1. Field of the Invention
The present invention is directed to an optical measurement system for determination of an object's profile or thickness, and more particularly to such an optical measurement system using two optical heads directing individual light beams to different points on the object's surface to measure distances of these points from a reference plane by triangulation for analyzing the surface profile or the thickness of the object based upon the measured distances of the two points on the object's surface.
2. Description of the Prior Art
In order to obtain a depth or height of a step on the surface of an object or thickness of an object by optical triangulation measurement, it has been proposed to use a pair of optical heads disposed to direct individual light beams to different points on the object's surface for measuring the positions of these points. The distances of these points are processed by triangulation and are analyzed to determine the object profile. For example, when the two heads are disposed to measure the positions of the points spaced along the object's surface for measuring individual perpendicular distances to the surface from a reference plane, the difference of the measured distances gives the height or depth of a step existing between these two points. On the other hand, when the optical heads are disposed on the opposite of the object to measure like perpendicular distances of the positions of two points on the opposite surfaces of the object from a reference plane selected to be within the thickness of the object, the addition of the measured distances gives a thickness of the object at these points.
In such optical measurement systems, the optical head is normally designed to have a photo-sensor which receives the light beam reflected on a point on the object's surface and provides an output which varies in proportion to the perpendicular distance of the point from a reference plane selected to be generally parallel to the object's surface. The output from the head is processed in an associated signal processing circuit so as to determine a true distance of the point from the reference plane. In this connection, when the two heads are connected to the individual signal processing circuits, there is a potential problem that the distances measured in these separate processing circuits may include individual deviations or discrepancies due to inherent variations in the circuits, for example, deviations in the temperature characteristics of certain elements consisting the circuits. Since these discrepancies are inherent to the individual circuits, they are difficult to be compensated for in obtaining the step in the object's Surface and the thickness of the object. Thus, no reliable analysis is not expected in this system having two optical heads connected respectively to the individual processing circuits.