METHOD AND DEVICE FOR MEASURING THREE-DIMENSIONAL SHAPES

Abstract: The invention relates to a method and a device for measuring of the shape of an object being measured, in order to provide a supplementary object connected to the shape of the object being measured, wherein the object being measured is provided on its surface with a pattern of fluorescent substances. The surface of the object with the pattern is illuminated with a first type of light (13a, 13b) with a wavelength which excites the fluorescent paint substance used. The pattern is registered by means of optical reading units sensitive to the excited fluorescent light with registration of the same parts of pattern from mutually different directions. The surface of the object is illuminated with a second type of light (15a-15d). The registration of the appearance of the surface of the object is performed during illumination with the second type of light. Calculation of position and shape of the parts of the pattern are made from the registrations of the pattern, and the position for the gradient changes in the surface shape of the object calculated from the registrations of the appearance of the surface, and a three-dimensional shape with interfaces, connected to the object being measured, at the gradient changes is calculated for the supplementary object.

Description: Method and device for measuring three-dimensional shapes Field of the invention The present invention relates to a method for contactless measuring of the three- dimensional shape of different types of objects. The invention also relates to a device for performing this method. Examples of objects of which the shape can be measured according to the invention are teeth, auditory meatuses, etc. Background to the invention A trend in dental care is that one no longer shall use restoration material which is shaped directly in the mouth, such as amalgams or plastic. Instead there is used, for example, ceramic material which is produced mechanically based on measured shape information. Consequently, there is an increasing requirement for determining the shape of dental objects. A determination of a shape can take place directly in the mouth, on castings or on models. During measuring directly in the oral cavity, the measuring is made difficult because the teeth are formed of a material which is semi-transparent to light, wherefore, for example, measurement of the contact position for laser light cannot take place with sufficiently high measuring accuracy. Saliva can also impair the measuring result. The patent DE 4034007 describes an example of such equipment. In order to improve the optical quality of the surface of the tooth, it is sometimes prepared with, for example, a fine chalk powder. In the beginning of the work which led to the present invention, a number of different optical methods were tested for measuring the three-dimensional shape of the dental objects. Tests included illuminating the whole of the object with striped light, sweeping a laser point over the surface and reproducing this on a position-sensitive detector, amongst others. It was shown that most of the methods had deficiencies with respect to measuring accuracy, flexibility with respect to the different types of objects to be measured, and costs for producing the measuring equipment. Stereo and photogrammetry are methods which are often used for measuring three- dimensional shapes. The methods require that the object being examined has an outer structure, the angular position of which can be compared from different observation positions. Dental objects and the human skin contain no, or indistinct, surface structures, which leads to problems for stereo methods. A possible way of solving the problem is to pattern the surface in order to facilitate later measuring of the shape. In our copending patent application PCT/SE96/01329 a way of patterning the object with a fluorescent pattern is shown. By illuminating the patterned object with light of a wavelength which excites the fluorescent pattern, and then registering this pattern with a camera unit which is sensitive to the fluorescent light, an extremely good picture of this fluorescent pattern is obtained. This invention is a further development of that method. Objects of the invention An object of the invention is to provide a method and a device for improving measuring compared with the above method. Another object is to provide an accurate measuring of limits where the surface normal of the surface of the object changes direction, such as by so-called preparation interfaces. A good determination of the preparation interfaces (the interfaces between unprepared surface and prepared surface) is of great significance for the final result, particularly during teeth repairs. Yet another object of the invention is to provide a method and a device which, compared with the prior art technique, more accurately and in a more cost-effective way measures the three-dimensional shape of, for example, dental objects. A further object is to effect the production of customer-adapted hearing aids which have a perfect fit to the auditory meatus, and where the transition between the auditory meatus and the ear drum is clearly discernable. The invention The above-mentioned objects are obtained by means of a method which has the characteristics stated in claim 1. Further development and a device for performing the method are given in the rest of the claims. By means of the method and the device according to the invention a good determination of the shape of the object being measured is obtained. However, a better determination of the positions of edges and interfaces where the inclination of the surface changes greatly, such as by the so-called preparation interfaces in dental applications, is desired. The method according to the invention described below gives the desired accurate measurement of edges and preparation interfaces. The electronic camera unit partly reproduces the object to be measured with an illumination adapted to the pattern provided on its surface. Then a picture is registered with illumination from one side. Thereafter a further picture is taken with illu- mination from the opposite side. Further registrations occur after the angles between the camera unit and the object have been changed. Starting from the stored picture information from the different reading positions, the position for the gradient changes in the shape of the surface such as preparation interfaces are calculated. For each pair of pictures taken with lateral light from the opposite sides, the pictures are electronically subtracted from each other, whereby a difference picture is obtained. Further difference pictures are generated, where the remaining pairs of illumination are used. By evaluation of difference pictures the angle for the surface normal for the different x,y-positions of the object being measured are calculated. Further registrations are made from a number of angular positions until all parts of the object have been registered from at least two advantageous angular positions. The computer software has special algorithms for processing and possibly filtering away measurement date which, during unfavourable angular conditions, have been collected near to preparation interfaces and edges, have been collected from positions where the object being measured has shadowed parts of the transmitter or receiver beam path, or have been collected in combinations of the above cases. Brief description of the figures The invention is described more closely below with reference to the accompanying drawings, where: Fig. 1 shows a view from above of a bored tooth in order to illustrate the principle of the invention. Fig. 2 shows a lateral view of a tooth bored for attachment of a crown. Fig. 3 illustrates schematically an embodiment of a device according to the invention. Figure 4 shows an embodiment of a constructed unit with light and camera. Detailed description of embodiments Fig. 1 shows from above a tooth 1 provided with a bore hole 3 which is coated with a fluorescent pattern of dots 2. The tooth 1 is illuminated from the side with a light source 5 with a first type of light. In Fig. 1 the light is incident so that the bore 3 is illuminated and the tooth itself is in shadow. A sharp edge 4 marks the transition between the bore 3 and the remaining part of the tooth. In addition to the cavity which is to be filled with a filling adapted to the shape of the hole, the position in space in three dimensions for this transition 4 must be determined accurately to obtain a completely even transition between the remaining tooth and the filling. This is especially required when a filling of porcelain is to be manufactured and placed in the bored hole 3. Fig. 4 shows from the side a ground down tooth which is to be provided with a crown. The grinding is executed so that a peak 7 is formed onto which the crown is to be fixed. The shape of the peak 7 must be accurately measured in order to form an attachment hollow in the crown, which must agree completely with the shape of the peak 7. Equally, it is extremely important that the transition 8 between the peak 7 and the remaining part of the tooth is measured so that the fit of the crown is such that the lower edge of the crown corresponds well with the transition 8 so that the transition between the crown and the remaining part of the tooth is smooth. Before the work of measuring the tooth in order to obtain the shape of the filling or the crown which is to be made, the object being measured 1 or 6, which will be described as being a tooth, is bored and cleaned. Naturally, the measuring can take place on any other object needing a filling such as, for example, a model of a tooth or a casting of a dental object, a part of a hearing aid which is well fitting to an auditory meatus, a repair in a desktop or in a wall or the like. During measuring of an auditory meatus it is important to detect the fastening of the ear drum, as knowledge of the volume between the inner part of the hearing aid and the ear drum is of considerable importance for adjustment of the sound quality of the hearing aid. On the surface of the object to be measured is applied a pattern of paint which preferably is fluorescent. However, it is possible instead to use a pattern of e.g. black or coloured dots. The application can, for example, be carried out by controlled spraying of the surface with the paint. The pattern can be of another type, but a pattern of dots is easy to produce and is therefore illustrated. Fig. 3 shows very schematically an embodiment for measuring the three-dimensional shape of an object to be measured 10, which is shown here provided with a central bore hole 11 with edges 11a. The object to be measured has a spray ed-on pattern of dots 12 of substances which preferably are fluorescent. A first illumination unit, comprising at least one light source 13a and 13b, emits light with a wavelength, e.g. blue, which excites the sprayed-on fluorescent substances 3 in the pattern of dots applied on the object to be measured (the tooth) 1. The light source should suitably offer an illumination of the area being investigated around, and in, the bored hole 11, which is as even and without shadows as possible. A camera unit 14 movable between two or more positions A and B, or alternatively several camera units (not shown), registers the position for the fluorescent dots 12. The whole or a part of the object is registered by one or several exposures with the electronic camera unit 14, which for this exposure is provided with a colour filter which blocks the excitation light but which allows through the fluorescent light. Each registered picture will look like a pattern of stars in the night sky and is stored in a memory, which will be described in connection with Fig. 4. A second illumination unit comprising several illumination units 15a, 15b, 15c, 15d thereafter emits sequentially from two or more positions light with a wavelength to which the camera unit 14 is also sensitive, e.g. red. Several camera registrations are now made with the object of emphasizing the borders (or discontinuities) in the surface of the measuring object such as preparation interfaces. The illumination from the light elements 15a-15d in this registration is obliquely incident light. Each illumination unit 15a-15d can, for example, be provided with a light diode or an end of an illuminating light conductor. The light should have a relatively well delimited lobe. The registrations are made sequentially, e.g. pairwise from opposite sides after each other and with peripheral changes between each registration in opposing pairs, so that in Fig. 1 the light sources are illuminated in the following sequence 15a, 15b, 15c, 15d. The position of the measuring unit can thereafter be displaced relative to the object being measured, whereby further camera registrations according to the above are performed. The measuring is finished when all interesting partial surfaces have been registered from a sufficient number of advantageous angles. The registrations of the object to be measured taken with obliquely instant light from different angular positions are used primarily for calculating the preparation interfaces but data can also be used in order to improve the accuracy of the shape measuring. The registrations of the dotted pattern are used in the main for electronic and/or computer-aided calculations of the shape of the object to be measured, preferably with so-called CAD-techniques. The filling or crown can thereafter be manufactured in so-called CAM-equipment which is supplied with the result of the CAD-proces- sing. This result can be transferred to some computer media, e.g. a computer dis- cette, and sent to a laboratory with CAM-equipment. The camera unit 14 can, for example, be of a type of stereo camera wherein two simultaneous registrations are made from two angular positions. The camera 14 can, for example, have a lens with two or more picturing optical elements, which either can each take a picture simultaneously or be provided with a shutter, which means that one picture at a time can be taken. Each picture of the object being measured is reproduced on at least one picture sensor which, for example, can be formed of a two-dimensional matrix of sensor elements. Alternatively, the camera can be designed so that the object being measured 10 is reproduced on a coherent bundle of fibres, wherein a picture sensor is arranged at the second end of the bundle of fibres. Each light source can also be connected to at least one light fibre which carries the light to the vicinity of the object being measured. In this way the light sources, sensors and electronics connected thereto can be placed at a distance from the object being measured. The measuring heads which are placed near to the object being measured can in this way be made small and easily positioned. The embodiment shown in Fig. 4 of a measuring instrument is especially usable for flexible placement near to an object being measured, which can be relatively difficult to access, for measuring of this, such as a tooth inside the mouth of a patient and especially for measuring of a hole bored in a tooth. In this way a measuring head 20 with "camera"-equipment can be placed at the end of a guide means 21. In Fig. 4 the guide means 21 is shown as a handle which is easily manoeuvrable by an operator and the outer end 20 of which with the "camera"-equipment is slightly bent to give good access in tight spaces. The guide means 21 can instead be controllably bendable and/or a possibly guidable arm can be connected with the outer end 20 itself in order to guide it, and the light fibre bundle 11 in this case can come directly from the end 20. The picture collecting end is placed inside the outer end 20 in Fig. 4. A fibre bundle 11 is bendable outside the rod 21 and coherently arranged between the end of the measuring head and an electronically readable picture sensor 22, such as a second position of impact detector unit, placed at a distance from the measuring head 20. The electronically readable picture sensor 22 is connected to evaluation circuits 23. In the case of investigation of teeth, the teeth are coated before the measuring with a coating of fluorescent particles which can be brushed off. This can take place by activating a spray painting equipment connected to the measuring head 20 and which comprises a controllable paint unit 24 connected by a hose 25 to at least one nozzle (not shown) on the measuring head 20. Appropriately there are means (not shown) on the measuring head in order to protect the lens during the spray painting operation controlled from the unit 24, such as extension of the nozzle or nozzles and/or temporary closing of the opening to the lens. It is, however, obvious that the application of the pattern on the object being measured can instead be performed separately and that an instrument without the parts 24,25 can be used for the measuring itself. The appropriate pictures are taken and signal-processed, whereafter the coating is brushed and rinsed away. The light units 13a, 13b are placed beside the measuring head 20. The light units 15a-15d, which are to provide incident light obliquely from the side, are, in the embodiment shown in Fig. 4, placed in a ring 26 which is held by a support on the measuring head 20, possibly rotatable for oblique positioning in relation thereto, as required. The evaluation circuit 23 is preferably a computer provided with software, which performs the illuminations of the different light units 13a,13b,15a-15d, the camera adjustments with the application of filters, and opening and shutting of suitable shutters for the different registrations. The registrations are processed in the evaluation circuit 23, which thereby calculates the three-dimensional position in space and the shape of the tooth which is of interest for the reproduction. Furthermore, the evaluation circuit 23 can perform the required CAD-calculation with the special calculation of the positions of edges between different parts of the object, for example, with photogrammetry or correlation between several registrations of the same points in several different registration angles. In order to obtain a calibration reference during measuring of the fluorescent pat- tern, a small plate 27, provided with another type of pattern in e.g. fluorescent paint, in connection with either the measuring head 20 or the ring 26 is so placed that it is inserted into the field of view of the camera unit and comes into the registered picture at the side of the measuring region. The plate 27 can thereby be placed directly against the object to be measured. The pattern on the plate 27 can, for example, comprise a scale as a calibration pattern. The importance of the calibration pattern is that it should be able to give a standardization and therefore should have a known geometry with known dimensions. The calibration pattern can instead of being placed on a place 27 be directly applied onto the object being measured at the side of the measuring region before measuring. In addition to preparation interfaces, such as 4 and 8, the angle for the surface normal for the different partial surfaces of the object being measured 1, 6, is calculated. The calculation of the position and shape of the pattern parts is made from the pattern registrations. The position for gradient changes in the surface shape of the object is calculated from the registrations of the appearance of the surface. A three- dimensional shape with borders connected to the object being measured at the gradient changes is calculated for the filling or crown or some other type of supplementary object. In particular, the information concerning gradient changes is used for correction of shape data calculated from earlier calculations, e.g. from registra- tions of dotted pattern. The computer software has special algorithms for processing and possibly filtering away measurement data, which have been collected during unfavourable angular conditions, have been collected close to the preparation interfaces 4, 8 and edges, have been collected from positions where the object being measured 1, 6 has shaded parts of the sender or receiver beam path, or have been collected in combinations of the above cases. The type of algorithms and calculations of CAD-type can be of different suitable conventional types and are not part of the invention itself and are therefore not described more closely. The calculations of the three-dimensional position in space of the position of the pattern parts are performed by triangulation between parts in picture registrations, taken from different angles, and occur by geometric calculations. Many modifications of the invention are possible within the scope of the accom- panying claims. Even if an object being measured in the shape of a tooth is illustrated, it is obvious that objects of other types than teeth can be measured, both of organic and inorganic character.

Claims: Claims 1. Method for measuring of the shape of an object being measured, for example, in order to provide a supplementary object in connection to the shape of object " 5 being measured, wherein the object being measured is provided at its surface with a pattern of fluorescent substances, characterized in that the surface of the object with the pattern is illuminated with a first type of light (13a,13b) with a wavelength which excites the fluorescent substance used, the pattern is registered by means of optical sensing units sensitive to the excited 10 fluorescent light with registration of the same pattern parts from mutually different directions, the surface of the object is illuminated with a second type of light (15a-15d), registering of the appearance of the surface of the object is performed during illumination with the second type of light, 15 calculation of position and shape of the pattern parts is made from the pattern registrations, and position for the gradient changes in the surface shape of the object is calculated from the registrations of the appearance of the surface, and that a three-dimensional shape with marked interfaces at the changes in the gradient is calculated. 20 2. Method according to Claim 1, characterized in that the illumination with the second type of light occurs with an oblique instance to the object being measured and sequentially from several directions with at least one registration for each illumination. 25 3. Method according to Claim 1 or 2, characterized in that illumination occurs sequentially pairwise with illumination sequentially from opposite sides and with one registration of the illumination from each side, and that these registrations are combined with each other, e.g. subtracted, in order to obtain gradient changes in 30 the registrations. 4. Method according to any of the previous claims, characterized in that the angle for the surface normal of the different partial surfaces of the object being measured are calculated from the registrations made with illumination with the second type of light (15a-15b), and correction of the shape data obtained by the registrations with the first type of light is performed with the surface normal angular calculations. 5. Method according to any of the previous claims, characterized in that the registrations with both types of light are performed from several directions in relation to the object being measured. 6. Device for measuring of the shape of an object being measured, for example, for providing a supplementary object connected to the shape of the object being measured, wherein the object being measured is provided on its surface with a pattern of fluorescent substances, characterized by a first illumination unit (13a,13b), which during activation emits light of a first type, which has a wavelength which excites fluorescent substances, a second illumination unit (15a-15d), which during activation emits light of a second type, a picture registering device (14), which from at least two positions registers the pattern on the surface of the object illuminated with a first type of light, and from at least two positions registers the surface of the object stereoscopically or with exposures from even more directions, a calculation unit (22), which from the pattern registrations calculates the position and shape of the parts of the pattern, and from the registrations of the appearance of the surface calculates positions of the changes in gradient in the shape of the surface of the object, and calculates a three-dimensional shape with interfaces connecting to the object being measured at the gradient changes. 7. Device according to Claim 6, characterized in that the picture registering device is provided with a stereo lens with two or more reproducing registering units and with controllable shutters in the lens so that the lens can be controlled to register one picture at a time. 8. Device according to Claim 6 or 7, characterized by a coherent bundle of fibres, in which the picture of the object being measured is reproduced in one end and a picture sensor device is placed in the other end. 9. Device according to any of Claims 6-8, characterized by a bundle of fibres each connected between a light source placed at a distance from the object being measured and a position near to the object being measured suitable for illumination thereof. 10. Device according to any of Claims 6-9, characterized in that the second type of illumination is placed in order to provide obliquely incident light towards the surfaces of the object being measured and comprises preferably several sequentially controllable light sources for illumination from several directions of the object being measured. 11. Device according to any of Claims 6-10, characterized in that the illumination of the first type is arranged to give an illumination as shadowless as possible of the object being measured.

Inventors: AAHLEN HANS, AAHLEN, HANS
Applicants: AAHLEN HANS, OPTRONIC CONSULT AB, AAHLEN, HANS
Application year: 1998
Main classification: G01B11
IPC codes: A61C9, G01B11