Abstract:
An optical system calibration system and method particularly suited for calibrating the optical slit planes in an ophthalmic diagnostic instrument. The system includes an illumination source projector, an illumination image receiver, and a calibration component all having known relative positions, orientations and physical and optical characteristics. The calibration component includes at least two separated, diffusely reflecting surfaces. Images of an exemplary slit illumination pattern projected onto the calibration component and formed on the diffusely reflecting surfaces are detected by the image receiver such as a video camera. Based upon camera image coordinates and triangulation parameters of the projector, the receiver, and the calibration component, the slit image positions on the image detector plane can be calibrated to the axially displaced, diffusely reflecting calibration component surface positions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     N/A. 
     FEDERALLY SPONSORED RESEARCH 
     N/A. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention generally relate to an optical system calibration system and method. More particularly, embodiments of the invention are directed to a system and method for calibrating an ophthalmic diagnostic instrument. Most particularly, an optical slit plane calibration apparatus and method are disclosed. 
     2. Description of Related Art 
     Eye models provide a valuable tool for assessing the optical, physical and biophysical characteristics of human eyes and those of other species. In order to construct accurate eye models, the shapes and locations of the various ocular surfaces as well as their functions and effects need to be known with a high degree of precision. Alignment has always been a serious issue in creating eye models from image data. Misalignment errors directly result in errors in both estimates of optical power and details about the reconstructed ocular surfaces. 
     Bausch &amp; Lomb Incorporated (Rochester, N.Y.) is an example of one company that has traditionally used a diagnostic slit beam to illuminate the eye. The slit beam is projected from about a 45 degree angle with respect to the instrument axis. A CCD camera is positioned along the axis. A subject&#39;s eye is aligned with the instrument axis and the subject&#39;s cornea is positioned at an imaging plane of the camera. The slit beam is scanned in a step-wise manner across the cornea for each eye measurement. The camera captures an image at each step of the slit beam position. By examining a series of these digital images of the slit beam illumination on various surfaces of the eye, three-dimensional models of the eye&#39;s surfaces can be obtained. In order to obtain accurate modeling results, it is desirable to obtain good characterization of the slit beam profile and accurate calibration of slit image displacement as measured on the camera detector. In particular, the relationship between the measured parameters and the physical displacement of the scattering surfaces of the eye that are intersected with the slit beam are determined at various locations along the slit beam optical path. The characterization and calibration just referred to will herein be referred to as slit plane calibration. 
     Slit plane calibration is typically performed with the use of an optical slit projection source, a camera/detector, and a test plate, having known position and orientation coordinates in a plane. The test plate has a scattering surface that is typically flat and painted black to reduce the amount of diffusely reflected light. This test plate is placed close to the imaging plane of the camera and the scattering surface is aligned normal to the instrument axis. The scattering surface is then positioned precisely at a number of axial positions and slit images are captured for the scan sequence at each axial position. Using triangulation techniques, one can analyze where the slit beam impinges on the test plate for a given calibration set-up and calibrate the slit beam width and optical path for that set-up from the known and measured data. This is repeated for additional set-ups to obtain full system calibration. 
     There are several drawbacks associated with the aforementioned calibration technique and apparatus. The precise placement of the test plates presents challenges in regard to mechanical alignment repeatability and mechanical alignment accuracy. Traditional alignment plates/fixtures can unknowingly be bent or otherwise deformed thus affecting alignment accuracy. The need for multiple physical alignment measurements and sets of images increases the opportunity for error. In addition, it is often necessary to position extra optical occluders along the path of the slit beam to reduce illumination intensity. 
     In view of the foregoing difficulties encountered with traditional mechanical alignment apparatus and techniques, the inventors have recognized a need for an apparatus and method that simplifies the calibration measurement, improves measurement accuracy and repeatability, is less time consuming and more technically efficient, is integratable with other diagnostic equipment, and offers other benefits and improvements over current apparatus and techniques that will be appreciated by persons skilled in the art. 
     SUMMARY 
     An embodiment of the invention is directed to an optical system calibration system. The calibration system includes an illumination projector (P) having an illumination projection axis lying in a plane. The projector is positioned at a selected reference location coordinate (x P ,y 0 ,z p ) in the plane and the projection axis has a known projection direction in the plane. The system includes an illumination imager (R) having an imaging axis lying in the plane. The receiver is positioned at a known receiver location coordinate (x R ,y 0 ,z R ) in the plane. The system further includes a calibration component (C) comprising at least a first at least partially diffusely reflecting surface disposed along the projection axis at a known first surface location coordinate (x C1 ,y 0 ,z C1 ) and a second at least partially diffusely reflecting surface disposed along the projection axis at a known second surface location coordinate (x C2 ,y 0 ,z C2 ) that is different than the first surface location coordinate. The system also includes a processing component operatively coupled to the system and adapted to determine a calibration indicia for the system. According to an aspect, the calibration component comprises at least a third at least partially diffusely reflecting surface disposed along the projection axis at a known third surface location coordinate that is different than the first and the second surface location coordinates. According to an aspect, one or more of the diffusely reflecting surfaces may also be optically transmissive to allow at least partial transmission of the illuminating light and certain diffusely reflected light, depending upon the calibration component geometry. According to an aspect, the calibration indicia describes a relationship between planar separation coordinate parameters (Δx, Δy) of at least two different diffuse illumination images on a detector in the imager and an axial separation distance (Δz) between the at least two respective diffusely reflecting surfaces of the calibration component. 
     Another embodiment of the invention is directed to method for calibrating an optical system. The method includes the steps of projecting an illumination pattern from a selected projection location, having a projection axis and known projection axis direction; illuminating, with the illumination pattern, a plurality of partially diffusing calibration component surfaces disposed along the projection axis at respective known calibration component surface locations; imaging the plurality of diffusely scattered illumination images from the plurality of diffusely reflecting surfaces of the calibration component on an image sensor positioned at a known imager location; and calibrating the optical system based upon the projector, imager, and calibration component surface location and direction parameters and sensor image parameters. According to an aspect, the relative position of either the projector or the camera can be changed along a line containing the known positions of the projector and camera. In this manner, the illumination pattern is scanned across the surface of the calibration component. Image edge and/or centroid coordinate mapping may not be linear for different scan positions, thus this calibration adjustment may be useful. 
     The system and method embodiments described herein may provide calibration data from a single set of images, may require only a single alignment, and may exhibit alignment accuracy that is independent of mechanical positioning variability of repeat measurements. 
     The system and method embodiments described herein are suitable for slit plane image calibration that is particularly useful for determining physical and optical parameters of an eye including, but not limited to the cornea, the anterior chamber, the crystalline lens, and the retina. 
     In various aspects, the system and method embodiments can include the use and operation of an ophthalmic wavefront sensor for further determining physical and optical parameters of an eye. 
     The foregoing and other objects, features, and advantages of embodiments of the present invention will be apparent from the following detailed description of the embodiments, which make reference to the several drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top plan view of a calibration system setup according to an embodiment of the invention; 
         FIG. 2  is right side cross sectional view of a calibration component according to an exemplary aspect of the invention; 
         FIG. 3  is an illustrative top plan view of a calibration component according to an exemplary aspect of the invention; 
         FIG. 4  is an illustrative top plan view of a calibration system according to an embodiment of the invention; 
         FIG. 5  is a perspective view of a calibration component according to an illustrative aspect of the invention; 
         FIG. 6  is a schematic diagram illustrating a calibration indicia relationship according to an exemplary aspect of the invention; 
         FIGS. 7A and 7B  show a top plan view and a side elevational view, respectively, of a calibration component according to an alternative exemplary embodiment of the invention; and 
         FIG. 8  is a schematic diagram of an exemplary system including a wavefront sensor according to an exemplary aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the invention is directed to a slit plane image calibration system  10  as illustrated in  FIG. 1 . The calibration system is particularly suited to calibrating slit beam-based ophthalmic instruments such as anterior chamber analyzers from Bausch &amp; Lomb Incorporated and Nidek Co., LTD, for example, or slit beam pachymeters (see, e.g., Snook U.S. Pat. No. 6,193,371). In these types of instruments, a slit beam is scanned across a subject&#39;s eye and a series of slit beam images is taken to obtain cross sectional viewing of the subject&#39;s cornea and to reconstruct corneal profiles, including an anterior surface profile, a posterior surface profile, and a corneal thickness profile. Thus embodiments according to the invention can be used to measure the slit beam profile and to determine the optical path of the slit beam at various scanning positions of the slit beam. The slit beam profile includes the slit beam width across the slit plane and the slit beam width variation along its propagation direction. The optical path includes the slit beam position, projection axis direction, and slit beam orientation with respect to the instrument axis and camera focal plane. Embodiments according to the invention are beneficial because the measurement accuracy and precision of a slit beam based optical pachymeter, for example, is highly dependent on the slit plane calibration. 
     The system  10  includes a slit illumination projector (P)  25  having an illumination projection axis  26  lying in the x-z plane  15  as shown. The projector is positioned at a selected reference location coordinate (x P ,y 0 ,z p ) along a reference line  17 . The projection axis  26  has a known direction (e.g., an angle defined in relation to a coordinate axis) in the plane relative to the reference line  17 . The projector may be a Scheimpflug slit beam projector or other suitable incoherent illumination source known in the art. An illumination imager (R)  35  having an imaging axis  36  lying in the plane is positioned at a known imager location coordinate (x R ,y 0 ,z R ) in the plane. A calibration component (C)  45  is located in the plane  15  at position coordinates (x C ,y 0 ,z C ), where z C  is measured perpendicular to the reference line  17 , and x C  is a position coordinate that locates the calibration component in the plane  15  spatially between the illumination projector and the illumination imager but not along the reference line  17 . A computational processor  50  is shown operatively connected to the imager  35 . The illumination projector and the imager can be moved relative to one another along imaginary reference line  17  as a means for scanning the illumination pattern across the calibration component surface, similar to the way the illumination pattern would be scanned across the subject&#39;s eye. Because the eye is a three-dimensional volume scatterer, it may be beneficial to translate the illumination pattern across the calibration surfaces to adjust for any nonlinearity between scan position and image edge and/or centroid detection. Reference is made to  FIGS. 1 and 6 , which illustrate exemplary scanning of the illumination pattern. 
     The calibration component  45  includes at least a first optically transmitting and partially diffusely reflecting surface  51  disposed along the projection axis  26  at a known first surface location coordinate (x C ,y 0 ,z C1 ) and a second optically transmitting and partially diffusely reflecting surface  53  disposed along the projection axis  26  at a known second surface location coordinate (x C ,y 0 ,z C2 ) that is different than the first surface location coordinate. In a particular aspect, the calibration component  45  includes at least a third optically transmitting and partially diffusely reflecting surface  56  disposed along the projection axis  26  at a known third surface location coordinate (x C ,y 0 ,z C3 ) that is different than the first and the second surface location coordinates. According to an exemplary aspect as illustrated in  FIG. 2 , the calibration component  45  comprises a plurality of stacked, optically transmitting plates  52 ,  55 , . . .  58  . . . , each having a given thickness (d 1 , d 2 , . . . ). At least either a front surface (e.g.,  51 ,  54 ,  57 ) or a back surface (e.g.,  53 ,  56 ,  59 ) of each of the plates is a diffusely reflecting surface. Each of the plates is an optically transmissive material such as BK7 glass, for example, of index of refraction n. The first (i.e., as positioned along the projection axis  26  to receive illumination from P) plate  52  has an optically diffuse front surface  51  and a rear surface  53  that coincides with the optically diffuse front surface  54  of second plate  55 . The surfaces  51  and  53 / 54  are parallely displaced a known, fixed distance d 1 . Second plate  55  has a rear surface  56  that may be made optically diffuse. Alternatively, as shown by the dotted lines, calibration component  45  includes a third plate  58  having an optically diffuse front surface  57 . The surfaces  53 / 54  and  56 / 57  are parallely displaced a known, fixed distance shown as d 2 . The calibration component thickness from front surface  51  to surface  56 / 57  is shown as d′ 2 . In a particular aspect, d 1  is greater than d 2 . This insures that secondary reflections from the multiple surfaces do not overlap or interfere with the primary diffusely reflected slit images  77 ,  78 ,  79  as illustrated in  FIG. 3 . Plate thicknesses may, alternatively, be equal. 
       FIG. 4  shows an illustrative schematic in which the incident slit illumination axis  26  is incident on the first front surface  51  of the calibration component at an angle α (with respect to line  17 ), forming first diffusely scattered slit image  77  at distance coordinate z C1 . The light is refracted slightly as shown in  FIG. 3  due to the index, n, of the plate  52 . The second and third surface diffusely scattered slit images  78 ,  79  are shown displaced along the x-direction on respective surfaces  53 / 53  and  56 / 57  located at distance coordinates z C2 , z C3 . The angle α, while illustrated to be about 45 degrees, can be essentially any angle between 0 to 90 degrees such that the camera  35  can image the diffusely scattered slit illumination images from the surfaces of the calibration component  45 . When the illumination pattern is scanned across the calibration component surface(s), the relative positions of the projector and the imager along reference line  17  will change while the angle α (i.e., the projection axis direction) remains, for the most part, the same. 
       FIG. 5  shows an illustrative perspective view of the calibration component  45 . In this exemplary aspect, the calibration component consists of three stacked/fused glass plates  52 ,  55  and  58 . Plate  52  includes an optically diffuse front surface that will be initially impinged by source illumination in the set-up as illustrated in  FIG. 4 . Plates  55  and  58  also have optically diffuse front surfaces whereupon the diffusely scattered illumination images are formed. 
     Based upon the position, orientation and separation coordinate parameters of the illumination projector, the calibration component surfaces, and the image sensor and the images formed thereon, the system processor can be programmed to determine a calibration indicia. In an exemplary aspect, the calibration indicia describes a relationship between a) the separation of corresponding coordinate parameters Δx IMAGE 1, IMAGE 2 , Δy IMAGE 1, IMAGE 2  between at least two different diffuse illumination images on the sensor plane in the imager and, b) an axial separation distance Δz C(n)  between at least two respective diffusely reflecting surfaces of the calibration component. This is diagrammatically illustrated in  FIG. 6 . Accordingly, once the system is so calibrated, the coordinates Δx 12 , Δy 12  of the diffusely reflected slit images (e.g., image  1 , image  2 ) from the diffusely scattering anterior and posterior surfaces of a subject&#39;s cornea formed on a flat camera sensor can be used to determine the depth-wise (axial) separation Δz C(n) , and thus the thickness, of the subject&#39;s cornea, for example. Stated differently, the Δx, Δy, and Δz parameters are the projection of a corresponding section of the slit beam on the three coordinates. Calibration according to an embodiment of the invention allows the determination of camera measured Δx, Δy values to a known Δz at a known position of x, y, and z of the slit beam. This then provides a scale factor for the Δx and Δy values on the camera plane for a Δz value of the slit beam. The scale factor is position dependent due to the vergence of the projected slit beam, the camera depth of focus, and because the slit beam angle varies slightly as it scans. Thus calibration is important to determine the scale factor between the Δx, Δy, and Δz parameters as a function of the slit position. 
       FIGS. 7A and 7B  show a top plan view and a side elevational view, respectively, of a calibration component  45 - 2  according to an alternative embodiment of the invention. The exemplary component as illustrated has a first diffusely reflecting optical surface  51 ′, a second diffusely reflecting optical surface  53 ′ disposed parallel to surface  51 ′ and displaced a known distance Δt 12  therefrom in a step-wise manner; and a third diffusely reflecting optical surface  56 ′ disposed parallel to surfaces  51 ′,  53 ′ and displaced a known distance Δt 23  from surface  53 ′ in a step-wise manner as shown. The component  45 - 2  may be translated in a direction parallel to the orientation of the diffusely reflecting surfaces to effect scanning of the slit beam illumination. Alternatively, the slit beam may be scanned as described above by moving the illumination projector. 
     Other alternative embodiments of a calibration component having different fixedly displaced, diffusely reflecting surfaces could include block-like or disc-like rotatable structures having a plurality of diffusely reflecting surfaces contained therein. 
     According to an exemplary aspect of the invention as illustrated in  FIG. 8 , the calibration system  10  described herein may be utilized in conjunction with an ophthalmic wavefront sensor  100 . The calibration system may suitably be a modifiable version of an eye topography system or an anterior corneal segment analyzer. An example of such a device is the Orbscan® IIIz ophthalmic analysis system. The combination of the calibration system  10 , a wavefront sensor  100  and a processor  50  may form an integrated ophthalmic diagnostic calibration and measurement system. 
     The foregoing description of the preferred embodiments of the invention have been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the invention embodiments be limited not by this detailed description but rather by the claims appended hereto.