Range-gated Active Imaging System Research Articles

Direct method of three-dimensional imaging using the multiple-wavelength range-gated active imaging principle.

The tomography executed with mono-wavelength active imaging systems uses the recording of several images to restore a three-dimensional (3D) scene. Thus, in order to show the depth in the scene, a different color is attributed to each recorded image. Therefore, the 3D restoration depends on the video frame rate of the camera. By using a multiple-wavelength range-gated active imaging system, it is possible to restore the 3D scene directly in a single image at the moment of recording with a video camera. Each emitted light pulse with a different wavelength corresponds to a visualized zone at a different distance in the scene. The camera shutter opens just once during the emission of light pulses with the different wavelengths. Thus, the restoration can be executed in real time with regard to the video frame rate of the camera. From an analytical model and from a graphical approach, we demonstrated the feasibility of this new method of 3D restoration. The non-overlapping conditions between two consecutive visualized zones are analyzed. The experimental test results confirm these different conditions and validate the theoretical principle to directly restore the 3D scene in a color image with a multiple-wavelength laser source, an RGB filter, and a triggerable intensified camera.

Influence of gating and of the gate shape on the penetration capacity of range-gated active imaging in scattering environments.

Range-gated active imaging is a well-known technique used for night vision or for vision enhancement in scattering environments. A lot of papers have been published, in which the performance enhancement of range gating has been demonstrated. However, there are no studies which systematically investigate and quantify the real gain brought by range gating, in comparison with a classical imaging system, in controlled smoke densities. In this paper, a systematic investigation of the performance enhancement of range-gated viewing is presented in comparison with a color camera representing the human vision. The influence of range gating and of the gate shape is studied. We have been able to demonstrate that a short-wave infrared (SWIR) range-gated active imaging system can enhance by a factor of 6.9 the penetration depth in dense smoke. On the other hand, we have shown that the combination of a short pulse with a short integration time gives better contrasted images in dense scattering media.

Theoretical and experimental comparison of flash and accumulation mode in range-gated active imaging

Range-gated active imaging has significantly been improved in the recent past. Due to constraints imposed by the different types of sensors, systems using laser diodes as illumination sources are working in “accumulation” mode and systems using solid-state lasers are working in “flash” mode. Consequently, the type of source (diode or solid-state laser) gives basic differences in the behavior of the two types of systems. We systematically investigated the theoretical and practical differences between the two modes to point out their advantages and weaknesses. Parameters such as image quality, sensitivity to day light or stray light, fog penetration capacity, sensitivity to turbulence, and laser safety are examined. For comparative experimental purposes, we have built a range-gated active imaging system that allows the investigation of both illumination methods on the same sensor. We have carried out precise comparative studies between the two acquisition methods. Some differences are pointed out that have to be taken into account when designing a range-gated active imaging system in function of the desired performances. In particular, this work will help to design a new range-gated underwater active imaging system.