Automatic angle compensator

Abstract: An automatic angle compensator comprising a liquid sealing container 4 having transparent liquid to form a free liquid surface 1 sealed therein, light projecting systems 21 and 22 for projecting light at a given angle to the free liquid surface, and an optical system for guiding the light beam, which is reflected by the free liquid surface and irradiated from the liquid sealing container, whereby optical axis of the light beam reflected by the free liquid surface is aligned with optical axis of the optical system regardless of whether the free liquid surface is turned by 0° or 90°, and automatic compensation can be achieved regardless of whether the system is positioned in horizontal or vertical directions.

Description: BACKGROUND OF THE INVENTION The present invention relates to an automatic angle compensator, which is used for survey instruments and measuring instruments to measure the change in the amount of inclination or to maintain optical axis of the instrument in vertical or horizontal direction or to form a irradiation plane of horizontal or vertical light beam. When various surveying operations are performed using survey instrument or measuring instrument, it is necessary to compensate reference plane of the survey or measuring instruments, or to compensate verticality of optical axis. In the past, automatic compensators have been known, in which a pendulum of lens or prism is suspended from two or three suspension lines and, when main unit of the survey or measuring instrument is inclined, the pendulum is braked by a braking mechanism such as magnetic braking mechanism to automatically compensate optical path. Or, by utilizing back reflection of transparent liquid, optical path is automatically compensated using an optical system such as anamorphic prism so that reflected light beam will have an optical axis with the same sensitivity with respect to the change in all inclined directions of the liquid surface. In the prior art as described above, however, the compensator can be used only when the entire system is installed approximately in horizontal or vertical directions, and it is difficult to use the compensator for both approximately horizontal and approximately vertical installing conditions. To use the compensator for the two conditions, a separately prepared optical system must be used, or a part of the optical system must be designed as replaceable so that it can be replaced to suit for horizontal or vertical applications. In such cases, the reproducibility becomes low and accuracy is reduced because the optical system must be removed or assembled. Also, there is the problem of higher cost in the components. EP-A-0,161,207 discloses a liquid prism for use in an inclination measuring apparatus. The prism comprises a container which contains a liquid for providing a free liquid surface which is perpendicular to the direction of gravity. An optical system is provided to guide a beam of light at a given angle onto the free liquid surface so that the beam is reflected from that surface. However, this system cannot be used for both approximately horizontal and approximately vertical installing conditions. EP-A-0,366,150 describes a datum beam projecting apparatus for use with surveying equipment. The apparatus includes plane-parallel glass to compensate for the tilt of the beam and to maintain the direction of the beam constant with respect to the tilt of the apparatus. However, there is no disclosure of an element including a free liquid surface, nor can the system be used for both approximately horizontal and approximately vertical installing conditions. According to the present invention, there is provided an apparatus for use in an automatic angle compensator as specified in claim 1. Some preferred features are specified in the dependent claims. It is an object of the present invention to provide an automatic compensator, by which it is possible to automatically compensate optical path by utilizing free liquid surface and to use the compensator for both approximately horizontal or vertical installing conditions without requiring a specially designed optical system or an addition mechanism to replace or rearrange a part of the optical system. To attain the above object, the automatic angle compensator of the present invention comprises a liquid sealing container with a transparent liquid to form a free liquid surface sealed therein, a first light projecting system for irradiating light beam at a given angle to the free liquid surface so that it is reflected by the free liquid surface, a second light projecting system arranged face-to-face to the first light projecting system with the free liquid surface therebetween and for irradiating light beam at a given angle to the free liquid surface so that the light beam is reflected by the free liquid surface when the entire system is rotated by 90°, and a profile projecting optical system for guiding and projecting the light beam reflected by the free liquid surface. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a drawing for explaining a first embodiment of the present invention; Fig. 2 is a drawing for explaining the first embodiment when it is rotated by 90 degrees; Fig. 3 is a front view of a liquid sealing container provided with a liquid movement preventing device and used for the present invention; Fig.4 is a cross-sectional side view of the liquid sealing container provided with the liquid movement preventing device; Fig.5 is a perspective view of the liquid movement preventing device; Fig.6 is a drawing for explaining an application example of the present invention; Fig.7 is a drawing for explaining relationship between beam spot and pinhole in an application example of the present invention; Fig. 8 is a drawing for explaining change of reflection angle of the reflected light beam in case the free liquid surface is inclined; Fig. 9 is a drawing for explaining change of reflection angle of the reflected light beam in case the free liquid surface is inclined; Fig. 10 is a drawing for explaining automatic angle compensation in case the light beam is reflected once by the free liquid surface; Fig. 11 is a drawing for explaining change of optical axis of the transmitted light beam with respect to anamorphic prism system; Fig. 12 (A) and Fig. 12 (B) each represents a drawing to show change of optical axis of the transmitted light beam with respect to an anamorphic prism system; Fig. 13 is a drawing for explaining change of reflection angle of the reflected light beam in case it is reflected twice by the free liquid surface; and Fig. 14 is a drawing for explaining change of reflection angle of the reflected light beam when the free liquid surface is inclined in case the light beam is reflected twice by the free liquid surface. DETAILED DESCRITPTION OF THE PREFERRED EMBODIMENTS In the following, description will be given on an embodiment of the present invention in connection with the drawings. In case light beam is irradiated to a free liquid surface at a given angle and it is totally reflected by the free liquid surface, sensitivity of change in reflection angle varies according to inclined direction of the liquid surface when the free liquid surface is relatively inclined with respect to the light beam. First, description will be given of a compensator, which comprises an optical system to equalize the sensitivity of change in reflection angle of the reflection optical axis in all directions and a beam expander for angular magnification adjustment, both arranged at predetermined positions on the reflection optical axis, and which can maintain the reflection optical axis in a constant direction regardless of inclination of the entire system. In case light beam is irradiated to a free liquid surface at a given angle and it is totally reflected by the free liquid surface, sensitivity of the change in reflection angle according to inclined direction of the liquid surface when the free liquid surface is relatively inclined with respect to the light beam. This is explained, referring to Fig. 8 and Fig. 9. In fact, the free liquid surface is maintained in horizontal direction and the direction of incident varies, while it is assumed that the direction of incident light beam is constant and that the free liquid surface is inclined to simplify the following explanation. In the figures, reference numeral 1 represents a free liquid surface, and it is supposed that an incident light beam 2 enters the free liquid surface at an angle of . It is further supposed that the free liquid surface 1 approximately agrees with a x-z coordinate plane formed by coordinate axes x and z and that a coordinate axis perpendicular to the coordinate plane is y. It is also supposed that optical axis of the incident light beam 2 is present in a coordinate plane, which is formed by the coordinate axis z and the coordinate axis y. If the free liquid surface 1 is inclined by an angle α around the coordinate axis x from the above condition, optical axis of the reflected light beam 3 moves within the x-z coordinate plane. and reflection angle is changed by ξ 1x within the y-z coordinate plane. In this case, the relation between the liquid surface displacement angle α and the reflection displacement angle ξ 1x is given by ξ 1x = 2α . There is no reflection displacement angle ξ 2x within the x-y coordinate plane. In the figure, reference numeral 14 represents a reflecting mirror. In contrast, if the free liquid surface 1 is inclined by an angle α around the coordinate axis z as shown in Fig. 15, the reflected light beam 3 is separated and moved from each of the x-y coordinate plane and the y-z coordinate plane. Therefore, a reflection displacement angle ξ 1z and a reflection displacement angle ξ 2x appear on the x-y coordinate plane and the y-z coordinate plane respectively. Further, the relation between the reflection displacement angle ξ 1z and the liquid surface displacement angle α of the free liquid surface 1 is given by: ξ 1z = cos -1 ( cos 2  cos2α + sin 2  ) ξ 2z = 90° - cos -1 ( (1 - cos2α ) sin cos ) For example, if it is supposed that α = 10' and  = 50° , ξ 2z = 1.7", the value of ξ 2z is negligible in terms of accuracy. Further, if it is supposed that refractive index of the liquid is n, optical axis after passing through the liquid is given by: ξ 1x' = 2nα ξ 1z' = n • cos -1 ( cos 2  cos2α + sin 2  ) Therefore, sensitivity to the liquid surface displacement angle α varies between the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z'. The difference of the sensitivity between the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z' is compensated by optical means to equalize the sensitivity. As a result, it is possible to obtain an optical axis, which is deflected always at a given ratio with respect to all directions. Further explanation will be given referring to Fig. 10. In this figure, reference numeral 4 represents a liquid sealing container provided on main unit of an instrument such as a survey instrument, and a free liquid surface 1 is formed by the liquid sealed in the liquid sealing container 4. A light beam emitted from a light source 6 is projected at a given angle via a collimator lens 5 toward the free liquid surface 1 so that it is totally reflected, and optical axis of the light beam is positioned within the y-z coordinate plane as already described. With the free liquid surface 1 not inclined, an anamorphic prism system 9 is arranged, which comprises a pair of wedge-like prisms 7 and 8 along optical axis of the reflected light beam 3, which has been totally reflected by the free liquid surface 1. After passing through the anamorphic prism system 9, the light beam is reflected by a reflecting mirror 14 in vertical direction, and the light beam reflected by the reflecting mirror 14 passes through a beam expander 12, which comprises convex lenses 10 and 11. If it is supposed that focal length of the convex lens 10 is f3 and focal length of the convex lens 11 is f4, the distance between the convex lenses 10 and 11 is set to f3 + f4. The anamorphic prism system 9 may be arranged in the optical path after light beam has been reflected by the reflecting mirror 14. When it is supposed in Fig. 10 that the preset incident angle  to liquid is 45° , inclination angle α of the instrument, i.e. inclination angle of the free liquid surface 1, is 10', and refractive index n of the liquid is 1.5, the reflection displacement angle ξ 1x' in case the free liquid surface 1 is inclined around x axis and the reflection displacement angle ξ 1z' in case the free liquid surface 1 is inclined around z axis are given by the equation (2) as: ξ 1x' = 30', and ξ 1z' = 21.213'. Therefore, the difference of sensitivity between the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z' is: ( ξ 1x'/ξ 1z') = 1.414. Thus, ξ 1x' = 2nα , ξ 1z' = 1.414nα ξ 1x' /ξ 1z' = 1.414 Next, the above sensitivity difference is optically compensated by the anamorphic prism system 9. Now, description is given on the anamorphic prism system 9 referring to Fig. 11 and Fig. 12. It is supposed that vertical angles of the wedge-like prisms 7 and 8, which constitute the anamorphic prism system 9, are a7 and a8 respectively, that relative angle between the wedge-like prisms 7 and 8 is b, that refractive index is ng, that incident light beam is Din and outgoing light beam is Dout, and that the wedge-like prisms 7 and 8 are identical with each other and vertical angles of these prisms are a = a7 = a8. Then, Magnification Map = (Din/Dout) = cos 2 a/(1-ng 2 • sin 2 a) Therefore, angular magnification is approximately 1/Map. To satisfy the relation: Map = 2n α/1.414nα = 1.414 it is supposed that vertical angle of the prism is a, relative angle of the wedge-like prisms 7 and 8 is b, and refractive index ng are adequately selected (e.g. a = 25.559°, and b = 40.653° if ng = 1.51 ). Then, the value of ξ 1x' after passing through the anamorphic prism system 9 is converted to: 2nα × 1.414 nα /2nα = 1. 414 nα , and ξ 1x'= ξ 1z' after passing through the anamorphic prism system 9. After passing the anamorphic prism system 9, optical axis of the reflected light beam 3 always has a uniform reflection displacement angle with respect to inclination of the free liquid surface 1 in all directions. Even when the free liquid surface 1 is inclined in any direction, it is possible to always obtain reflection displacement angle with the same sensitivity with respect to the inclination of the free liquid surface 1. Further, after passing through the anamorphic prism 9 and being reflected upward by the reflecting mirror 14, the light beam passes through the beam expander 12. In this case, if angular magnification of the beam expander 12 is 1/1.414n, the optical axis after transmitting is inclined by: (ξ 1x' = ξ 1z' = 1.414nα ) x 1 /1.414n = α and final optical axis after passing through the beam expander 12 always runs perpendicularly to the free liquid surface 1, i.e. it is maintained in vertical condition. If it is supposed that focal length of the convex lens 10 of the beam expander 12 is f3 and focal length of the convex lens 11 is f4, angular magnification Mex of the beam expander 12 is f3/f4. Accordingly, by selecting the values of f3 and f4, angular magnification Mex can be set to 1/1.414n. Next, in the embodiment shown in Fig. 10, the values of vertical angles a7 and a8 of the wedge-like prisms 7 and 8, relative angle b of the wedge-like prisms 7 and 8, and refractive index ng may be adequately selected so that the anamorphic prism system 9 may be rotated by 90° and M = 1/1.414. The anamorphic prism system 9 in the above optical system is generally used for the purpose of changing the beam of elliptical shape to circular shape. For example, in case a laser diode is used as the light source, cross-sectional shape of the light beam can be made closer to circular shape by means of the anamorphic prism system 9. (Beam of laser diode is in elliptical shape.) In general, instrument or device with such a laser diode incorporated in it often serves as a laser pointer or a laser marker, and it is desirable that the shape of the irradiated beam is closer to circular shape. Therefore, the compensation of optical axis using the anamorphic prism system 9 is very effective to obtain the beam in circular shape. As described above, it is possible to obtain optical axis, which is constantly deflected at a constant ratio with respect to all directions by means of compensating the difference of sensitivity of displacement angles between the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1x' with respect to the liquid surface displacement angle by optical means and equalizing the sensitivity. Next, referring to Fig. 13 and Fig. 14, description will be given on the difference of sensitivity of change in the reflection angle in case the light beam is irradiated at a given angle to a free liquid surface and is totally reflected on the free liquid surface, the emitted light beam is reflected by a reflecting member such as a mirror and is again irradiated to the free liquid surface and is totally reflected by the free liquid surface, and also the case where the light beam is totally reflected twice by the free liquid surface with respect to inclined direction of the liquid surface when the free liquid surface is relatively inclined with respect to the light beam. In Figs. 13 and 14, the same component as in Figs. 8 and 9 is referred by the same symbol. Description is given on the case where incident light beam 2 reflected by a beam splitter 15 enters the free liquid surface 1 at an angle of  and is totally reflected by the free liquid surface 1, and it is again reflected by a reflection mirror 16 and enters again the free liquid surface 1, and is totally reflected by the free liquid surface 1, and passes through the beam splitter 15 and is reflected by the reflecting mirror 14. It is assumed that the x-z coordinate plane formed by coordinate axes x and z are approximately agrees with the free liquid surface 1, and that coordinate axis perpendicular to the above coordinate plane is y. It is also assumed that optical axis of the incident light beam 2 is present within a coordinate plane formed by the coordinate axes z and y. If the free liquid surface 1 is inclined by an angle α around the coordinate axis x from the above condition, optical axis of the reflected light beam 3 is moved within the y-z coordinate plane, and the reflection angle is changed by ξ 1x within the y-z coordinate plane. In this case, the relation between the liquid surface displacement angle α and the reflection displacement angle ξ 1x is given by ξ 1x = 4α , and there is no reflection displacement angle ξ 2x within the x-y coordinate plane. In contrast, if the free liquid surface 1 is inclined by an angle α around the coordinate axis z as shown in Fig. 14, the reflected light beam 3 is separated and moved from each of the x-y coordinate plane and the y-z coordinate plane. Therefore, the reflection displacement angle ξ 1z and the reflection displacement angle ξ 2z appear on the x-y coordinate plane and the y-z coordinate plane respectively. Further, the relation between the reflection displacement angle ξ 1z and the liquid surface displacement angle α of the free liquid surface 1 is given by: ξ 1z = cos -1 { cos2α • sin 2 2 - cos2 • sin 2  - (sin 2 2α - cos 2 2α • cos2 ) cos 2  } ξ 2z = 90° - cos -1 [{ 1/2 ( sin 2 2α - cos 2 2α • cos2 - cos2) + cos2α • cos2 } sin2 ] For example, if it is supposed that α = 10' and  = 45° , ξ 2z = 3.49", the value of ξ 2z is negligible in terms of accuracy. Further, if it is supposed that refractive index of the liquid is n, an optical axis of the light beam after light beam passing through the liquid is given by: ξ 1x' = nξ 1x ξ 1z' = nξ 1z More concretely, if α = 10',  = 45°, and n = 1.5, ξ 1x' = 60' ξ 1z' = n cos -1 { cos2α - 1/2 (sin 2 2α) } = 42.426' ξ 1x'/ξ 1z' = 1.414 Thus, when  = 45° , the difference of sensitivity to the liquid surface displacement angle α with respect to the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z' is the same as the difference in case the light beam is reflected once by the free liquid surface 1. Therefore, the anamorphic prism system 9 can be used, which makes it possible to obtain reflection displacement angle of the same sensitivity to the inclination. Here, angular magnification Map of the anamorphic prism system 9 is given by: Map = 4nα / 2.829nα = 1.414 1 / Map = γ ap = 1 / 1.414 = 0.70721 Therefore, when the light beam passes through the anamorphic prism system 9, its optical axis is compensated as: ξ 1x' = 4nα × 1/ 1.414 = 2.829nα ξ 1z' = 2.829nα Further, if it is supposed that n = 1.5, magnification Mex of the beam expander 12, which is provided to maintain optical axis of light beam in vertical direction after light beam passes through the anamorphic prism system 9, may be given by: Mex = 2.829nα /α = 2.829n = 4.244 γ ex = 1 / Mex = 1/ 2.829n = 0.236 As described above, it is possible to obtain optical axis, which is constantly deflected at a constant ratio with respect to all directions by means of the difference of sensitivity of displacement angles between the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z' with respect to the liquid surface displacement angle by optical means and equalizing the sensitivity. In the following, description will be given on embodiments of the present invention provided with an automatic angle compensator as described above. Fig. 1 represents a first embodiment of the present invention. A liquid sealing container 4 with transparent liquid 20 sealed in it is provided. Also, there are provided a first light projecting system 21, which has an optical axis to irradiate light beam 25 to the free liquid surface 1 of the transparent liquid 20 in horizontal direction and at an angle of 45° , and a second light projecting system 22, which is arranged face-to-face to the first light projecting system 21 with the liquid sealing container 4 therebetween on the same optical axis as that of the first light projecting system 21. A liquid movement preventing device 23 is arranged inside the liquid sealing container 4. The liquid movement preventing device 23 has its rotating center at the point where optical axis of the first light projecting system 21 crosses the transparent liquid 20. As described later, the liquid movement preventing device 23 has a transparent liquid movement preventing plate 24 at a position a little below the free liquid surface 1 of the transparent liquid 20, and it is freely movable in order to keep the liquid movement preventing plate 24 always in horizontal position. After entering the liquid sealing container 4, the light beam 25 from the first light projecting system 21 passes through the transparent liquid 20 and the liquid movement preventing plate 24. It is then totally reflected by the liquid surface of the transparent liquid 20, passes through the liquid movement preventing plate 24 and the transparent liquid 20 and is emitted out of the liquid sealing container 4. The light beam 26 reflected by the free liquid surface 1 of the transparent liquid 20 is reflected by a reflecting mirror 14 toward vertical direction. Along optical path of the light beam 26 reflected by the reflecting mirror 14, an anamorphic prism system 9, a beam expander 12 and a pentagonal prism 27 are arranged. The pentagonal prism 27 is freely rotatable and emits the reflected light beam 26 in horizontal direction. As described above, the anamorphic prism system 9 equalizes reflection sensitivity in all directions, and the beam expander 12 ultimately adjusts the sensitivity. Therefore, after passing through the beam expander 12, optical axis of the light beam is always compensated in vertical direction regardless of inclination of the entire system. Accordingly, by rotating the pentagonal prism 27, it is possible to obtain a constant horizontal reference plane at all times. In other words, the present invention can be applied as a leveling instrument. Next, Fig. 2 represents the case where the entire system is rotated by 90° . The liquid movement preventing device 23 is rotated by 90° so that the liquid movement preventing plate 24 is kept at horizontal position. Because the free liquid surface 1 of the transparent liquid 20 is kept in horizontal direction with respect to the center of gravity, it follows the angle of 90° of the entire system and is rotated by 90°. The light beam coming from the second light projecting system 22 is totally reflected by the free liquid surface 1 of the transparent liquid 20, and optical axis of the reflected light beam runs along the same optical axis as the emitting optical axis before turned by 90° , optical axis of the light beam passing through the beam expander 12 is always kept in horizontal direction. By rotating the pentagonal prism 27, the irradiated reference plane is converted toward vertical direction, and it is possible to obtain a constant vertical reference plane regardless of inclination of the entire system. The transparent liquid 20 sealed in the liquid sealing container 4 is moved by external vibration, while this movement is suppressed by the liquid movement preventing device 23. Referring to Figs. 3 to 5, description is now given on the liquid movement preventing device 23 together with the liquid sealing container 4. In the liquid sealing container 4 with regular octagonal shape, the transparent liquid 20 is sealed, and the liquid movement preventing device 23 having rotating center in the free liquid surface 1 of the transparent liquid 20 is provided via a pendulum shaft 28. The liquid sealing container 4 comprises a transparent member made of a material such as glass, or at least a part of the liquid sealing container 4, which the light beam enters through and emits light beam, comprises a transparent member. The liquid movement preventing device 23 comprises a moving frame 29 with concave cross-section with the pendulum shaft 28 protruding from it and a pair of trapezoidal weights 30 suspended from lower surface of the moving frame 29. At the middle portion of the moving frame 29, a transparent plate to pass the light beam, i.e. the liquid movement preventing plate 24 made of glass plate, not to attenuate the light, is inserted. The liquid movement preventing plate 24 and upper surface of the moving frame 29 are on the same level, and the upper surface of the liquid movement preventing plate 24 is a little below the free liquid surface 1. The center of gravity of the liquid movement preventing device 23 is below the liquid movement preventing plate 24, and the surface of the liquid movement preventing plate 24 is maintained at horizontal position regardless of the posture of the liquid sealing container 4. After entering the liquid sealing container 4, the light beam passes through the transparent liquid 20 and the liquid movement preventing plate 24 and it is totally reflected by the free liquid surface 1. Then, it passes through the liquid movement preventing plate 24 and the transparent liquid 20 again and goes out of the liquid sealing container 4. When natural frequency of the liquid movement preventing device 23 is set to a value lower than natural frequency of the transparent liquid 20 sealed in the liquid sealing container 4, the movement of the free liquid surface 1 is suppressed by viscosity of the liquid and resistance of the liquid movement preventing plate 24. Because the center of rotation of the liquid movement preventing device 23 is at the reflection point of the light beam on the free liquid surface 1. the same condition can be maintained even in case the liquid sealing container 4 is rotated by 90° . If the light beam enters from the opposite direction as described above, the light beam is reflected in the same direction. Thus, angle compensation can be achieved when the automatic angle compensator is at both horizontal and vertical positions. Here, incident angle of the light beam from the first light projecting system 21 and the second light projecting system 22 to the free liquid surface 1 is approximately 45°. Thus, to totally reflect the light beam, refractive index of the liquid must be about 1.4 or more. A part of the reflected light may be utilized without reflecting totally. Also, the liquid movement preventing plate 24 and upper surface of the moving frame 29 may not be on the same level, and the weights 30 are not limited to trapezoidal shape and may be designed in rectangular, circular or other shapes. It is needless to say that the reflecting mirror 14 may be replaced with other type of reflecting member such as a prism. Fig. 6 represents an application example of the present invention. When each of the above embodiments is used in practical application, inclination of the entire system is usually limited. Therefore, it is necessary in practical application to detect whether it is within the limitation of the inclination as required or not. To meet the requirement, it is recommended to perform as follows: Description is given in the first embodiment shown in Fig. 1. A half-mirror 40 is arranged instead of the reflecting mirror 14, and the anamorphic prism system 9 is moved and is positioned between the half-mirror 40 and the liquid sealing container 4. The reflected light beam from the free liquid surface 1 is split to a reflected light beam 41 directed in vertical direction and a transmitted light beam 42. The transmitted light beam 42 passes through a convex lens 43 and a pinhole 45 formed on a shielding plate 44 and is received by a light receiving element 46. The pinhole 45 is arranged at the focal point of the convex lens 43. The diameter of the pinhole 45 is designed in a size corresponding to the range of limitation. When the entire system is inclined, optical axis angle is deflected in the reflected light beam from the free liquid surface 1. Because this arrangement comprises a cylindrical lens system 9 as already described, the reflected light beam shows uniform sensitivity in all directions with respect to inclination angle of the entire system. The transmitted light beam 42 passes through the convex lens 43 and the pinhole 45 and is received by the light receiving element 46. Because the pinhole is arranged at the focal point of the convex lens 43, the converged light beam moves along the pinhole 45. If the amount of moving is high, the light beam is shielded by the shielding plate 44. When this amount of moving exceeds the limitation of inclination as required, the diameter of the pinhole 45 is determined in such manner that the light receiving amount of the light receiving element 46 is less than the light quantity required (see Fig. 7). By monitoring the light receiving amount of the light receiving element 46, it is possible to judge whether the inclination of the entire system is within the limited angle. The pinhole 45 may be omitted, and the light receiving element 46 may be designed as a light receiving element such as CCD, and the position of the beam spot may be detected by the light receiving element 46. As described above, it is possible according to the present invention to automatically compensate optical path regardless of whether the system is in approximately horizontal position or in approximately vertical position without requiring a specially prepared optical system or an additional mechanism to replace a part of the optical system. This contributes to simple design of the system and higher working efficiency as well as high accuracy and reproducibility. Because reflection of the free liquid surface always maintained in horizontal direction is utilized, it can be easily assembled without requiring high grade assembling technique required for conventional pendulum type angle compensator. Instead of suspension of optical members used in the conventional pendulum type angle compensator, liquid is injected in the liquid sealing container. This leads to very easy work and elimination of variability in assembling accuracy due to individual workers. Since the liquid is perfectly sealed, there is no change over time and the system has good proof against changes in environment. External vibration or impact can be controlled by viscosity of the liquid, and no complicated braking device is required.

Claims: An apparatus for automatic angle compensation comprising: a liquid sealing container (4) with transparent liquid filled therein to form a free liquid surface (1); a light projecting system (21) for irradiating a light beam (25) at a given angle relative to the free liquid surface (1) so that it is reflected therefrom when the apparatus is in a first position, means (27) for producing one of a vertical or horizontal beam of light, and optical means (9,12,14) for guiding the light beam (26) reflected from the free liquid surface (1) towards said means (27) for producing one of a vertical or horizontal beam of light, characterised in that the light projecting system comprises a first light projecting system (21) for irradiating a light beam (25) at said given angle to the free liquid surface (1) so that it is reflected by the free liquid surface (1) when the apparatus is in its first position and in that a second light projecting system (22) arranged face-to-face to the first light projecting system (21) with the free liquid surface (1) therebetween, the second light projecting system (22) being for irradiating light at said given angle to said free liquid surface (1) when the entire apparatus is rotated from its first position to a second position by rotating it by 90° about an axis substantially perpendicular to a plane in which the light beams irradiated from the light projecting systems (21,22) lie, so that the light is reflected from the free liquid surface (1) onto the said optical means (9,12,14) from where it is guided towards said means (27) for producing one of a vertical or horizontal beam of light, which means (27) is operable in the second position to produce the other one of the vertical or horizontal beam of light. An apparatus as claimed in claim 1, wherein a light receiving element is provided to receive the light beam reflected by the free liquid surface (1) through a pinhole (45) in order to detect whether the entire system is within a limited inclination according to the light receiving condition of the light receiving element. An apparatus as claimed in any one of the preceding claims comprising a liquid movement preventing plate (24) rotatably arranged in the liquid and having a centre of rotation approximately aligned with the free liquid surface (1). An apparatus as claimed in claim 3, wherein the liquid movement preventing plate (24) is transparent and is located a little below the free liquid surface (1) and is provided with weights (30) to maintain it horizontal.

Inventors: ISHINABE IKUO, KODAIRA JUN-ICHI, OHTOMO FUMIO, SUGAI KIROO, ISHINABE, IKUO, KODAIRA, JUN-ICHI, OHTOMO, FUMIO, SUGAI, KIROO, ISHINABE, IKUO, C/O K.K. TOPCON, KODAIRA, JUN-ICHI, C/O K.K. TOPCON, OHTOMO, FUMIO, C/O K.K. TOPCON, SUGAI, KIROO, C/O K.K. TOPCON
Applicants: TOPCON CORP, KABUSHIKI KAISHA TOPCON
Application year: 1995
Main classification: G01C15
IPC codes: G01C9, G01C15, G01C5, G02B27