CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Application No. 60/201,489; filed May 3, 2000 now abandoned. 
    
    
     FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a lighting fixture for projecting a beam of light and for use for spot lighting in connection with theater stages, cinema and television studios and the like, the fixture comprising: 
     a light source arranged at one end of a housing having a light beam exit aperture at the opposite end thereof, the light source and aperture being arranged generally concentric with a longitudinal or optical axis of the lighting fixture, 
     light beam influencing means at least comprising one or more, preferably four, beam-shaping blades and preferably also comprising other light influencing means such as one or more lenses and/or an iris and/or a pattern or gobo, for influencing a light beam emitted by the light source and being arranged along the path of the light beam along said longitudinal axis through the housing from the light source to the aperture, and 
     adjustment means for adjusting the position of at least said one or more beam-shaping blades and preferably of all said influencing means relative to said longitudinal axis. 
     The purpose of a lighting fixture as defined above is to produce a well-defined light beam or light cone with a geometry, angle of conicity and focal point that may be altered manually or by remote control. 
     A lighting fixture will normally comprise a light source, a reflector, a beam-shaping gate with beam-shaping blades, a pattern or gobo, an iris, a focusing lens, a zoom lens and a color filter as well as a suspension structure allowing the lighting fixture to be pivoted vertically and horizontally. 
     The visible part of the light emitted by the light source is collected by the reflector and is sent towards the iris, the gobo and the beam-shaping gate as a parallel light beam. The infrared part of the radiation from the light source passes through the dichroic coating of the reflector and impinges on the inner surface of the housing surrounding the light source, the heat being transported to the outer surface of the housing having cooling ribs for emitting the heat to the surrounding atmosphere. 
     It is often necessary to be able to determine the geometry of the light beam, and this is achieved by means of the zoom lens varying the angle of conicity of the light cone and by shaping or cutting off the periphery of the light beam by means of the beam-shaping gate with beam-shaping blades so as to obtain geometrical figures such as squares, triangles, trapezoids etc. The lenses project the light out through the aperture of the housing opposite the light source and through the color filter at the front end of the lighting fixture. It is important that the different elements influencing the shape and other characteristics of the light beam function as precisely as possible even when being influenced by the heat radiated from the light source and not removed by means of the dichroic reflector. This entails that the location and the configuration of the adjustment means for the beam-shaping blades, the gobo and iris are such that any bending caused by the heat influence from the light beam be kept at a minimum. 
     Lighting fixtures of this type are often arranged in places where it is difficult to access them manually and it is therefore of great importance that the adjustment means for adjusting the above-mentioned beam influencing means be as easily accessed and as flexible as possible when manual operation of the adjustment means is required. 
     U.S. Pat. No. 5,345,371 discloses a lighting fixture of the type in reference where the four beam-shaping blades or shutters are slidingly insertable in slots from outside, the shutters being radially adjustable by gripping a holder for each shutter and sliding the shutter in or relative to the optical axis. The shutters may also be tilted manually to a certain extent. However, a further tilting possibility is achieved by allowing the portion of the fixture containing the shutters to be rotated as a unit around the axis. This is a complicated solution and needs manual access to all holders of the shutters as well as manipulation of the rotation means for rotating part of the fixture. Motorization for remote control of this design will be very complicated and costly. 
     U.S. Pat. No. 4,890,208 discloses a lighting fixture of the type in reference where four shutters are arranged for motorized displacement radially toward the optical axis and motorized tilting by means of rack and pinion mechanisms. This solution is complicated and has only limited tilting capability, i.e. displacement capability circumferentially around the axis. Furthermore this solution is not well suited for manual operation. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a lighting fixture of the type indicated, wherein access for manual operation is convenient and not dependent on the orientation of the lighting fixture, wherein motorization for remote control may be established in a simple and reliable manner and wherein the range of displacement circumferentially around the optical axis is as great as possible. 
     According to the invention this object is achieved by at least the adjustment means corresponding to said one or more beam-shaping blades and preferably all the adjustment means are arranged for rotation around said longitudinal axis and are connected to a respective influencing means such that rotation of the adjustment means around said longitudinal axis adjusts the position of the respective influencing means relative to said longitudinal axis. 
     Hereby the adjustment means may be accessed from practically any angle, and no limit to the adjustment possibilities in circumferential direction is inherent. 
     In the currently preferred embodiment the adjustment means comprise an annular body arranged with the axis thereof substantially coinciding with said longitudinal axis. This is a particularly simple and effective embodiment. 
     In the currently preferred embodiment of the invention the annular body comprises an outer rim configured for being engaged for applying rotational force thereto, the surface of said outer rim being provided with friction enhancing means such as roughening means, rubber surfacing, projections or teeth. Hereby manual and remote operation of the adjustment means is particularly simple and efficient. 
     Advantageously, the fixture further comprises one or more electrical motors connected to a respective drive wheel engaging said outer rim of a respective annular body for applying a rotational force thereto, and preferably the drive wheel is a gear having teeth, and the respective outer rim engaged by a respective gear is provided with teeth for meshing with the teeth of said gear when said gear rotates. 
     For use in remote control of the lighting fixture with pre-determined positions of the light influencing means, it is advantageous that the annular body be provided with a position indicating means for indicating the angular position of the annular body relative to said longitudinal axis. Hereby a reference point for the remote control operation is available, thereby eliminating errors and inaccuracies. 
     Advantageously, the position indicating means comprises an element that may be remotely sensed such as a magnet or a gap, and the fixture further comprises remote sensing means for sensing the angular position of said element relative to said longitudinal axis. 
     So as to obtain the greatest flexibility of adjustment and the greatest range of adjustment, the adjustment means for each of the one or more beam-shaping blades comprises radial adjustment means for adjusting the position of the blade radially relative to said axis, and circumferential adjustment means for adjusting the position of said blade circumferentially around said axis. 
     A particularly simple and efficient as well as accurate embodiment of the light fixture according to the invention is provided by the adjustment means for each of the one or more beam-shaping blades comprising two adjacent co-centrical annular bodies or rings each connected to one point of the blade such that relative rotation of the two rings alters the radial position of the blade. 
     In the currently preferred embodiment, the rings comprise guiding tracks recessed into the lateral surface of each ring facing the other ring, and each blade comprises a body extending generally transversely to said axis and two arms extending generally parallel to said axis, the arms each being provided with sliding connecting means for connecting the respective arm to each of the rings and being adapted for being slidingly received in a guiding track in each of said rings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following description, preferred embodiments of a lighting fixture according to the invention will be described in detail, solely by way of example, with reference to the accompanying drawings, where: 
     FIG. 1 is an isometric elevational view of a lighting fixture according to the invention for manual operation; 
     FIG. 2 is a partially cut-away view of the lighting fixture in FIG. 1 illustrating the internal configuration of the lighting fixture; 
     FIG. 3 is a schematic cross-sectional view of the lighting fixture of FIGS. 1 and 2, the cross-section being taken along a vertical plane containing the longitudinal or optical axis of the lighting fixture; 
     FIG. 4 is an enlarged scale view of the left-hand part of FIG. 3; 
     FIG. 5 is an isometric elevational view of the bottom half of the frame of the lighting fixture of FIGS. 1 and 2; 
     FIG. 6 is an exploded view of the beam-shaping blades and adjustment rings of the fixture in FIGS. 1 and 2; 
     FIG. 7 is an axial end view of the blades and rings shown in FIG. 6 in nested assembled condition; 
     FIGS. 8 and 9 are schematic axial end views corresponding to FIG. 7 illustrating the adjustment of the beam-shaping blades of FIGS. 6-7; 
     FIG. 10 is an illustration of the constructive principles of the guiding tracks in the adjustment rings for the beam-shaping blades; 
     FIG. 11 schematically illustrates an alternative embodiment of the beam-shaping blades and the adjustment mechanisms therefor; 
     FIG. 12 shows an isometric partly exploded view in larger scale of the position adjustment mechanism for the lenses shown in FIG. 2; and 
     FIG. 13 shows an enlarged view of a detail of the construction shown in FIG.  12 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIGS. 1-5, a lighting fixture  1  according to the invention is suspended in a suspension fitting  2  having an aperture  3  for fixing the fitting  2  pivotably to a support structure (not shown) in a theater, a television studio or the like. The fitting  2  is pivotably attached to the body of the lighting fixture  1  at  4 , the attachment point being adjustable by sliding the pivot attachment point  4  in a slit  5  in a frame  6  so as to compensate for change of balance because of insertion or removal of different elements in the lighting fixture  1 . 
     The lighting fixture  1  may thus be manually pivoted around two mutually substantially orthogonal axes allowing the direction of a light beam emitted by the fixture to be any desired direction. 
     If it is desired to be able to remotely control the direction of the beam, the pivoting action may be achieved by means of remotely controlled electrical motors in many different ways that will be obvious to those skilled in the art. 
     The frame  6  is generally U-shaped having two arms supporting the body of the lighting fixture  1  between said arms. A series of toothed rims  7 - 18  are arranged for rotation around a longitudinal or optical axis  19  (see FIG.  3 ). The teeth of the toothed rims are configured such that the teeth of a pinion of a drive unit may engage and mesh therewith if the light beam influencing facilities of the lighting fixture operated by rotation of the bodies  7 - 18  are to be motorized for remote control. 
     In the manually operated embodiment shown in FIGS. 1-4, the teeth of the toothed rims serve as a roughening element of the surface of the rim of each of the annular bodies  7 - 18  such that good frictional engagement between the fingers of a hand and the toothed rims or annular bodies  7 - 18  may be achieved for rotating the annular bodies  7 - 18  manually. 
     Such roughening of the rim surface may be achieved in many other ways such as scoring of the surface or coating with rubber or provision of small projections etc. 
     In such case and if motorization of the rotation of the bodies  7 - 18  is desired, then a frictional surface engagement of for instance the surface of a rubber coated drive wheel driven by an electrical motor with the roughened rim surface may be provided for instead of the meshing of the teeth of a pinion with teeth of the rim of the annular body. 
     A light source or lamp  20  emits a light beam composed of individual light beams such as illustrated at  20   a , the visual portion thereof being reflected by a dichroic reflector  21  through a focusing lens  22  and a zoom lens  23  and out of the lighting fixture through an aperture  24  in the housing  25  of the fixture  1 , the light beam  20   a  traveling through a color filter (not shown) arranged in four color filter holders  26  that may be pivoted around pivots  27  so as to allow a color filter to be inserted and removed in the holders  26  in any of four directions determined by the four holders  26 . Hereby the color filter may be inserted and removed from the best angle for manual access for a given orientation of the housing  25 . The entire light beam projected by the lighting fixture is of course composed of a plurality of light beams analogous to individual light beam  20   a.    
     The infra red portion of the light beam  20   a  is transmitted through the dichroic reflector  21  to cooling ribs  82  in a manner well known in the art so as to reduce the heat distortion of light beam influencing elements, as described below, that are arranged along the path of the light beam from the light source  20  to the exit aperture  24 . 
     These light beam influencing elements comprise an iris  28  connected to the annular body  7 , a pattern or gobo  29  connected to the annular body  8 , four beam-shaping blades  30 ,  31 ,  32  and  33  connected to the pairs of annular bodies,  9 - 10 ,  11 - 12 ,  13 - 14  and  15 - 16 , respectively, the focusing lens  22  connected to the annular body  17 , and the zoom lens  23  connected to the annular body  18 . 
     The annular bodies or rings  7 - 18  are connected in different manners to the respective light beam influencing elements  22 ,  23  and  28 - 33  so that the position of these elements may be altered relative to the axis  19 , and thus the light beam, by rotating the rings around said axis. The individual connections between the individual rings and the respective elements will be described more in detail below. 
     The feature of being able to alter the position of the light beam influencing elements, and particularly of the light beam shaping blades  30 - 33 , by means of rotating the corresponding rings allows the position alteration to be carried out manually from a convenient angle of approach for a given orientation of the housing  25 . As the rim surface of each of the rings  7 - 18  may be engaged manually at most of the extent of the circumference thereof, the manual adjustment of the position of a respective light beam influencing element may be performed from the most convenient angle of approach to the housing  25 . Furthermore, the manual adjustment may be carried out with one hand which is important, as the fixture is often located such that access with both hands is difficult and perhaps impossible. 
     Hereby the lighting fixture according to the invention does not have the disadvantages of known lighting fixtures where the adjustment means for adjusting the position of a light beam shaping blade may be very inconveniently located relative to the position of the person operating the lighting fixture so that the person for instance has to reach around the lighting fixture housing to access the adjustment means thereby risking being burned on the hot housing surface and rendering rapid and precise position adjustment difficult and perhaps impossible. 
     This advantage can also be obtained by rotational means other than rings with a rim surface for being engaged manually or mechanically. Elements having a plurality of radially extending spokes spaced circumferentially for being engaged at the ends thereof by fingers of a hand or a motorized driving means may also be used. A circumferentially disposed endless belt arranged for substantially circular movement around the longitudinal axis may also be utilized instead of the illustrated rings. All means allowing access along a major part of the circumference of the housing and rotational frictional engagement by fingers or a motorized drive unit may be used to allow such convenient access to the adjustment means for altering the position of the beam influencing elements. 
     The feature of altering the position of the light influencing elements by rotational means also entails simple and reliable establishment of a certain adjustment setting of a respective influencing means such that pre-programmed settings may be set up for certain lighting requirements knowing that it will be simple, quick and reliable to achieve such settings either manually or remotely under difficult conditions, for instance during the course of a theater show where adjustments in the dark are necessary. 
     A further advantage is obtained by the shown structure according to the invention in that the construction is such that no light is emitted from the interior of the fixture except through the aperture  24 , and all adjustments of the light beam influencing elements may be carried out without creating a light emission slit or aperture. Hereby, the disadvantage of all known lighting fixtures that light “leaks” therefrom is eliminated which is of great value, particularly for theater use. 
     Referring again to FIGS. 1-5, the frame  6  is constituted by two identical halves  6   a  and  6   b  abutting each other at  6   c . The rings or annular bodies  7 - 18  are rotatably and slidingly supported in annular grooves  34  in annular support rings  35  by means of annular projections or ridges  36  slidingly received in the annular grooves  34 . The support rings  35  are each constituted by half a ring fixedly attached to or made in one piece with one half of the frame  6 , for instance  6   a  (see FIG.  1 ). In other words each of the frame halves  6   a  and  6   b  is fixedly attached to or integral with a series of half rings  35  as shown in FIG. 5, where the bottom half  6   b  of the frame  6  is shown with the corresponding half rings  35 . 
     When assembling the lighting fixture  1 , the adjustment rings  7 - 18  with corresponding beam influencing elements  22 ,  23  and  28 - 33  are arranged in the bottom half  6   b  of the frame with corresponding half rings  35  such that the ridge  36  of each adjustment ring is received in the corresponding groove  34  of the respective half ring  35  of the bottom frame half  6   b . Thereafter the top half  6   a  of the frame  6  with corresponding half rings  35  is placed abutting the bottom half  6   b  at  6   c  such that the ridge  36  of each adjustment ring is received in the corresponding groove  34  of the respective half ring  35  of the top frame half  6   a . The adjustment rings  7 - 18  will thus be slidingly and rotationally supported along the entire circumference thereof by the corresponding rings  35 . 
     Each of the adjustment rings or annular bodies  7 - 18  may then be rotated manually or by means of suitable mechanical means by applying a tangential force to the rim of the respective adjustment ring whereby the ridge  36  thereof slides in the respective annular groove  34  of the respective support ring  35 . The material of the ridges  36  and the grooves  34  are chosen such that frictional sliding resistance is kept at a minimum. The support rings  35  may be made of cast aluminum, and the adjustment rings may be made of glass-fiber reinforced plastic. The ridges  36  are made of a low frictional material such as PTFE (marketed, for example, under the trademark “TEFLON”), a ring of said material being embedded in the lateral surface of the corresponding adjustment ring. Hereby the frictional sliding resistance between the low friction material and the cast aluminum will be low, and the adjustment rings may consequently be rotated by applying a relatively small tangential force to the rim thereof. 
     Each of the adjustment ring pairs  9 / 10 ,  11 / 12 ,  13 / 14  and  15 / 16  carries a respective light beam shaping blade  33 ,  32 ,  31  and  30 , respectively, by means of pairs of arms  33   a,b ,  32   a,b ,  31   a,b  and  30   a,b , respectively, held by the adjustment ring pairs in a manner described more in detail below. So that the two rings of each ring pair can rotate relative to one another, a low friction material ring  37  is arranged between each pair of adjustment rings as illustrated in FIGS. 4 and 6. 
     Referring now to FIGS.  4  and  6 - 9 , the arrangement of the four light beam shaping blades  30 - 33  will now be explained more in detail. 
     The blades  30 - 33  are nested as illustrated in FIGS. 4,  6  and  7 , each blade  30 - 33  being carried by a pair of opposed arms,  30   a - 33   a  and  30   b - 33   b , respectively. It is important that the blades  30 - 33  are located as axially close to each other as possible so as to achieve a sharp cut-off boundary of the light beam all around the circumference thereof which only can be achieved if the blades are arranged such that there is no substantial distance between them in the axial direction of the housing. This is particularly well illustrated in FIGS. 3 and 4 where it is evident that the spacing of the blades in the direction of the axis  19  is slight. 
     The arrangement shown also has the advantage that the axial distance between the beam-shaping blades  30 - 33  and the iris  28  as well as the gobo or pattern  29  is small so that a good sharpness or quality of the influence of the blades, the iris and the gobo on the light beam may be obtained simultaneously because of the small axial distance covered by all said elements. 
     The blades  30 - 33  are shaped as shown in FIGS. 6-8 having a generally elliptical planar body  38  with an aperture  39  having a periphery comprising a curved portion  40  and linear portions  41 ,  42  and  43 , said periphery serving as the beam cut-off edge of the blade body  38 . This is illustrated in FIG. 7 where the peripheries of the apertures  39  of the four bodies  38  of the blades  30 - 33  define the periphery of the beam shaping aperture  44 . A multitude of different shapes of the aperture  44  may be achieved by a combination of a rotation of the different blades  30 - 33  around the axis  19  with a displacement of said blades  30 - 33  radially relative to said axis  19 . 
     The radial displacement of the individual blades  30 - 33  is illustrated in FIGS. 8-9 where the periphery portion  42  of blade  33  is shown in FIG. 8 at the maximum radial distance from the axis  19  and in FIG. 9 at the minimum radial distance from said axis  19 . The rotational displacement is achieved by rotating the ring pair  9 / 10  carrying the blade  33  around the axis  19 . Combinations of the radial and the rotational displacement of each blade allow the creation of a great variety of peripheral shapes for the aperture  44 . 
     The elliptical shape of the  39  has been chosen to give a relatively stiff blade as well as a continuous and smooth outer perimeter of the body. Hereby the bodies of the blades will not interfere with one another when they are displaced relative to one another even though the axial spacing of the bodies is small. So as to avoid such mutual interference between the bodies as well as between the pairs of arms  30   a,b - 33   a,b  it is advantageous that the radial displacement of the bodies take place in such a manner that practically no flexing of the arms takes place during such displacement, i.e. that the distance between the ends of the arms of each pair is constant during such radial displacement and that no torsional forces are exerted on the arms during such radial displacement. 
     In the currently preferred embodiment of the invention shown in FIGS. 1-9, this is achieved as follows: 
     Each arm is provided with an angled end portion  45  having a guiding pin  46  extending therethrough and projecting from both opposed surfaces of the angled portion  45 . The plane of each end portion  45  is substantially parallel to the plane of the body  38  of the respective blade. 
     The rings of each pair of rings, for instance  15  and  16  in FIG. 6 or  9  and  10  in FIGS. 8-9, are identical, and one lateral surface of each ring is provided with a recessed circumferentially extending track  47  in the bottom of an annular circumferentially extending recess  48  and an elongate radially extending track  49  in the bottom of an annular circumferentially extending recess  50  identical to the recess  48  and arranged diametrically opposite the recess  48 . 
     The two rings  15 ,  16  in FIG.  6  and the two rings  9 ,  10  in FIGS. 8 and 9 are arranged abutting each other with the lateral surfaces thereof provided with the recesses  48  and  50  facing one another such that the recess  48  of the ring  15  (ring  9 ) faces and overlies the recess  50  of the ring  16  (ring  10 ), and the recess  50  of the ring  15  (ring  9 ) faces and overlies the recess  48  of the ring  16  (ring  10 ). Hereby annular channels  51  for receiving the angled end portions  45  of the arms are formed when the rings of a pair  9 / 10 ,  11 / 12 ,  13 / 14  or  15 / 16  are arranged abutting each other. 
     One of the two projecting ends of each guiding pin  46  of each end portion  45  is inserted in the circumferential track  47  of one ring of a pair of rings while the other projecting end is inserted in the radial track  49  of the other ring of said pair of rings. 
     The geometries of the tracks  47  and  49  are such that when one ring of a pair of rings is rotated relative to the other ring of the pair, then the respective body  38  of the blade carried by the pair of rings in question is displaced radially such that the distance between the pins  46  of the two arms of the respective blade remains constant and the arms are not subjected to any torsional stresses. 
     In FIGS. 8 and 9 the ring pair  9 / 10  is shown with the ring  9  abutting and overlying the ring  10 . In the illustration both rings are shown in full lines for the sake of clarity and to illustrate the relative positions of the tracks  47  and  49  of both rings. 
     In FIG. 8 the ring  10  has been turned 10 degrees clockwise such that the track  47  thereof shown at left in FIG. 8 is turned 10 degrees clockwise, while the ring  9  has been turned 10 degrees counterclockwise so that the track  47  thereof shown at right in FIG. 8 is turned 10 degrees counterclockwise. Consequently the track  49  of the ring  10  shown at right in FIG. 8 is turned 10 degrees clockwise while the track  49  of the ring  9  shown at left in FIG. 8 is turned 10 degrees counterclockwise. The angles clockwise and counterclockwise are given relative to an initial position where the body  38  is at the halfway position between FIG.  8  and FIG.  9 . The maximum periphery of the light beam is shown by the circle  52 . 
     In FIG. 9 the ring  10  has been turned 10 degrees counterclockwise such that the track  47  thereof shown at left in FIG. 9 is turned 10 degrees counterclockwise, while the ring  9  has been turned 10 degrees clockwise so that the track  47  thereof shown at right in FIG. 9 is turned 10 degrees clockwise. Consequently the track  49  of the ring  10  shown at right in FIG. 9 is turned 10 degrees counterclockwise, while the track  49  of the ring  9  shown at left in FIG. 9 is turned 10 degrees clockwise. 
     All intermediate positions between the two end positions shown in FIGS. 8 and 9 are achieved by rotating the rings  9  and  10  relative to one another the corresponding amount of degrees between zero and twenty. 
     A multitude of different beam periphery shapes may be achieved by displacing the blades  30 - 33  radially by rotating the two rings of the corresponding ring pair relative to one another and by displacing the blades circumferentially by rotating the two rings of a ring pair together. 
     In FIG. 7 one of infinitely many combinations of radial and circumferential positions of the four blades  30 - 33  is shown, whereby a beam  44  with the shown eight sided polygonal peripheral shape is achieved. 
     So as to achieve a distance between the two pins  46  at the ends of the two arms of each of the blades  30 - 33  that is the same for all radial displacements of the body  38  thereof, and so as to provide that no torsion of the arms takes place such that the body  38  is not subjected to any distorting forces, the shapes of the tracks  47  and  49  are configured accordingly as described in the following, with reference to FIG. 10 which illustrates the construction and calculation of the said shapes of the tracks  47  and  49 . 
     In FIG. 10 three pairs of mutually corresponding points on the curves  47  and  49  are constructed, the angles being exaggerated for the sake of clarity. 
     The construction of the curves is carried out according to the following: 
     A 1  is constant and equal to half the distance between the two pins  48  of a blade. 
     C 2 =A 1   
     Angle 1 =Angle 2   
     Angle 1 +Angle 2 =Angle 3   
     Both triangles are right-angled triangles 
     Angle  1  is the angle at which ring  1  is set, and Angle  2  is the angle at which ring  2  is set 
     By rotating ring  1  relative to ring  2 , Angle  3  is obtained. A center line is constructed from the center of the rings and horizontally to the left such that Angle  1 =Angle  2 . 
     Angle  1  and Angle  2  are used to construct two triangles. 
     A line is drawn along the center line, the line having a length equal to half the length between the two pins  46  of a blade. 
     This line forms the hypotenuse C 2  as well as the triangle side A 1  so that the other triangle side B 1  can be constructed by drawing a line from the right angle downwards and C 1  away from the center until the two lines intersect at a point. This point is on the curve to be constucted for configuring track  47 . 
     
       
           B 1=SIN(Angle 1)× A 1  Equation 1.1  
       
     
     
       
           C 1 =A 1/COS(Angle 1)  Equation 1.2  
       
     
     C 1  is now a radius which together with Angle  3  may used to construct the track by means of the equations 1.3: 
     
       
           Xtrack 47=COS(Angle 3)× C 1  
       
     
     
       
           Ytrack 47=SIN(Angle 3)× C 1  
       
     
     Or the equation 1.2 may be inserted in the equation 1.3: 
     
       
           Xtrack 47=COS(Angle 3)×( A 1/COS(Angle 1))  
       
     
     
       
           Ytrack 47=SIN(Angle 3)×( A 1/COS(Angle 1))  
       
     
     The X and Y axes are as indicated in FIG. 10 for each point constructed. 
     The track  49  in one ring extends in the radial direction to take up the radial displacement of the corresponding end of the pin  46  arising from the geometry of the track  47  in the other ring. 
     As it is the intersection point or triangle apex B 1 /C 1  that alters its position relative to the center of the rings, the shape of the track  47  is given by: 
     
       
           Xtrack 49=A1/COS(Angle 1)  
       
     
     
       
           Ytrack 49=0  
       
     
     such that the fixed distance is maintained between the ends of the pins  46  in corresponding points of tracks  47  and  49 . 
     Those skilled in the art will readily appreciate that it is possible to achieve displacement of beam shaping blades radially and circumferentially by means of rotating rings in many other ways. 
     Referring now to FIG. 11, an alternative way of arranging the beam shaping blades is shown schematically. Two adjustment rings  56 ,  57  similar to the adjustment rings  9 , 10  of FIGS. 8 and 9 are arranged abutting each other with a beam shaping blade  60  arranged therebetween and attached to the rings by means of two guiding pins  61  and  62 . The pin  61  is received in a recess in the lateral surface of the ring  57  facing the ring  56 , the recess having a shape that only allows rotation of the pin  61  therein. The pin  62  is received in a linear track  63  recessed into the lateral surface of the ring  56  facing the ring  57 . The pin  62  may slide in the track  63 . 
     The situation wherein the blade  60  maximally obstructs the beam of light  52  is shown in full lines while the situation wherein the blade  60  does not obstruct the beam  52  is shown in dotted lines. The fully obstructing position of the blade  60  is amended to the non-obstructing position thereof by rotating the rings  56  and  57  relative to one another, for instance as shown by rotating the ring  56  counterclockwise and maintaining the ring  57  in the same position. Hereby the pin  62  will be forced to slide in the track  63  while the pin  61  merely rotates such that the blade rotates around the pin  61 . In the shown example a rotation of the ring  56  counterclockwise 12 degrees will result in a rotation of 22 degrees of the blade  60 . 
     This arrangement of the beam shaping blades requires relatively stiff blades and/or relatively large axial spacing between the individual blades so that the blades will not interfere with or engage one another when being rotated. 
     Referring now to FIGS. 2,  3 ,  12  and  13 , the mechanism for displacing the focusing lens  22  and the zoom lens  23  along the longitudinal axis  19  is shown in partly exploded form. A holder  64  for the zoom lens  23  and a holder  65  for the focusing lens  22  are slidingly arranged in tracks  66  and  67 , respectively, in track rails so that the holders  64  and  65  may be displaced to and fro parallel to the longitudinal axis  19 . 
     A bracket  68  is connected to each of the holders  64  and  65 , only the bracket  68  for the holder  65  being visible. The brackets are each connected to a respective toothed belt  69  and  70  corresponding to the holders  65  and  64 , respectively. The toothed belts are mounted on pulleys  71  and  72  rotatably mounted on the track rails  66 ,  67 . 
     Each of the adjustment rings  17  and  18  (partly cut away for clarity in FIG. 12) are provided with lateral toothed portions  73  and  74 , respectively, for engaging the teeth of the toothed belts  69  and  70 , respectively, so that rotation of the ring  17  to and fro will cause displacement of the toothed belt  69  to and fro, and rotation to and fro of the ring  18  will cause displacement to and fro of the toothed belt  70 . Hereby, the lens holders  64  and  65  may be displaced to and fro along the tracks  66  and  67  by rotation to and fro of the rings  18  and  17 , respectively. 
     Hereby, a simple, precise and relatively silent displacement mechanism is achieved for adjusting the position of the lenses along the longitudinal axis. 
     When the lighting fixture  1  is oriented with the axis  19  thereof steeply inclined, i.e. pointing upwards or downwards steeply, the weight of the lenses, particularly the zoom lens  23 , will tend to force the lens up or down from the desired and adjusted position, especially if vibration of the fixture takes place. This tendency can be curtailed or eliminated by introducing an inertia or braking in the displacement mechanism. 
     However, if the inertia is present constantly, for instance a constant brake force applied to the toothed belts, then displacement of the lens will require additional tangential force applied to the rims of the rings  17  and  18 . Naturally, this is undesirable both for manual operation, requiring greater exertion of force by the operator&#39;s fingers, and for motorized operation, requiring a more powerful motor with attendant increases in costs and possibly noise. 
     The displacement mechanism according to the invention is provided with a braking function that only is effective when displacement of the lens is not taking place, i.e. the braking function is only in force when the rings  17  or  18  are not being rotated. The principles of the selective braking mechanism according to the invention and described in the following are of course also applicable in other applications where a displacement of an object with subsequent braking of the object in the displaced position is desirable. 
     The selective braking mechanism (FIGS. 12-13) according to the invention comprises the pulley  71 , a locking wheel  90 , a friction washer  91 , a friction spring  92 , a locking washer  93  and a locking sled  94 . The spring  92  presses the locking wheel  90  and the friction washer  91  against the pulley  71  so as to create a suitable friction between the locking wheel  90  and the pulley  71 . The locking sled  94  is arranged between the two parallel lengths of the toothed belt and for displacement to and fro in the plane of said toothed belt  70 , perpendicularly to said two parallel lengths. The locking sled is provided with locking teeth  94   a  and  94   b  for locking engagement with teeth at the rim of locking wheel  90  in a ratchet type action. If the locking sled  94  is in a central position, i.e. not displaced toward any of the two parallel lengths of the belt  70 , then the locking teeth  94   a  and  94   b  will not engage the teeth of the locking wheel  90  so no friction brake is applied to the belt  70 . 
     The dimension of the locking sled  94  perpendicular to the parallel lengths of the belt  70  is slightly longer than the distance between the common tangents of the pulleys  71  and  72  such that in the central position of the locking sled  94 , the locking sled will press against the parallel lengths of the belt  70 . 
     If tension is applied to one of the parallel lengths of the toothed belt  70  because of the weight of the lens, said length will be tightened and the parallel length will be loosened whereby the locking sled  94  will be displaced from the central position to a lateral position where the respective one of the locking teeth  94   a  and  94   b  will engage the ratchet teeth of the locking wheel  90 , thereby applying frictional braking forces to the pulley  71  through the friction washer  91 . 
     However, if tension in one of the parallel lengths of the belt  70  is caused by rotation of the ring  18  for axial displacement of the holder  64 , then the displacement of the locking sled  94  from the central position thereof will not cause engagement of one of the locking teeth  94   a  or  94   b  with the ratchet teeth of the locking wheel  90  as the ratchet effect will cause the respective locking tooth to “ratchet” over the ratchet teeth. 
     Hereby, a selective braking mechanism is achieved whereby the brake effect is operative, when the weight of the lens tries to rotate the respective adjustment rings, but the brake effect is inoperative when rotation of the respective ring is carried out to displace the lens axially. 
     It will be apparent to those skilled in the art that the principles of the above selective braking mechanism may be applied in all applications where a braking effect is required in one direction of force application and is not required in the opposite direction of force application. 
     The arrangement of the gobo or pattern  29  in the ring  8  and the iris  28  in the ring  7  need not be described herein as it will be apparent to those skilled in the art that this can be done in many ways well known in the art. 
     For remote control of the adjustment rings it will also be readily apparent to those skilled in the art that an electrical motor  100  with a pinion  101  for each ring may be arranged such that the teeth of the pinion  101  mesh with the teeth on the rim of the respective ring. The motors  100  for instance may be firmly attached to the frame  6  or be spring biased so that any irregularities in the mountings of the rings and thereby the toothed rims may be taken up. Magnetic markers  102  may be attached to the rings such that a sensing means  103  may sense the marker  102  and thereby precisely identify the position of the respective ring as a basis for the subsequent rotation thereof to a new setting of the respective beam influencing means.
A lighting fixture projects a light beam for spot lighting in theater stages, cinema and television studios and the like. The fixture includes a light source at one end of a housing having a light beam exit aperture at the opposite end thereof, the light source and aperture being arranged generally concentric with a longitudinal or optical axis of the lighting fixture. One or more beam-shaping blades, and preferably also other light beam influencing elements, such as one or more lenses, an iris, and/or a pattern or gobo, are arranged along the path of the light beam along the longitudinal axis through the housing from the light source to the aperture. The position of the beam-shaping blade or blades, and preferably of all the light beam influencing elements, is adjustable relative to the longitudinal axis. The fixture produces a well-defined light beam or light cone with a geometry, angle of conicity and focal point that may be altered manually or by remote control.