Direct range measurements offer considerable advantages over the triangulation methods both in terms of speed of operation and data-processing efficiency but tend to have a lower image resolution, particularly along the x-y plane, and to suffer from the wide dynamic range of the returned signal. This latter point leads to the requirement for sophisticated detection circuits, which adds to complexity and, to a lesser extent, cost but which helps to shift the sensor design emphasis towards hardware rather than software as in the case of triangulation-based 3-D vision sensors (such as stereo and structured-lighting ones) thereby allowing for the potentially higher speed of operation (e.g. the rangepics are measured not calculated thus avoiding a major step of the data manipulation software).
The majority of the research work in this field has been concentrated on scanning laser rangefinders, static rangefinders and ultrasonic range finders.
Scanning Optical Range Finder
In the area of optical 3-D devices, scanning laser rangefinder research has received wide attention during the last decade and spanned the field of robotics as well as those of aeronautics and military applications.
As shown in Figure 1, the technique relies upon mechanical scanning of the target object by a laser beam whose reflected light is processed to obtain the range data; the resulting depth map has, potentially, a resolution of less than a millimetre. This makes the scanning laser range finder an important 3-D vision sensor but its relatively high price and inherent mechanical design can be a drawback in applications where low cost and/or rugged sensors are required. This is thought to be one of the underlying reasons why scanning laser rangefinders have yet to find wide application in the robotics industry.
Figure 2 gives an example of the typical output provided by a laser rangefinder as measured by Lewis and Johnson in 1977, the lighter regions being closer to the sensor. Figure 2 (a) shows a range-picture of a chair, whereas Figure 2 (b) shows a range-picture of the same chair with a person sitting in it.