WO2000010047A1

WO2000010047A1 - Optical system for carrying out measurements on and/or for inspecting one or more objects over a given angle

Info

Publication number: WO2000010047A1
Application number: PCT/NL1999/000513
Authority: WO
Inventors: Bart Snijders
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 1998-08-14
Filing date: 1999-08-12
Publication date: 2000-02-24
Also published as: NL1009873C2; AU5388599A

Abstract

An optical system for carrying out measurements on and/or for inspection of one or more objects over a given angle, in particular over 360°, comprises an illumination system for illuminating at least a part of the space around the optical system, a reflector having a substantially conical reflector surface, and an image recording device for recording via this reflector surface an image originating from at least a part of the illuminated space. The optical measuring system further comprises correction means arranged between the reflector and the image recording device for compensating image defects. These correction means are formed by a lens of which at least one surface is substantially conical.

Description

Title: Optical system for carrying out measurements on and/or for inspecting one or more objects over a given angle .

The present invention relates to an optical system for carrying out measurements on and/or for inspection of one or more objects over a given angle, in particular over 360°, comprising an illumination system for illuminating at least a part of the space around the optical system, a first reflector having a substantially conical reflector surface, and an image recording device for recording via this reflector surface an image originating from at least a part of the illuminated space, which optical measuring system further comprises correction means arranged between the reflector and the image recording device for compensating, in particular astigmatic, image defects.

Such an optical system can be used both for performing measurements on and for inspection of the interior of pipes, such as pipelines, sewers, tunnels, heat exchangers, and boreholes, etc., and for determining the position of all kinds of objects located relatively close to each other, such as, for instance, the teats of a cow by means of a milking robot, whereby the optical system can be positioned on a robot arm between the teats of a cow. In most cases, measurement and inspection will take place over an angle of 360° around the optical system. Obviously, it is possible to carry out such measurements and inspections over a given angular segment . An optical system as indicated above is known from US- A-4,976,524. In that publication, the correction means are formed by an extremely complicated system of convex and concave lenses, which renders the optical system particularly costly.

The object of the invention is to simplify these correction means to a considerable extent and to provide an optical system which, without adverse effect on the accuracy of the system, is relatively inexpensive. To that end, according to the invention, the optical system such as it is described in the preamble is characterized in that the correction means are formed by a lens of which at least one surf ce is substantially conical . The invention is based on the idea that the virtual image of a real object which is formed by the conical reflector at different positions behind the conical reflector surface for the optical components in the radial and the tangential direction of the optical system, is positioned at the same or substantially the same position behind the conical reflector surface by a conical lens, so that the distance of the virtual image to the lens of the image recording device is rendered equal or substantially equal for the radial and tangential directions. This eliminates the astigmatism due to which either the radial object component is represented sharply or, at a different focusing, the tangential object component is represented sharply. In a preferred embodiment, the half apical angle of the conical reflector is approximately 45° and the diaphragm of the image recording device is so small that only light incident on the reflector virtually perpendicularly to the main axis of the optical system is detected. Further, the half apical angle of the conical lens is preferably about 80° and the refractive index of this lens is in the order of 1.4 to 1.5, while the conical lens is then arranged at a distance of 30 to 40 mm from the conical reflector. For measurement on and/or inspection of an object located at a certain distance from the main axis of the optical system, an appropriate illumination system for the purpose is present. According to the invention, to that end, a second conical reflector is present, which is part of the illumination system and which serves to reflect a light beam incident thereon in a circular space segment around the main axis of the optical system, such that the light that is diffused by an object located within this circular space segment can be detected by the image recording device via the first conical reflector.

For an accurate determination of the position of an object relative to the main axis of the optical system, the circular space segment is so directed that the position of the object which diffuses the light within this space segment is determinative of the position on the first reflector from which the object can be recorded by the image recording device. By calibration of the image recorded by the image recording device on the basis of an object whose position relative to the main axis of the optical system is known, a measure can be obtained on the basis of which the position of objects relative to this main axis in three dimensions can be determined.

The optical system can be used in various wavelength ranges, both in the ultraviolet and in the visible and the infrared spectrum range. In an embodiment which has proved suitable in practice, a laser is used as light source.

The invention will now be further explained with reference to the accompanying drawings, wherein:

Fig. 1A schematically shows an exemplary embodiment of the optical system according to the invention; Fig. IB shows a specific embodiment of a combination of reflector and correction lens which is applicable in this exemplary embodiment;

Figs. 2A and 2B show the formation of a virtual image by the conical reflector for the optical components in the radial direction;

Figs. 3A and 3B show the formation of a virtual image by the conical reflector for the optical components in the tangential direction, and the displacement of this virtual image by means of a conical lens; and Fig. 4 represents the image formation in the radial and tangential direction by the lens of the image recording device in the absence of the correcting conical lens. Fig. 1 shows a pipe 1 whose inside is to be inspected and whose diameter is to be determined. To that end, the optical system 2 according to the invention is lowered into the pipe 1. This optical system comprises a laser light source, not shown in the figure, whose emitted radiation is guided via a glass fiber 3 to the position to be inspected and measured. By means of a lens 4 at the end of the glass fiber 3, and two mirror elements 5 and 6, a light beam along the main axis 7 of the optical system 2 is obtained. This main axis 7 coincides at least substantially with the centerline of the pipe 1, which can be realized by making the housing of the optical system, not shown, of circularly symmetrical design and displacing it along the wall of the interior of the pipe 1 by means of, for instance, rollers. By means of a conical reflector 8, a light beam 9 is obtained which covers a circular space segment. By means of a lens 10, this light beam is strongly focused in a circle on the wall of the pipe, in Fig. 1 at F . Further, a further conical reflector 11 is present to guide the light of the light beam 9 diffused by the inner wall of the pipe 1 in the direction of an image recording device 12. To prevent astigmatic defects as a result of reflections by the conical reflector 11, between this reflector 11 and the image recording device a correction lens 13 is arranged which reduces these defects to a large extent. As will be elucidated hereinbelow, this correction lens has likewise a conical surface. In the preferred embodiment shown here, the conical reflector 11 has a half apical angle of 45°, while further the diaphragm of the image recording device is chosen to be so small that what will be detected is substantially a very narrow light beam which falls from the pipe wall in a plane perpendicular to the main axis 7 onto the conical reflector 11. The image obtained by the image recording device 12 can be processed in an image analyzing device 14 and, if desired, be displayed on a monitor forming part of this device. The perimeter of the inner wall of the pipe 1, as depicted in the drawing on the monitor, is represented by the circle Cm. By calibration with a known pipe inside diameter, the inside diameter of the pipe 1 can be derived from the image obtained, that is, the circle Cm. The light beam 9 runs at such an angle that when the optical system is lowered into a narrower pipe 1A, the light diffused by the pipe inside wall and detected by the image recording device 12 originates from a circle which, passing through G in Fig. 1, is located lower down. On the monitor, the perimeter of this smaller circle is depicted by the larger circle Ck. Similarly, when the optical system is lowered into a wider pipe IB, the light diffused by the pipe inside wall and detected by the image recording device 12 will originate from a circle which, passing through H in Fig. 1, is located further up. On the monitor, the perimeter of this larger circle is depicted by the smaller circle Cl . It will be clear that the range in pipe diameters is limited by the surface of the conical reflector 11, while more towards the limits of this range, the focusing of the light beam 9 on the pipe inner wall diminishes. Therefore, for pipes with a very great inside diameter, with respect to pipes of a very small diameter, the optical system will have to be adapted. When in the pipe a thickening in the wall occurs, such as the thickening 15 in the pipe 1, this can be observed on the monitor by an outwardly directed protrusion 16 in the circle Cm. The size and location of this protrusion 16 define the position and the size of the thickening 15 in the pipe 1. The conical reflector 11 and the correction lens 13 can also be designed as one whole. Such an embodiment is represented in Fig. IB, where the correction lens, for reasons of manufacturing technique, is arranged by the flat side thereof on a glass body 22 in which a conical recess for the conical reflector 11 has been milled out. As a result, a better defined positioning of the correction lens and the conical reflector relative to each other is obtained.

Referring to Figs. 2-4, the optics of the optical system, in particular the functioning of the conical reflector 11 and the correction lens 13, will be further explained.

Figs. 2A and 2B represent the image formation in a plane through the main axis 7 of the optical system 2. Fig. 2B shows, viewed in the direction of the main axis 7, the elevation of this image formation in the plane through the main axis 7. When assuming an object formed by the arrow A, there is an associated virtual image B which extends in the radial direction relative to the main axis, viz. at that location behind the conical reflector 11. When between this reflector and the image recording device the conical correction lens is placed, the latter will not influence the position of the virtual image B, since in the plane through the main axis 7 the lens has no effect other than that of a prism, that is, only the image formation by the lens of the image recording device is displaced a little in radial direction.

Figs. 3A and 3B represent the image formation in the plane V perpendicular to the main axis 7 of the optical system 2. When assuming an object formed by the arrow D, there is an associated virtual image E which extends in the tangential direction relative to the main axis and, in the situation shown, is located in the plane V. If the correction lens 13 is not present, the phenomenon of astigmatism occurs. What is obtained by the image recording device when focusing on the virtual image B is a sharp image in the radial direction but an unsharp image in the tangential direction, while, conversely, when focusing on the virtual image E a sharp image in the tangential direction but an unsharp image in the radial direction is obtained. This effect is shown in Fig. 4 for a spoke wheel 17. Via the lens 18, which is astigmatic in this example, of the image recording device, the image 19 is obtained upon focusing on the spokes and the image 20 upon focusing on the wheel. Through the presence of the conical correction lens 13, the virtual image E of the object D as obtained by the conical reflector 11 is shifted. Through the proper choice of the focus of the lens 13, it is possible, upon arranging this lens at a suitably selected distance from the conical reflector 11, to shift the virtual image E backwards, such that the position of the thus obtained virtual image E' coincides with the position of the virtual image B. By focusing, the image recording device 12 can now obtain a sharp image of both the radial and the tangential component of the virtual image of an object on the side of the conical reflector.

When the object arrow D crosses the main axis 7 at a greater distance, the virtual image E will be smaller and in Fig. 3B come to lie closer to the main axis 21 in the plane V. However, closer to the plane through the main axes 7 and 21, the curvature of the conical lens 7 is greater and the focal distance of this lens is smaller. Through a suitably chosen apical angle for the conical lens, it is found that for various distances of the virtual image E to the plane through the main axes 7 and 21, a shift of this virtual image to virtually the same position, corresponding to that of the virtual image B, can be obtained. In a concrete situation with a conical reflector having a half apical angle of 45°, a conical lens having a half apical angle in the order of 80° and a refractive index of about 1.4 to 1.5 is useful; the conical lens is then placed at about a distance of 30 to 40 mm from the conical reflector.

The present invention is not limited to the exemplary embodiment represented here with reference to the drawings, but encompasses all kinds of modifications thereof, naturally insofar as they fall within the scope of protection of the appended claims. Thus, the half apical angle of the conical reflector 11 may deviate from 45°, which then requires, however, at least in the case of great deviations therefrom, that the half apical angle of the correction lens 13 and the position of this lens relative to the reflector 11 be adapted. The correction lens 11 may on one side have a spherical surface, instead of a flat surface, giving it a certain optical strength as well. Also, both sides of the correction lens can have a conical surface.

Although a laser light source is preferred, other light sources may very well be used; even an infrared light source can be used, though it must be noted then that in that case an infrared camera is needed. Further, the optical system is not limited, in its application, to pipe inspection, measuring pipe diameters, determining welding seams in pipes or the condition of these welding seams, and the like; the optical system is suitable for observing all kinds of objects around the system, for instance, as already mentioned earlier, the position of the teats of a cow in a milking robot. Also, observation over 360° is not requisite; an observation over any angular segment is possible with this optical system. In particular the combination of a laser light source, optics with conical correction lens and an image recording device, such as these units have been described in the foregoing, has been found to result in an accurate measuring system. Objects at a distance to the main axis of the optical system of, for instance, 10 cm can be determined with an accuracy of less than 0.5 mm.

Claims

1. An optical system for carrying out measurements on and/or for inspection of one or more objects over a given angle, in particular over 360┬░, comprising an illumination system for illuminating at least a part of the space around the optical system, a first reflector having a substantially conical reflector surface, and an image recording device for recording via this reflector surface an image originating from at least a part of the illuminated space, which optical measuring system further comprises correction means arranged between the reflector and the image recording device for compensating image defects, characterized in that the correction means are formed by a lens of which at least one surface is substantially conical.

2. An optical system according to claim 1, characterized in that the half apical angle of the conical reflector is approximately 45┬░ and the diaphragm of the image recording device is so small that only light incident on the reflector virtually perpendicularly to the main axis of the optical system is detected.

3. An optical system according to claim 1 or 2 , characterized in that the half apical angle of the conical lens is about 80┬░ and the refractive index of this lens is in the order of 1.4 to 1.5, while further the conical lens is arranged at a distance of 30 to 40 mm from the conical reflector.

4. An optical system according to claim 1, 2 or 3 , characterized in that a second conical reflector is present, which forms part of the illumination system and which serves to reflect a light beam incident thereon in a circular space segment around the main axis of the optical system, such that the light that is diffused by an object located within this circular space segment can be detected by the image recording device via the first conical reflector.

5. An optical system according to claim 4, characterized in that the illumination system comprises a laser light source.

6. An optical system according to claim 4 or 5, characterized in that the circular space segment is so directed that the position of the object which diffuses the light within this space segment is determinative of the position on the first reflector from which the object can be recorded by the image recording device .

7. An optical system according to claim 6, characterized in that by calibration of the image recorded by the image recording device on the basis of an object whose position relative to the main axis of the optical system is known, a measure is obtained on the basis of which the position of objects relative to this main axis in three dimensions can be determined.