Showing papers by "Robert Henderson published in 2023"

Submerged single-photon LiDAR imaging sensor used for real-time 3D scene reconstruction in scattering underwater environments.

Abstract: We demonstrate a fully submerged underwater LiDAR transceiver system based on single-photon detection technologies. The LiDAR imaging system used a silicon single-photon avalanche diode (SPAD) detector array fabricated in complementary metal-oxide semiconductor (CMOS) technology to measure photon time-of-flight using picosecond resolution time-correlated single-photon counting. The SPAD detector array was directly interfaced to a Graphics Processing Unit (GPU) for real-time image reconstruction capability. Experiments were performed with the transceiver system and target objects immersed in a water tank at a depth of 1.8 meters, with the targets placed at a stand-off distance of approximately 3 meters. The transceiver used a picosecond pulsed laser source with a central wavelength of 532 nm, operating at a repetition rate of 20 MHz and average optical power of up to 52 mW, dependent on scattering conditions. Three-dimensional imaging was demonstrated by implementing a joint surface detection and distance estimation algorithm for real-time processing and visualization, which achieved images of stationary targets with up to 7.5 attenuation lengths between the transceiver and the target. The average processing time per frame was approximately 33 ms, allowing real-time three-dimensional video demonstrations of moving targets at ten frames per second at up to 5.5 attenuation lengths between transceiver and target.

Development of a high-speed line-scanning fluorescence lifetime imaging microscope for biological imaging.

Abstract: We report the development of a novel line-scanning microscope capable of acquiring high-speed time-correlated single-photon counting (TCSPC)-based fluorescence lifetime imaging microscopy (FLIM) imaging. The system consists of a laser-line focus, which is optically conjugated to a 1024 × 8 single-photon avalanche diode (SPAD)-based line-imaging complementary metal-oxide semiconductor (CMOS), with 23.78 µm pixel pitch at 49.31% fill factor. Incorporation of on-chip histogramming on the line-sensor enables acquisition rates 33 times faster than our previously reported bespoke high-speed FLIM platforms. We demonstrate the imaging capability of the high-speed FLIM platform in a number of biological applications.

Field programmable gate array compression for large array multispeckle diffuse correlation spectroscopy

Abstract: Abstract. Significance Diffuse correlation spectroscopy (DCS) is an indispensable tool for quantifying cerebral blood flow noninvasively by measuring the autocorrelation function (ACF) of the diffused light. Recently, a multispeckle DCS approach was proposed to scale up the sensitivity with the number of independent speckle measurements, leveraging the rapid development of single-photon avalanche diode (SPAD) cameras. However, the extremely high data rate from advanced SPAD cameras is beyond the data transfer rate commonly available and requires specialized high-performance computation to calculate large number of autocorrelators (ACs) for real-time measurements. Aim We aim to demonstrate a data compression scheme in the readout field-programmable gate array (FPGA) of a large-pixel-count SPAD camera. On-FPGA, data compression should democratize SPAD cameras and streamline system integration for multispeckle DCS. Approach We present a 192 × 128 SPAD array with 128 linear ACs embedded on an FPGA to calculate 12,288 ACFs in real time. Results We achieved a signal-to-noise ratio (SNR) gain of 110 over a single-pixel DCS system and more than threefold increase in SNR with respect to the state-of-the-art multispeckle DCS. Conclusions The FPGA-embedded autocorrelation algorithm offers a scalable data compression method to large SPAD array, which can improve the sensitivity and usability of multispeckle DCS instruments.

Time resolved SPAD micro-camera probe for wide-field FLIm in microendoscopy

Abstract: We propose a handheld single photon avalanche diode (SPAD) micro-camera probe for wide-field in-vivo fluorescence lifetime imaging (FLIm) applications. The presented probe includes a novel 3D stacked 1.4 mm × 1.4 mm SPAD array, an integrated excitation light source, and imaging optics. The spatial and temporal performance of the integrated system was characterised using a USAF test target and range of fluorescence lifetime beads.

A combined fluorescence lifetime and depth imaging system for medical imaging and surgical guidance

Abstract: Fluorescence lifetime imaging (FLIM) is a valuable technique which can be used to provide label free contrast between different tissue types and provide information about their molecular makeup and local environment. FLIM systems based on single photon avalanche diode (SPAD) arrays are increasingly being used in applications such as medical imaging due to their high sensitivity and excellent temporal resolution [1]. Additionally, SPAD arrays are also commonly employed for time of flight (ToF) imaging techniques such as light detection and ranging (LiDAR) [2]. Here we demonstrate a system which employs both of these modalities into a single instrument, allowing us to acquire both depth and widefield FLIM images simultaneously using a single 32 x 32 pixel SPAD array operating in time correlated single-photon counting (TCSPC) mode with 50 ps temporal resolution. Initial results show that we can correctly measure depths and distances of sample objects with < 1 cm resolution while maintaining excellent and consistent fluorescence contrast. Lifetime is consistent over a distance of 10 cm with a standard deviation of < 0.5 ns, showing that it is possible to decouple depth and lifetime data. We believe this work is the first demonstration of a widefield FLIM system capable of 3D imaging. The next step will be the addition of a miniaturized system [1] and future applications for this technology include fields such as surgical guidance, endoscopy and diagnostic imaging.

Handheld wide-field fluorescence lifetime imaging system based on a distally mounted SPAD array.

Abstract: In this work a handheld Fluorescent Lifetime IMaging (FLIM) system based on a distally mounted < 2 mm2 128 × 120 single photon avalanche diode (SPAD) array operating over a > 1 m long wired interface is demonstrated. The head of the system is ∼4.5 cm x 4.5 cm x 4.5 cm making it suitable for hand-held ex vivo applications. This is, to the best of the authors' knowledge, the first example of a SPAD array mounted on the distal end of a handheld FLIM system in this manner. All existing systems to date use a fibre to collect and relay fluorescent light to detectors at the proximal end of the system. This has clear potential biological and biomedical applications. To demonstrate this, the system is used to provide contrast between regions of differing tissue composition in ovine kidney samples, and between healthy and stressed or damaged plant leaves. Additionally, FLIM videos are provided showing that frame rates of > 1 Hz are achievable. It is thus an important step in realising an in vivo miniaturized chip-on-tip FLIM endoscopy system.

3D image acquisition with multispectral FLIM and compressed sensing (Conference Presentation)

Abstract: Precisely characterising and quantifying interactions between tumour cells and their environment to understand metastatic mechanisms requires a multi-dimensional, high-speed imaging system. To this end, we report on the development of a compressive full spectrum fluorescence lifetime microscope that exploits a novel SPAD line sensor and a DMD to enable monitoring of dynamic sub-cellular interactions. At no cost to its temporal performance, the hyperspectral nature of the system helps to improve unmixing and, crucially, can detect the small spectral changes in the emission of fluorescent probes and intrinsic fluorophores that can occur in complex environments.

Depth video super-resolution using a high-speed time-of-flight sensor

Abstract: Three-dimensional (3D) imaging captures depth information from a given scene and is used in a wide range of fields like industrial environments, smartphones and autonomous driving, among others. This paper summarises the results of a depth video super-resolution scheme that is tailored for single-photon avalanche diode (SPAD) image sensors, which produces 3D maps at frame rates > 100 FPS (32×64 pixels). Consecutive frames are used to super-resolve and denoise depth maps via 3D convolutional neural networks with an upscaling factor of 4. Due to the lack of noise-free, high-resolution depth maps captured with high-speed cameras, the neural network is trained with synthetic data using Unreal Engine, which is later processed to resemble the data outputted by a SPAD sensor. The model is then tested with different video sequences captured with a high-speed SPAD dToF, which processes frames at >30 frames per second. The super-resolved data shows a significant reduction in noise and presents enhanced edge details in objects. We believe these results are relevant to improve the accuracy of object detection in autonomous driving cars for collision avoidance or AR/VR systems.

Single-Photon Counting Receivers for 6G Optical Wireless Communications

Abstract: Optical wireless communication (OWC) offers several complementary advantages to radio-frequency (RF) wireless networks such as its massive available spectrum; hence, it is widely anticipated that OWC will assume a pivotal role in the forthcoming sixth generation (6G) wireless communication networks. Although significant progress has been achieved in OWC over the past decades, the outage induced by occasionally low received optical power continues to pose a key limiting factor for its deployment. In this work, we discuss the potential role of single-photon counting (SPC) receivers as a promising solution to overcome this limitation. We provide an overview of the state-of-the-art of OWC systems utilizing SPC receivers and identify several critical areas of open problems that warrant further research in the future.

Time-correlated Raman imaging with a SPAD line sensor

Abstract: Time-resolved Raman and fluorescence lifetime spectroscopy imaging yields new research insights with great potential in applications including biomedical diagnostics, carbon materials, and battery development. Single Photon Avalanche Diode (SPAD) arrays are ideal for such applications and we present to our knowledge the first time-resolved Raman images obtained with such sensors. Utilizing motorized and confocal scanning configurations we obtain near shot-noise limited performance, room temperature operation, millisecond spectral acquisition times, and simultaneous acquisition and discrimination of Raman and fluorescence with high spectral resolution and range. Detailed images and spectra from samples including calcite, diamond, and single-wall carbon nanotubes demonstrate the possibility of high-resolution time-resolved Raman and fluorescence imaging.

Development of a high-speed confocal line scanning FLIM microscope for live cell imaging (Conference Presentation)

Abstract: We report the development of a novel confocal line-scanning microscope capable of acquiring video-frame rate TCSPC-based FLIM. The system consists of a one-dimensional laser beam, which is optically conjugated to a 1024×16 single photon avalanche diode(SPAD) based line-imaging CMOS(1), with 23.78 μm pixel pitch at 49.31% fill factor. Incorporation of on-chip histogramming on the line-sensor facilitates the acquisition of up to 16.5 Giga-photon counts/s, enabling operation 66 times faster than our previously reported bespoke high speed FLIM platforms. We will demonstrate its use in live-cell imaging investigating the roles that PAK proteins play in regulation of cytoskeletal dynamics.

Visualising varnish removal for conservation of paintings by fluorescence lifetime imaging (FLIM)

Abstract: Abstract The removal of varnish from the surface is a key step in painting conservation. Varnish removal is traditionally monitored by examining the painting surface under ultraviolet illumination. We show here that by imaging the fluorescence lifetime instead, much better contrast, sensitivity, and specificity can be achieved. For this purpose, we developed a lightweight (4.8 kg) portable instrument for macroscopic fluorescence lifetime imaging (FLIM). It is based on a time-correlated single-photon avalanche diode (SPAD) camera to acquire the FLIM images and a pulsed 440 nm diode laser to excite the varnish fluorescence. A historical model painting was examined to demonstrate the capabilities of the system. We found that the FLIM images provided information on the distribution of the varnish on the painting surface with greater sensitivity, specificity, and contrast compared to the traditional ultraviolet illumination photography. The distribution of the varnish and other painting materials was assessed using FLIM during and after varnish removal with different solvent application methods. Monitoring of the varnish removal process between successive solvent applications by a swab revealed an evolving image contrast as a function of the cleaning progress. FLIM of dammar and mastic resin varnishes identified characteristic changes to their fluorescence lifetimes depending on their ageing conditions. Thus, FLIM has a potential to become a powerful and versatile tool to visualise varnish removal from paintings. Graphical Abstract