With long working distance laser imaging systems, the laser objective lens cannot be incorporated as an optical element of part viewing optics. LWD objectives are typically not chromatically corrected for the visible, therefore, it is difficult to obtain simultaneous focus of the laser beam and camera image. In addition, LWD objectives do not permit high magnification. The general approach to part viewing with LWD beam delivery is to employ a completely separate part viewing system.
One approach to LWD part viewing is to position the viewing system slightly off-axis to avoid obstructing the laser beam. This permits high contrast viewing using a zoom-enabled camera/microscope system. In addition, the camera can be positioned so that the large laser exposure areas are achievable without interfering with the camera. The main disadvantage to this approach is the parallax error inherent in off-axis viewing.
Another approach to LWD part viewing is to utilize a mirror with a central opening to permit on-axis viewing. The laser beam is aligned to pass through this opening without contacting the mirror. This eliminates parallax errors associated with off-axis viewing. One disadvantage to this technique is a loss of contrast in the part image due to the central obscuration in the viewing mirror. Another is that the exposure area of the laser beam is limited by the opening in the mirror. As a result, this technique is limited to applications in which the laser beam size is small.