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.
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.
A novel gated-viewing instrument is presented for vision enhancement in maritime search and rescue applications predominantly under limited visibility conditions at night. The compact device consists of a camera and an eye-safe NIR (near-infrared) illuminator and has a field of view of ≈7° x 6°, which is similar to field glasses. The detection range is 250 m for Lambertian reflectors, but is much larger if clothes with retro-reflectors are worn. A key challenge is the cost effectiveness of the instrument as potential users in the field of maritime search and rescue applications usually suffer from financial limitations. As a result, no image intensifier, but an off-the-shelf CMOS camera in accumulation mode with a reasonable quantum efficiency in the NIR region is used. The active illumination is based on a self-developed illuminator consisting of 7 pulsed vertical-cavity surface-emitting laser (VCSEL) arrays. The mean optical power is 7 W, the center wavelength is λ≈804 nm, and the light pulse width is ≈100 ns at a repetition rate of 345 kHz. Detailed simulations leading to the system design are presented together with respective characterization measurements of the camera and illuminator as well as first test measurements of the complete system.
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