Method for checking the alignment of a distal end of an optical fiber and test device for carrying out the method

Description

[0001] The invention relates to a method for testing the alignment and / or positioning of a distal end of an optical fiber that is a component of a medical instrument. The method is designed to check whether the distal end, and in particular a distal end face at the distal end of the optical fiber, is correctly aligned and / or positioned relative to a handle and / or an electrode of the medical instrument. The invention also relates to a testing device designed to carry out the method.

[0002] The method and testing device can, for example, be used at least randomly to test manufactured medical instruments to ensure their functionality and sufficient quality. They can also be used additionally or alternatively to test reusable instruments after use, cleaning, or sterilization.

[0003] US 2017 / 0 167 861 A1 describes a method for checking the alignment of an optical fiber relative to a lens, in particular a GRIN lens. Light is emitted from the fiber through the lens onto a projection surface. The fiber is then moved relative to the lens until the desired alignment is achieved. This is checked or detected by centering a light spot on the projection surface.

[0004] In the method known from US 2011 / 0 228 259 A1, the correct arrangement of one or more optical fibers on a coupling element connected thereto is checked using a sensor. For this purpose, light is emitted via the at least one optical fiber, the coupling element, and a sensor, and then it is determined whether the emitted light is incident on the sensor at the correct position. Furthermore, the intensity of the light incident on the sensor can also be checked to check the transmission through the arrangement of the at least one optical fiber and the coupling element. Reference is also made to document US2021 / 0236187 A1.

[0005] Based on the known prior art, it can be considered the object of the present invention to improve the testing of an alignment of a distal end of an optical fiber of a medical instrument using simple means.

[0006] This object is achieved by a method having the features of patent claim 1 and a testing device having the features of patent claim 13.

[0007] According to the invention, the alignment and / or positioning of the distal end of an optical fiber of a medical instrument relative to a handle and / or relative to an electrode of the medical instrument is checked in order to ensure its proper functioning.

[0008] The optical fiber of the medical instrument has a distal end face and a proximal end face. During intended use of the medical instrument, for example, during an electrosurgical procedure, light can be received at the distal end face and guided via the optical fiber to the proximal end. There, the light is at least partially decoupled at the proximal end face and can be evaluated, for example, to identify or classify the tissue treated with the medical instrument. The alignment and / or positioning of the distal end of the optical fiber is important to ensure sufficient light can be received.

[0009] The optical waveguide can optionally have at least one cladding layer surrounding the core of the optical waveguide. The proximal and distal end faces of the optical waveguide are each considered to be the end faces of the core of the optical waveguide without the at least one cladding layer.

[0010] During the method according to the invention, light is guided through the optical fiber to check the alignment of the distal end. It is coupled into the proximal end face by means of a light source. In a first test state, the light is coupled into the proximal end face asymmetrically and / or eccentrically relative to a center point of the proximal end face. The proximal end face is therefore not uniformly irradiated with light. Preferably, light is only coupled into a contiguous or non-contiguous surface area of ​​the proximal end face that is not rotationally symmetrical with respect to the center point of the proximal end face or whose geometric center of gravity is offset from the center point of the proximal end face. In particular, the proximal end face is not irradiated with light over its entire surface in a circular manner concentric with its center point.This first test condition is used to check the relative position and orientation of the distal end of the fiber optic cable relative to the handle and / or the electrode.

[0011] The light coupled at the proximal end face is at least partially radiated onto a projection surface at the distal end face, generating a light pattern on the projection surface. For example, the light pattern is a closed ring or an annular arc. The distal end face is in a defined position or orientation relative to the projection surface. This means that the electrode or handle of the medical instrument also has a defined position and / or orientation relative to the projection surface. Based on the light pattern generated on the projection surface, the relative position and orientation of the distal end relative to the handle and / or the electrode can be assessed. For this purpose, the generated light pattern is tested against at least one predefined test criterion. For example, one or more of the following test criteria can be used: Comparison of the actual shape or geometry of the light pattern with a target shape or geometry; comparison of an actual value of at least one dimension parallel to the projection surface with an associated target value.

[0012] When comparing target values ​​with actual values, the test criterion can specify a permissible deviation between the respective target value and the respective actual value.

[0013] If the evaluation shows that the light pattern meets at least one test criterion, the distal end of the fiber optic cable is correctly positioned and / or aligned. Otherwise, the medical instrument does not meet the specifications and can therefore be repaired and / or discarded.

[0014] The procedure can be carried out in the context of the production of medical instruments on a sample basis for a part of the medical instruments produced or for all medical instruments produced.

[0015] It is also possible to inspect a medical instrument after one or more uses to determine changes in the relative position or orientation of the distal end of the optical fiber relative to the handle and / or the electrode. Such changes can occur, for example, due to deformations and / or plastic changes at connection points, particularly adhesive joints, for example, as a result of sterilization of the medical instrument.

[0016] Optionally, the method and / or testing device according to the invention can be used to switch between the first test state and a further, second test state. This optional second test state serves to check the position of the electrode in or on the handle or the relative position between the electrode and the handle. For this purpose, light is emitted at the distal end of the optical fiber such that the electrode is located in the light path between the distal end face and the projection surface. This creates a shadow image of the electrode on the projection surface. The position and / or orientation of the shadow image relative to the light pattern on the projection surface can be evaluated to check the relative position between the electrode on the one hand and the handle and the optical fiber on the other. It can thus be checked whether the electrode is correctly positioned on the handle.

[0017] In the second test state, a continuous, preferably circular, planar light pattern can be generated on the projection surface. In the second test state and without taking the shadow image of the electrode into account, the light pattern on the projection surface forms, in particular, a fully illuminated, preferably circular light area. In the second test state, the light from the light source can be coupled symmetrically and / or concentrically and / or in the form of a continuous, preferably circular, full-area light spot relative to a center point of the proximal end face onto the proximal end face of the optical fiber. Additionally or alternatively, in the second test state, the optical fiber can be arranged between its distal end and its proximal end in such a way that mode mixing occurs.

[0018] The medical instrument is preferably a monopolar or bipolar electrosurgical instrument, which is used in particular for the coagulation and / or dissection of human and / or animal tissue of a patient.

[0019] As already mentioned, the test criterion or one of the test criteria used can be checking the deviation between an actual shape and a target shape of the light pattern. The actual shape and the target shape can be, for example, a circular ring or a circular ring arc. The actual shape can, for example, be checked to see whether it has the same roundness as the target shape or something similar. Additionally or alternatively, for example, the actual value of an inner diameter and / or the actual value of an outer diameter of the circular ring or the circular ring arc can each be compared with an associated target value. In general, one or more actual values ​​of a dimension can be compared with an associated target value for this dimension, for example the width or thickness of a circular ring or circular ring arc (difference between the outer diameter and the inner diameter).

[0020] Preferably, the light coupled into the proximal end face has a spectrum that also includes one or more of the light wavelengths that may occur during intended use, e.g., during an electrosurgical procedure on at least one tissue type.

[0021] The evaluation of the light pattern can be performed automatically or by an operator. In both cases, markings can be present on the projection surface to facilitate the application of at least one test criterion and, for example, indicate a target value for a dimension and / or a target shape for the light pattern.

[0022] In a preferred embodiment, the light pattern on the projection surface is captured by a camera. The projection surface can be provided on an at least partially transparent screen, so that the camera can capture the generated light pattern on the rear surface of the screen opposite the projection surface. The camera can also be directed at the projection surface. Preferably, the optical axis of the camera is arranged at right angles to the projection surface. The projection surface is, in particular, a flat surface.

[0023] In a preferred embodiment, the optical fiber is a multimode fiber (MMF). In this case, it is advantageous if the optical fiber is arranged between its distal end and its proximal end in such a way that mode mixing is avoided. To this end, the optical fiber is arranged sufficiently stretched or with sufficiently little curvature between the distal end and the proximal end. Preferably, it does not form any curved bends and / or loops between the proximal end and the distal end that would cause mode mixing. For example, the optical fiber can extend in a straight line between the proximal end and the distal end.

[0024] The proximal end face and / or the distal end face can be flat or convex. In both cases, the end face in question can be a polished surface. A flat end face can also be an unfinished fracture surface.

[0025] In a preferred embodiment, the light source generates a contiguous light spot in a surface region of the proximal end face, which is arranged eccentrically with respect to the center of the proximal end face. The centroid of the light spot or surface region is offset from the center of the proximal end face. The light spot or surface region can, for example, be circular or elliptical. Alternatively, the surface region can also be formed by several separate sections, for example several separate circular and / or elliptical light spots. In a further embodiment, the surface region can also be arranged in a ring shape, concentrically or eccentrically with respect to the center of the proximal end face.In any case, the surface area in which light is coupled into the optical fiber is inhomogeneous radially to the center of the proximal end face and / or inhomogeneous in the circumferential direction around the center of the proximal end face.

[0026] The testing device according to the invention comprises a light source, a projection surface, and a holder. The holder is configured to arrange a component of the medical instrument, which is connected to the distal end of the optical fiber, in a predetermined relative position or orientation relative to the projection surface. The light source is configured to couple light into the proximal end face asymmetrically and / or eccentrically. The testing device is configured to be able to carry out any embodiment of the method described above.

[0027] The evaluation of the light pattern can be performed automatically or by an operator. For automatic evaluation, the testing device can optionally include a camera. At least one image of the light pattern captured by the camera can be transmitted to a central unit or evaluation unit. In this case, the central unit is configured to automatically evaluate the at least one image of the light pattern based on the at least one test criterion. The central unit can optionally be configured to control the camera and / or the light source.

[0028] Advantageous embodiments of the invention will become apparent from the dependent claims, the description, and the drawings. Preferred embodiments of the invention are explained in detail below with reference to the accompanying drawings. The drawings show: Figure 1a schematic representation of an embodiment of a medical instrument and a device to which the medical instrument is connected during operation, Figure 2 a block diagram of an embodiment of a testing device for the medical instrument for testing the position and / or alignment of an optical fiber relative to a handle or an electrode of the medical instrument, Figure 3 a schematic diagram of a distal end face of the optical waveguide of the medical instrument from Figures 1 and 2 and a surface area in which light is coupled into the proximal end face of the optical fiber in a first test state and Figure 4 a schematic representation of a light pattern generated on a projection surface of the test device in the first test state, Figure 5a schematic diagram of the distal end face of the optical fiber of the medical instrument from Figures 1 to 3 and a surface area in which light is coupled into the proximal end face of the optical fiber in a second test state and Figure 6 a schematic representation of a light pattern generated on a projection surface of the test device in the second test state.

[0029] Figure 1 shows a system 10 comprising a medical instrument 11 and a device 12 to which the medical instrument 11 can be connected. The medical instrument 11 is an electrosurgical instrument and can be designed as a monopolar or bipolar medical instrument 11 with at least one electrode 13. The electrode 13 is arranged on a handle 14 of the medical instrument 11.

[0030] A cable 15 leads from the handle 14 with the electrode 13 to a connecting element 16. A connector 17 is provided on the device 12, to which the connecting element 16 can be connected. For example, at least one electrical and one optical connection is established between the device 12 and the medical instrument 11 by means of the connecting element 16 and the connector 17.

[0031] When the connection is established between the medical instrument 11 and the device 12, the electrode 13 is connected to an electrical energy source 18 of the device 12 via at least one electrical conductor. The at least one electrical conductor is part of the cable 15 and is not illustrated in the drawing.

[0032] In the cable 15, an optical fiber 20 also extends from a proximal end 21 on the connecting element 16 to a distal end 22 on the handle 14. The distal end 22 is fixedly attached to the handle 14 relative to the handle 14, preferably by a material connection, in particular an adhesive connection. Additionally or alternatively, the distal end 22 can also be connected to the handle 14 in a force-fitting and / or form-fitting manner.

[0033] At the proximal end 21, the light source guide 20 has a proximal end face 23 and at the distal end 22, the optical waveguide 20 has a distal end face 24. The optical waveguide 20 can have a core 18 and a cladding 19 ( Figures 3 and 5 ). The proximal end face 23 and distal end face 24 are considered, for example, to be the end faces of the core 18 without the sheath 19.

[0034] When the connection is established, an optical connection is established between the proximal end 21 and an evaluation unit 25 of the device 12 via the connecting element 16 and the connector 17.

[0035] The medical instrument 11 is designed, for example, as an electrosurgical instrument, wherein animal or human tissue 26 can be treated by means of the at least one electrode 13, for example by coagulation and / or dissection. Light L generated during the treatment of the tissue 26 is received at the distal end face 24 and guided via the optical fiber 20 to the proximal end 21. At the proximal end 21, the light L is at least partially transmitted via the connection 17 to the evaluation unit 25, where it can be evaluated, for example, to classify and / or identify the tissue 26.

[0036] In order to perform such an evaluation of the treated tissue 26, the optical waveguide 20, and in particular its distal end 22, must be arranged and / or aligned very precisely relative to the handle 14 or relative to the electrode 13 in order to capture sufficient light L generated during the treatment of the tissue 26. Otherwise, an evaluation or classification of the tissue 26 is not possible or only inadequately possible.

[0037] According to the invention, the medical instrument 11 is therefore checked to determine whether the alignment and / or positioning of the distal end 22 of the optical waveguide 20 relative to the handle 14 and / or the at least one electrode 13 is correct and corresponds to the specifications. For this purpose, a testing device 30 can be used, as shown in Figure 2is schematically illustrated. The test device 30 has a light source 31 for coupling light into the proximal end face 23 of the optical waveguide 20 and a projection surface 32. The projection surface 32 is arranged and aligned such that light emerging from the distal end face 24 of the optical waveguide 20 strikes the projection surface 32 and generates a light pattern M there ( Figure 4 ). The light pattern M can, for example, be a circular ring, as shown in Figure 4 shown, or alternatively only at least a section of it, i.e. at least one circular ring arc.

[0038] The projection surface 32 is, for example, a flat surface. In the exemplary embodiment, the projection surface 32 is provided on a screen 33 of the test device. The projection surface 32 can, for example, be arranged at right angles to the direction of extension of the electrode 13 and be arranged close to the free end of the electrode 13 or in contact with it.

[0039] A camera 35 of the testing device 30 is optionally arranged on the rear side 34 of the screen 33, opposite the projection surface 32. The screen 33 is at least partially transparent to the light from the light source 31, so that the camera 35 can capture the light pattern M generated on the rear side 34 of the projection surface 32. The camera 35 is communicatively connected to a central unit 36. An image of the light pattern M recorded by the camera 35 can be transmitted to the central unit 36. The central unit 36 ​​can be configured to control the light source 31 and the camera 35. By means of a holder 37 of the testing device 30, the handle 14 is brought into a defined position and / or orientation relative to the projection surface 32, for example. The holder 37 thus also defines the relative position and orientation of the distal end face 24 of the optical fiber 20 relative to the projection surface 32.

[0040] In the exemplary embodiment, the optical fiber 20 is a multimode fiber (MMF). The proximal end face 23 and the distal end face 24 are designed, for example, as flat surfaces and could also be convexly curved as a modification.

[0041] The light source 31 can, for example, be a semiconductor light source and generate light by means of a light-emitting diode or laser diode.

[0042] In a test state, which is referred to as the first test state to distinguish it from an optional further test state, the light of the light source 31 at the proximal end 21 is coupled into the optical waveguide 22 asymmetrically and / or eccentrically with respect to a center point P of the proximal end face 23, as is shown in a highly schematic manner in the Figures 2 and 3can be seen. In an embodiment described here, the light of the light source 31 is coupled into a surface area A of the proximal end face 23, which can, for example, have a circular or elliptical shape. The surface area A has a center of gravity S. The center of gravity S is arranged at a distance from the center point P of the proximal end face 23, as shown in Figure 3 As an alternative to the illustration in Figure 3 the center point P can also be arranged within the surface area A into which light from the light source 31 is coupled. In any case, the surface area A into which light from the light source 31 is coupled is selected such that light from the light source 31 is coupled unevenly into the proximal end face 23 radially to the center point P and / or in the circumferential direction around the center point P.

[0043] In a variation of the Figure 3In the example shown, the surface section A can be discontinuous, so that light from the light source 31 can be coupled into several spaced-apart sections of the surface area A in the proximal end face 23.

[0044] As it is in Figure 2 As schematically illustrated, the test device 30 can optionally include a coupling device 38 configured for optically coupling the optical waveguide 20 to the light source 31. The coupling device 38 can, for example, be configured to establish an optical connection between the connecting element 16 and the light source 31 such that the light from the light source 31—as described above—is coupled in asymmetrically or eccentrically with respect to the center point P of the proximal end face 23.

[0045] In order to avoid mode mixing during testing in the first test state, the optical waveguide 20 is preferably sufficiently straightened during testing and can extend substantially straight between the proximal end 21 and the distal end 22, as shown schematically in Figure 2 Alternatively, the optical waveguide 20 could also run in smaller curves with sufficiently small curvature to avoid mode mixing.

[0046] Due to the asymmetrical and / or eccentric coupling of light from the light source 31 into the proximal end face 23 in the first test state, the light emerging at the distal end face 24 generates the light pattern M on the projection surface 32, for example a light pattern M in the form of a closed circular ring with an outer radius ra and an inner radius ri ( Figure 4 ).

[0047] Based on the shape of the light pattern M and / or at least one dimension of the light pattern M in at least one spatial direction parallel to the projection surface 32, it can be checked in the first test state whether the relative positions of the relative alignment of the distal end 22 or the distal end face 24, on the one hand, relative to the electrode 13 and / or the handle 14, on the other hand, correspond to the specifications. For example, it can be checked whether the actual shape of the light pattern M corresponds to a desired shape of the light pattern M, for example whether the light pattern M has a circular ring shape or a circular arc shape. Additionally or alternatively, one or more parameters can be checked, such as an actual value for the inner radius ri and / or an actual value for the outer radius ra, which can each be compared with an associated desired value.

[0048] By applying at least one test criterion, it can be determined in the first test state whether the medical instrument 11 and in particular the fastening of the light source conductor 20 to the handle 14 sufficiently corresponds to the specifications and the medical instrument 11 therefore meets the quality requirements.

[0049] The first test condition ( Figures 2 to 4 ) can be the only test state of the method and / or the test device 30. Optionally, a further, second test state of the method and / or the test device 30 can be present ( Figures 5 and 6), whereby it is possible to switch between the first test state and the second test state. This optional second test state serves to check the fit of the electrode 13 in the handle 14 or the relative position between the electrode 13 and the handle 14. For this purpose, light is emitted at the distal end 22 of the optical fiber in such a way that the electrode 13 is located in the light path between the light emerging from the distal end face 24 and the projection surface 32. As a result, a shadow image B of the electrode can be seen on the projection surface 32 ( Figure 6 The position of the shadow image B relative to the light pattern M on the projection surface 32 can be evaluated to check the relative position between the electrode 13 on the one hand and the handle 14 and the optical fiber 20 on the other. It can thus be checked whether the electrode 13 is correctly positioned on the handle 14.

[0050] In the second test state, a continuous planar light pattern M is generated on the projection surface 32, for example ( Figure 6). Without taking into account the shadow image B of the electrode 13, the light pattern M in the second test state forms, for example, a fully illuminated circular light area on the projection surface 32. In the second test state, the light from the light source 31 can be coupled symmetrically and / or concentrically and / or in the form of a fully illuminated, continuous - preferably circular - light spot relative to the center point P of the proximal end face 23 onto the proximal end face 23 of the optical waveguide 20. Additionally or alternatively, the optical waveguide 20 in the second test state can be arranged between its distal end 22 and its proximal end 21 such that mode mixing occurs. For example, the optical waveguide 20 in the second test state can be arranged to deviate from its stretched position by being sufficiently curved or bent at least once.

[0051] The test in the first and / or second test state can, for example, be carried out randomly for newly manufactured medical instruments 11 or for all manufactured medical instruments 11. Additionally, it is also possible to test a medical instrument 11 repeatedly after its use and / or sterilization, so that it can be detected if changes such as plastic deformation or the like occur due to external influences, in particular temperature influences during sterilization, which result in the medical instrument 11 no longer meeting the requirements. For example, temperature influences can impair the adhesive bond between the distal end 22 of the optical fiber 20 and the handle 14, which can change the alignment or position of the distal end 22 relative to the handle 14.

[0052] The invention relates to a method and a testing device 30 for testing whether the relative position and relative alignment of a distal end 22 of an optical fiber 20 relative to an electrode 13 and / or a handle 14 of a medical instrument 11 corresponds to the specifications and is thus correct. For this purpose, light is coupled into a proximal end 21 of the optical fiber 20 and at least partially emitted at the distal end 22 of the optical fiber 20 onto a projection surface 32. At the proximal end 21, the light from the light source 31 is coupled eccentrically and / or asymmetrically with respect to a center point P of a proximal end face 23 of the optical fiber 20. This creates a light pattern M on the projection surface 32, in particular an annular light pattern M, which can be tested using at least one test criterion in order to check the correct arrangement or alignment of the distal end 22.The test can be performed automatically using a camera 35 and a central unit 36, or by an operator by observing the light pattern M on the projection surface 32. Optionally, in a further test state, it can be checked whether the electrode 13 is correctly positioned on the handle 14. List of reference symbols:

[0053] 10System 11Medical instrument 12Device 13Electrode 14Handle 15Cable 16Connector 17Connector 18Core 19Sheath 20Fiber optic cable 21Proximal end 22Distal end 23Proximal face 24Distal face 25Evaluation unit 26Tissue 30Test device 31Light source 32Projection surface 33Screen 34Rear 35Camera 36Central unit 37Mounting bracket 38Coupling device AFarea area BShadow image LLight MLight pattern PCenter point raOuter radius riInner radius SCenter of gravity

Claims

1. Method for testing an orientation and / or positioning of a distal end (22) of an optical fiber (20) of a medical instrument (11) relative to a handle piece (14) of the medical instrument (11) and / or relative to an electrode (13) of the medical instrument (11), comprising the following steps: - arranging a distal face (24) of the distal end (22) of the optical fiber (20) in a predefined orientation relative to a projection surface (32), - coupling light of a light source (31) into a proximal face (23) at a proximal end (21) of the optical fiber (20) opposite the distal end (22) in an unsymmetric and / or eccentric manner relative to a center (P) of the proximal face (23), - emitting at least a part of the coupled light from the distal face (24) at the distal end (22) of the optical fiber (20) onto the projection surface (32), and - evaluating a light pattern (M) created on the projection surface (32) based on at least one predefined test criterion for testing the orientation of the distal end (22) of the optical fiber (20).

2. Method according to claim 1, wherein the at least one test criterion comprises the test of a deviation between an actual shape of the light pattern (M) on the projection surface (32) and a predefined desired shape.

3. Method according to claim 1 or 2, wherein the actual shape and / or the desired shape of the light pattern (M) is a closed ring or ring arc.

4. Method according to any of the preceding claims, wherein the at least one test criterion comprises the test of a deviation between an actual value of a dimension of the light pattern (M) in at least one spatial direction parallel to the projection surface (32) and a predefined desired value of the dimension in this spatial direction.

5. Method according to claims 3 and 4, wherein the deviation between a desired value and an actual value of the inner radius (ri) and / or the outer radius (ra) of the ring or ring arc is tested.

6. Method according to any of the preceding claims, wherein an image of the light pattern (M) on the projection surface (32) is captured by means of a camera (35) and the image is transmitted to a central unit (36) for automatic evaluation based on the at least one test criterion.

7. Method according to claim 5, wherein the central unit (36) is configured to control the camera (35) and the light source (31).

8. Method according to any of the preceding claims, wherein the optical fiber (20) is a multi-mode fiber.

9. Method according to claim 7, wherein the optical fiber (20) is arranged between its distal end (22) and its proximal end (21) so that mode mixing is avoided.

10. Method according to claim 8, wherein the optical fiber (20) is substantially stretched between its distal end (22) and its proximal end (21).

11. Method according to any of the preceding claims, wherein the light of the light source (31) is coupled into a surface section (A) of the proximal face (23) of the optical fiber (20), wherein the centroid of an area (S) of the surface section (A) is offset relative to the center (P) of the proximal face (23).

12. Method according to any of the preceding claims, wherein the proximal face (23) and / or the distal face (24) is a planar surface.

13. Method according to any of the preceding claims, wherein in another test condition the electrode (13) is illuminated by light exiting the distal face (24) in order to create a silhouette (B) of the electrode (13) on the projection surface (32) and in order to test based on the silhouette (B) whether the electrode (13) is correctly arranged relative to the handle piece (14).

14. Test device (30) for carrying out the method according to any of the preceding claims comprising: - a light source (31) for coupling light into a proximal face (23) at a proximal end (21) of an optical fiber (20) unsymmetrically and / or eccentrically relative to its center (P), - a projection surface (32) and - a holder (37) for arranging a distal end (22) of the optical fiber (20) in a predefined relative position relative to the projection surface (32).

15. Test device according to claim 14, further comprising a camera (35) for capturing an image of a light pattern (M) created on the projection surface (32).