This chapter describes 2D and 3D LADAR systems that perform an imaging function in addition to ranging. A 2D system is one that captures an image of the target area between a minimum and maximum range. This process of selecting a set of ranges through which to form an image is referred to as gated viewing. A true 3D imaging system is one that forms images of the target area at multiple range gates. As with digital cameras or computer monitors, the size of the 3D image is determined by the size of the array containing the picture elements, or pixels (e.g., 256 x 256 indicates an image with 256 rows and columns each). For each pixel, the measured range provides the third dimension in the image. The basic function of 3D imagers is finding the range to each of the smaller elements or pixels that will be combined to make the final, multipixel image. The factors affecting the accuracy of single-range measurements were discussed in previous chapters. This chapter will discuss some additional factors that must be considered when producing an image using different imaging system concepts.
5.1 Single-Pixel Scanning Imagers
For a single-point imaging system used to build a 3D image, a single-pixel range is obtained for each laser pulse. This type of system is also referred to as a 1D LADAR system. Some mechanism must be employed to move or "scan" the aim point of the sensor over the area of interest, fill in the array, and create the image. The most common method of scanning the scene is with mirrors attached to mechanical actuators, such as electromagnetic coils, where the scan angle is proportional to the applied voltage.