Abstract:
A method and system to provide for increased signal intensity and improved signal quality in Fourier-domain optical coherence tomography (FDOCT) by capturing the real and virtual images of an object being imaged by FDOCT in sequence or consecutively.

Description:
[0001]    This application claims the benefit under 35 USC §119 (e) of U.S. provisional application Ser. No. 61/565,297 filed Nov. 30, 2011, incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present invention relates to a method and system for improving signal quality in contact lens imaging. 
       BACKGROUND 
       [0003]    Optical Coherence Tomography (OCT) is a non-contact imaging system and method which may be used for surface and thickness profiling of optical structures and assemblies, for example, contact lenses or contact lens subassemblies. Using special Fourier-domain OCT (FDOCT) with an ultra-long scan range, the entire cross-section of an object, for example a contact lens submerged in saline, can be imaged in a single sweep (B-scan). 
         [0004]    The current method of processing the raw image data from a B-scan requires the conversion of frequency to depth using fast Fourier transform (FFT). This usually results in a processed image containing two mirror images—a real image and a virtual image, depending on the pathlength of an internal reference beam. The pathlength of the reference beam may be adjusted continuously, and there are two particular positions such that one position results in a real image of the full cross-section while the other position results in a mirrored virtual image of the full cross-section. The virtual image is normally referred to as the complex conjugate artifact of FDOCT. In terms of signal value vs. pathlength, the virtual image appears as a mirror image of the real image, with the mirror point at zero pathlength. 
         [0005]    A characteristic of the above image capture system is the systemic (monotonic) change in signal intensity with scan depth. For the B-scan cross-section images, the pathlength of the reference beam translates into scan depth (in terms of optical path length of the scanning beam). For example, for a given beam focus position and for a given medium absorption level, the intrinsic signal degradation with depth is the dominating factor. If an object such as a contact lens is placed in a cuvette and is scanned from the top of the lens, the apex of the lens will appear near the top (depth=0) of a B-scan in the real image, resulting in a higher signal intensity at the apex. Conversely, the edge of the lens will appear near the top of the B-scan in the virtual image, resulting in a higher signal intensity at the edges, rather than the apex. As a result, when the real and virtual images are viewed together, the overall image quality is reduced, and neither individual image contains the optimal signal level for the entire sample. 
         [0006]    It is therefore desirable to provide for a method and system for capturing the real and virtual images of FDOCT in sequence or consecutively to provide for increased signal intensity at various portions of the object being imaged, for example at the apex and edge of an object such as a contact lens. 
       SUMMARY 
       [0007]    The present invention advantageously provides a method and system for improving the signal quality in a FDOCT image of a contact lens or other optical structures by collecting the real and virtual images in sequence. 
         [0008]    In an embodiment, a system is provided for acquiring the real and virtual images of an object. The system includes a Fourier-domain optical coherence tomography (FDOCT) imaging system having a reference beam, the reference beam having a pathway and defining two reference arm lengths; and also includes at least one of an optical chopper, linear stage, and fiber-optic switch disposed along a portion of the reference beam and modifying the pathlength of the reference beam. 
         [0009]    Also provided herein is a method for acquiring the real and virtual images of an object. In embodiments, the method includes the steps of providing a Fourier-domain optical coherence tomography (FDOCT) imaging system; generating a reference beam having two different pathlengths; modifying the pathlength of the reference beam; and acquiring a real image and virtual image of the object consecutively. 
         [0010]    In another embodiment, the method for acquiring the real and virtual images of an object includes the steps of providing a Fourier-domain optical coherence tomography (FDOCT) imaging system; generating a reference beam having two different pathlengths, wherein the pathlengths are pre-defined and calibrated; acquiring a real image and virtual images of the object consecutively, wherein the two images are superimposed or averaged to achieve more uniform signal level across the depth range. 
         [0011]    In embodiments using an optical chopper, the optical chopper is disposed along a portion of the reference beam pathway, and can include a movable refractive medium that is capable of movement into and out of the optical coherence tomography reference beam pathway. The movable refractive medium may be, for example, a glass lens. 
         [0012]    In other embodiments where the system includes a fiber-optic switch, the switch is capable of switching or splitting the reference beam into at least a first channel and a second channel. The system may additionally include a first movable or fixed mirror positioned within the first channel, and a second movable or fixed mirror positioned within the second channel. The switch can be capable of switching the reference beam between the first and second channel substantially continuously, and may do so at a rate of about 50 micro seconds or faster. Alternatively, the switch can operate to switch the reference beam manually at predetermined intervals. 
         [0013]    In other embodiments where the system includes a linear stage, the stage is disposed along a portion of the reference beam pathway. The linear stage can include a reference reflector, and the stage is then operable to move the reference reflector to change the pathlength of the reference beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
           [0015]      FIG. 1  is a schematic of a system for capturing both the real and virtual images of a contact lens using a FDOCT system constructed in accordance with the principles of the present invention; 
           [0016]      FIG. 2  is another schematic of a system for capturing both the real and virtual images of a contact lens using a FDOCT system constructed in accordance with the principles of the present invention; and 
           [0017]      FIG. 3  is yet schematic of a system for capturing both the real and virtual images of a contact lens using a FDOCT system constructed in accordance with the principles of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0018]    Now referring to the drawings in which like reference designators refer to like elements, there is shown in  FIG. 1  a system for capturing for the real and imaginary images of a object to be imaged using a FDOCT imaging system referred to generally as “10.” The system  10  and the methods discussed herein provide for adjusting the length of the reference beam to capture the real and virtual images of an object, for example, a contact lens, in sequence. In an exemplary embodiment shown in  FIG. 1 , the reference beam  12  is at least partially refracted when an exposed to an optical chopper  14 . The optical chopper  14  may be any device that includes a movable refractive medium  16 , for example glass, such as a glass lens, that may be positioned within at least a portion of the beam pathway  18 . The chopper  14  may include a wheel or other device that allows for the movement or rotation of the refractive medium  16  into and out of the OCT beam pathway  18 . For example, as shown in  FIG. 1 , the refractive medium  16  is substantially ovular in shape, but may be defined to be any shape or size and may be positioned along any position of the OCT beam pathway  18 , whether proximate or distal from the reference reflector. 
         [0019]    The refraction of the OCT beam  12  modifies the pathlength of the reference beam such that either a real or virtual image may be captured in sequence. For example, depending on the refractive index, thickness, and/or absorption level of the refractive medium  16 , either the real or virtual image may be captured when the refractive medium  16  is positioned within the reference beam pathway  18 . When the optical chopper  14  moves the refractive medium  16  outside of the reference beam pathway  18 , the real or virtual image may be captured such that the real and virtual images may be captured in sequence to provide for increased signal quality at either the smallest or the largest scan depth. The optical chopper  14  may be manually or automatically moved such that the reference arm length may be modified to capture both real and virtual images. Alternatively, two or more refractive mediums  16 , each having a different refractive index, thickness, and/or absorption may be coupled to the optical chopper  14  such that as each refractive medium is rotated within the beam path 18 different aspects, whether real or imaginary, of the image may be captured by varying the pathlength of the reference beam. 
         [0020]    It is further contemplated that a processor and database may be programmed to capture and store both the real and virtual images in a database and combine them for optimal signal quality. In particular, a processor may be utilized to rapidly average the real and virtual images in a combined image. 
         [0021]    Now referring to  FIG. 2 , in another embodiment, the system  10  may include a stage  20 , for example a linear stage, operable to move the reference reflector  16  (refractive medium, for example a mirror), to change the pathlength of the reference beam. In particular, the reflector  16  may alter the distance between two optical fibers within the reference beam  12  such that the real and virtual images may be captured in sequence and processed according to the embodiment in  FIG. 1 . As the stage  20  moves, the reference arm length changes, resulting in appropriate pathlength change in the reference beam, allowing for the capture of real and virtual images. 
         [0022]    Now referring to  FIG. 3 , in another embodiment, a fiber-optic switch  22  capable of splitting the reference beam may be included in the reference beam pathway  18  such that the reference beam pathway  18  may be split into two or more different pathways. For example, OCT beam  12  may be split by fiber-optic switch  22  into at least a first channel  24  and a second channel  26 . Within channel  24  or channel  26  the path length to a mirror may be different such that the reference arm length varies from the first channel  24  to the second channel  26  such that the real and virtual images can be captured depending on which channel the beam  12  is directed. In an exemplary embodiment, a first movable or fixed mirror  28  may be positioned within the first channel  24  and a second movable or fixed mirror  30  may be positioned within the second channel  26  to vary the reference arm length. The fiber-optic switch  22  may switch the OCT beam  12  between the first channel  24  and the second channel  26  (i.e., splits the beam back and forth between the first and second channel), either substantially continuously at, for example, a rate of approximately 50 micro seconds or faster, or manually at predetermined intervals, in either case such that the real and virtual images may be captured sequentially. 
         [0023]    It is further contemplated that the above system and method for consecutively capturing the real and virtual images of an object, for example contact lens, may be used with a dual-channel FDOCT system or any FDOCT system, and in particular with those having an extended depth range. 
         [0024]    The depth range of the real and virtual images can also be calibrated with pre-defined pathlengths of the reference beam, such that the two images are nearly exactly mirror images of each other, and then may be conveniently superimposed or averaged to achieve more uniform signal across the scan depth range. 
         [0025]    In any of the above embodiments, the real and virtual images may be captured automatically and combined by a processor for fast and accurate image processing. It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope and spirit of the invention, which is limited only by the following claims.