Optics: measuring and testing – By polarized light examination – With light attenuation
Patent
1995-07-11
1997-09-30
Rosenberger, Richard A.
Optics: measuring and testing
By polarized light examination
With light attenuation
G01B 1114
Patent
active
056731116
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method for measuring distance according to the preamble of claim 1.
The present invention also relates to an arrangement for measuring distance according to the preamble of claim 5.
STATE OF THE ART
Non-contact methods of measuring lengths, thicknesses and distances are used in many industrial applications. With these measurements a high measuring speed is often required in combination with high measuring precision. Amongst the measuring equipment which meets these requirements there are arrangements which are based on a method using a type of triangulation measurement with the aid of lasers.
The principle of triangulation measurement is well known and its use in a laser distance measuring device will therefore only be described briefly:
The monochromatic light from a laser source is focused into a narrow beam and illuminates a measurement area on a measured object. A receiver is placed close to the laser source, said receiver thus "seeing" the measurement area from the side. The receiver is provided with a detector which detects the light reflected from the measurement area and thereby also its position on the detector. When the distance between the measuring arrangement and the measured object changes the image of the measurement area will be displaced on the surface of the detector. Since the position of the detector with respect to the measurement beam is known, the distance between the measuring arrangement and the measured object can easily be calculated using known trigonometrical methods. The equipment can alternatively be calibrated by measurements of known distances
The monochromatic laser light allows good possibilities of generating a narrow and well-defined measurement beam. The measurement area which is illuminated by the beam will of course have a certain extension in space. When the position of the measurement area is to be determined, it is therefore always necessary to take the measurement to the same point within this. The capability of the measurement equipment to determine the location of this measurement point becomes the determining factor for the measurement precision which can be obtained.
In a well-focused laser beam the distribution oft he light intensity is symmetrical taken from the centre of the beam. It is therefore comparatively simple to determine the position of the centre of the beam by measuring the light intensity. This normally occurs by firstly determining the position of the points where the light intensity is 50% of the maximum. Since the middle point is positioned midway between these points its position can be easily determined therefrom. This middle point could therefore be used as the measurement point.
A problem occurs however when the position of the measurement point is to be determined by means of a detector which measures the light reflected from the measurement area. When monochromatic laser light impinges upon the surface it is diffusely reflected. Since the diffuse reflection is influenced by the surface finish of the measured object the reflection in different directions will vary randomly. The light from different points on the surface will therefore interfere. This means that the measurement area for an observer will appear "grainy", a phenomenon which is well known under the term "speckle". From a measurement point of view the phenomenon is serious since this means that the determination of the 50%-points--and thereby also the measurement point--will be randomly dependent on the appearance of the measurement area. This uncertainty in the determination of the measurement point thus limits the precision of the measuring system.
SUMMARY OF THE INVENTION
The object of the invention is therefore to define a method which reduces the effect of "speckle" and which thereby improves the measurement precision for laser distance measurement devices.
A further object of the invention is to arrive at a laser distance measurement device where the measurement precision is improved due to the fact that the inf
REFERENCES:
patent: 4973152 (1990-11-01), Brunk
patent: 5056922 (1991-10-01), Cielo et al.
patent: 5307207 (1994-04-01), Ichibara
Limab Laser & Instrumentmekanik AB
Rosenberger Richard A.
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