Diffraction Limited Photogrammetry With The NPL Centrax Camera System

The NPL Centrax camera has been designed for ultra-precise work in close range photogrammetry. For this type of application it is seldom satisfactory to rely on natural object features, such as surface texture or edges, and in most cases specially designed targets are used to define the points being studied. If these targets are made small, symmetric and self-luminous, then it becomes a useful alternative to dispense with conventional photographic imaging and to redesign the lens so that the images which it produces, although larger than usual, have their positions sharply defined by a central diffraction pattern. These central patterns are referred to as "axicon images"; each of them contains a small bright spot surrounded by several concentric diffraction rings of almost regular spacing and gradually decaying intensity (Figure 1).