There are three prior techniques to which the invention is directed.
The first technique calculates the range from time lag in which light or acoustic wave is projected, reflected and received. Though it is easy to measure the range to one point by using this technique, it is difficult to measure ranges to many points in a short time.
The second technique picks up one point having characteristics in one image formed in one camera and another point having the same characteristics in another image formed in another camera, and calculates the ranged from these two points according to triangulation. Because it is difficult to determine the other point having the same characteristics, it is not easy to measure range by using this second technique.
The last technique projects a spot or a slit of light from one side, extracts the image of that spot or slit formed in a camera on another side, and calculates the range to the image according to triangulation while the spot or slit is moved. There remain such problems as the existence of points which cannot be reached by the spot or slit, as it is occasionally difficult to extract the image of the spot or slit because of round brightness, and as it is difficult to measure range with high accuracy because the spot or the slit has some area.
These three prior techniques are detailed in "Visual Sensor and Environmental Recognition" printed in monthly magazine "Sensor Technology" published in January of 1982 in Japan.