Abstract:
A parcentric objective is described, including an objective lens configured to receive an incident ray from a field of view and to translate the incident ray into a translated incident ray, the objective lens substantially aligned across an optical axis, and a wedge prism configured to receive and deflect the translated incident ray into an exiting ray, the wedge prism rotated about the optical axis. A specimen review system is also described, including a specimen stage configured to receive specimens for viewing, a source of illumination providing illumination to the specimen stage, a review scope configured to review specimens positioned on the specimen stage, the review scope comprising a parcentric objective configured to resolve a field of view of the specimen stage.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to microscopy. More specifically, a parcentric objective and its use are disclosed. 
     BACKGROUND OF THE INVENTION 
     Maintaining a specimen within a field of view when switching or translating objectives may be affected by the degree of parcentricity. A specimen may shift from a field of view when, for example, objective lenses are translated across their optical axes. Switching or translating objective lenses can create deviations resulting in a specimen being lost from a field of view when, for example, a greater magnification objective is selected and translated. Objective lenses may be switched manually or automatically and are often translated along a single degree of freedom, such as along a straight line or an arc. Subsequently, maintaining parcentricity while translating objectives is problematic. 
     Conventional objectives have several problems with regard to parcentricity. For example, translating an objective to a higher power magnification objective typically shifts the center of the field of view. Such shifts can also cause users to incur significant time and effort attempting to fix and align an objective after translation. Current methods of correction for parcentricity during manufacturing require specially-trained technicians. Furthermore, the parcentricity error can grow due to mechanical wear—restoration of parcentricity requires service by technicians. Conventional objectives typically provide for only a single degree of freedom during operation, rendering correction of parcentricity errors impossible during operation. Alignment methods for parcentricity are available during manufacture, but they are not appropriate during operation. These alignment methods employ translation of an objective across its optical axis. These parcentricity methods are typically not performed by an operator of the microscope. Alignment of parcentricity requires considerable effort by a specially-trained technician. 
     Conventional methods for beam steering can be applied to the exiting light of an objective to achieve parcentricity but these existing methods are impractical. Specifically, a pair of Risley prisms can establish parcentricity, but this alignment method would not be appropriate for an ordinary operator. A Risley prism is a wedge of glass with a freedom of rotation about a normal to one of the faces of the prism. Alignment of Risley prisms also require a specially-trained technician. A pair of Risley prisms are impractical when applied to parcentricity of microscope objectives. 
     A pair of Risley prisms is often used to provide two degrees of freedom in beam steering applications. Risley prisms deflect rays in a range of deflection that is typically less than 5°. However, Risley prisms cannot be used to achieve a net deflection of zero due to finite differences between the wedge angles of the prisms. The finite difference between wedge angles creates a small circle of exclusion, resulting in a range of deflection in the shape of an annulus. This circle of exclusion is problematic in application to parcentricity. Further, when used with converging rays, Risley prisms create aberrations in the form of astigmatisms that are dependent upon the variable orientation of the prisms. When used with parallel rays of different colors conventional Risley prisms create aberrations known as lateral color. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the invention are disclosed in the accompanying drawings, and described in the following detailed description: 
         FIG. 1A  illustrates an exemplary side view of a parcentric objective; 
         FIG. 1B  illustrates an exemplary isometric view of a parcentric objective with rotation of wedge prism; and 
         FIG. 2  illustrates an exemplary isometric view of a parcentric objective with rotation of wedge prism and translation of objective lens. 
     
    
    
     DETAILED DESCRIPTION 
     The invention can be implemented in numerous ways, including as an apparatus, a process, and a system. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. 
     A detailed description of one or more embodiments is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured. 
       FIG. 1A  illustrates an exemplary side view of a parcentric objective. Here, several components are shown. Objective lens  10 , wedge prism  12 , field of view (of a specimen)  14 , incident ray  16 , exiting ray  18 , and distant aperture  20  are shown in this example. In other examples, more components than those shown may be used to implement a parcentric objective. The components are aligned along an optical axis, which is also coincident with a radial propagation vector of incident ray  16 . Objective lens  10  may be implemented to resolve field of view  14  using a variety of techniques and lenses (e.g., PLAN, PLAN achromat, PLAN apochromat). However, an infinity-corrective objective may also be used for improving performance. Exiting ray  18  is deflected from wedge prism  12 , in some examples, using a translated incident ray (not shown) between objective lens  10  and wedge prism  12 . In some examples, characteristics of exiting ray  18  may be affected by objective lens  10  and wedge prism  12 , including altering the degree of deflection from wedge prism  18 . Exiting ray  18  is deflected from wedge prism  12  using incident ray  16  and the optical axis of the parcentric objective. However, in other embodiments, incident ray  16  and exiting ray  18  may be aligned using a range of deflection, as described below in connection with  FIG. 2 . 
     In the configuration shown in  FIG. 1A , wedge prism  12  may be placed within an infinity-corrected zone of objective lens  10 . In other examples, wedge prism  12  may be placed differently (e.g., asymmetrically aligned with an optical axis). A specimen may be illuminated within field of view  14 , from which light rays may be directed similar to incident ray  16  into objective lens  10 . Incident ray  16  passes through objective lens  10  creating an infinitely-distanced image of field of view  14 . Exiting ray  18  enters distant aperture  20 , where its spatial position corresponds with its angular direction. Another lens, such as a tube lens (not shown) converts the angular direction of the exiting ray  18  into a spatial position within a nearby image (not shown) of field of view  14 . The nearby image of field of view  14  may be viewed by an observer through an ocular lens (not shown), or the nearby image of field of view  14  may be located on an image sensor (not shown) such as film or a CCD camera. 
       FIG. 1B  illustrates an exemplary isometric view of a parcentric objective with rotation of wedge prism  12 . After passing through objective lens  10 , incident ray  16  is deflected by wedge prism  12 , which shifts exiting ray  18  within distant aperture  20 . Rotation  22  of wedge prism  12  directs exiting ray  18  along the perimeter of circle of deflection  24  within distant aperture  20 . Rotation  22  of wedge prism  12 , provides a first degree of freedom along the circumference of circle of deflection  24 . 
       FIG. 2  illustrates an exemplary isometric view of a parcentric objective with rotation of wedge prism  12  and translation of objective lens  10 . In this example, circle of deflection  24  (not shown) may be translated along distant aperture  20  across range of translation  26 . Translation of objective lens  10  provides a second degree of freedom. If translation of objective lens  10  exceeds the diameter of circle of deflection  24  (not shown), then a continuous range of deflection  28  is created. Thus, exiting ray  18  may be deflected in a direction that corresponds to the direction of another exiting ray of a different (e.g., previous, lower power, translated, etc.) objective. In this embodiment, exiting ray  18  from objective lens  10  may be directed to distant aperture  20  within two degrees of freedom. Subsequently, range of deflection  28  is achieved in which a net deflection of zero is possible. In this example illustrated, rotation  22  is depicted in a clockwise direction. In other examples, rotation  22  may be in a direction other than clockwise. 
     Objective lens  10  may be translated across range of translation  26 . Range of translation  26  provides a range of incident rays as objective lens  10  is translated or switched. By translating objective lens  10 , exiting ray  18  may be directed in a curvilinear path. In contrast, when wedge prism  12  is rotated in a direction (e.g.,  22 ), exiting ray  18  may be directed along circle of deflection  24 , depending upon the degree of rotation  22 . When combined, range of translation  26  and rotation  22  produce range of deflection  28 , which is a convolution of a curvilinear path and a circle. Thus, exiting ray  18  may be directed against distant aperture  20  within range of deflection  28 , without an area of exclusion. In this example, range of deflection  28  may be constructed by directing exiting ray  18  within two degrees of freedom. Although rotation  22  is illustrated in a clockwise direction in this example, movement of wedge prism  12  may occur in directions other than those illustrated. Similarly, range of translation  26  may also be different than shown in the above example. Range of deflection  28  is a continuous area and may be achieved without aberrations. Additionally, range of deflection  28  does not display a circle of exclusion about zero translation as a pair of Risley prisms does. 
     With regard to aberrations, different materials may be used to implement wedge prism  12  to avoid lateral color. As an example, a dependency of deflection represents a property of wedge prism  12  that affects the creation of a consistent angular deflection of exiting ray  18 . Materials used to implement wedge prism  12  may affect the dependency of deflection, which may also be affected by the wavelength of exiting ray  18 . Chromatic aberrations may be avoided by using a monochromatic infinity-corrected objective with wedge prism  12 . The use of a monochromatic infinity-corrected objective lens is not affected by the planar geometry of the active surface of wedge prism  12 . Wedge prism  12  creates a consistent angular deflection of exiting ray  18 , while the radial position may shift without consequence to the image quality of the specimen directed at distant aperture  20 . Thus, a range of deflection  28  may be achieved without incurring a chromatic aberration. 
     In a polychromatic system, chromatic aberrations may be eliminated using an achromatic wedge which employs two or more glass types, for deflecting light rays at different wavelengths. Although prisms disperse light of different wavelengths across a range of angles, elimination or minimization of chromatic aberrations of this type may be achieved with an achromatic prism. Using an achromatic prism as the wedge prism  12  with objective lens  10  enables a net deflection of zero in range of deflection  28  without chromatic aberrations in a polychromatic system. Regardless of the type of system used to correct chromatic aberrations, parcentric objectives such as those described above may be used. 
     Imaging, specimen review, specimen marking, specimen analysis, and other cytological systems may be used with various embodiments of the above techniques. For example, objectives found in imaging systems may be replaced with parcentric objectives such as those described above. Automatic and manual imaging or specimen review systems enable the review, marking, and analysis of specimens. In some examples, a parcentric objective may be implemented with a specimen review, analysis, marking, or other system to ensure that a field of view of a specimen, for example, is maintained. Parcentric objectives may also be used with systems such as those described in U.S. Published patent application Ser. No. 10/008,379 to Maenle et al. (filed Nov. 5, 2001, published Sep. 25, 2003), which is incorporated by reference in its entirety. Other types of systems having components such as review scopes, specimen modules for viewing one or more specimens (e.g., a deck of prepared specimen samples for viewing in an imaging system), optical instruments, objectives, or other types of lenses may also be used to implement the above techniques. As an example, optical instruments having objective lenses may be implemented using a parcentric objective in accordance with the techniques described above. 
     Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.