Abstract:
A system for quickly aligning a test optic with various components of an optical metrology tool. A collimated target is presented to a beamsplitting reference surface located on a positioning system for holding and manipulating the test optic. Video images of the target and its reflection from the reference surface are displayed for analysis and visualization so that any tilt between the reference surface and the optical axis of the collimated beam can be removed to align the test optic. After alignment, the video based system is used to quickly measure and display in real-time a variety of performance characteristics of optical components such as lenses. The metrology system is under the control of a computer which uses a windowing software program to provide the user with a graphical user interface by which the various components of the system and test lenses may be aligned and characterized.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of the filing date of United States Provisional Patent Application No. 61/809,755 filed on Apr. 8, 2013 in the name of Daniel Orband and entitled “OPTICAL ALIGNMENT APPARATUS AND METHODOLOGY FOR VIDEO BASED METROLOGY TOOL”, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field Of The Invention: 
         [0003]    This invention generally relates to optical metrology apparatus and more particularly to apparatus and methodology for aligning an image forming optical system to a video metrology tool. 
         [0004]    2. Background of the Prior Art: 
         [0005]    To properly measure an optical system, one needs to bring its optical axis into alignment with the optical axis of a collimated beam provided in the metrology tool. While this requirement is easily stated, co-alignment of the optical axes of the test instrument and the unit under test (UUT) is usually an awkward and time consuming process in practice, typically achieved by putting a mirror on a reference surface of the UUT and attempting to reflect the incident collimated beam back onto the collimator target. The return image formed at the target is usually difficult to see even at visible wavelengths and is not visible to the human eye if in the infrared or deep ultraviolet. 
         [0006]    Well-known metrology instruments contain a light source, test target, refractive or reflective collimator, and an image analyzer. The image analyzer is generally comprised of a relay lens and two-dimensional video sensor, such as a CCD camera for the visible spectrum or microbolometer for the long-wave infrared (LWIR) spectrum. The optical system to be tested (unit-under-test or UUT) forms an image of the illuminated test target at an infinite conjugate. The image analyzer captures this image for analysis to determine properties and qualities of the UUT. An example of such instruments is described in detail in U.S. Pat. No. 5,661,816 which issued on Aug. 26, 1997 in the name of Stephen D. Fantone, et al. with the title “IMAGE ANALYSIS SYSTEM.” 
         [0007]    To properly characterize the properties of a UUT with a metrology tool requires that it be aligned with the collimated beam of the metrology tool such that there is no tilt between the optical axes of the UUT and that of the metrology tool. 
         [0008]    Accordingly, it is a primary object of the present invention to provide alignment apparatus and methodology by which an optical component to be measured in a video based metrology instrument can readily be aligned with respect to the other components comprising the system. 
         [0009]    Other objects will be obvious and others will appear hereinafter when the following detailed description is read in connection with the accompanying drawings. 
       SUMMARY OF THE INVENTION 
       [0010]    This invention applies to metrology instruments that contain a light source, test target, refractive or reflective collimator, and an image analyzer. The image analyzer is generally comprised of a relay lens and two-dimensional video sensor, such as a CCD camera for the visible spectrum or microbolometer for the long-wave infrared (LWIR) spectrum. The optical system to be tested (unit-under-test or UUT) forms an image of the illuminated test target at an infinite conjugate. The image analyzer captures this image for analysis to determine properties and qualities of the UUT. An example of such instruments is described in detail in U.S. Pat. No. 5,661,816 which issued on Aug. 26, 1997 in the name of Stephen D. Fantone, et al. with the title “IMAGE ANALYSIS SYSTEM.” 
         [0011]    The present invention comprises apparatus for quickly aligning a test optic with various components of the above-described optical metrology tool. Here, a collimated target is presented to a reference surface located on a positioning system for holding and manipulating the test optic. Video images of the target and its reflection from the reference surface are displayed for analysis and visualization so that any tilt or other misalignment between the reference surface and the optical axis of the collimated beam can be removed to align the test optic. After alignment, the video based system is used to quickly measure and display in real-time a variety of performance characteristics of optical components such as lenses. The metrology system is under the control of a computer which uses a windowing software program to provide the user with a graphical user interface by which the various components of the system and test lenses may be aligned and characterized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The structure, operation, and methodology of the invention, together with other objects and advantages thereof, may best be understood by reading the detailed description in connection with the drawings in which each part has an assigned numeral or label that identifies it wherever it appears in the various drawings and wherein: 
           [0013]      FIG. 1  is a diagrammatic elevational view of an embodiment of an alignment apparatus in accordance with the invention; 
           [0014]      FIG. 2A  is a diagrammatic representation of an image of a target formed directly by a video camera; and 
           [0015]      FIG. 2B  is a diagrammatic representation of the image of  FIG. 2A  and its image after having been reflected by a reference surface as an inverted and reverted image (white) of the target image (black). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Reference is now made to  FIG. 1  which diagrammatically shows an elevational view of the alignment apparatus  10  of the invention by which a unit under test  12  (UUT), such as a lens or the like, may be aligned prior to measurement in a video based metrology system  14  of the type described more fully in the aforementioned U.S. Pat. No. 5,661,816, which is incorporated herein in its entirety by reference. The present apparatus  10  and its associated alignment method require a specific type of alignment target  16  and a plane parallel beamsplitter  18 . The alignment target  16  should contain fine, transparent features, such as a crosshair, half circle, pinhole, or the like, placed against a reflective background. 
         [0017]    A target shown generally at  16  is preferably a flat opaque disk  15  with a transparent pattern in the form of a central pinhole or cross hair, or the like, that provides features allowing visualization of an axis or alignment center. The disk  15  has two sides, a front side  21  facing a light source  26  and reflective back side  20  facing a collimating lens  28 . The back side  20  of the disk  15  is usually coated with a material to increase the reflectivity of the disk  15 . If the disk  15  is uncoated, then the reflectivity and surface finish of the disk  15  determine the specularity of the reflection of the light that was directed back towards the target  16  by beamsplitter  18  and focused onto the back side  20  of the disk  15  by the collimating lens  28 , as more fully explained hereafter. 
         [0018]    If the diffuseness and or reflectivity of the disk  15  substrate is insufficient, then it is advantageous to coat the back side  20  of the disk  15  with a coating that will enhance its reflectivity. There are a number of ways this can be accomplished including, white paint, retro-reflective paint, and evaporated coatings such as Inconel®, chrome, aluminum, silver or gold. Each of these ways has its own advantage. For instance, a diffuse coating, such as white paint, ensures broad band reflection and truly diffuse reflection. The diffuseness of reflection is particularly important if the pupil of the beamsplitter  18  and lens under test  12  are decentered in the pupil of the collimator lens  28 . A retroreflective paint increases the amount of light that is directed directly back into the pupil of the beamsplitter  18  and lens under test  12 . The effectiveness of retro-reflective paints in increasing the amount of light directed back to its source is readily appreciated when comparing the improvement in road side signs coated with retroreflective paints with those with conventional paints. 
         [0019]    Targets are typically made of metal foil substrates using well-known techniques including laser cutting or chemical etching and other photolithographic techniques. Targets can be made on clear glass or other transmissive substrates coated with a substantially opaque coating such as aluminum, Inconel®, chrome, gold or other metal. It is important that this coating have an optical density of at least four to minimize any stray light passing through the system, as is usually required for accurate measurement of modulation transfer function. 
         [0020]    For the visible region or operating wavelengths, a crosshair with a pinhole in the center (chrome on glass) has been found successful and for operation in the infrared region of the spectrum, a quarter-open fishtail target (tungsten foil) has been used. Alternatively, in the infrared one option is a fishtail or pinhole target on a infrared transmissive substrate. 
         [0021]    If the target  16  is made with a substrate that has a smooth specular surface and if the pupil of the beamsplitter  18  and lens under test  12  are decentered relative to that of the collimator lens  28 , the light specularly reflected off the back side  20  of the target  16  may not be reflected back into the pupil of the beamsplitter  18  and the lens under test  12 . In this case, it is essential to use either a diffuse reflective coating (e.g., white paint)) or a retro-reflective paint. It is a primary intent of this coating to maximize the light reflected off the back  20  of the disk back into the pupil of the lens under test  12 . 
         [0022]    Care must be taken to make sure that any coatings on the back side  20  of the disk  15  do not encroach on the target itself and vignette the view of the target from the collimating lens  28 . 
         [0023]    The front side  21  of the target  16  may be coated with a reflective material to minimize the absorption of light and the associated rise in temperature of the target  16 . 
         [0024]    The alignment target  16  can also serve as the primary test target for the metrology tool or is aligned to other test targets in the system through means of a target wheel or mechanical fixturing of the target mounts (not shown, but otherwise well-known). 
         [0025]    The plane parallel beamsplitter  18  generally requires parallelism of its two surfaces  22  and  24  within 1 arc minute. For the visible spectrum, the beamsplitter  18  can be fabricated from optical glass containing a partially reflective coating on one side. For the long-wave infrared spectrum, the beamsplitter  18  can be fabricated from an infrared material containing a partially reflective coating or an uncoated piece of germanium. 
         [0026]    A light source  26  back-illuminates the alignment target  16 , such that bright features on a dark background are present. The alignment target  16  is located at the focal point of the collimator lens  28 . The plane parallel beamsplitter surface  24  is placed against a mechanical surface datum  25  on the front of UUT  12 . 
         [0027]    Collimated light  32  exiting the collimator lens  28  is first incident to the beamsplitter  18 . Some percentage of the light transmits through the beamsplitter  18  and some percentage reflects off the beamsplitter  18  back towards the target  16 , i.e., it is specularly reflected off the beamsplitter. The portion that transmits through the beamsplitter  18  is imaged by the UUT  12  and captured by the image analyzer  14  comprising a relay optic  34  and downstream camera  36  whose images are processed in a well-known manner via computer a computer  38  provided with programs containing suitable algorithms. This image of the alignment target  16  serves as our lens  28  and is reimaged onto the alignment target  16 . Since the alignment target  16  contains a reflective background (surface  20 ), this image is reflected back through the collimator  18  and imaged by the UUT  12 . A second alignment target image is captured by the image analyzer  14  and is depicted as the white or empty semicircle shown in  FIG. 2B  along with the previously described black reference semicircle. Only when the UUT  12  is aligned to the optical axis of the collimator lens  28  will the two alignment target images be coincident. In  FIG. 2B , the two images are vertically displaced with respect to the optical axis to depict a tilt between the optical axis of the UUT and that of the collimated beam. When no such tilt exists, the two half circle images would have their bases coincident to form a perfect circle centered on the optical axis of the collimator. The sensitivity of this alignment method is dependent upon the size of the features in the alignment target  16 , the focal lengths of the collimator and UUT, the magnification of the relay lens  34  in the image analyzer  14 , and the resolution of the camera in the image analyzer. 
         [0028]    To bring the two images into alignment, use may be made of a manually adjustable multi-degree of freedom precision stage or a computer controlled micromanipulator as designated generally at  40 . If the later, control signals and processing information may be communicated to computer  38  with a suitable cable  44  or via wireless connections. Computer  38  may also be used to identify the UUT  12  properties, issue commands, acquire and process data, and perform routine housekeeping functions. The metrology system may also be under the control of computer  38  which preferably uses a windowing software program to provide the user with a graphical user interface by which the various components of the system and test lenses may be aligned and characterized. 
         [0029]    The method of the invention can also be used to align purely mechanical mounts. For this purpose, the relay lens  34  in the image analyzer  14  is replaced with a decollimating lens such that the alignment target  16  is directly imaged onto the video sensor of camera  36  without the UUT  12  in place. In this case, the plane parallel beamsplitter  18  is placed against the surface datum on the mount. The procedure for viewing and aligning the two alignment target images is the same. 
         [0030]    Procedurally, the plane parallel beamsplitter  18  is placed against a reference surface  25  on the lens that should be perpendicular to the optical axis of the lens. Light incident on the plane parallel beamsplitter  18  passes through it undeviated while light reflected off one or more surfaces of the plane parallel beamsplitter  18  is reflected back onto the target  16 . Due to the symmetry of the system, the image formed is inverted and is further displaced by an amount proportional to the tilt of the reference surface  25  relative to the direction of the optical axis of the collimated beam  32 . 
         [0031]    If the UUT  12  and the plane parallel beamsplitter  18  are aligned so that the original target and the inverted and reverted image of that target are adjacent to each other, then the optical axis of the collimated beam is at the geometric center+of the two images (See bottom left white half circle above black half circle in  FIG. 2B . 
         [0032]    This procedure works very well with video based type systems such as that shown and described in the &#39;816 patent. It is also particularly useful in the infra-red where it is usually very difficult to determine the direction of the collimated beam. The use of a target that has a reflective surround allows the user to have a direct view of the target and a view of the inverted return image from the beamsplitter  18 . This makes alignment of the UUT  12  to the optical axis of the collimator  28  very easy to perform. One useful beamsplitter for use in the visible had a reflection/transmission ratio of 70/30, and an uncoated germanium window that was used for the infrared beamsplitter had an effective 60/40 ratio from Fresnel reflections. 
         [0033]    In the visible, a plane parallel piece of common glass can be used, but the low refractive index makes the inverted and reverted image difficult to see. This image can be enhanced by applying a reflective coating to one side of the plane parallel beamsplitter  18 . The relative brightness of the direct viewed target to the reflected image is: 
         [0000]        T   b /( R   b   ×R   t   ×T   b ) 
         [0000]    Which is equal to 1/(R b   ×R   t )
 
where T b  is the single pass transmission of the beamsplitter, R b  is the reflectance of the beamsplitter, and R t  is the reflectance of the target surround in the target.
 
         [0034]    Note that if a non-absorbing beamsplitter is used, R b =1−T b . To a first approximation, assume the second surface of the beamsplitter is antireflection coated with a transmission of 100%. In this case, the relative brightness of the two images is 1/R b , so that to match the brightness of the two images, the beamsplitter reflectance should be high. Unfortunately, the overall throughput would drop. Generally, a ratio of 10:1 works, so a reflectance of 10% provides sufficient light to be able to see the reflected target. Note that if the beamsplitter is truly parallel, then you can use the reflection from both surfaces for alignment. With common optical glasses, reflectivities of 4% per surface are common, so an uncoated beamsplitter can be used. In the infra-red, much higher refractive materials are common, and they provide even better reflection than low index glasses. 
         [0035]    The source  26  for illuminating target  16  may be any well-known source whose output encompasses the operating wavelengths of the UUT  12  under test. 
         [0036]    Other variants of the invention are possible, and those skilled in the relevant arts may make such changes based on the teachings of the disclosure. It is intended that all such variants be within the scope of the claimed invention.