Extending the 3D range of a short-wave infrared laser-gated viewing system capable of correlated double sampling
Primarily, a laser gated-viewing (GV) system provides range-gated 2D images without any range resolution within the range gate. By combining two GV images with appropriately overlapping range gates, 3D information within a part of these range gates can be obtained. The depth resolution is higher (super-resolution) than the minimal gate shift step size in a tomographic sequence of the scene. For a state-of-the-art system with a typical frame rate of 30 Hz, the time difference between the two required GV images is approximately 33 ms which may be too long in a dynamic scenario with fast moving objects. Therefore, in a previous work, we have applied this approach to the reset and signal level images of a short-wave infrared laser GV camera whose read-out integrated circuit is capable of correlated double sampling originally designed for the reduction of kTC noise (reset noise). This camera consists of a 640 x 512 avalanche photodiode focal plane array based on mercury cadmium telluride with a pixel pitch of 15 mm. The great advantage of this idea is the fact that these images are extracted from only one single laser pulse with a marginal time difference in between. This allows 3D imaging of fast moving objects. However, a drawback of this method is the very limited 3D range in which 3D reconstruction is possible. In this paper, we describe and discuss two measures to extend the 3D range. First, refining the algorithm for 3D reconstruction is investigated, particularly using a quadratic model instead of a linear model as in previous work. Second, we use an illumination laser with longer pulse duration than before to study the influence of laser pulse length on 3D range in real experiments. Based on these measured data, we simulate further temporal stretching of the laser pulse, to evaluate the potential of this approach to extend the 3D range.