Optics: measuring and testing – By polarized light examination – With light attenuation
Patent
1987-02-10
1988-08-16
Evans, F. L.
Optics: measuring and testing
By polarized light examination
With light attenuation
356376, G01B 1130
Patent
active
047640162
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an instrument for measuring the topography of a surface. By the topography of a surface is meant both the micro and the macro irregularities in the surface.
The present invention utilizes the known principle of transmitting a beam of light onto the surface being measured and receiving the light reflected by the surface and therewith determine the topography or irregularities of the surface.
Such instruments are known to the art. These known instruments, however, are encumbered with several drawbacks, one of which is that the incident light impinges on the surface at a significant angle, for example 45.degree., and/or that the light reflected from the surface is received in a directional sense having a corresponding angle to the surface.
When the incident light impinges on the surface at a significant angle thereto, an error is obtained in the positional reading in the plane of the surface, hereinafter referred to as the horizontal positional reading. When traversing a totally flat surface in a direction perpendicular to the surface, the light spot produced on the surface by the incident beam will be displaced across the surface. Correspondingly, the light spot is displaced relative to the surface as a result of the depth profile of the surface. Thus, when the depth profile of a surface is measured with an instrument with which the incident light forms a significant angle with the surface and the surface is traversed in the horizontal plane, it is not known exactly where a given profile depth prevails, and hence an error is created in the horizontal positional reading, as before mentioned.
Another serious disadvantage is that when the reflected light is received by an optical system in which the optical axis thereof forms a significant angle with the surface, e.g. 45.degree., the vertical dissolution will be relatively poor, owing to the fact that the projected surface of the light spot becomes large in relation to the profile depth. This greatly influences the total resolving power of the instrument.
Another drawback experienced when the incident light beam is permitted to impinge on the surface at a significant angle thereto, e.g. 45.degree., is that the mean size of the light spot over a surface to be measured becomes greater than the diameter of the incident light beam.
Present day instruments of this kind are intended to measure relatively flat surfaces, where the profile depth is of the order of micrometres. In this measurement range the aforedescribed drawbacks become highly significant.
These drawbacks are eliminated, or at least greatly alleviated, by means of the present invention, which provides an instrument with which very high resolution can be achieved.
Thus, the present invention relates to an instrument for measuring the topography of a surface, comprising a light transmitting device for transmitting a light beam onto the surface; a light receiving device for receiving light reflected by the surface; a detector for detecting the position of the received light relative to the optical axis of the light receiving device; and means for providing relative movement between on one hand the light transmitting and light receiving devices, and on the other hand the surface to be measured, the instrument being characterized in that the optical axis of the light transmitting device is arranged to form a small angle with the normal to the median plane of a workpiece surface to be measured, said angle being smaller than about 15.degree., and preferably smaller than about 5.degree.; in that the optical axis of the light receiving device forms a right angle, or substantially a right angle, with the optical axis of the light transmitting device; in that said detector is position-responsive; and in that an image magnifying lens is located in front of the detector.
The invention will now be described in more detail with reference to an exemplifying embodiment thereof illustrated in the accompanying drawing, in which
FIG. 1 illustrates schematically an instrument according to t
REFERENCES:
patent: 3625618 (1971-12-01), Bickel
Bengtsson Anders
Evans F. L.
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