Optics: measuring and testing – By polarized light examination – With light attenuation
Patent
1993-08-10
1995-01-10
Evans, F. L.
Optics: measuring and testing
By polarized light examination
With light attenuation
2502017, 250561, G01B 1124
Patent
active
053812360
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an optical sensor and, more particularly, to a sensor used to sense the range of one or more parts of an object so as to form an image thereof.
BACKGROUND ART
A variety of different optical sensors are available for providing an image of objects within the field of view of the sensor. One such system known as a `sweep focus ranger` uses a video camera with a single lens of very short depth of field to produce an image in which only a narrow interval of range in object space is in focus at any given time. By using a computer-controlled servo drive, the lens is positioned (or `swept`) with great accuracy over a series of positions so as to view different range `slices` of an object. A three-dimensional image of the object is then built up from these `slices`. The system detects which parts of the object are in focus by analysing the detected signal for high frequency components which are caused by features, such as edges or textured parts, which change rapidly across the scene. Because of this, the system is not suitable for imaging plain or smooth surfaces, such as a flat painted wall, which have no such features.
This limitation is common to all passive rangefinding techniques. One way to overcome the problem is to actively project a pattern of light onto the target objects which can then be observed by the sensor. If this pattern contains high spatial frequencies, then these features can be used by the sensor to estimate the range of otherwise plain surfaces. A particularly elegant way of projecting such a pattern is described in U.S. Pat. No. 4,629,324 by Robotic Vision Systems Inc.
In this prior art, the sensor detects those parts of the target object that are in focus by analysing the image for features that match the spatial frequencies present in the projected pattern. Various analysis techniques such as convolution and synchronous detection are described. However, such methods are potentially time consuming. In an extension of these ideas a further patent by the same company U.S. Pat. No. 4,640,620 describes a method which aims to overcome this problem by the use of a liquid crystal light valve device to convert the required high spatial frequency components present in the image into an amplitude variation that can be detected directly.
The present invention aims to provide a simpler solution to this problem.
DISCLOSURE OF INVENTION
According to one aspect of the present invention, there is provided an optical sensor comprising: a structured light source for producing a pattern of contrasting areas; a detector which comprises an array of detector elements having dimensions matched to the pattern produced by the light source; an optical system for projecting a primary image of the light source onto an object that is to be sensed and for forming a secondary image on the detector of the primary image thus formed on the object; adjustment means for adjusting at least part of the optical system so as to vary the focussing of the primary image on the object, the arrangement being such that when the primary image is in focus on the object, the secondary image on the detector is also in focus; and processing means for analysing signals produced by the detector in conjunction with information on the adjustment of the optical system, wherein the structured light source is adjustable so as to interchange the positions of contrasting areas of the pattern produced by the light source and in that the processing means is arranged to analyse the secondary images received by the detector elements with the contrasting areas in the interchanged positions to determine those parts of the secondary images which are in focus on the detector and thereby determine the range of corresponding parts of the object which are thus in focus.
According to another aspect of the invention, there is provided a method of determining the range of at least part of an object being viewed using an optical sensor comprising: a structured light source which produces a pattern of contrasting ar
REFERENCES:
patent: 4629324 (1986-12-01), Stern
patent: 4640620 (1987-02-01), Schmidt
patent: 5151609 (1992-09-01), Nakagawa et al.
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Technische Rundschau, vol. 79, No. 41, (1987), pp. 94-98; E. Senn: "Dreidimensionale Multipunktmessung Mit Strukturiertem Licht".
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Evans F. L.
Oxford Sensor Technology Limited
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