OPTICAL ELEMENT

Abstract: The invention concerns an optical element (1) which is used in an optical measuring system with a camera, whereby this optical element (1) contains a light source (2) with an active side (4) which lights up when an electric tension is applied via electric contacts (9, 13) provided to this end, whereby this light source is mounted against a translucent wall (3), such that the light of said active side (4) can be seen through this wall (3), and whereby this translucent wall (3) is formed of transparent fibres (5) placed side by side in a parallel manner.

Description: Optical element The invention concerns an optical element containing a light source with an active side which lights up when an electric tension is applied via electric contacts provided to this end, whereby said light source is mounted against a translucent wall, such that the light of the above-mentioned active side can be seen through this wall. According to the present state of the art, optical elements of this type contain what is called a light-emitting diode, also called LED chip, which is mounted on a metal housing and which is protected by means of a transparent epoxy layer. These existing elements are disadvantageous, however, in that it is impossible to obtain a homogenous distribution of the produced light intensity. Both the metal housing and the epoxy layer produce interfering phenomena. Thus, in particular in the case of a spherical epoxy layer, this layer may act as a lens. Further, internal reflections in the housing of the LED and in the epoxy layer result in an uncontrolled diffusion of the light coming from the LED. Temperature variations in the LED due to a varying ambient temperature or changing electric conditions provoke a variation in the emitted light spectrum, which results in an inaccurate perception of the optical element. These qualities are particularly disadvantageous when such an optical element is used in optical measuring systems where the position of the LED has to be determined by means of a camera. On the basis of the position of the LED is calculated the position and/or orientation of the structure upon which this LED is fixed. In these optical measuring systems, the optical centre of the LED is determined to allow for an accurate determination of the position, such as for example the centre of gravity of the distribution of the light intensity of the LED. In order to be able to precisely determine a position, it is important that the optical centre of the LED is independent of the angle of detection at which it is perceived, and that the light intensity is as high as possible. Usually, at least one camera and preferably three cameras are used in such an optical measuring system. A single LED or optical element is observed at different angles by these cameras. A mathematical processor which works in conjunction with these cameras processes the images which are thus formed by said cameras, and on the basis thereof calculates the position of the LED or the optical element. Thus, the LED's used according to the prevailing techniques are disadvantageous in that the optical centre of the LED varies as a function of the angle of detection, whereby said internal reflections reduce the light intensity, as a result of which the measuring distance and accuracy are relatively restricted. The invention aims to remedy these disadvantages by making use of an optical element in an optical measuring system, whereby the position of its optical centre, for example the centre of gravity of the distribution of intensity, is independent of the angle of detection, and which, moreover, has practically no internal reflections, so that a high light intensity is obtained. To this aim, the above-mentioned translucent wall is formed of transparent fibres provided next to one another in a parallel manner which extend diagonally in relation to the above-mentioned wall. Practically, said fibres form a plate and they extend almost perpendicular to the surface thereof. According to a preferred embodiment of the optical element according to the invention, said transparent fibres are glass fibres. According to a specific embodiment of the optical element according to the invention, a non-transparent coating is provided on said wall on the side of the above-mentioned light source, whereby a recess is provided in this coating on said active side, opposite to the light source. According to a special embodiment of the optical element according to the invention, a first part of the above-mentioned electrically conductive contacts consists of an electrically conductive coating which is provided on said wall on the side of the light source. According to an advantageous embodiment of the optical element according to the invention, a layer of conductive material is provided on the side of said light source, which extends opposite to said active side of the latter, forming a second part of the above-mentioned electric contacts. The invention also concerns a measuring group which is used in an optical measuring system containing several optical elements according to the invention. Further, the invention also concerns the use of the optical element according to the invention in an optical measuring system. Other particularities and advantages of the invention will become clear from the following description of a specific embodiment of the optical element according to the invention; this description is given as an example only and does not restrict the scope of the claimed protection in any way; the reference figures used hereafter refer to the accompanying drawings. Figure 1 is a schematic cross section of a part of an optical element according to the invention. Figure 2 is a schematic top view of the part concerned of an optical element according to the invention. Figure 3 is a schematic top view of a part of a measuring group according to the invention. In the different figures, the same reference figures concern identical or analogous elements. The optical element 1 according to the invention, as represented in figure 1, contains a light source 2 and a protective translucent wall 3. The light source consists of a conventional LED chip 2 with an active side 4. When an electric tension is applied to the chip 2, the active side 4 will emit light having a certain intensity. Further, the dimensions of such a chip 2 are for example 0.7 x 0.7 mm, and its thickness is 0.5 mm. De chip 2 is fixed to the above-mentioned wall 3 according to what is called the "flip chip" technique, known as such, whereby its active side 4 is directed towards said wall 3. Thus, the light which is produced by the chip 2 can be perceived through this wall 3. According to the invention, the wall 3 consists of transparent fibres 5 provided next to one another in a parallel manner. These fibres can be made of plastic, for example, but they preferably consist of glass fibres. Thus, the corresponding far ends of these fibres 5 are directed towards the active side 4 of the chip 2, whereas the other far ends of the fibres 5 together form a plane 6. As the light coming from the chip 2 is guided through the fibres 5, this light is perceived at said plane 6, which forms a homogenous, light-emitting surface, so that the light source 2 seems to be situated on the plane 6. The transparent fibres 5 preferably have a diameter of 3 to 40 μm, in particular a diameter between 5 and 15 μm, and in particular a diameter of almost 10 μm. Further, these fibres 5 preferably have a length between 0.2 and 1.5 mm. It was found that fibres 5 having a length of more than 20 mm are less interesting. Good results were obtained with fibres 5 having a length of almost 1 mm. As is shown in figure 1, said fibres 5 extend according to a flat plate 7 and thus form this plate 7 with said plane 6, whereby the fibres 5 extend almost perpendicular to the plane 6. The thickness of this plate 7 is almost equal to the length of the fibres 5 and preferably amounts to almost 1 mm. Such a plate is also called a "faceplate". Such fibres 5 and what are called "faceplates" are known as such and are manufactured among others by the company "Schott Fiber Optics, Inc.", 122 Charlton Street, Southbridge, MA 01550, U.S.A. In order to avoid that the light intensity which is perceived through the above-mentioned wall 3, in particular the plate 7, is not homogenous, it is provided with a non-transparent coating 8. In this coating 8 is provided a recess opposite to said light source. Thus is formed a diaphragm which makes sure, for example, that possible interferences in the light intensity occurring on the edge of the above-mentioned active side 4 are not perceived through the wall 3. In the embodiment of the optical element according to the invention, as is represented in figure 1, this coating 8 is provided on the side of the wall 3 where the chip 2 is situated. In order to be able to apply an electric tension onto the optical element 1 according to the invention, it is provided with electric contacts 9 and 13. A first part of these electric contacts is formed of an electrically conductive coating which, in the embodiment of the optical element 1 from figure 1, consists of the above-mentioned non-transparent coating 8. This coating 8 preferably consists of a metal layer. The coating 8 is connected via conductors 10 to four electric contact surfaces 11 which are situated inside said recess on the wall 3, and which make contact to four corresponding junctions 12 on the upper surface of the chip 2. Further, a second part of the electric contacts is formed of a layer of conductive material 13 provided on the side of the chip 2, which extends opposite to its active side 4 and which is electrically conductive. This electrically conductive layer 13, as is represented in figure 1, consists of a small metal plate. The layer 13 also has a good thermal conductivity, such that heat produced by the LED chip 2 is removed via this layer 13, as a result of which the chip 2 is cooled. This heat removal guarantees an excellent thermal stability of the chip, so that the produced light remains constant and homogenous, and its spectrum is almost invariable. According to a particularly interesting embodiment of the optical element according to the invention, which is not represented in the figures, means are fixed to said layer 13 which make it possible to cool and/or possibly heat the light source 2. These means comprise an electric component with what is called a Peltier element. Such electric components are known as such. This Peltier element is fixed to the layer 13 with its so-called "cold side", whereas its so-called "hot side" is provided with cooling fins which make it possible to dissipate the heat to the surroundings. Such means make it possible to keep the temperature of the light source at an almost constant level by sending an appropriate electric current through said Peltier element. Thus, according to the invention, is obtained an optical element with an almost constant temperature, and which consequently emits a constant colour spectrum, which allows for very accurate optical measurements. According to a variant of the embodiment, said means are mounted directly on the light source 2, and the above-mentioned layer 13 is not provided. Further, as represented- in figure 1, the chip 2 is laterally surrounded by a non-transparent and electrically insulating material 15, such as for example an epoxy resin. This insulating material extends between said plate 7 and the layer of conductive material 13, such that the chip 2 is entirely cut off from the environment. Figure 3 represents a measuring group 14 for an optical measuring system. This measuring group 14 contains four different optical elements 1 according to the invention, which are mounted on a single plate 7 in the above- described manner. Thus is obtained a measuring group 14 in which the mutual positions of the different optical elements 1 are fixed, and which has to be mounted on a structure in order to determine the position of this structure by means of an optical measuring system. Naturally, the measuring group 14 may contain more or less than four optical elements 1 according to the invention. The use of the optical element 1 according to the invention in an optical measuring system offers the advantage that, irrespective of the angle at which a camera observes the element, its optical centre is always situated at exactly the same absolute position. Thus, it is possible to perform very accurate measurements. On top of that is obtained a great light intensity, allowing for measurements at large distances. Further, it is possible to sterilise the optical element according to the invention, for example in an autoclave, without any danger of damaging it and without its optical qualities being affected. Thus, the optical element can be used in an environment where it is very important that the equipment is sterile. Naturally, the invention is not restricted the above-described embodiments of the optical element according to the invention. Thus, for example, the transparent fibres forming the above-mentioned plate at a certain angle, different from a right angle, can be provided in relation to the surface thereof. Possibly, also other light sources than a semiconductor chip can be used in the optical element according to the invention.

Claims: Claims 1. Use of an optical element (1) in an optical measuring system with a camera with which this optical element is perceived so as to determine its position, whereby said optical element contains a light source (2) with an active side (4) which lights up when an electric tension is applied via electric contacts (9,13) provided to this end, whereby said light source is mounted against a translucent wall (3), such that the light of the above-mentioned active side (4) can be seen through this wall (3), characterised in that this translucent wall (3) is formed of transparent fibres (5) placed side by side in a parallel manner. 2. Use of an optical element according to claim 1, characterised in that said fibres form a plate (7) and extend almost perpendicular to the surface (6) of the plate (7). 3. Use of an optical element according to claim 1 or 2, characterised in that said transparent fibres (5) are glass fibres. 4. Use of an optical element according to any of claims 1 to 3, characterised in that said light source (2) contains an electric semiconductor, in particular a chip. 5. Use of an optical element according to any of claims 1 to 4, characterised in that a non-transparent coating (8) is provided on the above-mentioned wall (3), preferably on the side of the above-mentioned light source (2), whereby a recess is provided in this coating (8) on said active side (4), opposite to the above- mentioned light source (2). 6. Use of an optical element according to any of claims 1 to 5, characterised in that a first part of the above-mentioned electric contacts consists of an electrically conductive coating (9) which is provided on said wall (3) on the side of the light source (2). 7. Use of an optical element according to claim 6, characterised in that said non- transparent coating (8) is formed of the conductive coating (9). 8. Use of an optical element according to claim 6 or 7, characterised in that, on the side of said light source (2) which extends opposite to said active side (4) of the light source (2), is provided a conductive layer (13) which forms a second part of the above-mentioned electric contacts. 9. Use of an optical element according to any of claims 1 to 8, characterised in that means are provided to keep the temperature of the above-mentioned light source (2) almost constant. 10. Use of an optical element according to any of claims 1 to 9, characterised in that the above-mentioned light source (2) is laterally surrounded by a non- transparent and electrically insulating material (15), such as for example an epoxy resin. 11. Optical measuring system with a camera which works in conjunction with an optical element (1), whereby said camera observes this optical element so as to determine its position, and whereby the optical element contains a light source (2) with an active side (4) which lights up when an electric tension is applied via electric contacts (9,13) provided to this end, and whereby this light source is mounted against a translucent wall (3), such that the light of said active side (4) can be seen through this wall (3), characterised in that this translucent wall (3) is formed of transparent fibres (5) placed side by side in a parallel manner.

Inventors: VAN DEN BOSSCHE ALEX, VAN DEN BOSSCHE, ALEX
Applicants: DEN BOSSCHE ALEX VAN, KRYPTON ELECTRONIC ENGINEERING, N.V. KRYPTON ELECTRONIC ENGINEERING, VAN DEN BOSSCHE, ALEX
Application year: 2001
Main classification: G02B6
IPC codes: G01B11, G02B6