IMPROVEMENTS RELATING TO COMPASSES

Abstract: A compass comprising optical means (30, 60, 90) arranged to give an image of a directionally relevant portion of a compass scale (26) which image is superimposed directly upon a view viewed through a viewfinder (31, 61, 91) so as that all the scale degree marks visible in the viewfinder accurately represent real angles to enable a bearing or bearings to be taken on any object or objects visible in the viewfinder, without requiring any object to be precisely centralised in the viewfinder. The optical means includes a reflector (33, 93), disposed between an upstanding rotatable wall (24, 68) (on which the scale is marked) and a pivot (17), to reflect light up into a lens device (32, 62, 92) which is focused on the relevant portion of the scale. The lens device directs a scale image focused at infinity into the viewfinder. The effective focal length of the lens device is approximately equal to the radial distance between the scale and the pivot.

Description: IMPROVEMENTS RELATING TO COMPASSES This invention concerns hand held compasses. There are known compasses of the kind comprising a housing; a compass card, which card is pivotally mounted in the housing; sight means to enable a v user to align the compass with a distant object; and optical means to enable the user to view a portion of a scale on the compass card for taking a bearing on said distant object. The optical means comprises a reflector disposed below a foresight of said sight means to reflect an image of said portion of the scale towards a rear sight of the sight means. In some known compasses of the kind described the reflector is a magnifying prism, and such compasses are thus known as primatic compasses. Some considerable skill is needed to take accurate bearings using known compasses in order to minimise errors due to various causes, e.g. failure to align the fore and rear sights properly with the distant object, failure to read the bearing from the exact centre of the reflector, and damage to or maladjustment of the relatively fragile sights. In British Patent Specification No. 1109172 there is proposed direction finding apparatus comprising a compass combined with a telescope or binoculars, in which a lens and a reflector are employed to project an image of part of the compass scale into the telescope or binoculars so that the image can be viewed by a person using the telescope or binoculars. However, this apparatus has disadvantages equivalent to those inherent in the aforementioned compasses, e.g. the telescope or binoculars must be aligned with the distant object being viewed so that the latter is precisely central in the field of view and the bearing must be read from the centre of the scale image; and has other inherent disadvantages which can give rise to errors and difficulties, e.g. parallax errors, blurring of the scale image if the apparatus is not horizontal, and insufficient control of the contrast between the image of the field of view and the scale image. What is needed is a relatively easy to use compass. According to the present invention there is provided a compass comprising a housing, a rotatable member pivotally mounted in the housing, direction seeking means to orientate the member to a given direction, optical means secured to the housing, and a scale provided on the rotatable member; characterised in that the optical means comprises a viewfinder of beamsplitting form, and a lens device focused on the scale to project an image of a portion of the scale to the viewfinder; in that the viewfinder and lens device are arranged so that said image is focused at infinity and is reflected by the viewfinder so as to be superimposed upon light passing through the viewfinder; in that the scale is calibrated in degrees; and in that the arrangement of the scale and lens device is such that the projected degree marks in the image represent real angular degrees superimposed upon the field of view visible through the viewfinder. The compass enables a user to take a bearing on an object in a distant view simply by looking at the view through the viewfinder and observing the position of the object on the image of the portion of the scale which is visible in the viewfinder. The need to align sights with the object and the need to look down from the foresight to the centre of the reflector are obviated (as is the need to centralise the object precisely in the field of view). Furthermore a compass of the known kind can give rise to errors and the scale can be difficult to read even if the sights are aligned so that the distant object is central in the sights, if the compass is not held in a horizontal plane, due to the housing and optical means being tilted relative to the compass card. Attempts have been made to minimise such errors by restricting the movement of the card to rotational movements about a predetermined axis which is fixed in relation to the housing, but such attempts have involved extra costs, and have introduced secondary disadvantages. To enable such errors to be reduced, the compass of the invention is preferably further characterised in that the rotatable member has an upstanding peripheral wall or wall portion upon which the scale is provided so that a nominally horizontal plane, through the point about which the rotatable member is pivoted, intersects said wall or wall portion at a circle centred on said point; in that the optical means further includes a reflector to reflect light from said portion of the scale towards the viewfinder; and in that a reflective or partially reflective face of the viewfinder is parallel with a reflective face of the reflector. In preferred forms of the invention the lens device is disposed between the reflector and the viewfinder, and has an effective focal length substantially equal to the radius of said circle; and the optical means is preferably arranged so that the apparent position of the effective optical centre (or the equivalent nodal point) of the lens device in respect of light from said portion of the scale is located at, or proximal to, said pivot point, whereby to further reduce errors and distortion of the image of the portion of the scale visible in the viewfinder. Light is preferably directed to the scale from the field of view, from adjacent to the field of view or in line with the field of view; and said reflector is preferably disposed so as to intersect said nominally horizontal plane at an angle of about 450 at a line within said circle so as to be interposed between the scale and the pivot point. Said angle is preferably 450 but 0 a small variation, e.g. of up to 5 , may be employed without incurring excessive distortion if the geometry of the scale and lens device is arranged to minimise distortion. The lens device or at least the scale proximal face thereof, said reflector and said scale are preferably immersed in liquid within the compass. In particular, there are herein disclosed forms of compass which each, in accordance with the invention, comprise housing means containing a rotatable member, a compass scale provided on said rotatable member, pointed pivot means in said housing means and carrying said rotatable member, direction seeking means connected to said rotatable member, optical means carried by said housing means, and illumination means secured to said housing means for illumination of a portion of said scale; wherein: :- (a) said rotatable member comprises upstanding wall means on which said compass scale is marked so as to face towards said pivot means; (b) said optical means comprises a viewfinder mounted upon the housing means, and a lens device mounted in the housing means between the viewfinder and said rotatable member and vertically above the rotatable member; (c) said optical means further comprises a reflector disposed vertically below said lens device and projecting angularly between said compass scale and said pivot means to reflect light, travelling from said portion of the scale towards the pivot means in a nominally horizontal direction, upwardly to the lens device; (d) said lens device is focused on said portion of the scale to project light from said scale into said viewfinder as an angularly accurate image focused at infinity; and (e) said viewfinder is optically formed to selectively transmit light from a field of view confronting the viewfinder and to reflect and superimpose light received from the lens device upon said transmitted light to provide combined light for viewing by a user. The invention will be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein: FIGURE 1 is a vertical sectional view of a compass according to the invention; FIGURE 2 is a diagram showing a view and image superimposed thereon as seen by a person using the compass; FIGURE 3 is a vertical sectional view of a second embodiment of compass of the invention; FIGURE 4 shows a vertical longitudinal part sectional view of a third embodiment of the invention; FIGURE 5 shows a vertical transverse part sectional view of the third embodiment; FIGURE 6 is a plan view of the underside of a top part of a housing of the third embodiment; and FIGURE 7 is a plan view of the underside of an intermediate part of the housing. Referring to FIGURE 1, the first embodiment of compass comprises a housing 10, a rotatable member 20, and optical means 30. The housing 10 comprises a case 11 which is closed by a transparent lid 12. A central pivot needle 13 is secured so as to be perpendicular to the base 14 of the housing 10. A light collecting window 15 is provided in a cylindrical side wall 16 of the housing. The rotatable member 20 comprises a circular bottom 21 having a raised central portion 22 providing an apex 23 seated on the needle 13 so that the member is rotatable about a pivot point 17; and comprises a peripheral wall 24 which is upstanding from the bottom 21. A magnet 25 or magnets are secured to the bottom 21. In this first embodiment the walls 16 and 24 are cylindrical. A compass card is marked or provided on an upper surface of the bottom 21. A compass scale 26 is provided on a portion 27 of the wall 24, and is arranged so that said portion 27 intersects a nominally horizontal plane (indicated by a broken line 28) through the pivot point 17 at a circle 29 centred on the pivot point 17. The scale 26 is readable from that side of the portion 27 which confronts (facg towards) the pivot point 17. The optical means 30 comprises a viewfinder 31, a lens device 32 and a reflector 33 which are supported by a mounting 34 secured to the lid 12. In the embodiment shown the viewfinder 31 is of beamsplitting form and is represented as a transparent planar member of semi-reflective form having a face or surface which is partially silvered with a suitable deposited metal, e.g. a multiplicity of discrete chromium deposits, but said planar member may be of a selectively reflective form having a face or surface which is dichroic, i.e. which reflects light of a specific wave length and is transparent to light of other wavelengths. The lens device 32 is represented by a biconvex lens, but may be of other form such as a planoconvex lens or a more complex multi-element lens or lens array. The reflector 33 is represented by a mirror, but may be a reflector prism. The reflector 33, or reflective face thereof, is centred in the plane 28 at 450 to the plane, is vertically below the lens device 32, is parallel with the viewfinder 31 or semi-reflective face thereof, and is positioned between the portion 27 and the pivot point 17. The lens device 32 has an effective focal length which is equal to the radius of said portion 27, and the lens device is positioned so as to be focused on said portion 27 via the reflector. If the case 11 is liquid filled for damping, the refractive index of the liquid is taken into account when determining the back focal length and the positioning of the lens device so that the latter is focused on said portion 27. The arrangement of the reflector 33 and lens device 32 is such that, with respect to the portion 27, the apparent position of the nodal point centre of the lens coincides with the pivot point 17, so that the lens projects an image of that part or portion of the scale 26 which confronts the reflector 33 to or into the viewfinder, said image being focused at infinity. In this embodiment the wall 24 is of a transparent material, the scale 26 being represented by transparent characters on an opaque background. The viewfinder 31 receives light from a view confronting the viewfinder along a path centred on the broken line 40, a portion of which light passes through the viewfinder, and the viewfinder also reflects a portion of the light from the window 15, which light travels along a path centred on the dotted line 41, so that said portions of light are combined and follow a path centred on the chain broken line 42 from the viewfinder for observation by an eye 44 of a user of the compass, who will observe the scale as bright or coloured characters superimposed clearly upon the view, as indicated in FIGURE 2 in which the characters are shown in broken lines and comprise numerals and two spaced apart horizontal r & s of striations 45 which represent actual degrees. When the compass is horizontal a distant horizon will be central between the rows. However, if the compass is tilted slightly in any direction the card will remain horizontal, and the characters will continue to indicate accurate bearings in relation to the horizon and will remain sharply defined whilst visible in the viewfinder. The compass affords a direct reading of the bearing, and can be used to give comparative simultaneous bearings of a plurality of objects in a given view in the viewfinder, e.g. the lighthouse 46 and the rock 47 which are about four degrees 0 apart, the rock having a bearing of about 355 . The second embodiment of compass shown in FIGURE 3 also comprises a housing 50, a rotatable member 55, and optical means 60. The housing 50 comprises a substantially spherical case 51 in which a nominally perpendicular central pivot point needle 13 is secured so as to extend upwards to near to the centre of the housing. A light collecting window 52 is provided in the wall of the housing. The rotatable member 55 comprises a spider carrier 56 having a raised central portion providing an apex 23 seated on the needle 13 so that the member is rotatable about a pivot point 17; and comprises a cylindrical peripheral wall 24 which is upstanding from the carrier 56. Parallel magnets 25 are secured to the bottom parts of the carrier. A compass scale 26 is provided on the wall 24, and is arranged so that said wall intersects the nominally horizontal plane 28 through the pivot point 17 at a circle centred on the pivot point 17. The optical means 60 comprises a viewfinder 61, a lens device 62 and a mirror reflector 33 which are supported by a mounting 63 which serves as a closure for an upper opening in the housing 50. In the second embodiment, the viewfinder 61 is represented as a dichroic prism cube member, but a semi-reflective prism member may be substituted, and is of beam-splitting form. The lens device 62 comprises two plano-convex lenses 64 and 65. The upper lens 64 abuts the prism member. The reflector 33 is arranged in the same manner as in the first embodiment and is parallel with the reflective dichroic face of the viewfinder 61. The lens device 62 has an effective focal length which is equal to the radius of said wall 24, and is positioned so as to be focused on said wall 24 via the reflector. The case 11 is liquid filled, and the refractive index of the liquid is also taken into account when determining the back focal length of the lens device, to give the correct optical distance between the bottom lens 65 and said wall 24 via said reflector 33. The arrangement of the reflector 33 and lens device 32 is such that, with respect to the wall 24, the apparent position of the relevant nodal point of the lens device coincides with the pivot point 17, so that the lens projects an image of that arcuate part or portion of the scale 26 which confronts the reflector 33 to or into the viewfinder, said image being focused at infinity. In this erffiodiment the wall 24 is of an opaque material, the scale 26 being represented by painted fluorescent characters on an opaque background. The window 52 is disposed above the level of the wall 24 and a front face 53 of the mounting 63 is silvered to reflect light from the window onto the scale 26 in line with the field of view through the viewfinder. The viewfinder 61 receives light, from the view confronting the viewfinder, along a path centred on the broken line 40 t a portion of which light passes through the viewfinder, and the viewfinder also reflects a portion of the light from the window 52, which light travels along a path centred on the dotted line 43, so that said portions of light are combined and follow a path centred on the chain broken line 42 from the viewfinder for observation by an eye 44 of a user of the compass, who will observe the scale as coloured bright characters superimposed clearly upon the view, as indicated in FIGURE 2. The use of a spider carrier reduces coupling between the liquid and the carrier, and the immersion of the lower face of the bottom lens 65 in the liquid enables an expansion space 54 to be left in the top of the housing 50. The third embodiment of compass shown in FIGURES 4 to 7 comprises a rotatable member 66, a housing 70, and optical means 90. The housing 70 includes a moulded opaque plastics top part 71 which serves as a mounting for a viewfinder 91 of the optical means and defines a sighting passage 72 of circular cross section. The underside 73 (FIGURE 6) of the top part 71 interlocks with a moulded opaque intermediate part 74 of the housing 70, and has a bottom opening 75 below the viewfinder 91 and above a lens device 92 of the optical means 90. The intermediate part 74 incorporates a hollow mounting 76. The lens device 92 is mounted in the mounting 76 and a reflector 93 is mounted at the bottom of the mounting 76. The intermediate part 74 is bonded to and serves as a closure for a lower case part 77 of the housing, and is provided with a screw plug 78 to close a filler opening 79 via which liquid can be inserted into the case part 77 when the top part 71 and plug 78 are removed. The lower case part 77 is dished and provides a central mounting 80 for a screw in pivot needle 81 and a threaded socket 82 for insertion of a needle retaining plug or a fastener for securing the housing tp a suppport or handle. A light collecting window 15 is secured in the wall of the case part 77. A betalight 83 is provided at the window. The rotatable member 66 comprises a moulded or cast bottom portion 67 incorporating a raised central portion 67A in which a jewel bearing 67B is seated to rest on the needle 81, and a peripheral flange 67C which supports a separately formed cylindrical transparent wall portion 68 on which the scale 26 is marked by transparent characters on an opaque background, as described hereinbefore with reference to FIGURE 1. Two magnets 25 are carried by the bottom portion 67. The lens device 92 comprises an upper planoconvex lens 94 and a lower plano-convex lens 95 separated by a small gap. The combined lenses have an effective focal length which is approximately eqtàl to the radial distance between the scale 26 and the pivot point 17 when the case is filled with liquid. The lenses 94 and 95 are arranged in relatio to the reflector 93 so that the distance travelled by the light from the scale to the flat undersurface 96 of the lens 95 via the reflector 93 is substantially equal to the appropriate back focal length of the combined lenses 94 and 95 in the liquid chosen to fill the compass case; and the nodal point of the lens device for such light is effectively at a physical distance measured along the light path from the scale) which is substantially equal to said radial distance multiplied by the refractive index of the chosen liquid. Optically, this embodiment provides for the user of a functional equivalent of the first embodiment with respect to the light paths 40, 41 and 42, except in that the eye visible to the viewer's eye 44 is circular instead of being rectangular as shown in FIGURE 2. The lens device is similarly focused on the scale to give a clear image in real degrees focused at infinity. In use, unwanted vertical movements of the rotatable member 66 are restricted by a stop 97 which extends from the hollow mounting 76 to close to the top of the central portion 67A. The invention is not confined to the precise details of the foregoing examples, and many variations are possible within the scope of the invention as defined by the appended claims. For example, in the first embodiment the compass card may be omitted and the lid may be opaque, and shaped to serve as an integral mounting for the elements of the optical means. In the second embodiment the paint for the characters is selected to transmit light of the wavelength reflected by the beamsplitter, but the window 52 may be replaced by the window 15 and transparent characters employed in the wall 24 as in the first embodiment. The wall 24 or the portion 27 may be curved about an arc 27A centred on the pivot point as indicated in broken lines in FIGURE 1; and the wall portion 68 may be shaped and radially enlarged as indicated in broken lines as 68A in FIGURE 5. The lens device may comprise lenses or a compound lens or lenses, and such embodiments are arranged so that the effective focal length of the lens device is substantially equal to the radius of the scale from the pivot point, and so that the apparent position of that nodal point which is effective for the portion of light from the scale reflected by the viewfinder is disposed at or near the pivot point, after taking into account the refractive index of any liquid used for filling the case. A beta-light or other artificial light source may be provided, e.g. in the window 15 or 52, to provide light for using the compass at night, or a light source could be substituted in place of the windows. A gyroscopic device may be used as an alternative to a magnet or magnets as a suitable direction seeking means for orientating the rotatable member.

Claims: CLAIMS 1. A compass comprising a housing (10,50,70), a rotatable member (20,55,66) pivotally mounted in the housing, direction seeking means (25) to orientate the member to a given direction, optical means (30,60,90) secured to the housing, and a scale (26) provided on the rotatable member; characterised in that the optical means comprises a viewfinder (31,61,91) of beamsplitting form, and a lens device (32,62,92) focused on the scale to project an image of a portion of the scale to the viewfinder; in that the viewfinder and lens device are arranged so that said image is focused at infinity and is reflected by the viewfinder so as to be superimposed upon light passing through the viewfinder; in that the scale is calibrated in degrees; and in that the arrangement of the scale and lens device is such that the projected degree marks in the image represent real angular degrees superimposed upon the field of view visible through the viewfinder. 2. A compass as claimed in Claim 1 characterised in that the rotatable member has an upstanding peripheral wall or wall portion (24,27,27A,68,68A) upon which the scale (26) is provided so that a nominally horizontal plane (28), through the point (17) about which the rotatable member is pivoted, intersects said wall or wall portion at a circle centred on said point. 3. A compass as claimed in Claim 2, characterised ip that the optical means further includes a reflector (33,93) to reflect light from said portion that a reflective or partially reflective face of the viewfinder is parallel with a reflective face of the reflector. 4. A compass as claimed in Claim 3, characterised in that the lens device is disposed between the reflector and the viewfinder, and has an effective focal length substantially equal to the radius of said circle. 5. A compass as claimed in Claim 3 or 4 characterised in that the optical means is arranged so that the apparent position of the nodal point of the lens device, in respect of light from said portion of the scale, is located at said pivot point or is proximal to said pivot point. 6. A compass as claimed in Claim 3, 4 or 5 characterised in that the said reflector is disposed so as to intersect said nominally horizontal plane at an angle of about 450 at a line within said circle so as to be interposed between the scale and the pivot point. 7. A compass as claimed in Claim 3, 4, 5 or 6 characterised in that the housing comprises a top part supporting the viewfinder, an intermediate part supporting the reflector and lens device, and a lower part supporting the pivot; and in that the top part is releasably connected to the intermediate part, and the latter is fixedly joined to the lower part. 8. A compass as claimed in any preceding claim, characterised in that light is directed to the scale from the field of view, from adjacent to the field of view or in line with the field of view. 9. A compass as claimed in Claim 3 or any one of Claims 4 to 8 as appended to Claim 3, characterised in that a scale proximal face (96) of the lens device, said reflector and said rotatable member are immersed in liquid within the housing 10. A compass comprising housing means (10,50,70) containing a rotatable member (20,55,66), a compass scale provided on said rotatable member, pointed pivot means (13,17,81) in said housing means and carrying said rotatable member, direction seeking means (25) connected to said rotatable member, optical means (30,60,90) carried by said housing means, and illumination means (15,52,83 secured to said housing means for illumination of a portion of said scale; wherein: (a) said rotatable member comprises upstanding wall means (24,27,27A,68,68A) on which said compass scale is marked so as to face towards said pivot means; (b) said optical means comprises a viewfinder (31,61,91) mounted upon the housing means, and a lens device (32,62,92) mounted in the housing means between the viewfinder and said rotatable member and vertically above the rotatable member; (c) said optical means further comprises a reflector (33,93) disposed vertically below said lens device and projecting angularly between said compass scale and said pivot means to reflect light (41,43), travelling from said portion of the scale towards the pivot means in a nominally horizontal direction, upwardly to the lens device; ; (d) said lens device is focused on said portion of the scale to project light (41,43) from said scale into said viewfinder as an angularly accurate image focused at infinity; and (e) said viewfinder is optically formed to selectively transmit light (40) from a field of view confronting the viewfinder and to reflect and superimpose light (41,43) received from the lens device upon said transmitted light (40) to provide combined light (42) for viewing by a user.

Inventors: BUDDEN, RAYMOND GEORGE, SAXBY, MICHAEL ERNEST, SCOTT, FRASER
Applicants: HALL & WATTS LIMITED
Application year: 1983
Main classification: G01C17
IPC codes: G01C17