Optics: measuring and testing – By polarized light examination – With birefringent element
Patent
1994-12-19
1996-12-10
Turner, Samuel A.
Optics: measuring and testing
By polarized light examination
With birefringent element
356 73, 356359, G01B 902
Patent
active
055836399
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
It is previously known to measure the flatness of polished metal surfaces by interference methods or by reflection methods. Both methods have specific limitations which makes it difficult to fully utilize them. The present invention combines both methods, which improves the accuracy and is easier to automatize than either method alone.
Interference methods utilize preferably monochromatic light, which is split in two optical rays, one reflected from the surface to be measured and the other reflected from a high quality flat reference surface. The two optical rays are merged and form an interference pattern of dark and light stripes, where the distance between stripes is reversely proportional to the angular misalignement and the ratio between the curvature of the stripes and their distance is proportional to the height profile. A limitation with interference methods is that if the angles in the optical paths are chosen to give great distance between stripes, the height profile can be measured with high accuracy but only along a few lines on the surface, and if the angles are chosen to give shorter distances between stripes, the height profile can be measured along several lines on the surface but with lesser accuracy. There is a considerable risk of overlooking local defects.
Reflection methods illuminate the surface from a point source of light, and in an image plane there is a continuous two-dimensional light distribution related to the flatness deviations for all points of the surface. The light distribution can be converted by mathemathical methods to a map of the deviation from flatness, and local smoothness defects such as scratches and grinding marks are clearly visible. For the mathematical conversion it is important to know the angular alignment of the surface very accurately.
SUMMARY
According to the invention an interference method is used to orient the surface with high accuracy and to give a general view of skewness and bulging of the surface, and thereafter a reflection method to give a detailed overall picture of grinding marks, scratches and other local deviations from smoothness. A device employing a combination of both methods will give comprehensive and accurate information about the flatness and smoothness deviation of the surface, and can be automated for checking large numbers of items from a production line.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 indicate in principle the location of the components employed, relative to the surface of the item to be checked. FIG. 1 shows a device where the item (10) to be checked is directly illuminated by a laser light source (11) as well as a non-coherent light source (21), and FIG. 2 shows a device where the light sources illuminate a virtual image (24) created by reflection of the real item (20).
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The laser light source (11) forms part of a laser interferometer also comprising a semi-transparent mirror (12), a reference surface (13) and a first image plane (14). The reference surface (13) can be an approved item of the same kind as the item (10) to be checked, or some other master surface of the desired shape. The laser beam is split by the semi-transparent mirror (12) into two rays, a first ray travelling from the laser light source (11) to the mirror (12), to the reference surface (13), through the mirror (12) to the image plane (14). The second ray travels from the laser light source (11) through the mirror (12) to the item (10), to the mirror (12) and to the image plane (14), where the two rays interfere to create a pattern of dark and light stripes. The distance between stripes depends on the distance between the points on the mirror (12) intersected by a normal to the surface of the item (10) and a normal to the reference surface (13), the distance between stripes becoming very large when the points on the mirror (12) coincide. The general direction of the stripes depends on how much the normal to the surface of the item (10) deviates from a plane co
REFERENCES:
patent: 4715709 (1987-12-01), Sekine et al.
patent: 4854708 (1989-08-01), Kafri et al.
Ingenjorsfirman Tomas Rostvall
Kim Robert
Turner Samuel A.
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