Method and Apparatus for Highly Accurate Distance Measurement With Respect to Surfaces

The invention relates to a method of determining the profile of a surface using contactless distance measurement, and to an apparatus for also determining the profile of a surface using contactless distance measurement, wherein a light pencil which has been scattered and reflected from the surface is split into two light pencils having substantially the same distribution of intensities in a plane transverse to the direction of propagation.

A method and an apparatus for determining the profile of a surface using contactless distance measurement are known from Ger. OS 34 35 033. According to that document the method of measuring heights and distances in the plane of an object may be carried out without having to focus the imaging optics if the arrangement follows the Scheimpflug rule. In this way, most importantly, the accuracy of the determination can be improved by linearly moving a light spot on the photodetector. The light spot which is the smallest which can be maintained in place is accordingly produced with an edge focused onto the surface to be measured and from there, is imaged on the photodetector. When a laser is used as the light source, a rough surface causes a high level of noise in the measured value, particularly in the form of numerous so-called "speckles" in the light spot being imaged on the photodetector. The resulting noise in the photodetector signal is greater when the light spot on the surface which is to be measured is made smaller. For roughness measurements, the diameter of the spot is a few to several microns; and for this reason a laser light should be used. However, the ability to form the image of a sharp edge is limited because of the smallness of the light spot.

It is known from Ger. Pat. 2,945,251 that changes in distance, and thereby surface profiles, can be measured very precisely using a triangulation method. For this purpose, a laser beam is directed obliquely on the surface to be measured. With the aid of optics oriented perpendicularly with respect to the surface, the light spot is imaged onto a position-sensitive photodetector. The light spot imaged onto the photodetector shifts proportionally to the change in distance of the sample. With the aid of a differential photodiode or a row of photodiodes acting as the photodetector means, the shift in the light spot and thereby the change in the distance from the laser to the surface to be measured is determined, e.g. by electrical means. The principal shortcoming of this method is that when a rough surface is measured using laser light, the light spot is broken up into numerous "speckles", leading to appreciable noise in the measured values.

Sensitive measurement systems have a small scope of measurement. It is therefore a general problem to bring the measuring surface into the existing measuring zone. This problem is, for instance, solved in Morita, U.S. Pat. No. 4,732,485, by using a supplemental measurement system for bringing the measurement surface into a correct position.

Another problem with the prior art is the effect of reflections on the measurement system. Since the diodes will measure any kind of intensity alteration, certain reflections from the measuring surface occurring, when not a surface roughness, but a depression or elevation, completely outside of the scope of measurement cannot be distinguished and will be seen as measured values.

The present invention begins with and then departs from the state of the art set forth above. A primary object of the present invention is to solve the foregoing problems associated with the prior art by reducing the laser noise or speckle noise in the measured values by using a triangulation method, such that even in the case of a very small light-spot diameter one can measure distance changes with high precision. This should open up the possibility of determining surface roughness using such measurements.

This problem is solved according to the invention by providing a method and apparatus for splitting a light pencil which has been scattered and reflected by the surface into two light pencils having substantially the same distribution of intensities in a plane transverse to their respective principal directions of propagation. Under these conditions the ratio of the intensities of the two partial pencils is dependent on the principal beam direction of the light pencil scattered and reflected from the surface. Accordingly, one or both of the partial pencils can then be detected by photodiode means and a resulting signal from the photodiode means evaluated as a measure of the roughness.

To illustrate the invention, reference is made to the following description of exemplary embodiments, and the drawings which depict them schematically, which embodiments and drawings serve for more detailed explanation.

FIG. 1 shows a first embodiment of the invention which a deflecting prism for angle-dependent splitting; and

FIG. 2 shows a second embodiment of the invention, having two deflecting prisms which are arranged and operate to deflect away from one another, and further having a sum-and-difference amplifier connected to the two photodetectors disposed downstream in the optical path.

FIG. 1 shows a laser diode 1 the collimated laser beam 2 of which is directed by imaging optics 3 to the test surface 4 which is to be measured. The scattered and reflected light pencil 5 is then converted to a substantially parallel light pencil 7 by means of a collimator 6. This pencil 7 then strikes a deflecting prism 8, which splits pencil 7 into two partial pencils (9, 10) having nearly identical distributions of intensity in a plane normal to their respective main beam direction, that is, the primary direction of propagation of each partial pencil. Regarding the main beam direction of the pencil 7, the angle of incidence on the hypotenuse surface of deflecting prism 8 is preferably set at the transition to total reflection. It is understood that the hypotenuse surface represents an interface between the dielectric medium which forms the prism 8 and a less dense dielectric medium surrounding the prism 8. It is further understood that the angle between the beam 2 and the surface 4 may be selectively adjusted. The ratio of the overall intensities (in each case summed over the cross section) of the two partial pencils (9, 10) is then dependent only on the angle of incidence, and therefore on the main beam direction of the pencil 5 comprised of light scattered and reflected from the surface. The reflected partial pencil 10 is then passed to a first photodetector 12, and/or the transmitted partial pencil is passed to a second photodetector 11. The electrical signal outputs from the two photodetectors (11, 12) (which may comprise, e.g., photodiodes or linear photodiodes or photo cameras or a photodiode array) may be employed in such a way that the signal of one of the two photo detectors (11 or 12) is maintained at a constant level (e.g. by changing the intensity of laser diode 1) and the signal of the other photodiode (11, 12) is measured and displayed. In this way, compensation is provided for changes in the reflectivity of the surface 4. Another possibility is to maintain constant the sum of the signal intensities of the two photodetectors and to measure and display the difference. In this case, the outputs from the two photo detectors (11,12) may be connected to a sum-and-difference amplifier.

FIG. 2 shows, similarly to the exemplary embodiment of FIG. 1, a laser diode 1, the collimated laser beam 2 from which is directed by imaging optics 3 onto the surface 4 of the test article which is to be measured, and the pencil 5 of scattered and reflected light is then transformed by a collimator 6 into a substantially parallel light pencil 7. Pencil 7 is then split into two equal partial pencils which are then deflected at the hypotenuse surfaces of two respective pairs of prisms (14, 15; 16, 17) disposed downstream in the optical path. The angle of incidence on the hypotenuse surfaces is preferably at the transition to total reflection. The prisms (14, 15; 16, 17) are adjusted such that when the angle of the light pencil 7 is changed, the angle of incidence of one of the partial pencils approaches the total reflection angle, while the angle of incidence of the other partial pencil progresses farther from the total reflection angle. In this regard, the prism pairs (14, 15; 16, 17) operate appositely with regard to one another. The two light pencils, each of which has been reflected twice, are each passed through a respective lens (18; 19) to a respective photodiode (20; 21). The transmitted part need not be utilized in the present exemplary embodiment. The signals are then processed as in exemplary embodiment 1. The output from the photodiodes (20;21), for example, can be connected to a sum-and-difference amplifier 30 as indicated above. The adjustability of the prism pairs affords free selection of the operation point on the transmission/reflection curve, thereby enabling optimal exploitation of the sensitivity of the method.

In another embodiment of the invention, at least one extra diode (as shown, for instance, as element 24) is positioned beside the first diode of the measuring system and therefore provides a signal when the measuring surface is outside of, or leaves, the scope of measurement of the first diode. Likewise, an extra diode 25 may be positioned next to the second diode. This simplifies the procedure of bringing the measuring surface into the right position within the scope of measurement and avoids measuring reflections as actual measuring values.

Whenever the light pencil moves out of the diode surface area the extra diode will provide a signal, making it possible to eliminate all measuring signals received from the measuring system during the out range situation.