In order for a machine to have the capacity to operate flexibly in a 3-D environment, it is indispensable for it to be equipped with space information acquisition capability, and tools for distance measurement are in turn indispensable for obtaining space information. Indeed distance measurement is basic and important not only for a robot, but also for science and technology in general. Many methods have been proposed for obtaining distance information, ranging from the mechanical contact type through optical and acoustic to electric and magnetic methods, and many are in practical use. Among them the optical method permits measurement of distance without contact and from a remote position, advantages which have led to it being widely applied. One of the most important principles for measuring distance is the triangulation principle. This permits determination of the position of an object from the distance between two observation points together with the angles in the triangle formed by these two points and a target point on the object. Based on this principle, the detection of one specific point in each of the two images obtained from two sets of image input equipment installed at two observation points permits determination of coordinate values in 3-D space. However, this extraction of the point in the second image corresponding to a specified point in the first image is a very difficult subject of study, and no universal method has been developed. To cope with this, active methods, which evade the problem by applying projection of laser light on the surface of an object to identify a bright point or bright line, are widely used. The special feature articles on obtaining 3-D optical information in this issue present some principles and new trial applications of distance acquisition methods for 3-D information, the optical method in particular. There are three reports on active method optical systems developed for robots. These include (1) a high speed measurement method applying space encoding which employs a liquid crystal lattice to project light in changing lattice patterns onto an object dynamically; (2) realization of high speed measurement through projecting and processing multiple light spots; (3) development of a visual sensor for disaster prevention use which can detect objects in flames and smoke utilizing projection of a CO2 gas laser. These are nearly at the level of operational use and are expected to become visual sensors for robots.
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