Abstract:
A telecentric zoom lens having telecentric forming aperture optically and preferably mechanically intermediate an objective lens and a zoom group. The aperture is fixed along an optical axis and selected to provide a constant speed on an image side of the lens system. The location of the aperture relative to the zoom group maintains telecentricity of the lens in object space.

Description:
FIELD OF THE INVENTION 
     The present invention relates to zoom lenses and more particularly, to a telecentric zoom lens having a telecentric forming aperture mechanically intermediate an objective lens and a zoom group. 
     BACKGROUND OF THE INVENTION 
     Telecentric optical systems are often employed in a measuring projector. The measuring projector allows a measurement of various values by illuminating the profile of an object to be measured, projecting the profile onto a screen, enlarging the projected image by a projection optical system and comparing the projection with a reference figure on the screen. For such systems, it is preferable that the projection be telecentric. That is, a principal ray passes through the optical system parallel with the optical axis. 
     However, the need exists for a telecentric zoom lens system having a reduced level of distortion in low tolerance applications. A further need exists for a telecentric zoom lens that has inherent color correction and reduced aberrations. A need also exists for a telecentric zoom lens that provides a constant speed on an image side of the lens. The need also exists for a method and apparatus of constructing such a telecentric zoom lens. 
     SUMMARY OF THE INVENTION 
     The present invention provides a telecentric zoom lens having a constant speed on an image side of the lens. Generally, the telecentric zoom lens includes a plurality of optical elements located along an optical axis. The optical elements include an objective lens having an object side and an image side; a zoom group moveable along the optical axis; and a telecentricity forming aperture mechanically intermediate the objective lens and the zoom group and thus optically intermediate the objective lens and the zoom group. Thus, the lens is telecentric in object space. In a further configuration, it is contemplated that the telecentricity forming aperture is a variable aperture. It is also contemplated, the telecentric zoom lens may include a first decollimator optically intermediate the objective lens and the telecentricity forming aperture. 
     More particularly, the present telecentric lens system includes a plurality of optical elements located on the optical axis, and specifically, a first collimator; a first decollimator, a zoom lens group, a second collimator, second decollimator, and a variable aperture optically intermediate the first decollimator and the zoom group. Further, it is contemplated that the zoom group may include a variator and a compensator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a first configuration of the lens system in a first position. 
     FIG. 2 is a side elevational view of a first configuration of the lens system in a second position. 
     FIG. 3 is a side elevational view of a first configuration of the lens system in a third position. 
     FIG. 4 is a side elevational view of a first configuration of the lens system in a fourth position. 
     FIG. 5 is a side elevational view of a first configuration of the lens system in a fifth position. 
     FIG. 6 is a side elevational view of a second configuration of the lens system in a first position. 
     FIG. 7 is a side elevational view of a second configuration of the lens system in a second position. 
     FIG. 8 is a side elevational view of a second configuration of the lens system in a third position. 
     FIG. 9 is a side elevational view of a second configuration of the lens system in a fourth position. 
     FIG. 10 is a side elevational view of a second configuration of the lens system in a fifth position. 
     FIG. 11 is a side elevational view of a third configuration of the lens system in a first position. 
     FIG. 12 is a side elevational view of a third configuration of the lens system in a second position. 
     FIG. 13 is a side elevational view of a third configuration of the lens system in a third position. 
     FIG. 14 is a side elevational view of a third configuration of the lens system in a fourth position. 
     FIG. 15 is a side elevational view of a third configuration of the lens system in a fifth position. 
     FIG. 16 is a side elevational view of a fourth configuration of the lens system in a first position. 
     FIG. 17 is a side elevational view of a fourth configuration of the lens system in a second position. 
     FIG. 18 is a side elevational view of a fourth configuration of the lens system in a third position. 
     FIG. 19 is a side elevational view of a fourth configuration of the lens system in a fourth position. 
     FIG. 20 is a side elevational view of a fourth configuration of the lens system in a fifth position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the present invention includes a telecentric zoom lens. For purposes of the description, the term “object” or “front side” of an optical element is used to describe those elements or surfaces that are nearer to the viewed object. The term “image” or “rear side” is used to describe those sides or surfaces of the optical element that are nearer the image that passes through the lens system. The term telecentric generally means a parallel relationship between the optical axis of the optical system and a principal ray entering or exiting the optical system. 
     The telecentric zoom lens includes a plurality of optical elements disposed along an optical axis A. The telecentric zoom lens includes an objective lens and a zoom group. The telecentric zoom lens system may include a first collimator FC, a first decollimator, FD a zoom group, ZG a second collimator SC, and a second decollimator SD along the optical axis. 
     The objective lens is the first collimator. The first collimator may be defined by variety of optical surfaces, as set forth in Tables 1-4. The first collimator causes the rays from an on axis point in the object plane to orient substantially parallel to the optical axis. 
     The first decollimator is disposed along the optical axis on the image side of the objective lens. The first decollimator causes the rays to converge on the image side of the first decollimator. 
     A telecentric aperture TA is located along the optical axis rearward of the first decollimator and located at a fixed position along the optical axis. The telecentric aperture is preferably variable. The range of variance in the telecentric aperture TA is sufficient to provide a constant speed on the image side. The variable telecentric-causing aperture TA may be any of a variety of aperture varying mechanisms such as leaves, plates or diaphragms. Either a motor or cam controls actuation of the variable aperture. By employing a variable aperture it has been found the speed of the lens system on the image side can be controlled. 
     The zoom group is located along the optical axis rearward of the telecentric aperture. The telecentric causing aperture is optically intermediate the zoom group and the objective lens. Although the telecentric causing aperture is also shown as mechanically and physically intermediate the zoom group and the objective lens, it is contemplated that the physical location of the telecentric causing aperture may be changed while retaining its being optically intermediate the objective lens and the zoom group. Although the zoom group may have a variety of configurations, it is contemplated the zoom group includes a variator and a compensator. The zoom group may be any combination of negative and positive elements. That is, the variator may be negative and the compensator negative; the variator negative and the compensator positive; the variator positive and the compensator negative; and the variator positive and the compensator positive. Configuration  1  &amp;  4 , wherein the variator is negative and the compensator is negative, and configuration  2  &amp;  3  wherein the variator is negative and the compensator is positive are preferred. Representative optical services of the variator and compensator parameters are set forth in Tables 1-4. 
     Relative motion of the zoom group components may be accomplished by any of a variety of known mechanisms such as stepper motors and linear actuators. A preferred mechanism is a mechanical cam. 
     The second collimator is optically rearward of the zoom group and specifically the compensator. Configurations  1 ,  3  and  4  include the second collimator. The second configuration does not include the second collimator as the output from the zoom group (compensator) is collimated. Preferably, the second collimator is constructed to reduce distortion. 
     The second decollimator is on the image side of the second collimator. Each of the four configurations includes a second decollimator. The second decollimator focuses the light to the image plane. 
     The optical elements may be formed from any appropriate material including plastics, resins as well as, glass and composites. A preferred material is glass. 
     It is further contemplated that each air interface surface of the lens elements may be provided with an anti-reflective coating. Typical anti-reflective coatings include magnesium fluoride. The objective lens may have a broadband anti-reflective coating. 
     Configuration  1  includes a negative-negative zoom group with a working f-number 20 in a camera format of ½ inch. 
     In the first configuration, surfaces  1  to  8  are the first collimator FC, surfaces  9  to  14  are the first decollimator FD, surfaces  16  to  18  are the variator V in zoom group, surfaces  19 - 21  are the compensator C in the zoom group, surfaces  22  to  26  are the second collimator SC and surfaces  27  to  31  are the second decollimator SD. 
     
       
         
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface 
                 Radius 
                 Thickness 
                 Nd 
                 V# 
                 Diameter 
               
               
                   
               
               
                 OBJ 
                   
                 77.19 
               
               
                  1 
                 −281.5500 
                 4.00 
                 1.45600 
                 90.34 
                 32.00 
               
               
                  2 
                 −57.2600 
                 0.50 
                   
                   
                 33.00 
               
               
                  3 
                 Infinity 
                 3.80 
                 1.61310 
                 44.36 
                 34.00 
               
               
                  4 
                 103.6500 
                 4.25 
                 1.45600 
                 90.34 
                 34.00 
               
               
                  5 
                 −95.2500 
                 0.50 
                   
                   
                 34.20 
               
               
                  6 
                 Infinity 
                 3.80 
                 1.61310 
                 44.36 
                 34.50 
               
               
                  7 
                 115.8000 
                 4.25 
                 1.45600 
                 90.34 
                 34.50 
               
               
                  8 
                 −119.6000 
                 39.00 
                   
                   
                 34.70 
               
               
                  9 
                 95.2603 
                 3.00 
                 1.61310 
                 44.36 
                 38.00 
               
               
                 10 
                 43.0696 
                 7.00 
                 1.49695 
                 81.60 
                 38.00 
               
               
                 11 
                 −175.2769 
                 0.50 
                   
                   
                 38.00 
               
               
                 12 
                 45.0330 
                 3.00 
                 1.61310 
                 44.36 
                 38.00 
               
               
                 13 
                 33.8820 
                 6.50 
                 1.45600 
                 90.34 
                 38.00 
               
               
                 14 
                 157.5993 
                 13.79 
                   
                   
                 38.00 
               
               
                 STOP 
                 Infinity 
                 11.00 
               
               
                 16 
                 −96.3272 
                 1.60 
                 1.80080 
                 35.00 
                 10.00 
               
               
                 17 
                 −11.0564 
                 1.00 
                 1.78574 
                 44.20 
                 10.00 
               
               
                 18 
                 29.4655 
                 10.97 
                   
                   
                 10.00 
               
               
                 19 
                 −97.0317 
                 1.00 
                 1.78574 
                 44.20 
                 10.00 
               
               
                 20 
                 5.4658 
                 3.25 
                 1.80080 
                 35.00 
                 10.00 
               
               
                 21 
                 26.7295 
                 51.75 
                   
                   
                 10.00 
               
               
                 22 
                 730.8634 
                 6.00 
                 1.61262 
                 58.70 
                 40.00 
               
               
                 23 
                 −56.0953 
                 0.50 
                   
                   
                 40.00 
               
               
                 24 
                 123.3747 
                 8.00 
                 1.48749 
                 70.21 
                 40.00 
               
               
                 25 
                 −52.4897 
                 3.00 
                 1.80610 
                 33.27 
                 40.00 
               
               
                 26 
                 −258.4570 
                 5.00 
                   
                   
                 40.00 
               
               
                 27 
                 697.4209 
                 5.00 
                 1.64839 
                 53.00 
                 41.00 
               
               
                 28 
                 −78.9875 
                 3.00 
                 1.80518 
                 25.39 
                 41.00 
               
               
                 29 
                 −223.3974 
                 0.50 
                   
                   
                 41.00 
               
               
                 30 
                 156.2192 
                 4.00 
                 1.72904 
                 54.70 
                 41.00 
               
               
                 31 
                 −1479.0530 
                 127.51 
                   
                   
                 41.00 
               
               
                   
               
             
          
           
               
                 IMA Zoom Data 
               
             
          
           
               
                   
                 0.75x 
                 1.33x 
                 2.00x 
                 4.22x 
                 7.50x 
               
               
                   
               
               
                 S15 
                 11 
                 21.317 
                 26.16429 
                 29.96268 
                 30.12291 
               
               
                 S18 
                 10.96782 
                 4.999894 
                 5.663429 
                 16.85393 
                 32.59501 
               
               
                 S21 
                 51.74994 
                 47.40086 
                 41.88991 
                 26.90104 
                 10.99979 
               
               
                   
               
             
          
         
       
     
     Configuration  2  has a negative positive zoom group with a working f-number of 20 and a camera format of ½ inch. 
     In the second configuration, surfaces  1  to  8  are the first collimator FC, surfaces  9  to  13  are the first decollimator FD, surfaces  15  to  19  are the variator V in the zoom group, surfaces  20 - 25  are the compensator C in the zoom group and surfaces  27  to  29  are the second decollimator SD. This coguration does not employ a second collimator SC as the compensator C pull out collimated light. 
     
       
         
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface 
                 Radius 
                 Thickness 
                 Nd 
                 V# 
                 Diameter 
               
               
                   
               
               
                 OBJ 
                   
                 77.34 
               
               
                  1 
                 −283.7479 
                 4.00 
                 1.45600 
                 90.34 
                 34.00 
               
               
                  2 
                 −57.4479 
                 0.50 
                   
                   
                 34.00 
               
               
                  3 
                 Infinity 
                 2.50 
                 1.61310 
                 44.36 
                 34.80 
               
               
                  4 
                 103.1262 
                 5.00 
                 1.45600 
                 90.34 
                 34.80 
               
               
                  5 
                 −92.3134 
                 0.50 
                   
                   
                 35.20 
               
               
                  6 
                 Infinity 
                 2.50 
                 1.61310 
                 44.36 
                 35.60 
               
               
                  7 
                 115.3065 
                 5.00 
                 1.45600 
                 90.34 
                 35.60 
               
               
                  8 
                 −124.7165 
                 35.80 
                   
                   
                 35.60 
               
               
                  9 
                 66.9775 
                 5.00 
                 1.55220 
                 67.03 
                 35.00 
               
               
                 10 
                 −93.1287 
                 3.00 
                 1.65412 
                 39.62 
                 35.00 
               
               
                 11 
                 182.9913 
                 0.50 
                   
                   
                 34.90 
               
               
                 12 
                 62.7445 
                 4.00 
                 1.49695 
                 81.60 
                 33.50 
               
               
                 13 
                 121.7067 
                 22.92 
                   
                   
                 33.50 
               
               
                 STOP 
                 Infinity 
                 0.00 
               
               
                 15 
                 15.9237 
                 2.00 
                 1.78785 
                 47.40 
                 11.00 
               
               
                 16 
                 5.1692 
                 2.50 
                 1.80080 
                 35.00 
                 9.00 
               
               
                 17 
                 12.2617 
                 1.30 
                   
                   
                 8.00 
               
               
                 18 
                 −17.6645 
                 2.00 
                 1.77236 
                 49.60 
                 9.40 
               
               
                 19 
                 59.5758 
                 41.51 
                   
                   
                 9.40 
               
               
                 20 
                 −255.2385 
                 4.50 
                 1.78574 
                 44.20 
                 27.80 
               
               
                 21 
                 −29.2736 
                 3.50 
                 1.84635 
                 23.80 
                 28.50 
               
               
                 22 
                 −63.5616 
                 0.50 
                   
                   
                 31.00 
               
               
                 23 
                 262.2000 
                 4.10 
                 1.69350 
                 53.33 
                 30.80 
               
               
                 24 
                 −37.2864 
                 2.50 
                 1.75487 
                 52.30 
                 29.30 
               
               
                 25 
                 −147.4509 
                 45.91 
                   
                   
                 31.50 
               
               
                 26 
                 Infinity 
                 3.00 
                   
                   
                 29.00 
               
               
                 27 
                 185.5856 
                 4.25 
                 1.61791 
                 63.40 
                 33.00 
               
               
                 28 
                 −57.9111 
                 2.50 
                 1.80610 
                 33.27 
                 33.00 
               
               
                 29 
                 −121.8555 
                 148.67 
                   
                   
                 33.50 
               
               
                   
               
             
          
           
               
                 IMA Zoom Data 
               
             
          
           
               
                   
                 0.80x 
                 1.44x 
                 2.50x 
                 4.50x 
                 7.90x 
               
               
                   
               
               
                 S14 
                 0.0000 
                 32.94485 
                 50.92825 
                 61.57181 
                 67.20934 
               
               
                 S19 
                 41.51364 
                 38.24532 
                 32.50898 
                 21.85882 
                 3.980362 
               
               
                 S25 
                 45.91399 
                 16.23737 
                 3.995184 
                 3.998487 
                 16.23795 
               
               
                   
               
             
          
         
       
     
     Configuration  3  has a negative positive zoom group with a working f-number of 20 in a camera format of ½ inch. 
     In the third configuration, surfaces  1  to  8  are the first collimator FC, surfaces  9  to  13  are the first decollimator FD, surfaces  15  to  17  are the variator V of the zoom group, surfaces  18 - 21  are the compensator C in the zoom group, surfaces  22  to  24  are the second collimator SC and surfaces  25  to  27  are the second decollimator SD. 
     
       
         
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface 
                 Radius 
                 Thickness 
                 Nd 
                 V# 
                 Diameter 
               
               
                   
               
               
                 OBJ 
                   
                 78.39 
               
               
                  1 
                 −293.2219 
                 4.00 
                 1.52855 
                 76.98 
                 36.00 
               
               
                  2 
                 −61.5946 
                 0.50 
                   
                   
                 36.00 
               
               
                  3 
                 Infinity 
                 2.50 
                 1.61310 
                 44.36 
                 36.00 
               
               
                  4 
                 104.8174 
                 5.00 
                 1.49695 
                 81.60 
                 36.00 
               
               
                  5 
                 −94.0966 
                 0.50 
                   
                   
                 36.00 
               
               
                  6 
                 Infinity 
                 2.50 
                 1.61310 
                 44.36 
                 36.00 
               
               
                  7 
                 72.8788 
                 5.00 
                 1.49695 
                 81.60 
                 36.00 
               
               
                  8 
                 −188.2986 
                 41.80 
                   
                   
                 36.00 
               
               
                  9 
                 73.2881 
                 5.00 
                 1.49695 
                 81.60 
                 34.00 
               
               
                 10 
                 −125.6413 
                 2.50 
                 1.80349 
                 30.40 
                 34.00 
               
               
                 11 
                 −462.3412 
                 0.50 
                   
                   
                 34.00 
               
               
                 12 
                 94.6181 
                 5.00 
                 1.48656 
                 84.47 
                 34.00 
               
               
                 13 
                 230.8875 
                 18.63 
                   
                   
                 34.00 
               
               
                 STOP 
                 Infinity 
                 3.56 
               
               
                 15 
                 −27.6135 
                 2.25 
                 1.80318 
                 46.38 
                 9.00 
               
               
                 16 
                 6.9900 
                 2.75 
                 1.85025 
                 32.17 
                 9.00 
               
               
                 17 
                 24.2769 
                 41.67 
                   
                   
                 9.00 
               
               
                 18 
                 −505.7763 
                 2.00 
                 1.79935 
                 42.20 
                 21.00 
               
               
                 19 
                 −118.3258 
                 5.00 
                 1.69680 
                 55.41 
                 21.00 
               
               
                 20 
                 −26.3939 
                 3.50 
                 1.80349 
                 30.40 
                 21.00 
               
               
                 21 
                 −47.4743 
                 32.89 
                   
                   
                 22.00 
               
               
                 22 
                 87.2833 
                 5.00 
                 1.79619 
                 43.20 
                 29.00 
               
               
                 23 
                 −53.8974 
                 2.75 
                 1.87800 
                 38.48 
                 29.00 
               
               
                 24 
                 140.8270 
                 1.00 
                   
                   
                 29.00 
               
               
                 25 
                 142.0000 
                 3.75 
                 1.58304 
                 59.40 
                 29.00 
               
               
                 26 
                 −62.2000 
                 2.25 
                 1.66663 
                 33.00 
                 29.00 
               
               
                 27 
                 −203.0000 
                 164.00 
                   
                   
                 29.00 
               
               
                   
               
             
          
           
               
                 IMA Zoom Data 
               
             
          
           
               
                   
                 080x 
                 1.42x 
                 2.53x 
                 4.50x 
                 8.00x 
               
               
                   
               
               
                 S14 
                 3.564761 
                 29.43371 
                 44.92315 
                 53.94762 
                 59.06097 
               
               
                 S17 
                 41.66636 
                 38.75392 
                 32.98541 
                 22.17358 
                 1.999626 
               
               
                 S21 
                 32.88991 
                 9.933404 
                 0.212473 
                 1.999916 
                 17.06044 
               
               
                   
               
             
          
         
       
     
     Configuration  4  has a negative-negative zoom group with a working f-number of 20 in a camera format: ½ inch. 
     In the fourth configuration, surfaces  1  to  8  are the first collimator FC, surfaces  9  to  13  are the first decollimator FD, surfaces  16  to  18  are the variator V in the zoom group, surfaces  19 - 21  are the compensator C in the zoom group, surfaces  22  to  27  are the second collimator SC and surfaces  28  to  30  are the second decollimator SD. 
     
       
         
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface 
                 Radius 
                 Thickness 
                 Nd 
                 V# 
                 Diameter 
               
               
                   
               
               
                 OBJ 
                   
                 77.19 
               
               
                  1 
                 −281.5500 
                 4.00 
                 1.45600 
                 90.34 
                 32.00 
               
               
                  2 
                 −57.2600 
                 0.50 
                   
                   
                 33.00 
               
               
                  3 
                 Infinity 
                 3.80 
                 1.61310 
                 44.36 
                 34.00 
               
               
                  4 
                 103.6500 
                 4.25 
                 1.45600 
                 90.34 
                 34.00 
               
               
                  5 
                 −95.2500 
                 0.50 
                   
                   
                 34.20 
               
               
                  6 
                 Infinity 
                 3.80 
                 1.61310 
                 44.36 
                 34.50 
               
               
                  7 
                 115.8000 
                 4.25 
                 1.45600 
                 90.34 
                 34.50 
               
               
                  8 
                 −119.6000 
                 45.00 
                   
                   
                 34.70 
               
               
                  9 
                 175.8591 
                 6.00 
                 1.48656 
                 84.47 
                 34.00 
               
               
                 10 
                 −68.5279 
                 4.00 
                 1.61293 
                 36.96 
                 34.00 
               
               
                 11 
                 −399.7037 
                 0.50 
                   
                   
                 34.00 
               
               
                 12 
                 66.4254 
                 4.00 
                 1.48656 
                 84.47 
                 34.00 
               
               
                 13 
                 −1219.1600 
                 17.31 
                   
                   
                 32.00 
               
               
                 STOP 
                 Infinity 
                 0.00 
               
               
                 15 
                 Infinity 
                 49.20 
               
               
                 16 
                 −62.6972 
                 2.25 
                 1.80400 
                 46.57 
                 12.00 
               
               
                 17 
                 17.6723 
                 2.75 
                 1.76182 
                 26.52 
                 12.00 
               
               
                 18 
                 71.3495 
                 22.97 
                   
                   
                 12.00 
               
               
                 19 
                 −58.4378 
                 4.00 
                 1.82223 
                 37.45 
                 10.00 
               
               
                 20 
                 −6.0108 
                 2.50 
                 1.81550 
                 44.54 
                 10.80 
               
               
                 21 
                 52.7099 
                 7.00 
                   
                   
                 13.00 
               
               
                 22 
                 735.5523 
                 6.25 
                 1.75500 
                 52.32 
                 38.00 
               
               
                 23 
                 −44.2291 
                 4.25 
                 1.80349 
                 30.40 
                 39.00 
               
               
                 24 
                 −61.6877 
                 0.50 
                   
                   
                 40.00 
               
               
                 25 
                 155.5579 
                 6.71 
                 1.61800 
                 63.33 
                 40.00 
               
               
                 26 
                 −52.9482 
                 3.50 
                 1.83400 
                 37.16 
                 40.00 
               
               
                 27 
                 15685.880 
                 5.00 
                   
                   
                 40.00 
               
               
                 28 
                 89.3700 
                 4.00 
                 1.80440 
                 39.59 
                 40.00 
               
               
                 29 
                 51.8700 
                 6.50 
                 1.52855 
                 76.98 
                 40.00 
               
               
                 30 
                 −195.0000 
                 153.11 
                   
                   
                 40.00 
               
               
                   
               
             
          
           
               
                 IMA Zoom Data 
               
             
          
           
               
                   
                 0.75x 
                 1.33x 
                 2.00x 
                 4.22x 
                 7.50x 
               
               
                   
               
               
                 S15 
                 11.0000 
                 27.8863 
                 37.0673 
                 47.0086 
                 49.1975 
               
               
                 S18 
                 24.7087 
                 10.1201 
                 5.0000 
                 9.1381 
                 22.9692 
               
               
                 S21 
                 43.4578 
                 41.1600 
                 37.0992 
                 23.0198 
                 6.9999 
               
               
                   
               
             
          
         
       
     
     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation of material to the teachings of the invention without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope and spirit of the appended claims.