Levelling instrument

Abstract: A tilt detector comprises a liquid sealing container (2) with a transparent liquid (1) sealed therein to form a free liquid surface (3), a light projecting system (12) for projecting a light beam (13) to the free liquid surface (3) at a given angle so that it is reflected by the free liquid surface (3), and an optical axis dividing means (17) for dividing an optical axis of the light projecting system. The detector further comprises a first light entry means (4) for allowing the light beam (13) from the light projecting system (12) to pass into the sealing container (2), a second light entry means (5) for allowing the light beams (13) divided by the dividing means (17) into the sealing container (2), a first light irradiating means (6) for irradiating the light beam (13) reflected by the free liquid surface (3) from the sealing container (2), and a second light irradiating means (8) arranged face-to-face to the first light irradiating means (6). A light beam (13) entering the liquid sealing container (2) from the first light entry means (4) is irradiated by the first light irradiating means (6), and when the system is rotated and hence turned down by 90°, the light beam (13) divided by the dividing means (17) enters the liquid sealing container (2) from the second entry means (5) and is reflected by the free liquid surface (3) rotated by 90° and is irradiated by the second irradiating means (8).

Description: The present invention relates to a levelling instrument, that is to say an instrument that can be used to establish a horizontal reference line or plane, and which can be used, for example, in a tilt detector such as the type used in surveying or measuring instruments for measuring changes of tilting, or for maintaining an optical axis of the instrument in vertical direction or in horizontal direction or for forming an irradiation plane of a horizontal or vertical light beam. When various surveying operations are performed using a surveying or measuring instrument, it is normally necessary to compensate for minor deviations in the orientation of the instrument relative to the horizontal and vertical axes. Automatic compensators for this purpose are known. For example, it is known to provide a lens or a prism suspended as a pendulum by two or three or more suspension lines braked by magnetic or other type braking mechanism so that when the main unit of the surveying or measuring instrument is titled the optical path of the instrument can be automatically compensated. Alternatively, see EP-A-0676618, the reflection of a light beam from the rear surface of a transparent liquid can be provided in such a manner that the reflected light beam has an optical axis which is sensitive to the tilting of the liquid surface used in every direction. The reflected light beam can then be used in an optical system such as anamorphic prism to provide an automatically compensated optical path regardless of whether the system disclosed is positioned in a horizontal or vertical direction. A further example of a prior art compensator is EP-A-0626561 disclosing an apparatus in which the variation of the reflection angle to the optical axis is equalised in all directions. However, the conventional techniques described above can only be used when the entire system is installed in an approximately horizontal or vertical position and it is difficult to use the compensator at both approximately horizontal or approximately vertical positions. To use the compensator for both purposes, it is necessary separately to provide an optical system for changing the optical path, or to design a part of the optical system as replaceable and to replace a part of the optical system to suit the approximately horizontal or approximately vertical positions where the compensator is used. In such cases, removal or assembling of the optical system leads to lower reproducibility and poor accuracy. Also, the problem of increased cost for the components arises. EP-A-0676618 represents the closest prior art under Article 54(3)EPC. To solve the above problems, it is an object of.the present invention to provide a levelling instrument utilizing reflection at a free liquid surface and which can be used for the automatic compensation of an optical path regardless of whether the system is at approximately horizontal position or at approximately vertical position, and without requiring any specially prepared optical system, or additional mechanisms replacing a part of the optical system. The levelling instrument can be used in a tilt detector for detecting the tilt angle of a system. According to the invention there is provided a levelling instrument of the type comprising a levelling instrument comprising a chamber partially filled with a transparent liquid, and a light source for generating a polarised light beam for reflection by the liquid surface; a polarised light beam splitter in the path of the light beam; and means which when the instrument is in a first orientation relative to the horizontal are arranged to provide to the beam splitter a beam of a first polarisation, the beam splitter being adapted to produce therefrom a first split beam to pass through a first transparent portion (4,51) of the chamber and exit via an associated first element (6,53); and which when the instrument is in a second angular orientation relative to the horizontal is arranged to provide to the beam splitter (17,59) a beam of a second polarisation, the beam splitter being adapted to provide therefrom a second split beam arranged to pass through a second transparent portion (5,52) of the chamber and exit via an associated second element (8,54). Preferably the levelling instrument of the present invention comprises a sealed container with a transparent liquid sealed therein to form a free liquid surface, a light projecting system for projecting light beam to the free liquid surface at a given angle so that it is reflected by the free liquid surface, an optical axis dividing means for dividing an optical axis of the light projecting system, a first light entry means for allowing the light beam from the light projecting system to pass into the container, a second light entry means for allowing the light beams divided by the dividing means to pass into the container, a first light irradiating means for irradiating the light beam reflected by the free liquid surface from the container, and a second light irradiating means arranged face-to-face to the first light irradiating means. The arrangement is such that when the system is turned through 90° the light beam, divided by the dividing means, enters the liquid sealing container from the second light entry means, is reflected by the free liquid surface rotated by 90°, and is then irradiated by the second irradiating means. The first light irradiating means comprises a first reflection member for reflecting the light beam which has first of all been reflected by the free liquid surface when the system is titled by nearly 0°, back towards the free liquid surface. Correspondingly, the second light irradiating means is a second reflecting member for reflecting the light beam which has first of all been reflected by the free liquid surface when the system is tilted by nearly 90° back towards the free liquid surface. The first light irradiating means also comprises a first emitting window in the container and the second light irradiating means is a second emitting window provided on the container. The light beam irradiated by the light projecting system is a polarized light beam having a defined direction of polarization, and there is further provided a polarized light converting means for projecting light beams having different polarized properties toward the optical path dividing mans when the system is tilted by nearly 0° and when it is tilted by nearly 90°. The optical path dividing means comprises a polarized light beam splitter so that in the light beam irradiated by the light projecting system, the direction of polarization is that of the defined polarized light. Similarly the dividing means is a polarized light beam splitter so that when the direction of polarization is rotated by a first birefringence member movably arranged on the optical axis between the light projecting system and the polarized light beam splitter, and when the entire system is turned through 90°, the movably arranged birefringence member is displaced from the optical axis. The optical axis of the light beam from the light projecting system is separated from the optical axis before the entire system has been turned through 90° by the polarized light beam splitter. The light beam reflected by the free liquid surface is irradiated from the liquid sealing container by the second light irradiating means. The polarized light converting means is a first birefringence member removably arranged in the optical path of the light projecting system depending upon the tilting of the system and for rotating the direction of polarization of the light beam when it is arranged in the optical path. The first birefringence member is a λ/2 phase plate, and the second birefringence member is a λ/4 phase plate. Embodiments of the invention will now be described, by way of example, with reference to the drawings, of which: Fig. 1 shows a first embodiment of the present invention; Fig. 2 shows the embodiment shown in Fig. 1 of the present invention rotated through 90°; Fig. 3 is an enlarged partial view of an alternative first entry/exit window or a second entry/exit window of embodiment shown in Fig. 1; Fig. 4 is a front view of a liquid movement preventing device which can be used in the present invention; Fig. 5 is a plan view of the liquid movement preventing device of Fig. 4; Fig. 6 is a side cross-sectional view of the liquid movement preventing device of Fig. 4; Fig. 7 shows a second embodiment of the present invention; Fig. 8 is an enlarged partial view of an alternative first entry/exit window or a second entry/exit window of the embodiment shown in Fig. 7; Fig. 9 shows a third embodiment of the present invention; Fig. 10 is a plan view of the embodiment shown in Fig. 9; Fig. 11 shows the embodiment shown in Fig. 9 rotated by 90°; Fig. 12 is a plan view of the embodiment shown in Fig. 9 rotated by 90°; Fig. 13 represents a tilt angle detector, incorporating a levelling instrument according to the present invention; Fig. 14 represents the tilt angle detector of Fig. 13 rotated by 90°; Fig. 15 shows an angle detecting unit of the tilt angle detector of Fig. 13; Fig. 16 shows a fourth embodiment of the present invention; Fig. 17 shows the embodiment shown in Fig. 16 rotated by 90°; Fig. 18 is a plan view of the liquid movement preventing device used in the fourth embodiment; Fig. 19 is a sectional view along the line A-A of the device of Fig. 18; Fig. 20 is a sectional view along the line B-B of the device of Fig. 19; Fig. 21 illustrates automatic angle compensation when light is reflected once by a free liquid surface; Fig. 22 illustrates the change of reflection angle of a reflection light beam when the free liquid surface is tilted; Fig. 23 illustrates the change of reflection angle of a reflection light beam when the free liquid surface is tilted; Fig. 24 illustrates the change of optical axis of a transmitted light beam with respect to an anamorphic prism system; Fig. 25(A) and Fig. 25(B) illustrates the change of optical axis of a transmitted light beam with respect to an anamorphic prism system; Fig. 26 illustrates the change of reflection angle of a reflected light beam reflected twice by the free liquid surface; and Fig. 27 illustrates the change of a reflection angle of a reflected light beam when the free liquid surface is tilted in case it is reflected twice by the free liquid surface. Fig. 1 represents a first embodiment of the invention. A container 2 is provided, in which a transparent liquid 1 is sealed to form a free liquid surface 3. The container 2 is designed as a short column of approximately octagonal shape. Of the planes forming the octagon, on one plane adjacent to the bottom plane, a first entry/exit window 4 is arranged, and a second entry/exit window 5 is arranged next to the first entry/exit window 4. On another plane adjacent to the bottom plane, a first reflection mirror 6 serving as a reflection member is provided, and a λ/4 phase plate 7 is disposed on a reflection surface of the first reflection mirror 6. A second reflection mirror 8 serving as a reflection member is disposed at a position opposite to the first reflection mirror 6" and a λ/4 phase plate 9 of a birefringence member is disposed on a reflection surface of the second reflection mirror 8. A liquid movement preventing device 10 is movably arranged inside the liquid sealing container 2. The liquid movement preventing device 10 has a moving shaft, which is located in the free liquid surface 3 and is coincident with the center of the liquid sealing container 2. As described later, the liquid movement preventing device 10 has a transparent liquid movement preventing plate 11, which is positioned slightly below the free liquid surface 3 of the transparent liquid 1 and is hence immersed in the liquid. The center of gravity of the liquid movement preventing device is located below the moving shaft so that it is freely rotatable and the liquid movement preventing plate 11 is held at horizontal position. A light projecting system 12 is provided which projects a light beam 13 at a given angle, preferably at more than 45°, with respect to the horizontal free liquid surface 3, so that it is reflected by the free liquid surface 3 of the transparent liquid 1. The light projecting system 12 comprises a light source unit 14, i.e. a semiconductor laser for emitting approximately linearly polarized laser beam having a direction of polarization perpendicular to the paper surface, a collimator lens 15 transforming the light beam coming from the light source unit 14 into parallel beam, a λ/2 phase plate 16 of a polarized light converting means rotatably suspended and made of a birefringence member and used for converting the direction of polarization, and a polarized light beam splitter 17. The polarized light beam splitter 17 allows a linearly polarized light component which is parallel to the paper surface to pass and reflects a linearly polarized light component which is perpendicular to the paper surface. A first reflection mirror 18 for reflecting the light beam 13 toward the polarized light beam splitter 17 is arranged adjacent to the polarized light beam splitter 17, and a second reflection mirror 19 is arranged at a position opposite to the first reflection mirror 18 with the polarized light beam splitter 17 therebetween. The second reflection mirror 19 reflects the light, which has been reflected by the first reflection mirror 18 and has passed through the polarized light beam splitter 17, in the vertical direction in Fig. 1. Along the optical axis of the light beam reflected by the second reflection mirror 19 are arranged an anamorphic prism system 20, a beam expander 21, and a pentagonal prism 23 rotatable around the optical axis of the light beam reflected by the second reflection mirror 19. As described later, the polarized light beam splitter 17, the first reflection mirror 18 and the second reflection mirror 19 constitute a projection optical system, which projects a reflection light beam emitted through the first entry/exit window 4 via the first reflection mirror 6 and a reflection light beam 30 emitted through the second entry/exit window 5 via the second reflection mirror 8 both in vertical direction. Next, a description will be given of the operation of the system. After being emitted from the light source unit 14, the light beam 13 passes through the λ/2 phase plate 16 so as to provide a linearly polarized light beam, the direction of polarization being parallel to the paper surface. As already described, the polarized light beam splitter 17 allows the linearly polarized light, which is parallel to the paper surface, to pass. Thus, the light beam coming from the light source unit 14 passes through the polarized light beam splitter 17 and enters the transparent liquid 1 via the first entry/exit window 4, and it further passes through the liquid movement preventing plate 11 and is reflected by the free liquid surface 3 towards the first reflection mirror 6. The light beam is reflected by the first reflection mirror 6, passes back through the liquid movement preventing plate 11 is reflected again by the free liquid surface 3 and is irradiated through the first entry/exit window 4. On the reflection surface of the first reflection mirror 6 is a λ/4 phase plate 7 and since the light beam 13 passes through the λ/4 phase plate 7 twice in the process of being reflected by the first reflection mirror 6, the direction of polarization of the reflected light beam 30 will now be perpendicular to the paper surface. The reflected light beam 30 passes through the first entry/exit window 4 and enters the polarized light beam splitter 17, and because the reflected light beam 30 is now linearly polarized in a direction perpendicular to the paper surface, it is reflected by the polarized light beam splitter 17 to the second reflection mirror 19 where it is again reflected into a vertical direction. The free liquid surface 3 is always maintained in horizontal direction even when the entire system of the present embodiment is tilted, and the light beam 13 is always reflected by the horizontal plane. Therefore, when the incident angle of the light beam 13, with respect to the free liquid surface 3, is changed depending upon the tilting of the entire system, the optical axis of the light beam reflected by the second reflection mirror 19 will correspondingly be tilted with respect to the vertical direction. The anamorphic prism system 20 and the beam expander 21 compensate this tilting so that the incident light beam to the pentagonal prism 23 is always vertical. Correspondingly the light emitted by the pentagonal prism 23 is horizontal thus providing a horizontal reference line or a horizontal reference plane if the pentagonal prism 23 is rotated. A description is now given of the automatic compensation system of the present embodiment and referring to Figs. 21-27. Fig. 21 is a simplified drawing to schematically illustrate the arrangement of the present embodiment, where the light beam is reflected once by the free liquid surface 3. The same components as in Fig. 1 are referred by the same symbols. In Fig. 22, it is supposed that the incident light beam 13 enters the free liquid surface 3 at an angle . It is supposed that x-z co-ordinate plane formed by co-ordinate axes x and z is approximately consistent with the free liquid surface 3 and that the co-ordinate axis perpendicular to the co-ordinate plane is y. It is also supposed that the optical axis of the incident light beam 13 is present in the co-ordinate plane formed by the co-ordinate axes z and y. If the free liquid surface 3 is tilted around the co-ordinate axis x at an angle α, the optical axis of the reflected light beam 30 moves in the y-z co-ordinate plane, and the reflection angle is changed by ξ 1x in the y-z co-ordinate plane and is turned to the reflection light beam 30'. In this case, the relationship between the liquid surface displacement angle α and the reflection displacement angle ξ lx is given by: ξ = 2α, and no reflection displacement angle ξ 2x occurs in the x-y co-ordinate plane. In the figure, reference numeral 19 represents a reflection mirror. In contrast, if the free liquid surface 3 is tilted around the co-ordinate axis z at any angle α as shown in Fig. 23, the reflection light beam 30 is separated and moved from the x-y co-ordinate plane and the y-z co-ordinate plane. Therefore, a reflection displacement angle ξ 1z and a reflection displacement angle ξ 2z appear on the x-y co-ordinate plane and the y-z co-ordinate plane respectively. Further, the relationship between the reflection displacement angle ξ 1z and the liquid surface displacement angle α of the free liquid surface 3 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° Then ξ 2z = 1.7", and the value of ξ 2z is negligible in terms of accuracy. Further, if it is supposed that refractive index of the liquid is n, the optical axis of the light beam after passing through the liquid is given by: ξ 1x' = 2nα ξ 1z' = ncos -1 (cos 2  cos2α + sin 2 ) Therefore, sensitivity to the liquid surface displacement angle α is different from the cases of the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z'. This difference in sensitivity from the reflection displacement angles ξ 1x' and ξ 1x' is corrected by optical means to equalize the sensitivity. In so doing, it is possible to obtain an optical axis, which deflects always at a given ratio with respect to all directions. Further description is given now, referring to Fig. 21. In the figure, reference numeral 2 represents a sealed container arranged in a main unit of an instrument such as a survey instrument. In the container, a liquid provides a free liquid surface 3. A light beam, emitted from a light source unit 14, is projected onto the free liquid surface 3 at a given angle via a collimator lens 15 so that it is totally reflected, and so that the optical axis of the light beam is positioned within the y-z co-ordinate plane as described above. With the free liquid surface 3 not tilted, the reflection light beam 30 totally reflected by the free liquid surface 3 is reflected in vertical direction by a polarized light beam splitter 17 and a second reflection mirror 19, and an anamorphic prism system 20 comprising a pair of wedge-like prisms 24 and 25 is arranged along the optical axis of the reflection light beam 30. The light beam passing through the anamorphic prism system 20 is allowed to pass through a beam expander 21, which comprises convex lenses 26 and 27. If it is assumed that focal length of the convex lens 26 is f3 and focal length of the convex lens 27 is f4, the distance between the convex lenses 26 and 27 is set to: f3 + f4. The anamorphic prism system 20 may be arranged along the optical path before the light beam is reflected by the reflection mirror 19. In Fig 21, it is to be supposed that the present incident angle  to the liquid is 50°, the tilt angle of the instrument, i.e. tilt angle a of the free liquid surface 3, is 10', and that the refractive index n of the liquid is 1.4. Then, from the equation (2), the reflection displacement angle ξ 1x', when the free liquid surface 3 is tilted around x-axis, and the reflection displacement angle ξ 1z', when the free liquid surface 3 is tilted around z-axis, are obtained respectively as: ξ 1x' = 28' and ξ 1z = 18'. Therefore, the difference of sensitivity between the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z' is (ξ 1x'/ξ 1z') = 1.555. Thus, under this condition: ξ 1x' = 2nα, ξ 1z' = 1.286nα ξ 1x'/ξ 1z' = 1.555 Next, the anamorphic prism system 20 optically corrects the sensitivity difference. Referring to Figs. 24 and 25 a description is now given of the anamorphic prism system 20. In this case it is to be supposed that prism apex angles of the wedge-like prisms 24 and 25, which constitute the anamorphic prism system 20, are a24 and a25, the relative angle of the wedge-like prisms 24 and 25 is b and the refractive index is ng, and the incoming light beam is Din and outgoing light beam is Dout. It is also to be supposed that the wedge-like prisms 24 and 25 are identical with each other and that apex angle a = a24 = a25. Then, Magnification Map = (Din/Dout) = cos 2 a/(1-ng 2 Sin 2 a) Thus, the angular magnification is approximately 1/Map. Therefore, if it is supposed that the prism apex angle is a and the relative angle of the wedge-like prisms 24 and 25 is b and the refractive index ng is selected (e.g. in case ng = 1.51, a = 27.732° and b = 44.793°) so that the relationship: Map = 2nα/1.286nλ = 1.555 exists, ξ 1x', after passing through the anamorphic prism 20, is converted to: 2nα x 1.286 nα/2nα = 1.286 nα. After passing through the anamorphic prism system 20, ξ 1x' = ξ 1z'. After passing through the anamorphic prism system 20, the optical axis of the reflection light beam 30 always has uniform reflection displacement angle with respect to the tilting of the free liquid surface 3 in all directions. Thus, even when the free liquid surface 3 is tilted in any direction, the reflection displacement angle of the same sensitivity is always obtained with respect to the tilting. Further, after the reflection light beam 30 passes the anamorphic prism system 20 and when the light beam reflected upward by the second reflection mirror 19' passes through the beam expander 21, if angular magnification of the beam expander 21 is 1/1.286n, the optical axis after the light beam has passed is tilted by: (ξ 1x' = ξ 1z' = 1.286n α) x 1/1.286n = α and final optical axle axis after passing through the beam expander 21 always runs perpendicularly to the free liquid surface 3, i.e. the optical axis is maintained in vertical direction. If it is supposed that the focal length of the convex lens 26, which constitutes the beam expander 21, is f3, and the focal length of the convex lens 27 is f4, the angular magnification Mex of the beam expander 21 is f3/f4. By selecting the values of f3 and f4, it is possible to have angular magnification of 1/1.286n. Next, in the arrangement example shown in Fig. 21, the anamorphic prism system 20 may be rotated by 90° and the prism apex angles a24 and a25 of the wedge-like prisms 24 and 25, the relative angle b of the wedge-like prisms 24 and 25, and the refractive index ng may be appropriately selected to obtain M = 1/1.555. The anamorphic prism 20 in the above optical system is generally used for the purpose to turn a beam of oval shape to circular shape. For example, when laser diode is used as light source, cross-sectional shape of the light beam can be made closer to circular shape by the anamorphic prism system 20. (The shape of the beam of laser dioxide is oval.) In general, an apparatus or a system where such laser diode is incorporated, often fulfils the function as a laser pointer or a laser marker, and it is desirable that the irradiated shape of the light beam is closer to circular shape. Therefore, it is also very effective for obtaining the beam of circular shape to correct the optical axis by the use of the anamorphic prism system 20. As described above, the difference of sensitivity of the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z' with respect to the liquid displacement angle is corrected by the optical means to equalize the sensitivity. As a result, it is possible to obtain an optical axis which is always deflected at a constant rate in all directions. Next, referring to Figs. 26 and 27, description will be given of the difference of sensitivity of the changes of reflection angle in the following two cases, i.e. (i) where the light beam is incident upon the free liquid surface at a given angle and is totally reflected by the free liquid surface, and the emitted light beam is reflected by a mirror and is again irradiated into the free liquid surface and is totally reflected by the free liquid surface, and (ii) where the light beam is totally reflected twice by the free liquid surface with respect to the tilting direction of the liquid surface when the free liquid surface is relatively tilted with respect to the light beam. Considering the case where the incident light beam coming from the beam splitter 17 impinges the free liquid surface 3 at an angle of , is totalling reflected by the free liquid surface 3, is reflected by the first reflection mirror 6, is totally reflected again by the free liquid surface 3, passes through the beam splitter 17 and is finally reflected by the mirror 19, it can be supposed that the free liquid surface 3 is approximately consistent with the x-z co-ordinate plane formed by the co-ordinate axes x and z and that the co-ordinate axis perpendicular to the co-ordinate plane is y. It can also be supposed that the optical axis of the incident light beam is present in the co-ordinate plane formed by the co-ordinate axes z and y. If the free liquid surface 3 is tilted around the co-ordinate axis x at an angle of α from this condition, the optical axis of the reflected light beam 30 moves in the y-z co-ordinate plane, and the reflection angle is changed by ξ 1x within the y-z co-ordinate plane. In this case, the relationship between the liquid surface displacement angle α and the reflection displacement angle ξ 1x is given by: ξ 1x = 4 α, and no reflection displacement angle ξ 2x occurs in the x-y co-ordinate plane in this case. In contrast, if the free liquid surface 3 is tilted around the co-ordinate axis z at an angle α as shown in Fig. 27, the reflection light beam 30 is separated and moved from the x-y co-ordinate plane and the y-z co-ordinate plane respectively. Accordingly, the reflection displacement angle ξ 1z and the reflection displacement angle ξ 2z appear on the x-y co-ordinate plane and y-z co-ordinate plane. Further, the relationship between the reflection displacement angle ξ 1z and the liquid surface displacement angle α of the free liquid surface 3 is given by: ξ 1z = cos -1 {cos2αsin 2 2 - cos 2sin 2  - (sin 2 2α - cos 2 2αcos2) cos 2 } ξ 2z = π/2 - cos -1 [{1/2 (sin 2 2α - cos 2 2α cos2 - cos2) + cos2αcos2} sin2] For example, if α =10' and  = 50°, then ξ 2z = 3.43", and the value of ξ 2z is negligible in terms of accuracy. Further, if it is supposed that refractive index of the liquid is n, the optical axis after the light beam has passed through the liquid is given by: ξ 1x' = nξ 1x ξ 1z' = nξ 1z More concretely, if it is supposed at α = 10',  = 50° and n = 1.4, then: ξ 1x' = 56' ξ, 1z' = 35.994' ξ 1x'/ξ 1z' = 1.555 In this way, under the condition that  = 50°, the difference of sensitivity to the liquid displacement angle α with respect to the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z' is the same as the case where the light beam is reflected once by the free liquid surface 3. Therefore, the anamorphic prism system 20 can be used, which always provides the reflection displacement angle of the same sensitivity to the tilting. Here, the magnification Map of the anamorphic prism system 20 is expressed as: Map = 4nα/2.572nα = 1.555 1/Map = γ ap = 1/1.555 = 0.64309 Therefore, after the light beam passes through the anamorphic prism system 20, correction is made as: ξ 1x' = 4nα x 1/1.555 = 2.572nα ξ 1z' = 2.572nα Further, if it is supposed that n = 1.4, the magnification Mex of the beam expander 21 to maintain the optical axis in vertical direction after the light beam has passed through the anamorphic prism system 20 is given as: Mex = 2.572n α/α = 2.572n = 3.601 γ ex = 1 Mex = 1/2.572n = 0.278 As described above, the difference of sensitivity between the reflection displacement angle ξ 1x' and the reflection displacement angle ξ 1z' with respect to the liquid displacement angle is corrected by the optical means to equalise the sensitivity. As a result, it is possible to obtain an optical axis, which always deflects at a constant rate in all directions. As already described, the anamorphic prism system 20 equalises the reflection sensitivity in all directions and the beam expander 21 ultimately adjusts the sensitivity. Accordingly, after passing through the beam expander 21, the optical axis of the light beam is always compensated in vertical direction regardless of the tilting of the entire system. Therefore, the light beam emitted from the pentagonal prism 23 is directed in horizontal direction, and a constant horizontal reference plane can be always obtained by rotating the pentagonal prism 23. In other words, the present invention can be applied to a levelling instrument. Next, Fig. 2 represents the entire arrangement of the present embodiment turned through 90°. The liquid movement preventing device 10 is rotated by 90° so that the liquid movement preventing plate 11 is held in horizontal position by the gravitational force. Because the free liquid surface 3 of the transparent liquid 1 is always held in horizontal direction with respect to the center of gravity, it is rotated by 90°, following the rotation by 90° of the entire system. Similarly , the λ/2 phase plate 16 is also rotated by 90°, and it is displaced out of the path of the light beam 13 by this rotation. As a result, the light beam entering the polarized light beam splitter 17 has linearly polarized light perpendicular to the paper surface, and it is reflected by the polarized light beam splitter 17 and is further reflected by the first reflection mirror 18. Then, it enters the container 2 through the second entry/exit window 5, passes through the transparent liquid 1 and the liquid movement preventing device 10 and is reflected by the free liquid surface 3. After being reflected by the free liquid surface 3, the light beam is reflected by the second reflection mirror 8. Because the second reflection mirror 8 is provided with a λ/4 phase plate 9, the reflection light beam 30 passes through the λ/4 phase plate 9 twice. As a result, the direction of polarization is converted, and it runs in parallel to the paper surface. The reflection light beam 30 reflected by the second reflection mirror 8 is reflected by the free liquid surface 3, passes through the liquid movement preventing device 10 and is emitted through the second entry/exist window 5. The second reflection mirror 8 serves as a second light irradiating means for irradiating the light beam again, which has entered the liquid sealing container 2. After being emitted through the second entry/exit window 5, the reflection light beam 30 is reflected by the first reflection mirror 18, passes through the polarized light beam splitter 17 and is reflected in horizontal direction by the second reflection mirror 19. By passing through the anamorphic prism system 20 and the beam expander 21, the reflection light beam 30 emitted from the beam expander 21 is always maintained in horizontal direction regardless of the tilting of the entire system, and the plane formed by light beam rotated and irradiated by the pentagonal prism 23 always provides a constant vertical reference plane. Also, it is possible to achieve automatic angle compensation at horizontal position or at vertical position. The first entry/exit window 4 and the second entry/exit window 5 are separately provided in the above, but they may be integrated as shown in Fig. 3. The polarized light beam splitter 17 may be another optical path dividing means such as a half-mirror. If a half-mirror is used, there is no need to use the λ/2 phase plate 16. If the transparent liquid 1 sealed in the liquid sealing container 2 is moved by external vibration, the movement is suppressed by the liquid movement preventing device 10. Referring to Figs. 4 to 6, the liquid movement preventing device 10 will be described. The liquid movement preventing device 10 has a projection 32 comprising a shaft hole 31, and the projection 32 is movably engaged with a pendulum shaft (not shown) provided on the liquid sealing container 2. The pendulum shaft is arranged at a position, which is approximately coincident with the portion of the free liquid surface 3 to reflect the light beam 13, and the liquid movement preventing device 10 is movable around this reflecting portion. The liquid movement preventing device 10 is designed in semi-circular shape with flat top surface and has a cross-section concaved upward. At the center of the top surface is a rectangular transparent plate 33 which allows the light beam to pass, in particular a transparent glass which does not attenuate the light. The upper surface of the liquid movement preventing device 10 is positioned slightly below the free liquid surface 3, and the center of gravity is set at such position that the device is always maintained in horizontal direction. If a natural frequency of the liquid movement preventing device 10 is lower than a natural frequency of the transparent liquid 1 sealed in the container 2, the movement of the free liquid surface 3 is reduced due to viscosity of the liquid and resistance of the liquid movement preventing device 10. If the center of movement in the liquid movement preventing device 10 is at the portion of the free liquid surface 3 to reflect the light beam, the same condition can be maintained when the container 2 is rotated by 90°, and when the light beam enters from the opposite direction as described above, the light beam is reflected in the same direction. A material having high thermal conductivity, e.g. copper material. is used for the liquid movement preventing device 10, and it is designed with large surface area. As a result, it promotes heat transmission toward the interior of the liquid when sudden change occurs in environmental temperature and this prevents the change in distribution of refractive indices and increases the stability of the light beam. Referring now to Fig. 7, a description is given of a second embodiment of the invention. In Fig. 7 the same components as in Figs. 1 and 2 are referred by the same symbols. In the first embodiment described above, the λ/4 phase plate 7 and the λ/4 phase plate 9 are provided on reflection surfaces of the first reflection mirror 6 and the second reflection mirror 8. In the second embodiment, the λ/4 phase plate 7 is closely fitted to the first entry/exit window 4, and the λ/4 phase plate 9 is closely fitted to the second entry/exit window 5. In this embodiment, too, the light beam entering the free liquid surface 3 and the light beam reflected by the free liquid surface 3 pass through the first entry/exit window 4 and the second entry/exit window 5 twice, i.e. then pass through the λ/4 phase plates 7 and 9 twice, even when the entire system is in the condition shown in Fig. 7, or when the entire system is rotated by 90°. Therefore, the direction of polarization is changed by 90 c . Similarly to the first embodiment, a horizontal reference plane and a vertical reference plane are formed by the light beam emitted from the pentagonal prism 23 regardless of the tilting of the entire system, and an angel of compensation can be achieved when the automatic angle compensator is at horizontal position or at vertical position. As shown in Fig. 8, the first and second entry/exit windows 4 and 5 and the λ/4 phase plates 7 and 9 may be put together integrally. Fig. 9 and Fig. 10 each represents a third embodiment of the present invention. In Figs. 9 and 10, the same components, as in Figs. 1 and 2 are referred by the same symbols. In this embodiment, see Figs. 9 and 10, the light beam is irradiated to the free liquid surface 3 at an angle in vertical direction with respect to the paper surface, and the incident optical axis and the reflected optical axis are deviated in a direction perpendicular to the paper surface. Behind the light source unit 14, and along the reflection optical axis, is a fourth reflection mirror 35. Similarly, a fifth reflection mirror 36 is disposed behind the first reflection mirror 18 and along the reflection optical axis. In this embodiment, the liquid movement preventing device 10 is not shown. Also, in this embodiment, the λ/4 phase plates 7 and 9, used in the first embodiment, are omitted. In operation of the third embodiment, the light beam 13 emitted from the light source unit 14 passes through the λ/2 phase plate 16 and is turned to linearly polarized light beam in parallel to the paper surface. As described above, the polarized light beam splitter 17 allows the linearly polarized light in parallel to the paper surface to pass. Thus, the light beam coming from the light source unit 14 passes through the polarized light beam splitter 17 and enters the transparent liquid 1 via the first entry/exit window 4 and is reflected by the free liquid surface 3 toward the first reflection mirror 6. The reflection light beam 30 is incident upon the first reflection mirror 6 at a given angle and is reflected. Then, it passes long an optical axis different from that of the incident light beam 13 and is reflected by the free liquid surface 3. The reflection light beam 30 is irradiated from the first entry/exit window 4 and is deviated from the polarized light beam splitter 17 and reaches the fourth reflection mirror 35 and is reflected by the fourth reflection mirror 35 in vertical direction. After being reflected by the fourth reflection mirror 35, the reflection light beam 30 passes through the anamorphic prism system 20 and the beam expander 21 and is emitted in horizontal direction from the pentagonal prism 23. The light beam emitted from the pentagonal prism 23 is directed in horizontal direction, and the horizontal reference plane can be obtained by the emitted light beam. By rotating the pentagonal prism 23, the horizontal reference plane is formed by the light beam. Figs. 11 and 12 each represents the case where the entire system is rotated by 90°. The λ/2 phase plate 16 is rotated by 90° and as a result is displaced from the path of the light beam 13. Therefore, the light beam entering the polarized light beam splitter 17 has linearly polarized light perpendicular to the paper surface. It is reflected by the polarized beam splitter 17 and is then reflected by the first reflection mirror 18 and enters the liquid sealing container 2 via the second entry/exit window 5. Then, it passes through the transparent liquid 1 and is reflected by the free liquid surface 3. After being reflected by the free liquid surface 3, it is reflected by the second reflection mirror 8. The reflection light beam 30 reflected by the second reflection mirror 8 is deviated in a direction perpendicular to the paper surface from the optical axis of the incident light beam. It is further reflected by the free liquid surface 3 and is emitted from the second entry/exit window 5. The reflection light beam 30 emitted from the second entry/exit window 5 is deviated from the first reflection mirror 18 and reaches the fifth reflection mirror 36 and is reflected in horizontal direction by the fifth reflection mirror 36. Because the fourth reflection mirror 35 and the fifth reflection mirror 36 are deviated in a direction perpendicular to the paper surface in Fig. 11, the reflection light beam 30 reflected by the fifth reflection mirror 36 is not shielded by the fourth reflection mirror 35. After passing through the anamorphic prism system 20 and the beam expander 21, the reflection light beam 30 emitted from the beam expander 21 is always maintained in horizontal direction regardless of the tilting of the entire system, and the plane formed by light beam which is rotated and irradiated from the pentagonal prism 23 always forms a constant vertical reference plane. Also, in the third embodiment, angle compensation can be achieved when the automatic angle compensator is at horizontal position or at vertical position. In the third embodiment, the route of an outward trip of the light beam is different from its route on a return trip, and it does not pass through the polarized light beam splitter17 on the return trip, and this prevents attenuation of the light. Next, a description will be given of the tilt angle detector shown in Figs. 13 and 14, and which incorporates a levelling instrument according to the present invention. In this embodiment, the same components as in Fig. 1 are referred by the same symbols. When embodied as a tilt detector, the pentagonal prism 23 of the instrument shown in Fig. 1 is replaced by a photodetector 38. In addition, a condenser lens 41 for forming an image on the light receiving surface of the photodetector 38 is disposed so that the reflection light beam 30 emitted from the anamorphic prism system 20 can be received by the photodetector 38. As shown in Fig. 15, the photodetector 38 has its light receiving surface divided into light receiving sections 38a, 38b, 38c and 38d around the optical axis. The light receiving sections 38a and 38c are arranged in x-axis direction, and the light receiving sections 38b and 38d are arranged in z-axis direction. Outputs from the light receiving sections 38a and 38c are inputted to a comparative amplifier 39, and outputs from the light receiving sections 38b and 38d are inputted to a comparative amplifier 40. Outputs from the comparative amplifiers 39 and 40 are inputted to a computing unit (not shown). From the outputs of the two amplifiers. the position of the reflection light beam 30 is detected and the tilt angle of the liquid surface of the free liquid surface 3 and the tilt angle of the entire system are calculated. A device such as CCD is used as the light receiving element so that the co-ordinates of the position of the reflected light on the photodetector 38 can be determined and used to calculate the tile angle. When the free liquid surface 3 is tilted in Fig. 13, a displacement angle corresponding to the tilting of the free liquid surface 3 appears on the optical axis of the reflection light beam 30 as explained referring to Fig. 26, and the optical axis is moved. Also the anamorphic prism system 20 corrects sensitivity of displacement angle to the titling of the free liquid surface 3 in all directions. Therefore, the position of the reflection light beam 30 on the light receiving surface of the photodetector 38 moves to a position, which matches the tilting of the free liquid surface 3. Thus, the tilt angle of the entire system can be detected according to the output from the photodetector 38 without correcting the output of the photodetector 38. When a device such as CCD is used as the light receiving element and the tilt angle is calculated by obtaining the co-ordinates of position of the light beam on the photodetector 38, and if it is supposed that the distance between the photodetector 38 and the condenser lens 41 is f and that the angle between the optical axis of the reflection light beam 30 entering the condenser lens 41 and the optical axis of the condenser lens 41 is , and the displacement on the light receiving surface of the photodetector 38 can be given by f tan  can be detected from the signal of the photodetector 38. Therefore, the angle  which depends upon the free liquid surface 3, can be reversely calculated from the displacement f tan . Fig. 14 represents the present embodiment with the entire system turned through 90°. The operation of this embodiment shown in Fig. 14 is the same as that of Fig. 2 in that the reflection light beam 30 is emitted from the anamorphic prism system 20. Using the present invention it is possible to detect tilt angles of the entire system both near 0° and 90° when the entire system is turned through 0° and the entire system is turned through 90°. Instead of the anamorphic prism system 20 an electrical correction may be made to the output from the photodetector so that the sensitivity of the photodetector 38 is equalized in all directions. Also, instead of a polarized light beam splitter 17 any other optical dividing means may be used such as a half-mirror. If a half-mirror is used, there is no need to use the λ/2 phase plate 16. A description is now given of the fourth embodiment shown in Figs. 16 and 17. In the fourth embodiment, the light beam is reflected once by the free liquid surface. On lower side of the container 2, in which the transparent liquid 1 is sealed to form the free liquid surface 3, there are provided a first sealed window 51 serving as a first light entry means to guide the light beam 13 toward the transparent liquid 1, and a second window 52 serving as a second light entry means adjacent to the first window. Also located in the container are a first exit window 53 serving as a first light transmitting means for transmitting the light beam reflected from the free liquid surface to the outside of the container 2. and a second exit window 54 serving as a second transmitting means are arranged on the liquid sealing container. The first exit window 53 is located to one side of the second entry window 52, whilst the second exit window 54 is disposed to the side of the first entry window 51. Inside the container 2 is a liquid movement preventing device 55 immersed in the transparent liquid 1, and rotatable about an axis which is coincident or nearly coincident with the free liquid surface 3. As described later, the liquid movement preventing device 55 has a transparent liquid movement preventing plate 68 at a position somewhat below the free liquid surface 3 of the transparent liquid 1, and the center of gravity of the liquid movement preventing device 55 is located at such position that the liquid movement preventing plate 68 is always kept at horizontal position. A light projecting system 56 is arranged opposite to the first and second entry windows 51 and 52. The light projecting system 56 comprises a light source unit 57, a collimator lens 58, and a polarized light beam splitter 59 arranged sequentially toward the first entry window 51. A ½ wave plate 60 of birefringent material is rotatably mounted between the polarized light beam splitter 59 and the collimator lens 58. When the y-axis of the entire system is directed in vertical direction as shown in Fig. 16, the ½ wave plate 60 interacts the light beam from the light source unit 57, but when z-axis of the entire system is directed in vertical direction, as shown in Fig. 17, the center of gravity of the plate is rotated so that the plate is displaced away from the optical path of the light beam. A semiconductor laser having approximately linearly polarized light is used as the light source in the light source unit 57, and the direction of polarization is set in such manner that the light beam is turned to linearly polarized light vibrating in parallel to x-axis shown in Fig. 16. The plane of polarization of the ½ wave plate 60 is set in such manner that the direction of axis is rotated by 90° and runs in parallel to y-axis when the incident light beam 13 from the light source unit 57 passes through the ½ wave plate 60. A reflection mirror 61 is arranged opposite to the polarized light beam splitter 59, and the reflection mirror 61 reflects the light beam reflected by the polarized light beam splitter 59 towards the second entry window 52. The polarized light beam splitter 59 is so designed that it allows the incident light beam 13, which has passed through the ½ wave plate 60, i.e. the incident light beam 13 vibrating in parallel to y-axis, to pass and reflects the incident light beam 13, which does not pass through the ½ wave plate 60, i.e. the incident light beam 13, which vibrates in parallel to x-axis. After entering the transparent liquid 1 through the first entry window 51, the incident light beam 13 passes through the liquid movement preventing plate 68 of the liquid movement preventing device 55 and impinges the free liquid surface 3 at a given angle, e.g. at 50°, as shown in Fig. 16. After being reflected by the free liquid surface 3, the reflected light beam 30 passes through the liquid movement preventing plate 68 and is transmitted through the first exit window 53. The incident light beam 13 entering the transparent liquid 1 via the second entry window 52 also passes through the liquid movement preventing plate 68 of the liquid movement preventing device 55 and impinges the free liquid surface 3 at a given angle, e.g. at 50°, as shown in Fig. 17. The light beam 30 reflected by the free liquid surface 3 passes back through the liquid movement preventing plate 68 and is externally transmitted through the second exit window 54. There are provided a reflecting mirror 19, which reflects the reflected light beam 30 transmitted through the first exit window 53 in the vertical direction, and an anamorphic prism system 20 between the first exit window 53 and the reflection mirror 19. Sequentially arranged along the optical axis of the reflected light beam 30 reflected by the mirror 19 are a beam expander 21, a polarized light beam splitter 63, and a rotary pentagonal prism 66 rotatable by rotating means such as a motor. A reflection mirror 64, arranged opposite to the second exit window 54, guides the reflected light beam 30 towards the polarized light beam splitter 63. A beam expander 65 is disposed between the polarised light beam splitter 63 and the reflection mirror 64 so that the optical axis of the reflection light beam 30 reflected by the polarized light beam splitter 63 agrees with the optical axis of the light beam, which has passed through the polarized light beam splitter 63. The polarized light beam splitter 63 is so designed that it allows the light beam vibrating in parallel to z-axis to pass and reflects the light beam vibrating in parallel to x-axis. The operation of the above embodiment is now described. The incident light beam 13, which is emitted from the light source unit 57, passes through the ½ wave plate 60 so as to be vibrated in the y-axis direction. It then passes through the polarized light beam splitter 59 and enters the transparent liquid 1 via the first entry window 51 and is totally reflected by the free liquid surface 3. The point of reflection of the beam by the surface of the transparent liquid 1 is coincident or nearly coincident with the axis of rotation of the liquid movement preventing device 55. After being reflected by the free liquid surface 3 of the transparent liquid 1, the reflected light beam 30 is transmitted externally by the first exit window 53 and passes through the anamorphic prism system 20. Then, it is reflected in vertical direction by the reflection mirror 19 and further passes through the beam expander 21 and the polarized light beam splitter 63 and is finally emitted in horizontal direction by the rotary pentagonal prism 66. As described above, the anamorphic prism system 20 equalizes reflection sensitivity on the free liquid surface 3 in all directions and the beam expander 21 ultimately adjusts the sensitivity. Therefore, the optical axis of the light beam passing through the beam expander 21 is always compensated in vertical direction regardless of the tilting of the entire system. Accordingly, the reflection light beam 30 emitted from the rotary pentagonal prism 66 forms the horizontal reference line. By rotating the rotary pentagonal prism 66, it is possible to obtain a constant horizontal reference plane at all times. Fig. 17 represents the entire system rotated by 90°. The liquid movement preventing device 55 is also rotated by 90° so that the liquid movement preventing plate 68 is held in a horizontal position. Similarly, the free liquid surface 3 of the transparent liquid 1 remains horizontal. The ½ wave plate 60 also rotates by 90° and so is now displaced from the optical path of the incident light beam 13. Therefore, the incident light beam 13 having vibration in x-axis direction enters the polarized light beam splitter 59, and is reflected toward the reflection mirror 61 by the polarized light beam splitter 59. After being reflected by the reflection mirror 61 and entering via the second entry window 52, the incident light beam 13 is totally reflected by the free liquid surface 3. In this case, the point of reflection of the incident light beam 13 on the transparent liquid surface is coincident or nearly coincident with the axis of rotation of the liquid movement preventing device 55. After being reflected by the free liquid surface 3, the reflected light beam 30 is transmitted externally via the second exit window 54. It is then reflected by the reflection mirror 64, passes through the beam expander 65 and is further reflected in horizontal direction by the polarized light beam splitter 63. It therefore runs along the same optical axis as the optical axis of the reflected light beam before the system is turned through 90°. Because the entire optical system is turned through 90°, the optical axis of the reflected light beam 30 reflected by the polarized light beam splitter 63 is kept always in the horizontal direction. By rotating the rotary pentagonal prism 66, the irradiated and formed reference plane is changed to the vertical direction, so that it is possible to obtain a reference plane which is always maintained in the vertical direction in the z-y plane, regardless of the tilting of the entire system. In this case, compensation of the optical axis, when the entire system is turned through 90°, as shown in Fig. 17, is done in the vertical direction in the y-z plane; however, the optical axis is not compensated with regard to the x-y plane. Because the system is rotated and matches the reference when the vertical irradiation plane is aligned with the reference point, it is more convenient that the movement in rotating direction of the irradiation plane is slow when the system is installed. The first entry window and the second entry window may be integrated as shown in Fig. 16 or may be separate. By defining the direction of polarization in this manner, it is possible to suppress attenuation of light in the beam splitter as much as possible. Movement of the transparent liquid 1 sealed in the container 2 due to external vibration is suppressed by the liquid movement preventing device 55. Referring to Figs. 18 to 20, the liquid movement preventing device 55 is a modified version of the liquid movement preventing device 10 shown in Figs. 4 to 6. The liquid movement preventing device 55 comprises a liquid movement preventing plate 68 and a plate support 69. The liquid movement preventing plate 68 is provided horizontally on the upper surface of the plate support 69. The axis of rotation of the plate support 69 is located slightly above the liquid movement preventing plate 68. The center of gravity of the liquid movement preventing device 55 is located in the plane containing the axis of rotation and which is perpendicular to the liquid movement preventing device 68, and is located below the liquid movement preventing plate 68. A recess 70 is formed on upper surface of the plate support 69 and spanned by the liquid movement preventing plate 68. The liquid movement preventing plate 68 is made of transparent glass material to prevent attenuation of light. On the lower surface of the plate support 69 are parallel grooves are formed in the direction of rotation in the form of a comb, and an adequate resistance is generated in the transparent liquid 1. A material having high thermal conductivity such as copper is used as the material of the plate support 69, and its surface area is increased by designing it in comb-like shape. Thus, the thermal conduction between the moving plate support 69 and the transparent liquid 1 is improved. The natural frequency of the liquid movement preventing device 55 in the transparent liquid 1 is made lower than natural frequency of the transparent liquid 1. Thus, by the viscosity of the liquid and the resistance between the transparent liquid 1 and the moving plate support 69, the vibration of the liquid movement preventing device 55 is suppressed, and stability of the free liquid surface 3 is maintained. Even when sudden change occurs in environmental temperature, temperature difference rarely occurs because the moving plate support 69 increases heat transmission toward inside the transparent liquid 1. This prevents the distribution of refractive indices and raises stability of the light beam. It is needless to say that the reflection mirror 19 can be replaced with other reflection material such as prism. As described above, it is possible according to the present invention to automatically compensate optical path regardless of whether the system is at horizontal position or at vertical position without requiring a specially prepared optical system or a surplus mechanism to replace a part of the optical system. As a result, the system is simple and easy to use and working efficiency is improved. At the same time, accuracy and reproducibility are increased. Because reflection of the free liquid surface is utilized, which is always maintained in horizontal direction, complicated assembling technique as required in the conventional pendulum type angle compensator is not needed and the system can be easily assembled. Instead of suspending optical components in the conventional pendulum type angle compensator, it will suffice only to infuse the liquid in the liquid sealing container. This means very simple work and less variation in the assembling accuracy due to individual difference. Because the liquid is completely sealed, no change occurs over time and resistance to environmental condition is high. To external vibration and impact, the liquid movement is controlled by utilization and viscosity of the liquid, and no complicated movement control mechanism is required.

Claims: A levelling instrument comprising a chamber (2) partially filled with a transparent liquid (1), and a light source (12,57) for generating a polarised light beam (13) for reflection by the liquid surface (3); a polarised light beam splitter (17,59) in the path of the light beam; and means (16) which when the instrument is in a first orientation relative to the horizontal are arranged to provide to the beam splitter (17,59) a beam of a first polarisation, the beam splitter being adapted to produce therefrom a first split beam to pass through a first transparent portion (4,51) of the chamber and exit via an associated first element (6,53); and which when the instrument is in a second angular orientation relative to the horizontal is arranged to provide to the beam splitter (17,59) a beam of a second polarisation, the beam splitter being adapted to provide therefrom a second split beam arranged to pass through a second transparent portion (5,52) of the chamber and exit via an associated second element (8,54). A levelling instrument as claimed in claim 1 in which the associated first and second elements (6, 53; 8,54) are provided on substantially opposing sides of the chamber. A levelling instrument as claimed in claim 1 or claim 2 in which in the second angular orientation the chamber is rotated through 90° from its orientation in the first angular orientation. A levelling instrument according to any one of the preceding claims wherein the incident angle of the polarised light beam (13) to the free liquid surface is more than 45°. A levelling instrument according to any one of the preceding claims wherein the first associated element (6,53) is a first reflection member for reflecting the polarised light beam reflected by the liquid surface (3) back towards the liquid surface when the levelling instrument is in the first angular orientation, and the second associated element (8,54) is a second reflection member for reflecting the polarised light beam reflected by the liquid surface back towards the liquid surface when the levelling instrument is in the second angular orientation. A levelling instrument according to any of claims 1 to 4 wherein the first associated element (6,53) is a first transparent portion arranged on the chamber (2) and the second associated element (8,54) is a second transparent portion arranged on the chamber. A levelling instrument according to any one of the preceding claims, wherein a first birefringence member (16) is arranged to be positioned in the optical path of the light source (12,57) when the instrument is in one of the first and second angular orientations and out of the optical path when the instrument is in the other angular orientation for providing beams of different polarization. A levelling instrument according to claim 7, wherein the first birefringence member (16) is a λ/2 phase plate. A levelling instrument according to claim 7 or claim 8 when dependent on claim 5, wherein a respective second birefringence member (7, 9) for altering the direction of polarization is provided between the liquid surface (3) and each of the first reflection member (6,53) and the second reflection member (8,54). A levelling instrument according to claim 7 or claim 8 when dependent on claim 5, wherein a respective second birefringence member (7, 9) for altering the direction of polarization is provided between the beam splitter (17,59) along a first optical path and the liquid surface (3) and between the beam splitter along a second optical path and the liquid surface. A levelling instrument according to claims 9 or 10, wherein the second birefringence member is a λ/4 phase plate. A levelling instrument according to any one of the preceding claims, wherein there is further provided a photodetector (38) for detecting tilting of the instrument and to which the polarised light beam (13) reflected by the liquid surface (3) and emitted from the chamber (2) is guided. A levelling instrument according to any one of the preceding claims, wherein the optical axes of the first and second split beams exiting the chamber (2) are made identical with each other by an aligning beam splitter (63). A levelling instrument according to claim 13, wherein the aligning beam splitter (63) is provided separate from the beam splitter (17,59) provided in the path of the polarised light beam (13). A levelling instrument according to any one of the preceding claims, wherein the incident angle of the polarised light beam (13) to the liquid surface (3) is selected such that the light beam is totally reflected by the liquid surface. A levelling instrument according to any one of the preceding claims, wherein a liquid movement preventing plate (11,68) is located with an axis of rotation substantially coincident with the point of incidence of the light beam (13) on the liquid surface (3).

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