In the maritime domain, fog represents a significant challenge, e.g. for the detection of castaways in search and rescue missions or obstacle avoidance/warning systems, particularly at night. In scattering environments, conventional active imaging techniques often struggle due to backscattering resulting in low contrasts or completely saturated pixels from backscattering at fog particles (water droplets). The effective suppression of atmospheric backscatter in foggy environments by using Gated-Viewing technology has been demonstrated in previous studies. However, another alternative of blocking backscattered light is using polarization if the scattering event preserves polarization, which is the case for water droplets (maritime fog). In the present study, we modified an existing, unpolarized light emitting Gated-Viewing instrument and added the option to emit and receive linear polarized light through installation of polarization filters. All images taken with this modification are acquired in Continuous Exposure (CE) mode, i.e. without gating. The objective was to investigate whether polarization can serve as an alternative to Gated-Viewing in the maritime domain where backscatter is expected to largely preserve polarization. In initial field tests the efficiency of the modified (polarized) instrument in suppressing retroreflections under clear weather conditions was evaluated. Further tests with artificial fog showed that the modified instrument could effectively suppress backscatter, achieving a contrast loss comparable to that observed in clear weather conditions.
Two compact and portable SWIR active imaging instrument configurations aiming at vision enhancement in indoor applications are tested and compared, working at 1300 nm and 1550 nm, respectively. Both configurations are in-house developments, but based on a limited number of standard and commercially available components (cameras, LEDs). The instruments provide images (640×512, resp. 1280×1024 pixels) at a rate of ca. 17 Hz (live stream) that can be displayed either directly on an integrated display or send via (wireless) network. Key specifications (optical power, field of view, heat development) have been characterized in laboratory tests. The performance of the two system configurations in terms of vision enhancement is compared both practically (field tests) and theoretically (Mie scattering theory). The 1300 nm illuminator has almost double power compared to the 1550 nm illuminator. However, Mie calculations predict more backscatter and less transmission through fog and smoke, which is highly depending on the particle size. Field tests using artificial fog and an in-house developed transmissiometer have been performed to validate the findings from modeling and found a vision enhancement in the order of one magnitude due to use of SWIR (instead of the Visible) for use in typical environments for which the instruments are designed for. A substantial additional improvement in terms of vision enhancement could be achieved by using polarized light and polarization optics to reduce the backscatter signal. In contrast to other research studies, this vision enhancement is not based on polarization difference imaging but on reducing the backscatter component only, enabling a robust and simple system design.
A portable short-wave infrared (SWIR) sensor system was developed aiming at vision enhancement through fog and smoke for support of emergency forces such as fire fighters or the police. In these environments, wavelengths in the SWIR regime have superior transmission and less backscatter in comparison to the visible spectral range received by the human eye or RGB cameras. On the emitter side, the active SWIR sensor system features a light-emitting diode (LED) array consisting of 55 SWIR-LEDs with a total optical power output of 280 mW emitting at wavelengths around λ = 1568 nm with a Full Width at Half Maximum (FWHM) of 137 nm, which are more eye-safe compared to the visible range. The receiver consists of an InGaAs camera equipped with a lens with a field of view slightly exceeding the angle of radiation of the LED array. For convenient use as a portable device, a display for live video from the SWIR camera is embedded within the system. The dimensions of the system are 270 x 190 x 110 mm and the overall weight is 3470 g. The superior potential of SWIR in contrast to visible wavelengths in scattering environments is first theoretically estimated using the Mie scattering theory, followed by an introduction of the SWIR sensor system including a detailed description of its assembly and a characterisation of the illuminator regarding optical power, spatial emission profile, heat dissipation, and spectral emission. The performance of the system is then estimated by design calculations based on the lidar equation. First field experiments using a fog machine show an improved performance compared to a camera in the visible range (VIS), as a result of less backscattering from illumination, lower extinction and thus producing a clearer image.
KEYWORDS: Signal to noise ratio, Backscatter, Visibility, Sensors, Reflectivity, Visibility through fog, Signal attenuation, Gated imaging, Maritime surveillance
Maritime search and rescue operations (SAR) are highly affected by harsh environmental conditions and darkness (night time operation). Especially at low visibility and high humidity scenarios like fog, mist or sea spray, gated-viewing offers an active-imaging solution to effectively suppress atmospheric back-scatter and enhance target contrast. The presented TRAGVIS gated-viewing system is designed to fill the needs in SAR operations: at least 185 m detection range at a minimum FOV of 7°x6° and operates in the NIR at 804 nm emission wavelength, combining a high repetition rate VCSEL illuminator with an accumulation mode CMOS camera. The performance of the demonstrator in a wide range of different visibility fog events and different sets of system parameters has been evaluated by analysing the target signal, contrast and signal to noise ratio SNR as a function of the optical depth OD, which was measured by an atmospheric visibility sensor. As the back-scattered signal (suppressed by the camera shutter) overcomes the target signal of a 41% reflectivity target at OD > 4, it was found, together with a low target signal, to be the major reason for the drop of contrast after a vision enhancement up to OD ≈ 3. A limitation of the system to approximately OD = 5.3 is estimated, as the image shows a decent contrast of 10%, but at an SNR of only ∼ 2.2. The highest potential for improvements was found in an optimised placement of the illuminator with respect to the receiver and scene geometry.
Two Gated-Viewing instruments of different design, but similar mean optical power, were compared during a field test: The TRAGVIS sensor is an experimental, scientific development which was designed for particular needs of maritime search and rescue operations. The instrument uses pulsed VCSELs in the NIR, and a CMOS camera in multi-integration mode. As designed for distances < 400 m, a fixed focal length (wide angular FOV of ≈ 9° ) is used, and the repetition rate is high, while the pulse energy is low. The MODAR is a commercial multi-sensor platform comprising a Gated-Viewing instrument designed for security operations (e.g. police) both on sea and on land. Aiming at distances up to several kilometers, both camera and laser illumination are equipped with zoom optics, and the repetition rate is small, while the pulse energy is high. In contrast to TRAGVIS, an image intensifier is used. TRAGVIS and MODAR were compared in terms of signal-to-noise ratio (SNR) and image contrast using Lambertian reflectors at different distances. TRAGVIS was found to perform better than MODAR at distances < 350 m, but its performance decreases with distance while MODAR’s performance stays constant as a result of the laser and camera zoom. When used in ungated (continuous exposure) mode, TRAGVIS shows > 5 times larger SNR than in gated mode, and almost one order of magnitude larger SNR than MODAR due to the lack of an image intensifier. This demonstrates the instrument’s ability to be used for both, Gated-Viewing as well as simple active illumination mode. However, for the same reason (image intensifier) MODAR’s shutter suppression, which is crucial for reducing the back-scatter signal and therefore vision enhancement, was found to be at least 5-6 times better than that of TRAGVIS.
Our contribution presents a high bandwidth platform that implements traffic aggregation and switching capabilities for the Cherenkov telescope array (CTA) cameras. Our proposed system integrates two different data flows: a unidirectional one from the cameras to an external server and a second one, fully configurable dedicated to configuration and control traffic for the camera management. The former requires high bandwidth mechanisms to be able to aggregate several 1 gigabit Ethernet links into one high speed 10 gigabit Ethernet port. The latter is responsible for providing routing components to allow a control and management path for all the elements of the cameras. Hence, a simple, efficient, and flexible routing mechanism has been implemented avoiding complex circuitry that impacts in the system performance. As a consequence, an asymmetric network topology allows high bandwidth communication and, at the same time, a flexible and cost-effective implementation. In our contribution, we analyze the camera requirements and present the proposed architecture. Moreover, we have designed several evaluation tests to demonstrate that our solution fulfills the CTA project needs. Finally, we illustrate the general possibilities of the proposed solution for other data acquisition applications and the most promising futures lines of research are discussed.
The Gamma-ray Cherenkov Telescope (GCT) is one of the telescopes proposed for the Small Sized Telescope (SST) section of CTA. Based on a dual-mirror Schwarzschild-Couder design, which allows for more compact telescopes and cameras than the usual single-mirror designs, it will be equipped with a Compact High-Energy Camera (CHEC) based on silicon photomultipliers (SiPM). In 2015, the GCT prototype was the first dual-mirror telescope constructed in the prospect of CTA to record Cherenkov light on the night sky. Further tests and observations have been performed since then. This report describes the current status of the GCT, the results of tests performed to demonstrate its compliance with CTA requirements, and the optimisation of the design for mass production. The GCT collaboration, including teams from Australia, France, Germany, Japan, the Netherlands and the United Kingdom, plans to install the first telescopes on site in Chile for 2019-2020 as part of the CTA pre-production phase.
J. L. Dournaux, A. Abchiche, D. Allan, J. P. Amans, T. P. Armstrong, A. Balzer, D. Berge, C. Boisson, J.-J. Bousquet, A. Brown, M. Bryan, G. Buchholtz, P. Chadwick, H. Costantini, G. Cotter, L. Dangeon, M. Daniel, A. De Franco, F. De Frondat, D. Dumas, J. P. Ernenwein, G. Fasola, S. Funk, J. Gironnet, J. Graham, T. Greenshaw, B. Hameau, O. Hervet, N. Hidaka, J.A. Hinton, J.M. Huet, I. Jégouzo, T. Jogler, T. Kawashima, M. Kraush, J. Lapington, P. Laporte, J. Lefaucheur, S. Markoff, T. Melse, L. Mohrmann, P. Molyneux, S. Nolan, A. Okumura, J. Osborne, R. Parsons, S. Rosen, D. Ross, G. Rowell, C. Rulten, Y. Sato, F. Sayède, J. Schmoll, H. Schoorlemmer, M. Servillat, H. Sol, V. Stamatescu, M. Stephan, R. Stuik, J. Sykes, H. Tajima, J. Thornhill, L. Tibaldo, C. Trichard, J. Vink, J. Watson, R. White, N. Yamane, A. Zech, A. Zink
The GCT (Gamma-ray Cherenkov Telescope) is a dual-mirror prototype of Small-Sized-Telescopes proposed for the Cherenkov Telescope Array (CTA) and made by an Australian-Dutch-French-German-Indian-Japanese-UK-US consortium. The integration of this end-to-end telescope was achieved in 2015. On-site tests and measurements of the first Cherenkov images on the night sky began on November 2015. This contribution describes the telescope and plans for the pre-production and a large scale production within CTA.
A. Brown, A. Abchiche, D. Allan, J.-P. Amans, T. Armstrong, A. Balzer, D. Berge, C. Boisson, J.-J. Bousquet, M. Bryan, G. Buchholtz, P. Chadwick, H. Costantini, G. Cotter, M. Daniel, A. De Franco, F. de Frondat, J.-L. Dournaux, D. Dumas, G Fasola, S. Funk, J. Gironnet, J. Graham, T. Greenshaw, O. Hervet, N. Hidaka, J. Hinton, J.-M. Huet, I. Jégouzo, T. Jogler, M. Kraus, J. Lapington, P. Laporte, J. Lefaucheur, S. Markoff, T. Melse, L. Mohrmann, P. Molyneux, S. Nolan, A. Okumura, J. Osborne, R. Parsons, S. Rosen, D. Ross, G. Rowell, Y. Sato, F. Sayede, J. Schmoll, H. Schoorlemmer, M. Servillat, H. Sol, V. Stamatescu, M. Stephan, R. Stuik, J. Sykes, H. Tajima, J. Thornhill, L. Tibaldo, C. Trichard, J. Vink, J. Watson, R. White, N. Yamane, A. Zech, A. Zink, J. Zorn
The Gamma-ray Cherenkov Telescope (GCT) is proposed for the Small-Sized Telescope component of the Cherenkov Telescope Array (CTA). GCT's dual-mirror Schwarzschild-Couder (SC) optical system allows the use of a compact camera with small form-factor photosensors. The GCT camera is ~ 0:4 m in diameter and has 2048 pixels; each pixel has a ~ 0:2° angular size, resulting in a wide field-of-view. The design of the GCT camera is high performance at low cost, with the camera housing 32 front-end electronics modules providing full waveform information for all of the camera's 2048 pixels. The first GCT camera prototype, CHEC-M, was commissioned during 2015, culminating in the first Cherenkov images recorded by a SC telescope and the first light of a CTA prototype. In this contribution we give a detailed description of the GCT camera and present preliminary results from CHEC-M's commissioning.
Markus Lauscher, Pedro Assis, Pedro Brogueira, Miguel Ferreira, Thomas Hebbeker, Luís Mendes, Christine Meurer, Lukas Middendorf, Tim Niggemann, Mário Pimenta, Johannes Schumacher, Maurice Stephan
A sophisticated technique to study ultra-high-energy cosmic rays is to measure the extensive air showers they
cause in the atmosphere. Upon impact on the atmosphere, the cosmic rays generate a cascade of secondary particles,
forming the air shower. The shower particles excite the atmospheric nitrogen molecules, which emit fluorescence
light in the near ultraviolet regime when de-exciting. Observation of the fluorescence light with suitable
optical telescopes allows a reconstruction of the energy and arrival direction of the initial particle. Due to their
high photon detection efficiency, silicon photomultipliers (SiPMs) promise to improve current photomultipliertube-
based fluorescence telescopes. We present the design and a full detector simulation of an SiPM-based
fluorescence telescope prototype, together with the expected telescope performance, and our first construction
steps. The simulation includes the air showers, the propagation of the fluorescence light through the atmosphere
and its detection by our refracting telescope. We have also developed a phenomenological SiPM model based on
measurements in our laboratories, simulating the electrical response. This model contains the photon detection
efficiency, its dependence on the incidence angle of light and the effects of thermal and correlated noise. We have
made a full performance analysis for the detection of air showers including the environmental background light.
Moreover, we will present the RandD in compact modular electronics using photon counting techniques for the
telescope readout.
A sophisticated method for the observation of ultra-high-energy cosmic rays (UHECRs) is the
fluorescence
detection technique of extensive air showers (EAS).
FAMOUS will be a small
fluorescence telescope, instrumented with silicon photomultipliers (SiPMs) as highly-sensitive
light detectors. In comparison to photomultiplier tubes, SiPMs promise to have a higher photon-detection-efficiency. An increase in sensitivity allows to detect more distant and lower energy showers which
will contribute to an enrichment of the current understanding of the development of EAS and the chemical
composition of UHECRs.
A sophisticated technique to measure extensive air showers initiated by ultra-high-energy cosmic rays is by
means of fluorescence telescopes. Secondary particles of the air shower excite nitrogen molecules of the atmosphere,
which emit fluorescence light when they de-excite. Due to their high photon detection efficiency (PDE)
silicon photomultipliers (SiPMs) promise to increase the sensitivity of todays fluorescence telescopes which use
photomultiplier tubes - for example the fluorescence detector of the Pierre Auger Observatory. On the other
hand drawbacks like a small sensitive area, a strong temperature dependency and a high noise rate have to be
managed.
We present plans for a prototype fluorescence telescope using SiPMs and a special light collecting optical system
of Winston cones to increase the sensitive area. In this context we made measurements of the relative PDE
of SiPMs depending on the incident angle of light. The results agree with calculations based on the Fresnel
equations. Furthermore, measurements of the brightness of the night sky are presented since this photon flux is
the main background to the fluorescence signals of the extensive air showers. To compensate the temperature
dependency of the SiPM, frontend electronics make use of temperature sensors and microcontrollers to directly
adjust the bias-voltage according to the thermal conditions. To reduce the noise rate we study the coincidence
of several SiPMs signals triggered by cosmic ray events. By summing up these signals the SiPMs will constitute
a single pixel of the fluorescence telescope.
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