Patent Publication Number: US-2013250243-A1

Title: Retinal imaging device including position-sensitive optical tracking sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is filed under 35 U.S.C. 111(a) as a continuation of International Patent Application No. PCT/US12/068079, filed Dec. 6, 2012, which international application designates the United States and claims the benefit of U.S. Provisional Application Ser. No. 61/568,323 filed Dec. 8, 2011. 
    
    
     BACKGROUND 
     The present disclosure relates to ophthalmic devices and their methods of operation including, for example, retinal imaging systems, fundus cameras, and other types of surgical and non-surgical ophthalmic devices where the human eye is under direct or indirect observation. More specifically, the present disclosure is directed towards improving the manner in which optical alignment can be achieved in such devices, making it easier to acquire clear, high-resolution images that are less subject to vignetting, shadowing, and other types of optical deficiencies. 
     BRIEF SUMMARY 
     According to the subject matter of the present disclosure, optical systems and methods for tracking the pupil of a patient and automatically aligning the illumination and imaging optics of a retinal imaging device to the pupil are provided. Such systems and methods can be employed using relatively low cost, non image-forming optical tracking sensors and can be utilized to achieve optimum image acquisition operations. 
     In accordance with one embodiment of the present disclosure, a retinal imaging device is provided comprising an optical stage, one or more off-axis illumination sources, a field-limited optical system, a position-sensitive optical tracking sensor, a retinal image sensor, and a tracking controller. The off-axis illumination sources are configured to direct an illumination beam onto a cornea of an eye under examination where the illumination beam undergoes both specular and diffuse reflection. The field-limited optical system defines a detection envelope θ and primary optical axis extending from a cornea of an eye under examination through the detection envelope of the field-limited optical system. The off-axis illumination sources are displaced from the primary optical axis by a displacement angle ω that exceeds the angle of the detection envelope θ. The extent to which the displacement angle ω exceeds the angle of the detection envelope θ is sufficient to exclude a majority of specular reflections of the illumination beam from a cornea of an eye under examination and to include a significant portion of the diffuse reflections of the illumination beam from a cornea of an eye under examination. The field-limited optical system is configured to direct diffuse reflections included in the detection envelope θ to an input face of the position-sensitive optical tracking sensor and the tracking controller is configured to utilize an intensity distribution signal from the position-sensitive optical tracking sensor to control an optical alignment function of the optical stage, relative to a cornea of an eye under examination. 
     In another embodiment of the present disclosure, a retinal imaging device is provided comprising an optical stage, one or more illumination sources, an optical system, a position-sensitive optical tracking sensor, a retinal image sensor, and a tracking controller. The illumination sources are configured to direct an illumination beam onto a cornea of an eye under examination where the illumination beam undergoes both specular and diffuse reflection. The position-sensitive optical tracking sensor comprises a non-image forming sensor configured to generate a signal indicative of the relative positioning of relatively low and high intensity portions of an optical signal incident on the sensor, in at least two dimensions. The optical system is configured to direct diffuse reflections from a cornea of an eye under examination to an input face of the position-sensitive optical tracking sensor and the tracking controller is configured to utilize an intensity distribution signal from the position-sensitive optical tracking sensor to control an optical alignment function of the optical stage, relative to a cornea of an eye under examination. 
     Although the concepts of the present disclosure are described herein with primary reference to an improved retinal imaging device that includes a cost effective, optical hardware-based automatic pupil tracking and instrument alignment apparatus, it is contemplated that the concepts will enjoy applicability to any ophthalmic device where the human eye is under direct or indirect observation. For example, and not by way of limitation, it is contemplated that the concepts of the present disclosure will enjoy applicability to handheld, portable retinal imaging devices and, more generally, to retinal imaging systems, fundus cameras, auto-refractors, corneal topographers, scanning laser ophthalmoscopes, optical coherence tomographers, direct ophthalmoscopes, etc. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The following detailed description of specific embodiments of the present disclosure can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
         FIG. 1  is a schematic representation of a retinal imaging device with an optical hardware-based pupil tracking and instrument alignment apparatus as described by the present disclosure; 
         FIG. 2  is a schematic illustration of the iris and cornea of the eye; 
         FIG. 3  illustrates the specularly reflective nature of the cornea and the diffusely reflective nature of the iris; 
         FIG. 4  illustrates the detection envelope of the retinal imaging device of  FIG. 1 ; 
         FIGS. 5 and 6  illustrate off-axis illumination sources according to embodiments of the present disclosure; and 
         FIGS. 7A ,  7 B and  7 C illustrate the manner in which a linear array sensor can be utilized to generate a signal indicative of the relative positioning of relatively low and high intensity portions of an optical signal incident on the sensor, in at least two dimensions. 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIGS. 1 and 2 , it is noted that a retinal imaging device  300  can be aligned coarsely with an eye  100  to be imaged via mechanical positioning of a fixed optical stage  200 . The fixed optical stage  200  may be the handle or grip area of a handheld retinal camera or other imaging device. Alternatively, the fixed optical stage  200  could be the mechanical attachment point to a standard xyz joystick positioning device, as is often utilized in fixed-station fundus cameras and other conventional ophthalmic instrumentation, including retinal imaging devices or fundus cameras. 
     One or more off-axis illumination sources  110  can be configured optically, mechanically, and electrically to generate an intensity profile, which may be uniform or non-uniform and is directed as an illumination beam  112  onto the cornea  102  of the eye  100 . At the eye  100 , the illumination beam  112  selectively undergoes both specular and diffuse reflection as indicated in  FIG. 3 . In areas of the cornea  102  that are backed by the pupil  106  as opposed to the iris  105 , individual illumination rays primarily transmit through the cornea interface, which is substantially clear relative to areas of the cornea  102  that are backed by the iris  105 . A transmitted beam  113  travels through the optical media of the inner eye—the direction of transfer determined by the laws of refraction. Reflected rays form reflected illumination beams  114 . The direction of travel of these individual reflected rays, which are referred to herein as specular reflections, is governed by the Law of Reflection. The magnitude of these specular reflections is a fraction of the magnitude of the original incident rays of the illumination beam  112 —the exact value of which can be determined by Fresnel&#39;s Equation which governs the interaction of electromagnetic waves at the interface of dielectric materials. In areas of the cornea  102  that are backed by the iris  105 , transmitted rays become incident upon the surface of the iris  105  where diffuse reflectance occurs. In diffuse reflectance, incident rays possessing unique directions of travel are reflected into a broad range or distribution of directions of travel and also form a reflected illumination beam  114 . These reflected rays are referred to herein as diffuse reflections. These two types of reflections, namely, specular and diffuse reflections, are illustrated schematically in  FIG. 3 . 
     According to particular embodiments of the present disclosure, the primary ophthalmic lens  120 , the beamsplitter  130 , and the focusing lens  140  collectively define a field-limited optical system that is configured to exclude a majority of the specular reflections of the illumination beam  112 , i.e., those portions of the reflected illumination beam  114  that originate solely from areas of the cornea  102  that are not backed by the iris  105  or another diffuse reflecting background material, and to include a substantial portion of the diffuse reflections of the illumination beam  112 , i.e., those portions of the reflected illumination beam  114  that originate from areas of the cornea  102  that are backed by the iris  105  or another diffuse reflecting background material. As such, the field-limited optical system of  FIG. 1  can be designed such that the illumination beam  112  and the reflected illumination beam  114 , as defined and constrained by the off-axis illumination sources  110 , the primary ophthalmic lens  120 , the beamsplitter  130 , and the focusing lens  140 , allow the formation of a very unambiguous optical representation of the iris  105  and pupil  106  of the eye  100 . More specifically, a relatively small portion of reflected rays originating from the area of the cornea  102  that is backed by the pupil  106  will be reflected back in directions that fall within the detection envelope of the field-limited optical system. Accordingly, the optical intensity corresponding to the pupil  106 , when processed by the focusing lens  140  with support from the primary lens  120  and beamsplitter  130 , will have a relatively low intensity (relatively dark). By contrast, in areas of the cornea  102  that are backed by the iris  105 , diffuse reflection ensures that a relatively large portion of reflected rays originating from the iris  105  will fall within the detection envelope of the field-limited optical system. Therefore, the optical intensity corresponding to the iris  105  will have a relatively high intensity (relatively bright). 
     The present inventors have further recognized that, in the near-IR region of the electromagnetic spectrum, e.g., between 700 nm and 1100 nm, the wavelength-specific absorption behavior of typical iris pigments is minimized. Accordingly, in the near-IR, the reflectivity of the respective irises of most patients will be very similar and it is contemplated that near-IR sources will be particularly well-suited for use with the field limited optical system described above to make the generally circular iris  105  a very robust target to track using a relatively simple position-sensitive optical tracking sensor  150 . 
     In the embodiment as indicated in  FIG. 1 , the off-axis illumination sources  110  are configured as two or more discrete elements located peripheral to the primary lens  120 . Any number of off-axis illumination sources  110 , including a single device, can be envisioned to be suitable in creating the illumination beam  112 . Although in the illustrated embodiments, the off-axis illumination sources  110  appear as a pair of off-axis sources  110  (see  FIG. 1 ) or a circular array of off-axis sources  110  (see  FIG. 5 ), one advantageous implementation takes the form of a substantially continuous ring of light  110 ′ that extends unbroken around the complete periphery of the central primary lens  120  (see  FIG. 6 ). 
     Referring to  FIG. 4 , more important than the number of off-axis illumination sources  110  deployed is their geometric placement relative to the other optical elements such as the primary lens  120 , beamsplitter  130 , and focusing lens  140 . As is noted above, to create the advantageous discrimination between the pupil  106  and the iris  105 , it is often advantageous to position the off-axis illumination sources  110  in positions which place any and all specular reflections occurring at the surface of the cornea outside the detection envelope θ of the field-limited optical system. This detection envelope θ is illustrated schematically in  FIG. 4  as corresponding to the input acceptance angle of the lens  120  relative to the centroid of the corneal surface of the eye  100 . In  FIG. 4 , the off-axis illumination sources  110  are displaced from the primary optical axis  125  by a displacement angle ω that exceeds the angle of the detection envelope θ. In this way, the binary dark/light intensity representation of the circular pupil  106  against the iris  105  is maintained within the reflected illumination beam  114 . 
     In practice, care should be taken to ensure that the displacement angle ω exceeds the angle of the detection envelope θ by an amount that is sufficient to keep a majority of the specular reflections from the surface of the cornea  102  from falling within the detection envelope θ and achieve sufficient contrast in the dark/light intensity representation within the reflected illumination beam  114 . Conversely, the degree to which the displacement angle ω exceeds the angle of the detection envelope θ cannot be so large as to exclude a significant portion of the diffuse reflections of the illumination beam  112 , i.e., those originating from areas of the cornea  102  that are backed by the iris  105  or another diffuse reflecting background material, from the detection envelope θ. Although the detection envelope θ is illustrated in  FIG. 4  as being defined by the primary lens  120 , it could alternatively be defined by one or more other optical constraints in the field-limited optical system of the present disclosure. For example, and not by way of limitation, the detection envelope θ could be defined by the primary ophthalmic lens  120 , the beamsplitter  130 , the focusing lens  140 , the position-sensitive optical tracking sensor  150 , or combinations thereof. 
     In one embodiment of the retinal imaging device  300 , near-IR LEDs are used to implement the off-axis illumination sources  110 . There are commercially-available near-IR LEDs available that emit at several different wavelengths. These near-IR LEDs are offered in a variety of different types of both standard and custom optomechanical packages. Near-IR LEDs are robust and are generally easy to spatially-deploy. Additionally, they operate using low voltage DC power. Although LEDs are described as an optimum choice, other off-axis illumination sources  110  could also be used within the retinal imaging device  300  within the spirit of this disclosure. These alternate illumination sources include visible light LEDs, lamps such as halogen, metal halide, and xenon, as well as fiber optic-coupled lamps or LED sources. 
     As the reflected transmission beam  114  moves away from the eye  100  and in the direction of the retinal imaging device  300 , it first encounters the primary lens  120 . In the illustrated embodiment, the pupil tracking apparatus is implemented co-linear with the retinal imaging optics. In  FIG. 1 , the retinal imaging optics are schematically indicated by the presence of the primary lens  120 , a retinal imaging lens  160 , a focus coupler  220 , and an image sensor  170 . The operation of retina illumination and imaging optics within fundus cameras is well known in the art. As such, details related to the retinal image forming portion of the retinal imaging device  300  are, for the most part, omitted from this discussion. 
     The primary lens  120  can be optimized to generate an indirect image of the retina surface  107  somewhere between the primary lens  120  and the retina imaging lens  160 . This indirect image is then relayed onto the image sensor  170  by the retina imaging lens  160 . In the embodiment shown in  FIG. 1 , a beamsplitter  130  is used to re-direct the reflected illumination beam  114  away from the main optical pathway used for retinal imaging (retina illumination and imaging beam pathways omitted for clarity). Beamsplitters  130  as indicated in  FIG. 1  are well known in the art. These devices are typically designed to allow a portion of the incident irradiation to pass through while reflecting, minor-like, the remainder of the electromagnetic radiation. Of particular applicability to the present disclosure are beamsplitters of the type that selectively transmit or reflect irradiation based on wavelength. These types of beamsplitters, known as dichroic beamsplitters, can be configured to transmit at high-efficiencies light irradiation up to a design transition wavelength while reflecting longer wavelength irradiation. It is contemplated that operation of the basic retina imaging function is advantageously performed with visible and near-IR illumination out to about 850 nm. Efficient LEDs exist that emit electromagnetic radiation out to 940 nm and beyond. In one contemplated embodiment, the off-axis illumination sources  110  are 940 nm LEDs and the beamsplitter  130  is designed to transition from transmission to reflection somewhere around 900 nm. Implemented in this way, the functions of pupil tracking and retina imaging are cleanly split from the retinal rays at the beamsplitter  130 . 
     After reflecting off of beamsplitter  130 , the reflected illumination beam  114  is brought to a focus by the focusing lens  140 . Focusing lens  140  works in combination with the optical power applied to the illumination beam  114  by the primary lens  120  to bring a relatively high-contrast intensity distribution representing areas of the eye corresponding to the pupil  106  and the iris  105  into focus onto the active surface of the position-sensitive optical tracking sensor  150 . Suitable tracking sensors  150  include, but are not limited to, linear array sensors such as the S5668 series 16-element Si photodiode linear array available from Hamamatsu Photonics K.K., quadrant sensors such as a low dark current quadrant photodiode available from Pacific Silicon Sensor, Inc, or any other type of position-sensitive optical sensor that can be used to generate a signal that indicates the relative positioning of relatively low and high intensity portions of an optical signal incident on the sensor, in at least two dimensions. 
     Regardless of the type of position-sensitive optical tracking sensor  150  is used, the electrical signals that are generated by the position-sensitive optical tracking sensor  150  can be communicated to a tracking controller  210 , which is in communication with an alignment actuator  190  coupled to the optical stage  200 . The tracking controller  210  can consist of, in part or in whole, analog amplifiers suitably configured to provide the appropriate sum, difference, comparison, and other signals indicative of the intensity profile at the tracking sensor  150 . Additionally, the controller  210  could include a variety of other simple electronic components including mixed-signal and discrete electrical components, programmable logic devices, microcontrollers, microprocessors, power amplifiers, and motor control circuits. All of these components and their application in actuator control circuits and assemblies are well documented in the art. The output of the controller  210  comprises electrical signals that are suited to drive the specific type of actuators contained within the alignment actuator  190 . 
     According to embodiments of the present disclosure that utilize a tracking sensor that produces a one-dimensional intensity profile, as is the case with the linear array sensor  150  illustrated in  FIGS. 7A-7C , it is contemplated that a pupil tracking optical system can be configured to generate a signal that indicates the position on an input face of the sensor  150  of relatively low and high intensity portions of an optical signal incident on the sensor  150 , in two dimensions. More specifically, the linear array sensor array  150  comprises a linear array of sensor elements  152  and the off-axis positioning of the illumination sources creates a beam spot characterized by a unique one-dimensional intensity profile I. This intensity profile I is derived from the off-axis configuration of the illumination sources  110  and from the optical characteristics of the cornea  102  and underlying iris  105  and includes a relatively low intensity portion  154  that is surrounded, or at least bounded on one or more sides, by a relatively high intensity portion  156 . 
     As is illustrated schematically in  FIGS. 7A-7C , the tracking controller  210  can be programmed to implement a relatively simple processing scheme to provide an indication of the centerpoint of the intensity profile I and control an alignment actuator  190  of the optical stage  200  to effect movement of the profile centerpoint along the linear array  150  of sensor elements  152  until the profile centerpoint reaches an “aligned” position. This transition to a first aligned position is illustrated as the intensity profile I moves from an unaligned position in  FIG. 7A  to the aligned position of  FIG. 7B , where the system optics are aligned, in one dimension, with the pupil  106  under examination. Once the beam spot is tracked to a target location on the sensor array  150  in one dimension, i.e., a location corresponding to the center of the pupil  106 , tracking control can be shifted to adjust the position of the beam spot in a second dimension, i.e., transversely across the linear array  150  of sensor elements  152 . This adjustment will typically be along an axis that is perpendicular to the linear axis of the sensor array  150  and is illustrated schematically in  FIG. 7C . Again, the tracking controller  210  can be programmed to implement a relatively simple processing scheme to provide an indication of the transverse centerpoint of the intensity profile I and control the alignment actuator  190  of the optical stage  200  to effect movement of the transverse centerpoint across the linear array  150  of sensor elements  152  until the profile centerpoint reaches an “aligned” position, where the system optics are aligned, in a second dimension, with the pupil  106  under examination. Once the beam spot is tracked to a target location along this additional axis, i.e., a location corresponding to the center of the pupil along a second dimension, dual axis alignment of the optical system is achieved. 
     According to embodiments of the present disclosure that utilize a tracking sensor that produces a two-dimensional intensity profile, as is the case with a quadrant array sensor comprising at least four sensor elements arranged symmetrically about a common sensor centroid, it is contemplated that the tracking controller  210  can be programmed to utilize signals indicative of the symmetry of the intensity profile across the sensor elements to control the alignment actuator  190  of the optical stage  200  to affect movement of a profile centroid towards the sensor centroid and align the optical system with the pupil under examination. For example, a relatively simple processing scheme of summing the signal coming from the individual detector quadrants while at the same time calculating the difference in the signal generated from two opposed detector elements can be a very robust method of generating appropriate 2-axis alignment control signals. 
     Generally, the alignment actuator  190  would be configured to move in at least two spatial dimensions as referenced to the fixed optical stage  200 , either independently or simultaneously. The alignment actuator  190  is used to respond to pupil tracking information provided by the position-sensitive discrete optical sensor arrangement  150  and controller  210  by physically aligning the optical tube  180  of the retinal imaging system  300  with the pupil  106  and iris  105  of the eye  100 . By doing this automatically, the critical fine alignment of the device is no longer limited by the positioning skills of the operator. By providing automatic closed-loop alignment at response times shorter than typical human eye or hand jitter response times, the technology of the present disclosure facilitates proper operation of the retinal imaging device  300  allowing improved image quality due to improvements in lighting uniformity and image focus actuation. 
     There are many different methods of supplying a suitable alignment actuator  190  that are known in the art. The alignment actuator  190  can generally be configured to provide motion in two or more independent axes. The Cartesian coordinates x and y defined to form a plane that generally is parallel to the iris  105  is one useful manner in which to configure the alignment actuator  190 . Additionally, a third axis, z, of automated motion defined to be generally parallel to the reflected illumination beam  114  is advantageous in providing additional alignment fidelity. Alternately, the alignment actuator  190  could equally be configured to provide tilt and pitch actuation, or in 3 dimensions, tilt, pitch and roll actuation of the optical tube  180  relative to fixed optical stage  200 . 
     Referring to the elements of  FIGS. 1 ,  2 , and  3 , a contemplated method of providing an automated pupil tracking and instrument alignment function in support of the general operation of an improved retinal imaging device includes the following steps, which may be taken in succession:
         (1) Coarse position the retinal imaging device  300  relative to the eye  100 ;   (2) Illuminate the complete area of the pupil  106  and iris  105  with one or more off-axis illumination sources  110 ;   (3) Receive the reflected illumination beam  114 ;   (4) Focus the reflected illumination beam  114  onto the active surface of a position-sensitive optical tracking sensor  150  via the focusing lens  140 ;   (5) Communicate the output of the position-sensitive optical tracking sensor  150  to a processing unit  210 ;   (6) Calculate the motion control drive signals required to keep retinal imager  300  properly aligned on the centroid of the pupil  106  or iris of the eye  100 ;   (7) Communicate motion control drive signals to the alignment actuator  190 ; and   (8) Automatically enact the necessary motion to align the optical tube  180  and retinal imaging device  300  to the eye  100 .
 
The aforementioned steps may be taken in succession or may be condensed or expanded without departing from the scope of the present disclosure.
       

     It is noted that recitations herein of “at least one” component, element, etc., should not be used to create an inference that the alternative use of the articles “a” or “an” should be limited to a single component, element, etc. 
     It is noted that recitations herein of a component of the present disclosure being “configured” in a particular way, to embody a particular property, or to function in a particular manner, are structural recitations, as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “configured” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component. 
     It is noted that terms like “preferably,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to identify particular aspects of an embodiment of the present disclosure or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment of the present disclosure. 
     For the purposes of describing and defining the present invention it is noted that the terms “substantially” and “approximately” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terms “substantially” and “approximately” are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
     Having described the subject matter of the present disclosure in detail and by reference to specific embodiments thereof, it is noted that the various details disclosed herein should not be taken to imply that these details relate to elements that are essential components of the various embodiments described herein, even in cases where a particular element is illustrated in each of the drawings that accompany the present description. Rather, the claims appended hereto should be taken as the sole representation of the breadth of the present disclosure and the corresponding scope of the various embodiments described herein. Further, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present disclosure are identified herein as preferred or particularly advantageous, it is contemplated that the present disclosure is not necessarily limited to these aspects. 
     It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the present invention, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.”