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.
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