from 4.24 mm to 5.04 mm. The paper outlines the optical designs, the performance, and the pros and cons of the dual-prism CASSI (DP-CASSI) system. The performance of the system is illustrated by TraceProTM ray tracing, to allow researchers in the field to repeat or to validate the results presented in this paper.
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    Modelling of a new X-ray backscatter imaging system: simulation investigation
    (IS&T Society for Imaging Science and Technology, 2022-04-04) Selvagumar, Senthurran; Yuen, Peter W. T.; Soori, Umair; Piper, Johnathan; James, David; Andre, David; Richardson, Mark A.
    X-ray backscatter imaging is a powerful technique for medical, aerospace, and security applications. Conventionally, a pinhole is commonly used for focusing x-ray, but there is always a desire to enhance the signal-to-noise-ratio (SNR) and optical throughput compared to a single pinhole. The main aim of this paper is to present a new x-ray backscatter imaging system which was inspired by a Twisted Slit collimator system called the Vortex Collimator and compare the optical throughput and the imaging performance with that of the Twisted Slit' collimator1,2 and the Pinhole imaging systems for axial point sources, where the pinhole system was used purely for comparison purposes. All the comparisons were performed through Ray tracing (Trace-Pro) simulation software. This work shows that the Vortex design yields ~4% higher SNR/optical throughput than that of the Twisted Slit collimator, and ~42.5% higher transmittance. Furthermore, the opening of the Vortex Collimator was increased and reduced to observe the performance, resulting in about ~1% transmittance increment when the opening was increased. Also, thicknesses of the Vortex Collimator and Twisted Slit collimator were increased and reduced and found that reducing the thickness seems to increase the system's throughput marginally.
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    Monte Carlo simulation results for anaemia detection in the skin
    (Cranfield University, 2020-01-20 08:27) Kallepalli, Akhil; James, David
    This research was presented at SPIE Photonics West 2020 (February 2020) with the article titled: Quantification and influence of skin chromophores for remote detection of anemic conditions (https://doi.org/10.1117/12.2545784) Monte Carlo simulations were performed using Virtual Tissue Simulator (VTS) and Monte Carlo Command Line (MCCL) tools. The target was a two-layer model of the epidermis and dermis layers of the skin. The optical attenuation properties quantifying the absorption and scattering in these layers are set according to two parameters: - Amount of melanin in the epidermis - Amount of haemoglobin in the blood-perfused dermis The optical models are simulated at six wavelengths for 6 skin types (Fitzpatrick's scale) and two blood conditions, healthy and anaemic. This results in a total of 72 simulations The simulation calculates the optical interaction of 10 6 photons with the various combinations of optical properties. The resulting outputs show the absorption and fluence in the tissue model, and reflected and transmitted energy.
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    Monte Carlo simulation results for full finger models based on ultrasound image data
    (Cranfield University, 2021-06-02 16:35) Kallepalli, Akhil; James, David; Richardson, Mark
    Monte Carlo simulations were performed using Virtual Tissue Simulator (VTS) and Monte Carlo Command Line (MCCL) tools. The target was a tissue model specific to individual participants, including all anatomical components in a finger. The thickness and depth measurements were ascertained using an 18 MHz ultrasound probe. This implementation is proposed as a low-cost and novel method for patient-specific photo-therapy. The optical models of 12 participants are assessed at four wavelengths of interest. The optical interaction with the tissue layers is calculated with 10 6 photons. The absorption, reflection, fluence and transmission are measured and quantified in the experiments.
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    Non-Parametric Spatial Spectral Band Selection methods
    (2021-05) Torres, Ruben M; Yuen, Peter W. T.; James, David
    This project is about the development of band selection (BS) techniques for better target detection and classification in remote sensing and hyperspectral imaging (HSI). Conventionally, this is achieved just by using the spectral features for guiding the band compression. However, this project develops a BS method which uses both spatial and spectral features to allow a handful of crucial spectral bands to be selected for enhancing the target detection and classification performances. This thesis firstly outlines the fundamental concepts and background of remote sensing and HSI, followed by the theories of different atmospheric correction algorithms — in order to assess the reflectance conversion for band selection — and BS techniques, with a detailed explanation of the Hughes principle, which postulates the fundamental drawback for having high-dimensional data in HSI. Subsequently, the thesis highlights the performances of some advanced BS techniques and to point out their deficiencies. Most of the existing BS work in the field have exhibited maximal classification accuracy when more spectral bands have been utilized for classification; this apparently disagrees with the theoretical model of the Hughes phenomenon. The thesis then presents a spatial spectral mutual information (SSMI) BS scheme which utilizes a spatial feature extraction technique as a pre-processing step, followed by the clustering of the mutual information (MI) of spectral bands for enhancing the BS efficiency. Through this BS scheme, a sharp ’bell’-shaped accuracy-dimensionality characteristic has been observed, peaking at about 20 bands. The performance of the proposed SSMI BS scheme has been validated through 6 HSI datasets, and its classification accuracy is shown to be ~10% better than 7 state-of-the-art BS algorithms. These results confirm that the high efficiency of the BS scheme is essentially important to observe, and to validate, the Hughes phenomenon at band selection through experiments for the first time.
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    Progress towards recalibration of spectrographs
    (IEEE, 2019-03-07) Kallepalli, Akhil; Soori, Umair; James, David; Richardson, Mark A.
    The spectral resolution of a spectrograph depends on the input slit width, the diffraction grating grooves and the number of imaging sensor/detector pixels. Due to the proprietary nature of spectrograph designs, recalibration by end-users can be challenging. Most calibration procedures currently published are applicable to in-house instruments or spectrographs with access to the internal specifications. Narrowing the input slit improves the resolution but also reduces the throughput of the imaging system. We attempted to recalibrate an Offner-based spectrograph by using a larger detector plane (an imaging system with a larger sensor), to vary the distance along the focal plane; and by utilising lens optics. Basic experiments were conducted by varying the distance from the exit window and inserting a lens to magnify the spectrograph output onto the larger detector plane. We concluded that the calibration could not be achieved using simple optics within the scope of our experiments. This article addresses a gap in literature that does not present the research community with the unsuccessful steps that are not applicable to similar problem statements. The alternative would be to rely on reflective optics, but this approach may reduce portability.
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    Signal to noise ratio of a coded slit hyperspectral sensor
    (MDPI, 2022-10-26) Piper, Jonathan; Yuen, Peter W. T.; James, David
    In recent years, a wide range of hyperspectral imaging systems using coded apertures have been proposed. Many implement compressive sensing to achieve faster acquisition of a hyperspectral data cube, but it is also potentially beneficial to use coded aperture imaging in sensors that capture full-rank (non-compressive) measurements. In this paper we analyse the signal-to-noise ratio for such a sensor, which uses a Hadamard code pattern of slits instead of the single slit of a typical pushbroom imaging spectrometer. We show that the coded slit sensor may have performance advantages in situations where the dominant noise sources do not depend on the signal level; but that where Shot noise dominates a conventional single-slit sensor would be more effective. These results may also have implications for the utility of compressive sensing systems.
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    Visible-band nanosecond pulsed laser damage thresholds of silicon 2D imaging array
    (MDPI, 2022-03-25) Westgate, Christopher; James, David
    Laser-induced camera damage thresholds were measured for several sensors of three different sensor architectures using a Q-switched Nd:YAG laser in order to determine their pulsed laser-induced damage thresholds. Charge coupled device (CCD), front-side illuminated complimentary metal-oxide semiconductor (FSI CMOS), and back-side illuminated (BSI) CMOS sensors were assessed under laboratory and outdoor environments by increasing the focused laser intensity onto the sensors and recording the sensor output. The damage sites were classified qualitatively into damage types, and pixel counting methods were applied to quantitatively plot damage scale against laser intensity. Probit-fits were applied to find the intensity values where a 95% probability of damage would occur (FD95) and showed that FD95 was approximately the same under laboratory conditions for CCD, FSI CMOS, and BSI CMOS sensors (mean 532 nm FD95 of 0.077 ± 0.01 Jcm−2). BSI CMOS sensors were the most robust to large-scale damage effects—BSI sensor kill was found at approximately 103 Jcm−2, compared to 10 Jcm−2 for FSI CMOS, and between ~1.6 and 2.7 Jcm−2 for CCDs.