Optics: measuring and testing – By polarized light examination – With light attenuation
Patent
1996-05-08
1997-09-09
Evans, F. L.
Optics: measuring and testing
By polarized light examination
With light attenuation
356383, 356384, G01B 1102
Patent
active
056662041
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for optical shape measurement of oblong objects.
The invention also concerns an apparatus for implementing the method.
When the shape of oblong objects such a boards is measured optically, a problem is caused by the geometrical constraint that the shape of the objects to be measured usually differs substantially from the scan aspect ratio (e.g., square) of the measurement apparatus. If the entire object being measured can be accommodated in the scan area, the relative measurement accuracy for its width dimension will be inferior to the relative measurement accuracy for its length dimension by an order of magnitude.
Attempts have been made to overcome this problem by adapting a multiplicity of cameras in a row along the longitudinal direction of the object being measured. Such an embodiment is, however, handicapped by the great number of equipment required and the difficulty of joining the pictures together.
Another prior-art technique is to record the object moving past the scan area of the imaging system as a series of sequential pictures. This arrangement in turn is hampered by the difficulty of arranging a sufficiently long, undisturbed travel for the object being measured.
FI Pat. No. 68,910 discloses equipment capable of overcoming several problems in optical measurement methods and providing an essentially improved apparatus for optical shape measurement of oblong objects.
The equipment described in cited FI Pat. No. 68,910 is based on distorting the picture by means of a mirror or lens having a different radius of curvature in the different main directions so that the measurement camera "sees" the width and length dimensions of the object being measured as having an order of equal magnitude, while the image field of depth is retained by using an elongated slit aperture in the camera optics. With the help of the curved mirror, even the image of a substantially long object can be distorted so as to fit in the scan area of the measurement camera as the camera sees the object image reflected from the curved mirror shorter than in nature. Moreover, such distorted imaging with object length contraction makes the relative measurement accuracy equal in both the width and length dimensions of the object (e.g., yielding a relative measurement error of 1% of object width and 1% of object length even if the object length would be 10-fold as compared with the object width).
The equipment described in cited FI Pat. No. 68,910 comprises a horizontal measurement platform on which sawn lumber to be measured is fed one by one. Above the measurement platform is adapted a mirror aligned to reflect the image of the object lying on the measurement platform to the lens of the camera. The camera is connected to image processing equipment for processing the image information rendered by the camera into control information suited for, e.g., optimizing the lumber sawing process. The lengthwise contracted image of the object is available to the operator on a monitor screen.
The reflective surface of the mirror has a different radius of curvature along its longitudinal and lateral axes so that the object image reflected to the camera lens is lengthwise contracted relative to its width dimension. This is attained by making the mirror unidirectionally curved so that the contour of the reflective surface is curved along the mirror axis parallel to the longitudinal axis of the object being measured, while the mirror surface along the mirror axis parallel to the transverse axis of the object is straight. This arrangement serves for contracting the apparent length of the object being measured to the same order of magnitude on the image plane of the camera.
To keep the image field of depth equal in the directions of both principal axes, the camera optics is provided with a narrow slit aperture which in the described embodiment is aligned horizontal and transverse to the longitudinal axis of the object being measured. The effect of such an aperture is that the light rays imaging longitudinally the obje
REFERENCES:
patent: 4179707 (1979-12-01), Sjodin
patent: 4770537 (1988-09-01), Koskenohi
Cimmon OY
Evans F. L.
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