Optics: measuring and testing – By polarized light examination – With light attenuation
Patent
1987-12-30
1990-07-31
Evans, F. L.
Optics: measuring and testing
By polarized light examination
With light attenuation
2504581, G01B 1106
Patent
active
049445936
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention is directed to a volume-measuring method for surface depressions.
DESCRIPTION OF THE PRIOR ART
In measurements at surfaces or, respectively, surface depressions, a distinction must be made between roughness (order of magnitude: <=5 .mu.m), ripple (10-100 .mu.m) and shape (<=10 .mu.m). The dissertation by G. Thurn, "Automated Surface Testing by Computer-Assisted Measurement of the Scattered Light Distribution", Technical University of Berlin, 1984, provides an overview of the standard mechanical and optical methods for the identification thereof.
Mechanical sensing with a diamond sensor covers roughness, ripple and shape in accord with the filter characteristics of the measuring arrangement employed but is relatively slow (sensing rate =<0.5 mm per second) and leaves traces behind on the surface.
The optical methods which are disclosed, for example, by Brodmann, "Surface Roughness Measurement of Turned and Ground Surfaces with a Light Scattering Instruement", third International Conference on Metrology and Properties of Engineering Surfaces, Middlesborough, Apr. 10 through 12, 1985; or Baker, L.R., "Comparator Technique to Measure Micro Height Variations", third Conference on Metrology and Properties of Engineering Surfaces, Middlesborough, Apr. 10 through 12, 1985; or Vorburger, T.V., "Optical Measurement of Surface Roughness", third Conference on Metrology and Properties of Engineering Surfaces, Middlesborough, Apr. 10 through 12, 1985, have the advantages of a non-contacting and fast sensing but, due to the short light wavelength employed, are only suitable for roughness measurements. A height profile or, respectively, statistical parameters such as height distributions or, respectively, surface angle distributions are obtained, Thomas, T.R., "Characteristic of Surface Roughness, Precision Engineering", pages 97ff, IPC Business Press 1981, being referenced in this respect.
A case of volume measurement of surface depressions which often occurs in practice arises in the manufacture of printing forms for rotogravure printing. The surface of this printing form, usually an engraved or etched impression cylinder, comprises a plurality of small cups (depressions) distributed in accord with the later printing raster which are separated from one another by webs. The rotogravure process with this printing form is executed such that the small cups are filled with ink, the excess ink is removed with a ductor that is carried by the webs, and the printing material is brought into contact with the surface of the impression cylinder, whereby the printing ink is transferred from the small cups onto the printing material.
The transferred ink volume determines the tint value of the respectively printed point, for which reason the volumes of the individual cups are a matter of concern in the manufacture of the printing form. When engraving these cups with a diamond stylus, the cup volume was previously to identified by measuring the cup diagonals on the copper surface, whereby the volume was identified with approximation formulas from the shape of the diamond stylus and the geometry of the printing raster. In accord with a known measuring procedure, a microscope is employed for this purpose, this microscope being placed on the surface of the printing form. This measurement, wherein a respective plurality of cups are measured for a plurality of different gray tints wastes valuable time, for example on the order of half an hour. The production outage connected with this time outlay makes it obvious that a measuring principle be sought which makes dynamic measurements possible. This demand is further intensified in that the shape of the engraved cups is not established by the geometry of a stylus given a more recent electron beam engraving method as was the case, for example, when engraving the cups with the diamond stylus. A direct measurement is therefore desirable for identifying the engraved volume.
SUMMARY OF THE INVENTION
It is therefore an object of the p
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Kalstroem Reiner
Schulz-DuBois Erich
Dr. -Ing. Rudolf Hell GmbH
Evans F. L.
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