ZOOM LENS SYSTEM HAVING IMAGING AND NON-IMAGING RANGES

Abstract: A project zoom les system is used in imaging and non-imaging modes for entertainment and architectural illumination, providing 8:1 zoom ratio in an imaging mode and 20:1 zoom ratio in a non-imaging mode. When coupled with a variable aperture iris, beam size control is obtained with 120:1 ratio. The lens system comprises four lens groups: a first group having a positive refractive power, a second group having a negative refractive power, a third group having a negative refractive power, and a fourth group having a positive refractive power. In the imaging mode, the third group is adjusted axially to vary magnification while the second group is adjusted axially to focus on the object. In the non-imaging mode, the second and third groups are brought into close proximity with each other and adjusted axially as a unit to vary magnification. The fourth group remains fixed and the first group can be adjusted axially to focus the image at different throw lengths.

Description: ZOOM LENS SYSTEM HAVING IMAGING AND NON-IMAGING RANGES FIELD OF THE INVENTION The present invention relates to stage lighting instruments having variable magnification (zoom) lenses, and particularly to a zoom lens system operable over imaging and non-imaging ranges. BACKGROUND OF THE INVENTION Stage lighting instruments and other similar luminaires are used to provide controllable illumination of persons and/or objects. Among the controllable parameters are beam size and intensity of a light beam used for illumination. Another controllable parameter is the optical focus of a field stop, aperture, or image pattern generator used in a spot light or image projector. Two ways of controlling beam size include the use of an iris diaphragm having an aperture of adjustable diameter, and the use of a zoom lens system having variable magnification power. A beam size iris controls beam diameter by acting as a field stop having an aperture of adjustable diameter, and varies the size of the beam by cutting off light rays which do not pass through the aperture thereby decreasing the total amount of light which reaches the illuminated area. A zoom lens system controls beam diameter by changing the divergence angle of a bundle of light rays entering the lens system thereby concentrating or deconcentrating the light beam such that the total amount of light reaching the illuminated area is not changed. A zoom lens, therefore, varies the energy density in a projected spot of light as the diameter of the spot is varied, while a beam size iris varies the diameter of the projected spot without altering the energy density thereof. Two-lens systems (two groups of lenses, each group having one lens) afford variable magnification and focus, such as those shown in U.S. Patent No. 3,594,566 to Kneisley, and U.S. Patent No. 4,519,020 to Little. A triplet system (three groups of lenses, one lens in each group) is disclosed in U.S. Patent No. 5.029,992 to Richardson. Other lighting projectors featuring adjustable focus optical systems include U.S. Patents No. 2,076,240 and 2,1 14,963 to Levy, U.S. Patent No. 2.650,292 to Strong, U.S. Patent No.2,909,652 to Pratt, U.S. Patent No. 2,950,382 to Hatch, U.S. Patent No. 4,101,957 to Chang, U.S. Patent No. 4,232,359 to Leon, U.S. Patent No. 4,338,654 to Logothetis, U.S. Patent No. 4,462,067 to Altman, U.S. Patent No. 4,739,456 to Little, U.S. Patent No. 5,295,056 to Peck, and U.S. Patent No. 5,404,283 to Yantz et al. Examples of four group lens systems include U.S. Patent Nos. 5.282,089; 5,223,979; 5,579,172; 5,583,700; and 5,606,459. As recognized by Tejima in U.S. Patent No. 5,283,599, image projectors using a zoom lens as the projection lens must necessarily effect a zoom adjustment in which the focal position is kept constant, even if the zooming is carried out by the zoom lens system and a back focus adjustment is made to keep the image pattern generator 10 coincident with the focal position obtained by the zoom adjustment. A typical system utilizing five lens groups disclosed in U.S. Patent No. 3,192,829 to Yamaji maintains focus over a wide range of magnifications by moving three of the lens groups with respect to the remaining two fixed lens groups. Some other zoom lens systems '5 designed for projection applications are disclosed in U.S. Patent No. 3,267.803 to Macher and Klemt, U.S. Patent No.4,063,800 to Iizuka et al, and U.S. Patent No.4,118,108 to Muszumanski. However, the restriction in the above references that the image be focusable throughout the range of variable magnification tends to limit 20 the range of magnification available from a particular zoom lens design. The development of high quality projection lens systems having variable magnification has, for some time, been impeded by limitations as to cost, size, and particularly the overall length of the lens system. Whereas generally the ratio of overall length (from the front edge of front lens to the back edge of back lens) to 25 longest focal length is usually 1.5x or greater, it has become desirable to develop a very short zoom lens system approaching l .Ox length and having about an 8:1 range of magnification change (zoom ratio), which can be produced inexpensively and withstand the rigors of high power density and portability. 30 SUMMARY OF THE INVENTION An embodiment of the present invention provides a zoom lens for a stage lighting instrument, the lens operating over a certain range of magnification in which an image can be projected in focus, the lens further operating over another range in 35 which the projected image is not focused, thereby providing a lens system effecting variable magnification in a spot light operating mode and effecting variable beam divergence in a wash light operating mode. In an embodiment of the present invention, a zoom lens is provided having an 8: 1 range of magnification in which a projected image can be focused, i.e. be disposed within the depth of field of the object plane; the zoom lens also having a 20:1 range of magnification irrespective of image focus. The zoom lens system according to one aspect of the present invention comprises four groups of lens elements: a first group having a positive refractive power, a second group having a negative refractive power, a third group having a negative refractive power, and a fourth group having a positive 10 refractive power. The zoom lens system according to one aspect of the present invention in an imaging mode (where the lighting instrument is projecting an image), movement of the second lens group (Group 2) controls focus while movement of the third lens group (Group 3) rapidly controls magnification and in a non-imaging mode, * ■• * Group 2 and Group 3 are moved together in fixed relationship with each other to provide a much wider angle of beam divergence control. In an aspect of practicing an embodiment of the present invention, in a stage lighting instrument having variable magnification optics, a method of operating said 0 optics in which a zoom ratio of 8: 1 is obtained and image focus is maintained, and in which a zoom ratio 20:1 is obtained and image focus is not maintained, comprising the steps of: maintaining axial position of a projection lens group throughout; moving an objective lens group with respect to said projection lens group to focus a light beam at a desired distance; moving a variator lens group with respect to an objective lens 5 group to obtain a desired magnification power; moving a compensator lens group with respect to said variator lens group to obtain a desired image focus; and moving said compensator lens group in a fixed relationship with said variator lens group with respect to said objective lens to obtain a desired magnification power irrespective of 0 image focus. In an aspect of an embodiment of the present invention, a zoom lens system comprising: a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a negative refractive power, and a fourth lens group having a positive refractive power, said lens system satisfying the following conditions: /m = VFwFn (condition 1) 0.38 < ^- < 0.71 (condition 2) - 0.68 < ^- < - 1.26 (condition 3) - 0.37 < ^- < -0.69 m 10 (condition 4) 1.07 < ^- < 1.98 /m where /i is the effective focal length (efl) of said first lens group, /ii is the efl , < - of said second group, /iii is the efl of said third group, and /iv is the efl of said fourth group. Fw is the efl of said lens system at a wide-angle, Fn is the efl of said lens system at a narrow angle, and /m is a reference value as defined above with respect to magnification. 20 BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a zoom lens system according to a first embodiment of the present invention; Figure 2 is a set of graphs plotting aberration curves of a zoom lens according to a first embodiment at wide angle; Figure 3 is a set of graphs plotting aberration curves of a zoom lens according to a first embodiment at medium angle; Figure 4 is a set of graphs plotting aberration curves of a zoom lens according to a first embodiment at narrow angle; 30 Figure 5 is a cross-sectional view of a zoom lens system according to a second embodiment of the present invention; Figure 6 is a set of graphs plotting aberration curves of a zoom lens according to a second embodiment at wide angle; 35 Figure 7 is a set of graphs plotting aberration curves of a zoom lens according to a second embodiment at medium angle; Figure 8 is a set of graphs plotting aberration curves of a zoom lens according to a second embodiment at narrow angle; Figure 9 is a schematic side elevation of a lamp head assembly including a zoom lens system; Figure 10 is a schematic side elevation of a stage lighting instrument including a zoom lens system; Figure 11 is a graph plotting the variables D12, D14 and D19 at various angles of beam divergence in the imaging mode according to a second embodiment; Figure 12 is a graph plotting the variables D12, D14, and D19 at various focal lengths according to a second embodiment in the non-imaging mode; Figure 13A-13D illustrate the relative positions of the lens groups according to a first embodiment of the present invention in an imagining mode; Figure 14A-14D illustrate the relative positions of the lens groups according to a first embodiment of the present invention in a non-imaging mode; Figure 15 is a cross-sectional view of the zoom lens system according to the second embodiment of the present invention in the non-imagining, wide-angle mode; Figure 16A-16D illustrate the relative positions of the lens groups according to a second embodiment in an imaging mode; and Figure 17A-17D illustrate the relative positions of the lens groups according to a second embodiment in a non-imaging mode. DETAILED DESCRIPTION In accordance with one aspect of the present invention, and as shown in Figure 1, a zoom lens system preferably comprises 13 lens elements arranged in four groups. Group 1, an objective group, preferably comprises six lens elements L1-L6 and has a positive refractive power. Group 2, a compensator group, preferably comprises one lens element L7 and has a negative refractive power. Group 3, a variator group, preferably comprises three lens elements L8-L10 and has a negative refractive power. Group 4, a projection group, preferably comprises three lens elements L11-L13 and has a positive refractive power. The lens system operates in an imaging mode in which Group 3 moves linearly to vary magnification and Group 2 moves non-linearly to compensate for image shift. The lens system also operates in a non-imagining mode in which Group 3 is brought close to Group 2 and the two groups are moved together to vary beam divergence. The lens system uses routine optical glasses made of commonly available materials such as borosilicate, flint, dense flint and barium glasses and avoids the use of exotic and expensive materials such as Lanthinum crowns and flints. The lens system is mounted in a stage lighting instrument, as shown in Figure 9 and Figure 10, having a lamp 1 coupled with a reflector 2 to project a beam of light that illuminates an object 3, an image of which is projected by the lens system 5. The object may be a light pattern generator or gobo, such as the type disclosed in U.S. Patent No. 4,779,176; or an electronic display device of the type disclosed in U.S. Patent No. 5,282,121; or the object may be an iris diaphragm having a variable-diameter aperture such as disclosed in U.S. Patent Nos. 3,594,566; 4,338,654; and 4,462,067. Of the lens system, Group 1 is located closest to the object and Group 4 is located farthest from the object. The object 3 is disposed coincidently with optical axis OA and is generally coplanar with an object plane 4. The object plane is located at a back focal distance from the lens system 5. Lens Group 1 is supported by a motorized carriage assembly illustratively comprising supporting bracket 21 coupled to lead screw 24, which is driven by a reversible electric motor 27. Lens Group 2 is supported by a similar motorized carriage assembly illustratively comprising supporting bracket 22 coupled to lead screw 25, which is driven by a reversible electric motor 28. Lens Group 3 is supported by another motorized carriage assembly illustratively comprising supporting bracket 23 coupled to a lead screw 26, which is driven by reversible electric motor 29. The motors 27, 28, and 29 are controlled by an electronic controller 10 via control lines 11, 12, and 13. The controller 10 may comprise a local de-multiplexer circuit such as that shown in U.S. Patent No. 4,392,187 (incorporated herein by reference). Alternatively, controller 10 may comprise a local processor-based circuit such as that shown in U.S. Patent No. 4,980,806. Motors 27, 28 and 29 are preferably stepper motors controlled by a local . processor type of controller including suitable motor drive circuits for energizing the motors. The controller preferably includes a look-up table stored in local memory, and includes a processor that, by referring to the look-up table, transforms input control signals representing desired magnification and focus into signals for energizing motors 27, 28 and/or 29 to position the lens groups properly. Separate look-up tables may be provided in memory, the tables containing values representing lens group positions in an imaging mode and in a non-imaging mode. These tables are preferably downloaded to a local processor system from a control console system, and thereafter maintained in local memory in a manner similar to the down-loading of local processor operating system as described in U.S. Patent No. 4,980,806 mentioned above. The lamp 1, reflector 2, object 3, lens system 5, controller 10, and motorized carriage apparatus 20 are enclosed within a housing having an exit aperture 7. As shown in Figure 10, the lamp head housing 6, is mounted for rotation within a yoke assembly 8. The yoke assembly 8 is mounted for rotation about a mounting tube 9. The mounting tube 9, includes a hook 16 for attachment to a pipe 17. Rotation of the lamp head housing 6 is effected by a motor drive assembly 14 mounted in the yoke, while rotation of the yoke assembly 8 is effected by a motor drive assembly 15, also mounted in the yoke. Preferably the exit aperture 7 of the lamp head housing 6 is filled with the last lens element L13 of lens Group 4. The lens system is operable, in an imaging mode, to project an image of the illuminated object located in an object plane at a back focal distance behind lens element LI . The Group 4 may be fixed in axial position at the nose of a stage lighting instrument, with the remaining groups mounted in axially-translatable mounts coupled to motorized drives for adjusting the axial positions of those groups. In particular, Group 3 may be adjusted to vary the magnification power of the lens system over a wide range e.g. about 8:1, while Group 2 may be adjusted to focus the lens system on the object. The overall length of the lens system as shown in Figures 1 and 9 is approximately 1 1.1 inches and the focal length of the system is preferably adjustable from about 1.251 inches at wide angle (shortest focal length) to about 10.00 inches at narrow angle (longest focal length), yielding a ratio of overall length to longest focal length of 11.1 / 10.0 = 1.1 lx, and a ratio of longest focal length to shortest focal length about of 7.9936:1 (zoom ratio = 8:1). The prescription for a first embodiment of the lens system is given in Table 1 , in which the values for F, R and D are in inches. Table 1. First Embodiment F=1.251 NA=0.36 2w=38 degrees D0=1.500 Rl= 3.7755 Dl = 0.565 Nl=1.6204 Vl=60.32 LI R2= -3.7755 D2= 0.586 R3=-1.1235 D3= 0.200 N2= 1.6200 V2=36.37 L2 R4=- 1.9667 D4= 0.684 R5=-8.0411 D5= 0.588 N3=1.6204 V3=60.32 L3 R6=-2.097 D6= 0.020 R7= Infinity D7= 0.200 N4=1.7174 V4=29.51 L4 R8=2.790 D8= 0.265 R9=4.618 D9= 0.677 N5=1.6204 V5=60.32 L5 R10=-4.618 D10= 0.084 Rl 1=2.685 Dl 1= 0.643 N6=1.5638 V6=60.80 L6 R12=- 11.949 D 12= Variable R13=-9.784 D13= 0.150 N7=1.5488 V7=45.41 L7 R14=5.966 D 14= Variable R15=-6.561 D15= 0.300 N8= 1.6204 V8=36.37 L8 R16=-2.338 D16= 0.125 N9=1.5488 V9=45.41 L9 Rl 7=1.900 D17= 0.611 R18=-1.652 D18= 0.125 N10= 1.5481 V10=45.75 L10 R19=-23.000 D19= Variable R20=- 10.451 D20= 0.499 Nl 1= 1.6204 VI 1=60.32 Ll l R21=-3.668 D21= 0.021 R22=1 1.114 D22= 0.800 N12= 1.5231 V12=58.57 L12 R23= -5.256 D23= 0.220 N13= 1.7174 V13=29.51 L13 R24=- 12.878 D24= 240.0 Focal Length 1.251 2.50 5.00 10.00 (Imaging mode) Variable Dl 2 0.136 1.537 1.022 0.050 Variable D14 3.193 1.298 0.223 0.575 Variable Dl 9 0.411 0.905 2.495 3.113 Focal Length 0.989 2.256 5.0261 10.452 (Non-imaging mode) Variable D12 3.4036 2.004 1.045 0.050 Variable D14 0.200 0.2000 0.200 0.200 Variable Dl 9 0.1364 1.5365 2.4949 3.113 F is the focal length. NA is the numerical aperture (tangent of the half angle for the marginal axial ray which is on the object side). 2w is the total field angle. DO is the distance from the object to be projected to the first surface (leftmost in the Figures) of the most upstream lens. The N values are the respective indices of the refraction of each lens and the V numbers are the Abbe numbers for each lens. The listed R numbers represent the radii of the respective front and back surfaces of each lens; the D numbers represent the respective distances from one lens surface to the next downstream lens surface with D24 representing the distance from the most downstream lens surface to the surface to be illuminated, i.e. the throw length. Note that for the imaging mode case illustrated above, the distance D24 represents 20 feet from the nose of the luminaire to the surface illuminated by the luminaire; for example, a projection screen. For a given throw length, represented by D24, the relative positions of Group 1 and Group 4, and thus the distance DO from the object plane to the first surface of lens LI (radius Rl), remain constant. If the luminaire is redirected towards another surface to be illuminated, at a different throw length, the value of D24 will have changed and the value of DO must also change thereby requiring adjustment of Group 1 since Group 4 is fixed in the nose of the luminaire. As shown in Figures 13A-13D, the zoom lens system in the imaging mode holds Group 1 in fixed relationship with Group 4 throughout the range of focal lengths (f/1.251 through f/10.00). Group 3 moves linearly, being closest to Group 4 at wide- angle (Figure 13 A), moving towards Group 1 at intermediate angles (Figures 13B and 13C) and being closest to Group 1 at narrow angle (Figure 13D). In a non-imaging mode, and as shown in Figures 14A-14D, variable D14 is made small such that lens Group 3 is brought close to Group 2, and then Group 2 and Group 3 are moved as a unit to vary beam divergence over a 20:1 ratio although image focus cannot be maintained at all angles. This provides a very wide range of beam divergence angles for wash light applications in which a wide range of beam angle adjustment is desired but image projection is not a requirement. As shown in Figure 14A, Group 2 and Group 3 are closest to Group 4 at wide angle. As shown in Figures 14B and 14C, Group 2 and Group 3 maintain their relative separation and move as a unit towards Group 1 to decrease the angle of beam divergence. As shown in Figure 14D, Group 2 and Group 3 are closest to Group 1 at narrow angle, and still maintain the same relative separation. If the object is an iris diaphragm having a variable aperture, varying from 0.9 inches to 0.100 inches for example, having a ratio of largest aperture to smallest aperture of 9:1, then the lens system coupled with the variable-aperture iris is capable of beam angle adjustments over a range of 9x20=180; a combined ratio of 180:1 in a non-imaging mode. The variable aperture is effective to vignette a beam illuminating the aperture and produces a small spot of light projected by the lens system to an appreciable distance (20 feet or more). At that distance, the spot of light is small enough that any fuzziness at the edge thereof is not distinguishable by a member of an audience so the problem of maintaining focus in the non-imaging mode is minimized. When the lens system of the first embodiment is focused at a throw length of 20 feet, the systems yields high optical performance exhibiting aberration characteristics illustrated in Figures 2-4. Figure 2 shows aberration curves for the first embodiment zoom lens system at wide angle. Figure 3 shows aberration curves for the first embodiment zoom lens system at medium angle. Figure 4 shows aberration curves for the first embodiment zoom lens system at narrow angle. While the first embodiment lens system according to the present invention yields good optical performance with respect to imagery, provides a wide zoom ratio, as is physically short compared to its longest focal length, the system still contains three pounds of glass (considering that lens LI 3 is 4.5 inches in diameter in this particular stage lighting application) with two-thirds of that mass being located in Group 4 at the forward end of the luminaire. In a zoom lens system for an automated, pan & tilt luminaire, it is desirable to minimize the mass located at the extremes of the lamp head structure so as to minimize the inertial load on the pan and tilt motors. Therefore, a second embodiment lens system according to the present invention is hereafter disclosed. In accordance with another aspect of the present invention, and as shown in Figure 5, a second embodiment zoom lens system preferably comprises 13 lens elements arranged in four groups. Group 1, an objective group, preferably comprises six lens elements L1-L6 and has a positive refractive power. Group 2, a compensator group, preferably comprises one lens element L7 and has a negative refractive power. Group 3, a variator group, preferably comprises three lens elements L8-L10 and has a negative refractive power. Group 4, a projection group, preferably comprises three lens elements L11-L13 and has a positive refractive power. The lens grouping is the same as the first embodiment disclosed above, and the second embodiment lens system also uses routine optical glasses made of commonly available materials, avoiding the use of exotic and expensive materials. The second embodiment lens system is operable in an imaging mode and a non-imaging mode, with the lens groups mounted for axial movement as described above for the first embodiment lens system. The Group 4 lenses may be fixed in axial position at the nose of a stage lighting instrument, with the remaining groups mounted in axially-translatable mounts coupled to motorized drives for adjusting the axial positions of those groups. As in the first embodiment, Group 3 may be adjusted to vary the magnification power of the lens system over a wide range while Group 2 may be adjusted to focus the lens system on the object. The lens system operates in an imaging mode in which Group 3 moves linearly to vary magnification and Group 2 moves non-linearly to compensate for image shift as shown in Figures 16A-16D. The variable air spaces D12, D14, and D19 in the imaging mode are plotted in a graph shown in Figure 11. Note that from the graph that the distance D19, representing the separation of Group 3 from Group 4, increases linearly from wide angle at the left of the graph to narrow angle at the right of the graph. Also note that the distance D14, representing the separation of Group 3 and Group 2, decreases from two inches at wide angle to less than one-tenth inch and then begins to increase again approaching narrow angle; thereby exhibiting non-linear motion. The lens system also operates in a non-imaging mode in which Group 3 is . brought close to Group 2 and the two groups are moved together to vary beam divergence as shown in Figures 17A-17D. The variable air spaces D12, D14, and D19 in the non-imaging mode are plotted in a graph shown in Figure 12. Note from the graph that the distance D19 representing the separation of Group 3 from Group 4 increases from wide angle at the left of the graph to the narrow angle at the right of the graph, while the distance D14, representing the separation of Group 2 from Group 3 remains small and constant throughout most of the range of adjustment, but varies somewhat towards wide angle. The overall length of the lens system is approximately 9.2 inches and the focal length of the system is adjustable from about 1.25 inches at wide angle to about 10.00 inches at narrow angle, yielding a ratio of overall length to longest focal length of about 9.2 / 10.0 = 0.92x, and a ratio of longest focal length to shortest focal length (zoom ratio) of about 8:1. A prescription for the second embodiment of the lens system is given in Table 2, in which the values for R and D are again given in inches. Table 2. Second Embodiment F=1.250 NA=0.31 2w=38 degrees D0=1.500 Rl=3.2161 Dl=0.500 Nl=1.6204 Vl=60.32 LI R2=-3.2161 D2=0.279 R3 =-0.9319 D3=0.200 N2=1.7618 V2=26.53 L2 R4=Infinity D4=0.188 R5=Infinity D5=0.600 N3=1.6204 V3=60.32 L3 R6=-1.4382 D6=0.005 R7=6.6702 D7=0.220 N4=1.7847 V4=26.76 L4 R8=3.1350 D8=0.173 R9=22.198 D9=0.375 N5=1.6700 V5=47.11 L5 R10=-2.822 D10= 0.010 Rl 1=2.822 Dl l= 0.375 N6=1.6700 V6=47.11 L6 R12=-22.198 D 12= Variable Rl 3=54.609 D13= 0.125 N7=1.7618 V7=26.53 L7 R14=2.480 D 14= Variable Rl 5=3.993 D15= 0.352 N8=1.7847 V8=26.76 L8 R16=-1.652 D16= 0.120 N9= 1.6700 V9=47.11 L9 R17=1.652 D17= 0.480 R18=-1.299 D18= 0.150 N10= 1.7847 V10= 26.76 L10 R19=-4.485 D19= Variable R20=-10.643 D20= 0.513 Nl l= 1.6204 VI 1= 60.32 Ll l R21=-3.993 D21= 0.010 R22= Infinity D22= 0.853 N12= 1.5168 V12= 64.17 L12 R23=-3.093 D23= 0.250 N13= 1.7059 V13= 30.30 L13 R24=-4.679 D24= 230.0 Focal Length 1.25 2.50 5.00 10.00 (Imaging mode) Variable D12 1.466 1.496 1.111 0.005 Variable D14 1.952 0.593 0.055 0.615 Variable Dl 9 0.005 1.333 2.257 2.803 Focal Length 0.896 2.1981 5.0834 10.00 (Non-imaging mode) Variable D12 3.318 1.990 1.066 0.005 Variable Dl 4 0.100 0.100 0.100 0.615 Variable Dl 9 0.005 1.334 2.257 2.803 In the imaging mode case illustrated above, the distance D24 represents just over 19 feet from the nose of the luminaire to the surface to be illuminated by the luminaire (ie, a projection screen). When the lens system of the second embodiment is focused at a throw length of 19 feet 2 inches, the systems yields high optical performance exhibiting aberration characteristics illustrated in Figures 6-8. Figure 6 shows aberration curves for the second embodiment zoom lens system at wide angle. Figure 7 shows aberration curves for the second embodiment zoom lens system at medium angle. Figure 8 shows aberration curves for the second embodiment zoom lens system at narrow angle. The second embodiment lens system of the present invention, compared to the first embodiment, is more compact although less fast, but has better image correction, deeper focus, and provides a bigger, larger diameter projected light beam. Satisfactory results are obtained with either of the above lens system, and may be obtained with other lens systems in which the specified values of R, D, N, and V do not vary by more than plus or minus 20% of the values given in Table 1 or Table 2. Generally, the lens system first and second embodiments satisfy the following conditions wherein for a given ratio of Fn:Fw (where Fn=focal length at narrow angle and Fw=focal length at wide angle), a reference value /m is defined as the square root of (Fw x Fn): /m = vFΛvFn (condition 1) 0.38 < ^- < 0.71 /m 10 (condition 2) - 0.68 < - - < -1.26 m (condition 3) - 0.37 < ^- < -0.69 15 (condition 4) 1.07 < ^- < 1.98 m where /i is the effective focal (efl) length of Group 1, /ii is the efl of Group 2, /iii is the efl of Group 3, and /iv is the efl of Group 4. 20 In operation, the lens system of the present invention is useful in at least three modes of operation when mounted in a luminaire such as a stage lighting instrument. In this discussion, a beam of light has a "hard edge" characteristic when the periphery of the beam has a distinct sharpness or contrast rather than a gradual transition from 25 illuminated area to un-lighted area. A beam of light further has a "soft edge" characteristic when the periphery of the beam has a gradual transition from illuminated area to un-lighted area. A "general illumination" usage is defined as, but not limited to, illumination which is clear of structure (no projected image) and is by default „ „ chromatically white, although color is frequently modified with a color filter system. In general illumination, the lens system of the first and second embodiments of the present invention are operable in three modes. First mode, zooming a hard-edged beam between extremes using the lens system alone. Second mode, zooming a hard- edged beam between extremes using the lens system in combination with a variable- 35 aperture iris. Third mode, zooming a light beam between extremes using the transition between imaging and non-imaging range of the lens system. In the first case, the lens system may perform a hard-edge zoom from either extreme-wide to extreme-narrow, or vice versa, while holding constant the aperture of a beam-size iris, and obtain a range of magnification of the projected spot of light of 8:1. In the second case, the lens system performs an 8:1 zoom of the iris aperture, in cooperation with the iris which performs a 9:1 range of aperture diameter adjustment, to obtain a range of projected spot sizes of 72: 1. In the third case, a maximum range of spot sizes is obtained when the smallest spot is the smallest projected image of the smallest iris aperture and the largest spot is the widest beam divergence angle obtained from the lens system in non-imaging mode with the iris fully open. The smallest spot must be at the smallest magnification, and must also be a sharply focused image of the iris at minimum aperture since any blurring or de-focusing will only increase the size of the spot or beam, divergence angle. A transition is made between the imaging mode, in which Group 3 (variator) controls magnification and Group 2 (compensator) controls focus, and the non-imaging mode in which the motions of the variator and compensator are decoupled from their image-mode relations and are moved linearly in fixed relationship. Linear motion of the variator and compensator in fixed relationship produces a non-linear rate of change of beam divergence angle, which is not suggested by the typical calculation of overall focal length of the lens system in the non-imaging mode. As shown in Figure 15, Group 2 and Group 3 lenses are moved in fixed relationship towards Group 4 in accordance with Table 2 Second Embodiment in the non-imaging mode and an image is formed within Group 3. At the positions shown, the image formed within Group 3 is located forward of the primary focus of Group 4, which is to say behind Group 4 at approximately 25% of the distance to the primary focus. This condition causes rapid non-linear (wide-angle) divergence of the light beam through Group 4. Whereas generally the zoom ratio for imaging applications is defined as the ratio of the longest focal length divided by the shortest focal length, this calculation is deceptive for non-imaging applications. The present invention make use of a nonlinear beam expansion caused by the decoupling of the imaging motion table that controls the positions of Group 2 (compensator) and Group 3 (variator) into a linear movement of the Group 2 and Group 3 combination towards Group 4. During this movement, the rate of change of the beam divergence angle is very non-linear because the refracted beam is rapidly changed from a converging set of rays to an intermediate image plane located well in front of the primary focus of Group 4, causing very wide- angle refraction. Those skilled in the art will appreciate that various substitutions, omissions, modifications and changes may be made in the method and apparatus of the present invention without departing from the scope or spirit thereof. Accordingly, it is intended that the foregoing description be considered merely exemplary of the present invention and not a limitation thereof. Therefore, the above-described embodiments are merely examples of how the invention may be carried out and other ways may also be possible which are within the scope of the following claims.

Claims: THE CLAIMS o 1. In a stage lighting instrument having variable magnification optics, a method of operating said optics in which a zoom ratio of 8: 1 is obtained and image focus is maintained, and in which a zoom ratio 20: 1 is obtained and image focus is 5 not maintained, comprising the steps of: maintaining axial position of a projection lens group throughout; moving an objective lens group with respect to said projection lens group to focus a light beam at a desired distance; n moving a variator lens group with respect to an objective lens group to obtain a desired magnification power; moving a compensator lens group with respect to said variator lens group to obtain a desired image focus; and moving said compensator lens group in a fixed relationship with said variator lens group with respect to said objective lens to obtain a desired magnification power irrespective of image focus. 2. A zoom lens system comprising: a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens 0 group having a negative refractive power, and a fourth lens group having a positive refractive power, said lens system satisfying the following conditions: /m = VFwFn « (condition 1) 0.38 < ^- < 0.71 5 /m (condition 2) - 0.68 < - - < - 1.26 0 (condition 3) - 0.37 < ^- < -0.69 (condition 4) 1.07 < ^- < 1.98 5 where /i is the effective focal (efl) length of said first lens group, /ii is the efl of said second group, /iii is the efl of said third group, and /iv is the efl of said fourth group, Fw is the efl of said lens system at a wide-angle. Fn is the efl of said lens system at a narrow angle, and /m is a reference value as defined above with respect to magnification.

Inventors: STEELE RICHARD K, STEELE, RICHARD, K.
Applicants: VARI LITE INC, VARI-LITE, INC.
Application year: 1998
Main classification: G02B15
IPC codes: F21S8, F21S10, G02B9, G02B15