GB2269453A - Optical inspection device. - Google Patents

Abstract

An inspection device such as endoscope 1 is provided with an additional light source for illuminating a viewed object with a collimated beam which provides a light pattern of known dimensions superimposed on the object. A recorded image of the object can be analysed to determine the absolute size of features of the object, by comparison with the dimensions of the light pattern. The pattern may be a disc or a grating and its distortion may be used to estimate the orientation of the illuminated surface. Fibre optics may guide the light to the object and back to a CCD camera. <IMAGE>

Description

OPTICAL INSPECTION DEVICE The present invention relates to optical inspection devices, such as endoscopes, for viewing inaccessible features, for example internal bodily organs, and the like.

More particularly, the invention relates to an endoscope or similar device with means for readily calculating absolute dimensions of the objects being viewed.

Over the past few years endoscopes have been developed, primarily in the medical field, to a very high degree of sophistication and of extremely small diametral size, so that objects as small as arterial valves or blood clots can be investigated by passing an optical fibre along a body canal to the site of the object to be viewed.

Endoscopes are now widely used in many surgical and investigative procedures, for example, heart operations, vocal cord surgery and investigation, etc. Similar devices have industrial applications.

However, a problem exists in many procedures which involve the use of endoscopes in that, while it is possible to record images through such devices and thus make relative measurements of the dimensions of features of the objects, it is not possible to make accurate absolute measurements of features of the object being viewed. Such measurements are often very desirable as they can indicate the true scale of a particular problem and enable a better matching of corrective treatment to the problem.

Alternative techniques, involving x-ray analysis, for example, may be necessary in order to determine actual dimensions.

In order to try to overcome these difficulties various devices have been proposed which include means for projecting a light pattern onto the object to be viewed and then measuring the distance of the object from the source and/or measuring the position of features of the light pattern in order to enable calculation of the angles and dimensions of parts of the object. Examples of such devices are shown in US-A-4 834 070, DE-A-2 847 561, Systems and Computers in Japan, USA, Vol.18, No,12, December 1987, pages 11-17, and Optical Engineering, USA, Vol.30, No,12, December 1991, pages 1981-4.

These known systems all suffer from the disadvantage that a simple measurement of surface features of the object to be viewed is not readily achievable and the systems proposed have considerable complexity.

There is a need therefore to be able to make absolute measurements of features viewed through an endoscope or similar optical inspection device in a way which avoids additional procedures other than the viewing of the object through the device and simple comparative measurement.

According to the present invention therefore there is provided an optical inspection device comprising means for recording an image of an object viewed through the device; and means for projecting a collimated light beam of fixed dimensions through the device to form a pattern of fixed size on the object, whereby a recorded image of the object can be analysed for size or angle by comparison of features of the object with the dimensions of the light spot.

The basic concept underlying the invention is that a pattern of fixed size, independent of distance, is generated on a surface to be examined and viewed through the conventional viewing system, so that, by making one or several measurements on the observed image of this pattern, the physical size of any part of the surface observed can be calibrated by reference to the known size of the projected pattern. The pattern may be of various types, for example a uniform disk or 'spot', a pair of crossed lines, a grating, as appropriate.

In practice, for a given system, one or more optical fibres form transmission channels to carry the image back to an external detector, so that, at the same time, one channel would serve to convey the image back to the sensor while, preferably, another channel is used to transmit light from a source of light (for example a laser) to the target. An optical system is employed to ensure that such channels function correctly, the sensor receiving an image of the target, and the transmitted light generating an image of a pattern on the target, this image being of size which is known, because of the collimation of the projected beam.

The fibres to channel the information may be contained with a single or multiple rods, or may be formed from a central channel surrounded by another channel or a combination of channels in any appropriate geometry.

Preferably a laser light source is used. Mono- or multimode fibres may be used, but the use of mono-mode can provide enhanced parallelism in the resultant beam to improve accuracy of collimation and hence of the measurments taken by the device.

The optics of the system are such that, for the receiving sensor, an image is transmitted back from the appropriate volume, but at the same time, the second transmission channel directs a pattern onto a target with the specified desired characteristics. This may employ a single lens or if needed a prism or multiple lenses and prisms, in an appropriate configuration, where the ends of the fibres or channels may be at different distances from the optical system. By modification of the light path, a collimated beam can be projected to produce a pattern of known dimensions. The pattern is then recorded with the remainder of the image and can be used to provide an accurate measurement of dimensions of the object viewed.

Analysis may be carried out by software appropriate for the task, in order to simplify the taking of measurements.

The term "transmission channel" is used to mean any light transmission system, such as, preferably, an optical fibre or the like, through which an object can be viewed remotely.

The term "pattern" is used herein to indicate the image of a collimated beam of light falling on an object so that the beam has superimposed on it a certain shape, or is composed of a number of such shapes, such as a circle, a rectangle, a line, crossed lines, a grid, etc.

The inspection apparatus preferably includes a camera, such as a CCD camera, or the like, and/or the means for recording the images of objects to be viewed may include a video recorder or similar and means may be provided for digitising the images recorded, storing them in a computer memory, and, if desired, manipulating them. Alternatively, a CCD camera or similar may be directly connected to an image processor located on an expansion card in a computer.

An endoscope usually includes a means for illuminating an object to be viewed, so that an image can be recorded.

A variety of systems for illuminating the object viewed have been proposed, for example, using stroboscopic light source. An important novel. feature of the invention proposed here is the generation of an additional, collimated, light beam with specific characteristics. The normal optics of a conventional system enable a region of size which varies strongly as a function of distance, to be viewed and illuminated. This invention proposes a modification of this optical system, such that an additional beam, with completely different characteristics, is also generated. Such a beam cannot be generated using any current endoscopic device (such as those mentioned in the introduction above) without the modification to the optical system proposed. Current devices use a single optical fibre for the optical channel, both to view the object and to illuminate it. The addition of further optical channels, for the generation of an additional beam can overcome the problems with the prior art.

An advantage of the present device is that no external additional measurement or complex calculation is required, the determination of the size of object viewed being determined solely from the instrument itself. In practice many applications require rapid access to data, but it can be very difficult to adjust some control, for example, the focal distance, while attempting simultaneously to record the image as may require to be done with prior art devices.

The advantage of the present approach is that no additional manipulation of the instrument is required, a long depth of focus can be used and the absolute size of objects determined, independent of distance between endoscope and object.

The apparatus will find particular utility in cardiology, urology, and laryngology, to name but three areas of medical interest, but will also have application to non-medical technologies.

One example of apparatus according to the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of the apparatus; Figure 2 is an image recorded by the apparatus; and, Figure 3 is a schematic diagram of the system in a clinical application.

The components of the apparatus are shown in figure 1.

The channels (optical fibres) carry the information from the observed object in the 'image read out area' back to the sensor. The light needed to generate the pattern to be superimposed on the object is viewed along the beam path labelled 'spot beam'. The pattern can be generated either by a mask placed in an appropriate plane in the optical system, or may be generated prior to transmission along the optical channel.

The apparatus is shown in figure 3 in use in the investigation of laryngeal problems and comprises an endoscope 1 which includes a laser light source, a bundle of optical fibres forming a fibre optic transmission channel 2 with an appropriate objective 3, a CCD camera 4, a video recorder 5, a digitiser 6 and a computer 7.The human larynx 8 is shown being viewed and an image 9 recorded in the CCD camera 4 is recorded in the video recorder 5 and the recorded image 10 is then passed to the digitiser 6 which is preferably an expansion card which can be located in one of the so-called slots provided for expansion within the computer 7 An image of the larynx can be displayed on the computer monitor 11 and absolute measurements of features of the vocal cords taken by means of software specially written for the task, for example involving the use of a mouse to mark the extent of the projected spot and other features which are to be measured using the calibration resulting from measurement of the spot.

Within the device, the image captured is digitized and then manipulated by image processing software. The image is then stored in~the form of a matrix of pixels. The software then identifies the pattern, and measures details of the phantom in terms of distances in pixels. The pixel size is then calibrated from knowledge of the absolute size of details of the pattern, so that any other object, initially measured in terms of pixels may have its size and dimensions expressed in terms of absolute size such as centimetres, millimetres etc.

The software has the ability to estimate the orientation of the target, for example whether it is tilted with respect to the target beam, from the distortion observed in the observed pattern. As an illustration, a pattern being a uniform circular beam, when hitting a titled plane will generate an ellipse with measurable long and short axes, and orientation. From this information, the slope of the plane being illuminated may be calculated, and the size estimation corrected for a resulting distortion. As an example, the shortest diameter of an ellipse observed for a circular spot illuminating a tilted plane equals the diameter of the circular spot. More complex patterns, for example, a grid, may be used to improve this estimate, or to correct more complex distortions of the plane with respect to the viewing angle.

It is envisaged, as an illustration, that a beam of 3mm diameter could be projected from an endoscope used for laryngeal inspection, so that, given that the image is of the order of 30mm diameter and is captured by a CCD on an array of 512x512 pixels, measurement to an accuracy of about lOO m could be achieved.

Claims (6)

1. An optical inspection device comprising means for recording an image of an object viewed through the device; and means for projecting a collimated light beam of fixed dimensions through the device to form a pattern of fixed size on the object, whereby a recorded image of the object can be analysed for size or angle by comparison of features of the object with the dimensions of the light spot.

2. An optical inspection device according to claim 1, wherein the image of the object and pattern is captured by digital or analogue means, so that, at any time thereafter, a measurement of the size of image of the pattern of known size can be used to determine the absolute size of the object or features of the object.

3. An optical inspection device according to claim 1 or claim 2, wherein the means for recording the image is a CCD camera and a video recorder.

4. An optical inspection device according to any of claims 1 to 3, wherein the collimated beam of light is provided by a laser beam projected through a fibre-optic channel.

5. An optical inspection device according to any of claims 1 to 4, further including a computer system arranged to detect the projected pattern and assess the size of the object with respect to the known size of the pattern, such that an absolute measure of size of the object can be obtained.

6. An optical inspection device according to claim 1, substantially as described with reference to the accompanying drawings.