Abstract:
In a flow cytometer, an objective lens for focusing an input laser-radiation beam including at least four different laser-radiation wavelengths in a common plane includes only three singlet lens-elements. Two of the elements are cylindrical elements arranged as a cylindrical telescope for shaping and reducing the size of the input laser-beam. The third element is a spherical element arranged to focus the reduced size beam in the common plane. In one example, all three elements are made from the same optical material.

Description:
PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 62/363,532, filed Jul. 18, 2016, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to projection lenses. The invention relates in particular to anamorphic projection lenses for focusing two or more laser-beams having different wavelengths into a common elliptical focal-spot. 
       DISCUSSION OF BACKGROUND ART 
       [0003]    A basic principle of flow cytometry is the passage of cells in a fluid-stream through a focused laser-beam so that the cells can be detected, identified, counted, and sorted. Cell components are fluorescently labelled and then excited by the laser-beam to emit light at varying wavelengths. The fluorescence can then be measured to determine the amount and type of cells present in a sample. Up to thousands of particles per second can be analyzed as they pass through the fluid-stream. 
         [0004]    Several detectors are carefully placed around the fluid-stream at the point where the fluid passes through the focused beam. The suspended particles or cells, which may range in size from 0.2 micrometers (μm) to 150 μm, pass through the focused beam and scatter the radiation. The fluorescently labelled cell components are also excited by the focused laser-beam and emit light (fluorescence) at a longer wavelength than that of the laser-beam. The fluorescence is then detected by the detectors. The detectors measure a combination of scattered and fluorescent light. Measurement data is then analyzed, using special software, by a computer that is attached to the flow cytometer. 
         [0005]    It is generally accepted that the above described flow cytometry process is more flexible and more accurate the more light-wavelengths that are included in the laser-beam. In practice, this is accomplished by combining component beams from different lasers along a common path to provide a combined beam that is focused into the fluid-stream. Diode-laser modules are typically used for providing the component beams. Commercially available diode-laser modules can provide laser radiation at selected fundamental wavelengths in a range from the near ultraviolet (UV) the near infrared (NIR). 
         [0006]    An increasing number and range of wavelengths presents significant problems in the design and construction of an optical objective for focusing the combined laser-beam into the fluid-stream. It is generally accepted that for focusing two significantly different wavelengths at a common location (focal plane) a focusing objective must include at least two lens elements having different, for example high and low, spectral dispersion. An objective arranged to focus two different wavelengths (red and blue) in a common focal plane is generally referred to as an achromatic objective. 
         [0007]    If three significantly different wavelengths, for example, red, green, and blue wavelengths, are to be focused at a common location, a focusing objective must include at least three lens elements having different spectral dispersion. An objective arranged to focus three significantly different wavelengths (red, green, and blue) in a common focal plane is generally referred to as an apochromatic objective. 
         [0008]    In either achromatic or apochromatic objectives individual (singlet) lens elements of different spectral dispersion may need to be “cemented” together in a form referred to by practitioners of the lens design art as “doublets” or “triplets”. This could provide a problem in including UV wavelengths in a flow cytometer, as optical cements (adhesives) may be degraded by the UV radiation 
         [0009]    Based on conventional optical design wisdom, it can be expected that as more laser-radiation wavelengths, for example four or more, are included in a flow cytometer, the more complex and expensive will be the objective required to focus the wavelengths into the fluid-stream. This could result in the cost and complexity of a focusing objective determining a practical upper limit to how many laser-radiation wavelengths could be used in a flow-cytometer. 
         [0010]    There is need for a simple focusing objective, capable of focusing four or more laser radiation wavelengths in a common focal plane, but wherein the number of different optical materials (glasses) required is less than the number of different wavelengths to be focused by the objective in the common focal plane. Preferably the focusing objective should not include any cemented doublet or triplet elements. 
       SUMMARY OF THE INVENTION 
       [0011]    In one aspect of the present invention, an objective lens for focusing an input beam of laser-radiation in a focal plane, comprises first, second, and third optical elements in consecutive numerical order in a propagation direction of the beam. The first optical element is a cylindrical element having a focal length f CL1 . The second optical element is a cylindrical element having a focal length f CL2 . The third optical element is a spherical element having a focal length f FFL . The focal lengths f CL1 , f CL2 , and f FFL  are related by an equation f CL1 −f CL2 =G*f FFL , where G is between about 0.7 and about 1.4, and preferably between about 0.9 and about 1.1. 
         [0012]    In another aspect of the present invention, optical apparatus comprises at least first, second, third, and fourth lasers delivering respectively first, second, third, and fourth component laser-beams at respectively first, second, third, and fourth wavelengths. A beam combiner is arranged to combine the first, second, third, and fourth component laser-beams into a combined beam. An objective lens is provided including only three singlet optical elements. The objective lens is arranged to receive the combined laser-beam and focus the combined laser-beam such that the component laser-beams thereof are all focused about in a common focal plane. 
         [0013]    The terminology “about in a common focal plane” recognizes that laser-beams, when focused, are focused into smallest diameter (focal spot) of what is termed a “beam waist” by practitioners of the art. The beam converges on one side of the focal spot and diverges on an opposite side of the focal spot. The waist is further characterized by a “Raleigh range” which is the distance from the focal spot to a point where the beam diameter is equal to √2 times the smallest diameter. “About in a common focal plane”, in this description and the appended claims, means that the focal spots of the component beams are within a Rayleigh range of that common focal plane. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1A  and  FIG. 1B  are respectively horizontal and vertical views of one preferred embodiment of a three-element anamorphic focusing objective in accordance with the present invention in including a positive cylindrical lens element, a negative cylindrical lens element, and a spherical lens element listed in the direction of propagation of radiation through the lens, with five input beams being focused, each beam having a different wavelength from any other. 
           [0015]      FIG. 1C  is a view perpendicular to the views of  FIG. 1A  and  FIG. 1B , schematically illustrating details an elliptical focal-spot produced by the objective of  FIGS. 1A and 1B  from the input beams. 
           [0016]      FIG. 2A  and  FIG. 2B  are respectively horizontal and vertical views of one preferred example of a beam-combiner, combining the outputs from five laser-radiation sources into a collinear bundle of beams for focusing by the objective of  FIGS. 1A and 1B . 
           [0017]      FIG. 3  is a graph schematically illustrating calculated paraxial 1/e 2  beam-diameter as a function of working distance shift for one example of the objective of  FIGS. 1A and 1B  focusing 640 nm, 561 nm, 488 nm, 405 nm, and 355 nm collimated beams from the beam combiner of  FIGS. 2A and 2B  into an elliptical focal spot with a 6:1 axis ratio. 
           [0018]      FIG. 4  is a graph schematically illustrating calculated horizontal waist shift from vertical focus as a function of wavelength for various values of the focal length of positive cylindrical lens element of  FIGS. 1A and 1B . 
           [0019]      FIG. 5  is a graph schematically illustrating calculated paraxial 1/e 2  beam-diameter as a function of working distance shift for another example of the objective of  FIGS. 1A and 1B  focusing 640 nm, 561 nm, 488 nm, 405 nm, and 355 nm collimated beams from the beam combiner of  FIGS. 2A and 2B  into an elliptical focal spot with a 4:1 axis ratio. 
           [0020]      FIG. 6  schematically illustrates another embodiment of the inventive focusing objective, similar to the embodiment of  FIGS. 1A and 1B  but wherein the negative cylindrical element is replaced by a positive cylindrical lens element. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Turning now to the drawings,  FIGS. 1A and 1B  schematically illustrate a preferred embodiment  20  of an achromatic anastigmatic anamorphic focusing objective in accordance with the present invention.  FIG. 1A  is a view designated arbitrarily as a “horizontal” view. This is a view in a y-z plane defined by arbitrarily assigned Cartesian optical axes x, y, and z, where z is the propagation-axis (propagation direction) of light through the lens.  FIG. 1B  is a view designated arbitrarily as a vertical view, i.e., perpendicular to the view of  FIG. 1A , i.e., a view in an x-z plane defined by the Cartesian optical axes. 
         [0022]    Objective  20  includes cylindrical lens elements CL 1  and CL 2 , having optical power in only the y-axis. Elements CL 1  and CL 2  are followed in the propagation-axis by final focusing element FFL. Element FFL has equal optical power in both the x-axis and the y-axis and be referred to generally as a rotationally symmetrical element. A spherical optical element is preferred for production cost considerations. 
         [0023]    In this embodiment, element CLI has positive optical power in the y-axis (horizontal plane), and element CL 2  has negative optical power in the y-axis. The elements preferably have a plane surface and a curved surface as depicted in the drawing and are preferably arranged with such that radiation is incident first on the curved surface of each element in the direction of propagation. i.e., in the z-direction. 
         [0024]    Objective  20  is configured to bring nominally collimated, collinear beams of different wavelengths to a focus in a working plane P. Here the wavelengths, for purposes of this description, are 640 nanometers (nm), 561 nm, 488 nm, 405 nm, and 355 nm. The invention, however, is applicable to other wavelengths in the same or a different range. Ideally, the horizontal focus (working distance f H  from the FFL) and the vertical focus (working distance f v  from the FFL) should ideally be exactly the same, for all of the input wavelengths. This is not achievable in practice, but can be closely approximated as described further herein below. The diameters of the input beams are preferably about linearly related to the beam-wavelength as indicated in the drawing 
         [0025]    As summarized above, focal lengths f CL1 , f CL2 , and f FFL  are related by an equation f CL1 −f CL2 =G×f FFL , where, f CL1 , f CL2 , f FFL  are the focal lengths of elements CL 1 , CL 2 , and FFL, respectively, and G is between about 0.7 and about 1.4, and preferably between about 0.9 and about 1.1. Efforts to analytically determine the factor G in the equation f CL1 −f CL2 =G*f FFL  were unsuccessful. Values of G between about 0.7 and 1.4 and between about 0.9 and 1.1 were determined empirically, and used to define a set of initial optical designs which could be readily optimized to provide a practical, functional optical design. Such optimization can be carried out using commercially available ray-tracing software. In examples described herein, optimizations were carried out using ZEMAX available from Zemax, LLC, of Kirkland, Wash. 
         [0026]    Continuing with reference to  FIGS. 1A and 1B , objective  20  is designed to focus input wavelengths into an elliptical focal spot having a width (major-axis) W H  in the horizontal (y-z) plane, and a width (minor-axis) W V  in the vertical (x-z) plane, as depicted in  FIG. 1C . The ratio W H :W V  is determined primarily by the ratio of the focal lengths of elements CL 1  and CL 2 . In an above-described flow cytometer, the minor-axis would be aligned with the flow-direction. 
         [0027]      FIGS. 2A and 2B  are respectively horizontal-plane and vertical-plane views schematically illustrating a preferred arrangement for combining beams of different wavelengths for input into inventive objective  20 . Lasers L 1 , L 2 , L 3 , L 4 , and L 5  provide radiation at respectively the 640 nm, 561 nm, 488 nm, 405 nm, and 355 nm wavelengths. Afocal beam-expanding telescopes T 1 , T 2 , T 3 , T 4 , and T 5  are associated with lasers L 1 , L 2 , L 3 , L 4 , and L 5  respectively for forming the beams into required sizes relative to the laser wavelength and to collimate the beams. Here again, the near-linear relationship of beam diameter to wavelength is evident. Each telescope, in turn, can be adjusted to adjust the collimation of the expanded laser-beams. Mirror M 1 , and dichroic mirrors M 2 , M 3 , M 4 , and M 5  are arranged as a beam-combiner to combine the beams into a collinear bundle for input into objective  20 . Note that  FIG. 2B  is seen in the direction  2 B- 2 B of  FIG. 2A  and depicts only the mirrors forming the beam combiner, and the objective lens. 
         [0028]      FIG. 3  is a graph schematically illustrating calculated paraxial 1/e 2  beam-diameter as a function of working distance shift for an example of objective  20  focusing 640 nm, 561 nm, 488 nm, 405 nm, and 355 nm collimated beams from combiner  30  of  FIGS. 2A and 2B . The term “working distance shift” is the difference between the calculated beam-waist position and the position of working plane P of  FIGS. 1A and 1B . 
         [0029]    Specifications of this example of objective  20  are listed in Table 1 below. EFL is the effective focal length of a lens element. Elements CL 1  and CL 2  have an EFL in the y-axis only. All lens elements are assumed to be made from fused silica. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Center 
                 Distance to 
               
               
                   
                   
                 EFL 
                 Thickness 
                 Previous Lens 
               
               
                   
                 Lens 
                 (mm) 
                 (mm) 
                 (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CL1 
                 42.3 
                 4 
                 N/A 
               
               
                   
                 CL2 
                 −7.05 
                 2 
                 32.5 
               
               
                   
                 FFL 
                 50 
                 4 
                 1 
               
               
                   
                   
               
             
          
         
       
     
         [0030]    The EFLs of elements CL 1  and CL 2  are selected to provide an elliptical focal spot with a 6:1 ratio of major to minor axes. The beam-waist positions in the vertical plane for all wavelengths are coincident, by design, in the working plane. In the horizontal plane, the working distance shifts for the 355 nm, 405 nm, 488 nm, 561 nm, and 640 nm beams are 0.23 mm, −0.19 mm, −0.19 mm, −0.3 mm, and 0.16 mm, respectively. These shifts are relatively small compared with the Rayleigh range of the horizontal beam-waists, which is about 10.0 mm for this 6:1 ratio example, and, accordingly, not visible on the graph. Because of this, objective  20  can be realistically considered as astigmatic. 
         [0031]    The height of the vertical beam waists at locations beyond the focus provides an indication of input beam sizes at different wavelengths. By way of example it can be seen that at a distance of −1.5 mm from the focus, the vertical beam waist diameters for 640 and 355 mm are about 120 micrometers (μm) and 68 μm respectively, with ratio of the wavelengths and beam diameters being the same at about 1.8. It is emphasized here that for any given ratio of horizontal-to-vertical ratio of focal spot-size, independent of the focal length of lens element FFL, there is only one optimum value of the focal length of lens element CL 1 , for which an equation f CL1 −f CL2 ≈F FFL  is satisfied. 
         [0032]    By way of demonstration,  FIG. 4  is a graph schematically illustrating calculated horizontal waist shift from vertical focus as a function of wavelength for various values of f CL1  for a 6:1 spot-size ratio and f FFL  of 50 mm. All lenses elements are assumed to be made from fused silica. 
         [0033]    It can be seen that a value for f CL1  of 42.3 mm provides that horizontal and vertical foci are essentially coincident in the range of wavelengths from 350 nm to 650 nm. The focal length of CL 2  (f CL2 ) for providing a spot-size ratio of exactly 6:1 is 7.05 mm (negative) which provides for f CL1 −f CL2 =49.35 mm, i.e., 0.987*f FFL . 
         [0034]    Given that some shifts are relatively small compared with the Rayleigh range of the horizontal beam-waists, as discussed above with reference to the graph of  FIG. 3 , other values of f CL1  can provide an acceptable, if not necessarily optimum result. By way of example, an acceptable result may be obtained with f CL1 , having any value between about 30 mm and about 50 mm. It is this consideration that was used to empirically establish preferred values of the factor G in the equation f CL1 −f CL2 =G*F FFL  discussed above. 
         [0035]    In discussions presented above, a spot-size ratio (horizontal to vertical) of 6:1 is assumed. The inventive focusing objective is not limited, however, to that particular ratio. Exemplary specifications for other ratios are provided in TABLE 2, still, of course, with only the three singlet lens elements CL 1 , CL 2 , and FFL. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Ratio of EFL 
                 Optimum EFL 
                 Optimum EFL 
                 EFL of FFL 
               
               
                   
                 CL1:CL2 
                 of CL1 (mm) 
                 of CL2 (mm) 
                 (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  2:1 
                 31.69 
                 −15.85 
                 50 
               
               
                   
                  4:1 
                 39.03 
                 −9.78 
                 50 
               
               
                   
                  6:1 
                 42.3 
                 −7.05 
                 50 
               
               
                   
                  8:1 
                 44.45 
                 −5.56 
                 50 
               
               
                   
                 10:1 
                 46.69 
                 −4.66 
                 50 
               
               
                   
                   
               
             
          
         
       
     
         [0036]    The lens element material of each specification is assumed to be fused silica. It should be noted that the focal lengths of the cylindrical lens elements is referenced at a wavelength of 488 nm, while the focal length of element FFL, here assumed to be an “off the shelf” available element is referenced at a wavelength of 587.6 nm. 
         [0037]      FIG. 5  is a graph schematically illustrating calculated paraxial 1/e 2  beam-diameter as a function of working distance shift for the inventive objective having the 4:1 specification of TABLE 2, focusing 640 nm, 561 nm, 488 nm, 405 nm, and 355 nm collimated beams from combiner  30  of  FIG. 2 . The center thicknesses of elements CL 1 , CL 2 , and FFL are 4 mm, 2 mm, and 4 mm, respectively. Axial spacing between elements CL 1  and CL 2  is assumed to be 26.5 mm. Axial spacing between elements CL 2  and FFL is assumed to be 1.0 mm. 
         [0038]    It can be seen that the horizontal beam waists are significantly tighter than in the 6:1 ratio example of  FIG. 3 . In the horizontal plane, the working distance shifts for the 355 nm, 405 nm, 488 nm, 561 nm, and 640 nm beams are 0.08 mm, −0.07 mm, −0.07 mm, −0.01 mm, and 0.06 mm, respectively. Accordingly alignment of the horizontal and vertical beam-waists waists is well within a Rayleigh range of the horizontal beam-waists, which, here, is on the order of about 2.0 mm. 
         [0039]    While in foregoing examples of the inventive focusing objective the final focusing element FFL has an EFL of 50 mm this should not be considered as limiting. The FFL may have other EFL values without departing from the spirit and scope of the present invention. Examples are presented in TABLE 3 below for a 6:1 ratio of CL 1 :CL 2 . 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Ratio of EFL 
                 Optimum EFL 
                 Optimum EFL 
                 EFL of FFL 
               
               
                   
                 CL1:CL2 
                 of CL1 (mm) 
                 of CL2 (mm) 
                 (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 6:1 
                 25.25 
                 −4.21 
                 30 
               
               
                   
                 6:1 
                 33.92 
                 −5.65 
                 40 
               
               
                   
                 6:1 
                 42.30 
                 −7.05 
                 50 
               
               
                   
                 6:1 
                 51.71 
                 −8.62 
                 60 
               
               
                   
                 6:1 
                 65.64 
                 −10.94 
                 75 
               
               
                   
                   
               
             
          
         
       
     
         [0040]    In the 75 mm EFL example, the working distance shifts for the 355 nm, 405 nm, 488 nm, 561 nm, and 640 nm beams in the horizontal plane are 0.28 mm, −0.23 mm, −0.23 mm, −0.03 mm, and 0.19 mm, respectively. These shifts are comparable with those discussed above for the 50 mm EFL example of TABLE 1 and  FIG. 3 . 
         [0041]    In all examples of the inventive focusing objective described above, all three lens elements are assumed to be fused-silica elements. Comparable optical performance is available, however, if all three elements CL 1 , CL 2 , and FFL are made from the same different glass, or if one or more of the elements are made from one glass, and the remaining element or elements are made from another glass. 
         [0042]    By way of example TABLE 4 lists a specification of the inventive focusing objective in which all three elements are made from N-BK7 glass. The effective focal lengths of CL 1  and CL 2  are selected to provide a 6:1 spot-size ratio. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                   
                 Center 
                 Distance to 
               
               
                   
                 Lens 
                 EFL 
                 Thickness 
                 Previous Lens 
               
               
                   
                 (Glass) 
                 (mm) 
                 (mm) 
                 (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CL1 (N-BK7) 
                 42.6 
                 4 
                 N/A 
               
               
                   
                 CL2 (N-BK7) 
                 −7.1 
                 2 
                 32.9 
               
               
                   
                 FFL (N-BK7) 
                 50 
                 4 
                 1 
               
               
                   
                   
               
             
          
         
       
     
         [0043]    In the example of TABLE 4, the working distance shifts for the 355 nm, 405 nm, 488 nm, 561 nm, and 640 nm beams in the horizontal plane are 0.27 mm, −0.23 mm, −0.23 mm, −0.03 mm, and 0.19 mm, respectively. These shifts are nearly identical with those discussed above for the 50 mm EFL example of TABLE 1 and  FIG. 3 . 
         [0044]    TABLE 5 lists a specification of the inventive focusing objective in which elements CL 1  and CL 2  are made from N-BK7 glass, and element FFL is made from fused silica (SiO 2 ). Here again, the effective focal lengths of CL 1  and CL 2  are selected to provide a 6:1 spot-size ratio. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                   
                 Center 
                 Distance to 
               
               
                   
                 Lens 
                 EFL 
                 Thickness 
                 Previous Lens 
               
               
                   
                 (Glass) 
                 (mm) 
                 (mm) 
                 (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CL1 (N-BK7) 
                 45.3 
                 4 
                 N/A 
               
               
                   
                 CL2 (N-BK7) 
                 −7.55 
                 2 
                 35.1 
               
               
                   
                 FFL (Fused SiO 2 ) 
                 50 
                 4 
                 1 
               
               
                   
                   
               
             
          
         
       
     
         [0045]    In the example of TABLE 5, the working distance shifts for the 355 nm, 405 nm, 488 nm, 561 nm, and 640 nm beams in the horizontal plane are 0.063 mm, −0.066 mm, −0.038 mm, −0.010 mm, and 0.027 mm, respectively. These shifts are much less than those discussed above for the 50 mm EFL example of TABLE 1 and  FIG. 3 . 
         [0046]    In all examples of the inventive focusing objective discussed above, the cylindrical lens element CL 2  is a negative-power element, i.e., an element having a negative focal length. The inventive objective, however, can be equally effective if element CL 2  has a positive optical power in the y-axis, i.e., has a positive focal length. 
         [0047]    By way of example,  FIG. 6  schematically illustrates an embodiment  21  of the inventive focusing objective in which the second cylindrical element along the z-axis has positive optical power. This element is designated as element CL 2 ′ to distinguish the element from element CL 2  in  FIG. 2  In this drawing, only a horizontal (y-z plane) view is depicted, and only the longest-wavelength (640 nm) and shortest-wavelength (355 nm) input beams are depicted for simplicity of illustration. The 355 nm input beam is preferably smaller than the 640 nm input beam in linear relation to the wavelength, as discussed above. Input beams are focused into a working plane P. 
         [0048]    An exemplary specification for objective  21  is provided in TABLE 6. All three lens elements are assumed to made form fused-silica. EFLs of CL 1  and CL 2 ′ are selected to provide a 6:1 spot-size ratio. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                   
                 Center 
                 Distance to 
               
               
                   
                   
                 EFL 
                 Thickness 
                 Previous Lens 
               
               
                   
                 Lens 
                 (mm) 
                 (mm) 
                 (mm) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CL1 
                 59.2 
                 4 
                 N/A 
               
               
                   
                 CL2{grave over ( )} 
                 9.9 
                 2 
                 65 
               
               
                   
                 FFL 
                 50 
                 4 
                 1 
               
               
                   
                   
               
             
          
         
       
     
         [0049]    In the example of TABLE 6, the working distance shifts for the 355 nm, 405 nm, 488 nm, 561 nm, and 640 nm beams in the horizontal plane are 0.57 mm, −0.52 mm, −0.50 mm, −0.07 mm, and 0.43 mm, respectively. These shifts are comparable with those discussed above for the 50 mm EFL example of TABLE 1 and  FIG. 3 . It can be seen however that the length of the objective is increased by a need for an extended spacing between CL 1  and CL 2 ′ to accommodate the positive focal length of CL 2 ′. 
         [0050]    In all of the examples of the inventive focusing objective described above all of the lens elements are either plano-convex or plano-concave. This is particularly advantageous in minimizing production costs for the elements. As the design does not require any cemented elements the objective can be used to focus ultraviolet radiation without any degradation of lens elements. Further, the number of available focal length selection options offers a possibility that one or more of the lens elements could be an “off the shelf” lens element from a catalog optics supplier. This is particularly true of the spherical element FFL. 
         [0051]    It should be noted that the exemplary wavelengths for radiations being focused used throughout the above presented should not be considered as limiting. Those skilled will recognize from the description that the objective can be designed to focus other wavelengths without departing from the spirit and scope of the present invention. 
         [0052]    In summary, the present invention is described above with reference to preferred embodiments and examples thereof. The invention, however, is not limited to the embodiments and examples described and depicted herein, rather the invention is limited only by the claims appended hereto.