Gated viewing in the atmosphere: a study of performance limits

Abstract

The main purpose of the work presented here is to study the potential for an active imaging system for target recognition at long distances. This work is motivated by the fact that there are a number of outdoor imaging needs where conventional passive electro optical (EO) and infrared (IR) imaging systems are limited due to lack of photons, disturbing background, obscurants or bad weather. With a pulsed illuminating source, several of these problems are overcome. Using a laser for target illumination, target recognition at 10's of km can be achieved. Powerful diode pumped lasers and camera tubes with high spatial and time resolution will make this technique an interesting complement to passive EO imaging. Beside military applications, civilian applications of gated viewing for search and rescue, vehicle enhanced vision and other applications are in progress. To study the performance limitations of gated viewing systems due to camera, optics and the atmosphere an experimental system was developed. Measurements up to 10 km were made. The measurements were taken at the wavelength 532 nm. To extrapolate the results to future system performance at an eye safe wavelength, 1.5 µm nm, a theoretical performance model was developed. This model takes into account the camera and atmospheric inßuence on resolution and image quality, measured as a signal-to-noise-ratio, SNR. The result indicates turbulence inßuence, in agreement with the modelling. Different techniques were tested for image quality improvement and the best results were obtained by applying several processing techniques to the images. Moreover, the tests showed that turbulence seriously limits the resolution for horizontal paths close to the ground. A tactical system at 1.5 µm should have better performance than the used 532 nm in atmospheric-limited applications close to ground level. The potential to use existing laser range Þnders and the eye safety issue motivates the future use of 1.5 µm for gated viewing.

Speed Measurement of the Moving Targets Using the Stepping Equivalent Range-Gate Method

“…Range-gated technology is widely used in array imaging lidar systems, which uses a controllable pulse laser and a selective intensified-charge-coupled device (ICCD) that can be opened and closed at high speed to separate the reflected and backscattering light from targets at different distances [ 10 , 11 ] so that the laser pulse signals reflected from the target to be measured will reach the camera and be imaged in exactly the selective time of the ICCD. During lidar imaging, poor imaging quality is due to backward-scattering by atmospheric, water and other media in the optical path.…”

Overview of range gated imaging at FOI

“…Using laser illumination at 1.5 m, this range can be extended to about 25 km. Both image tube technology 1 and avalanche photodiode arrays 2 are being considered for range gated imaging 3 . The gating gives a possibility to extract the target from the background but also to reduce the effect of the optical background light during the short gate.…”

Image quality for range-gated systems during different ranges atmospheric conditions

“…Using laser illumination at 1.5 µm, this range can be extended to about 25 km. Both image tube technology 1 and avalanche photodiode arrays 2 are being considered for range gated imaging 3 . The gating gives a possibility to extract the target from the background but also to reduce the effect of the optical background light during the short gate.…”