WO2000021475A1

WO2000021475A1 - Laser system with projected reference pattern

Info

Publication number: WO2000021475A1
Application number: PCT/US1999/023851
Authority: WO
Inventors: Paul R. Yoder, Jr.; William B. Telfair
Original assignee: Irvision, Inc.
Priority date: 1998-10-14
Filing date: 1999-10-14
Publication date: 2000-04-20
Also published as: AU1203600A

Abstract

An apparatus for projecting a pattern into the field of view of an optical system, such as a microscope, comprising a laser for generating a laser beam, projection optics for generating an optical beam carrying the reference pattern, injection optics for combining the optical beam with the laser beam so that both beams propagate in the same direction; and reflection optics for receiving the optical beam and the laser beam and for reflecting the optical beam towards the optical system in order to cause the reference pattern to be visible in the field of view of the optical system. Such an apparatus may be used in conjunction with devices which track and compensate for movement of an object. One application of the instant apparatus pertains to the projection of a pattern for use as an optical alignment reference during eye surgery. In particular, the instant apparatus provides compensation for involuntary eye movements which may misalign the eye with respect to the reference pattern and with respect to the treatment laser beam. A method for projecting a pattern into the field of view of an optical system is also disclosed.

Description

LASER SYSTEM WITH PROJECTED REFERENCE PATTERN

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending application Serial No.

08/969,128, filed on November 11, 1997, which is a continuation-in-part of application Serial No. 08/549,385, filed on October 27, 1995, now U.S. Patent No. 5,785,822.

BACKGROUND OF THE INVENTION

The present disclosure relates generally to an apparatus and method for projecting a pattern into the field of view of an optical system, such as a microscope. Such an apparatus and method also may be used in conjunction with devices which track and compensate for movement of an object. One particular application involves the projection of a pattern for use as an optical alignment reference during eye surgery.

Surgical procedures are known which aim to correct refractive disorders of a human eye through ablation of the cornea of the eye using laser radiation. Such procedures include

Photorefractive Keratectomy (PRK), Phototherapeutic Keratectomy (PTK), and Laser In Situ Keratomileusis (LASIK). Typically, according to these procedures, laser pulses are scanned in sequence over centralized circular areas of the cornea to cause localized tissue ablation (what may be called "scanning laser" ablation) or are used to simultaneously irradiate similar centralized circular areas of the cornea (commonly referred to as "wide area ablation"). The treated areas are typically between 6 and 9 mm in diameter.

Typically, the above-referenced well known scanning techniques for corneal sculpting involve rapidly moving a relatively small spot of laser radiation over a specific central portion of the corneal surface in a predefined pattern. This allows selective removal of tissue at various points within the scanned region, thereby cumulatively re-shaping the surface of the cornea into the desired geometry in a predictable fashion.

A problem which has plagued the an is that, during corneal refractive surgery, the eye which is receiving the laser pulses is subject to various involuntary and voluntary movements. The movements of the eye vary in type and in degree and may even occur simultaneously. For example, one type of involuntary eye movement is known as a "saccade". Saccades generally involve rapid eyeball rotations of up to 600 deg/sec and occur typically on a 10-30 msec time scale with amplitudes ranging from 1 to 10 degrees. See Bahill et al. Invest. Ophthalm. Vis. Sci., 21 , 1 16, 1981. A second type of involuntary eye movement involves tremors. Tremors may occur at rates of 10 to 200 Hz and with amplitudes on the order of 0.5 arc min. See Carpenter, Movements of the Eyes, 2^nc* ed., 1988 and Findlay, "Frequency Analysis of Human Involuntary Eye Movement," Kybernetik, 8, 207, 1971. Another type of involuntary eye movement involves drifts which can occur at velocities of about 4 arc min/sec and with significantly larger amplitudes than tremors. See Ditchburn, Eye Movements and Visual Perception, 1973. Studies of eye movements, such as one reported by Bahil et al (referenced above), indicate that extremely high accelerations of up to 40,000 deg/sec² may be involved in the fastest movements.

Eye movements often lead to misalignments, i.e., decentrations, of all or portions of the ablated region on the comea. Decentrations are particularly harmful in the above mentioned surgical procedures since they may result in irregular astigmatism, glare phenomena, decreased visual acuity and lower contrast sensitivity. Such eye movements cause uneven distribution of tissue ablation patterns and must be minimized in order to achieve requisite surface smoothness. Implementation of such improved means for suppressing eye motion, while important in wide area ablation, is especially important in scanning laser delivery systems, which require precise execution of specific scanning algorithms, and spot placement accuracy on the order of 5 to 50 μm. It is standard practice during comeal laser surgery for the patient's head to be securely restrained so movements of the eye being treated result only from roll of the eyeball within its socket. These movements cause the center of the comea to shift position in the vertical and/or horizontal directions, usually by no more than 5 mm.

In some prior art apparatus for comeal surgery, the eyeball itself is further immobilized by clamping, suction rings or other means, such as stitching the eye to an eyelid retractor (called a speculum), such as that disclosed in U.S. Patent No. 5,556,417 to Scher. However, even this further immobilization of the eye is not completely effective in suppressing all involuntary eye movements. These physical constraints also may be uncomfortable for the patient and may lead to infection, as in the case where invasive techniques such as stitching are used. The availability of a technique for tracking movements of the eye and compensating therefor such as is disclosed in Parent Application Serial No. 08/969,128 eliminates the need for immobilization of the eye during laser surgery.

Most apparatus for performing PRK, PTK, and LASIK laser surgery on the human comea employ a microscope as the primary means for the surgeon to observe the patient's eye during phases of the surgery. The microscope also is used as an alignment reference when centering the eye to the proper location with respect to the laser beam. In the latter mode, visual reference marks, such as crosshairs or a pattern of concentric circles, typically are provided within the field of view of one eyepiece of the microscope to indicate where the center of the comea should be located. A glass plate, called a reticle, with the appropriate paπem etched or photographically reproduced on one surface thereof is located at the focal plane of the eyepiece to provide this reference. These marks then appear as black (i.e., opaque) lines superimposed on the image of the eye. For example, as shown in Figure 1 , a simple crosshair pattern 20a may be used. In this figure, the pattern is centered to the pupil 21 and limbus 22. The limbus is the approximately circular boundary between the colored iris 23 and the white sclera 24. Note that the iris and limbus may not be precisely centered with respect to each other in human eyes. Reference numbers 28 and 29 indicate the upper eye lid and the lower eye lid. respectively.

If. during the surgical procedure, the patient's eye moves for any reason, such as described above, the eye is seen by the surgeon to be decentered. The surgeon must then move the patient's eye back into alignment with the reference pattern. This is, in most cases, done by commanding the patient's motorized chair to move in the direction that restores proper alignment. Should this corrective action not be taken, the ablated areas on the comea produced before and after eye movement will not be superimposed.

Some tasks that the surgeon must perform in connection with the surgery require very close examination of the surgical site. These tasks are done with the microscope set at high magnification. Ideally, the field of view presented to the surgeon should be unobscured so details of the eye are not hidden. An opaque reticle pattern would, in such cases, be undesirable. Since the conventional reticle cannot easily be removed when not needed or wanted, at least one manufacturer of laser surgical equipment VISX, Incorporated (Santa Clara, CA) has developed and incorporated an apparatus for projecting the reticle pattern into the microscope field to provide the needed reference pattern as a bright-line pattern superimposed on the visual image of the eye.

Figure 2 shows a general representation similar to the VISX apparatus, which projects a bright-line pattern into the field of a microscope. A laser 1 emits a laser beam 2 which is focused by a lens 3 as beam 2a and reflected by a beam combiner plate 4 as beam 2b onto the cornea 5 of the patient's eye 19. The beam combiner plate 4 is located below the microscope objective 12 to provide the means for reflecting the treatment laser beam 2b onto the patient's comea 5 while allowing that comea to be observed at all times through the beam combiner plate 4. A reference pattern beam 1 la is generated by projection optics 6 comprising an opaque mask 9 with transparent lines which is back-illuminated by a light source 7 and imaged by a lens 10. Condensing lens 8 images the source 7 at a large distance. The reference pattern beam 1 la is reflected by the beam combiner plate 4 as beam 1 l b into the objective 12 of the microscope 13 to generate the bright-line image observed by the surgeon. It is noted that this general type of optical system for projecting a pattern into the field of an optical instrument is frequently used in military applications for direction of weapon fire or for displaying information to the user. In aircraft applications, such systems are termed "heads-up displays."

Newer types of surgical apparatus intended for performing the above-mentioned surgical procedures on the comea, such as that disclosed in Parent Application Serial No. 08/969,128, filed on November 1 1 , 1997, include means for tracking the patient's eye and compensating for involuntary motions of the eye by deflecting the treatment laser beam so that it remains centered with respect to the comea. However, when observed through the microscope, the eye still appears to move with respect to a reference pattern because the reference pattern is fixed - - i.e., it does not move with respect to the microscope. Although the surgeon may not need to realign the eye, since the eye really is still aligned to the laser beam, a misaligned reference pattern can be disconcerting.

The above described systems do not enable compensation for involuntary eye movements which may misalign the eye with respect to a reference pattern and the treatment laser beam. Moreover, the above prior art systems do not enable the observed reference pattern to remain centered with respect to the actual location of the eye. As will become apparent, the instant apparatus and method represents a substantial improvement in this regard.

SUMMARY OF THE INVENTION

One embodiment of the instant disclosure pertains to an apparatus for projecting a reference pattern into the field of view of an optical system comprising a laser for generating a laser beam; projection optics for generating an optical beam carrying the reference pattern; injection optics for combining the optical beam with the laser beam so that both beams propagate in the same direction; and reflection optics for receiving the optical beam and the laser beam and for reflecting the optical beam towards the optical system in order to cause the reference pattern to be visible in the field of view of the optical system.

Another embodiment of the instant disclosure pertains to an apparatus for performing a surgical procedure by directing a laser beam upon an eye of a patient using a mirror, wherein the eye has a feature and a visual axis associated therewith, wherein the feature is illuminated with ambient light, wherein the laser beam has an optical axis associated therewith, wherein a surgeon uses a microscope having a field of view associated therewith to view the eye during the surgical procedure, the apparatus comprising a laser for generating the laser beam; projection optics for generating an optical beam carrying a reference pattern, wherein the optical beam is associated with the optical axis; injection optics for combining the optical beam with the laser beam so that both beams propagate in the same direction; reflection optics for receiving the optical beam and the laser beam and for reflecting the optical beam towards the microscope in order to cause the reference pattern to be visible in the field of view of the microscope; means for compensating for movement of the eye during the surgical procedure, comprising: illumination means for illuminating at least the feature of the eye with a tracking light; detection means for detecting an image of the feature and for outputting signals corresponding to movement of the eye, wherein the signals have a first component due to the tracking light and a second component due to the ambient light; filter means for filtering the second component from the signals and for outputting the first component of the signals so that the tracking light is discriminated from the ambient light; logic means for receiving the filtered signals and for generating tracking signals based thereon; and means for directing the laser beam and optical beam upon the eye based on the tracking signals to maintain a substantially centered condition between the optical axis of the laser beam and the optical beam, and the visual axis of the eye.

A further embodiment of the instant disclosure pertains to a method for projecting a reference pattern into the field of view of an optical system comprising combining an optical beam carrying the reference pattern with a laser beam; and reflecting the optical beam _towards the optical system to cause the reference pattern to be visible in the field of view of the optical system.

Yet a further embodiment of the instant disclosure pertains to a method for projecting a reference pattern into the field of view of an optical system, comprising injecting an optical beam carrying the reference pattern into the path of a laser beam; deflecting the optical beam carrying the reference pattern and the laser beam to maintain alignment of the beams with a patient's eye that is moving; and reflecting the laser beam and a first portion of the optical beam carrying the reference pattern towards the patient's eye.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGURE 1 is a schematic of a patient's eye with a superimposed dark-line reference (crosshair) pattern from a glass reticle in the microscope eyepiece as found in the prior art.

FIGURE 2 is a schematic which depicts a prior art apparatus for projecting a bright line reticle pattern into the microscope field.

FIGURE 3 is a schematic of one embodiment of an apparatus according to the instant application.

FIGURE 4 illustrates one embodiment of the pattern projected into the microscope according to the instant application.

FIGURE 5 is a schematic of a second embodiment of an apparatus according to the instant application which includes an optional eye tracking apparatus.

FIGURE 6 is a schematic of an image of a patient's eye formed on the detector elements of an eye tracking system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of an apparatus for projecting a reference pattern into the field of view of an optical system is shown in Figure 3. Here, the optical system is a microscope; however, other optical systems such as telescopes may be used in applications not involving surgery on the eye. This apparatus provides a bright-line ring pattern 20b as depicted in Figure 4 that is centered on the laser pattern, even when the eye moves within the microscope field. Of course, other reference patterns may be used as desired.

Figure 4 illustrates the pattern projected into the optical system of the apparatus of Figure 3. This pattern is shown as black lines rather than bright lines for pictorial clarity. All lines are produced at substantially the same scale as the patient's comea. The pattern consists of three circles of radii 4, 6, and 9 mm with four radially-oriented straight-line marks at 90 degree intervals and shorter straight-line marks at 15 degree intervals above the horizontal centerline. The straight line marks may be used as guides for the surgeon to perform astigmatic refractive correction using the laser apparatus. In one embodiment, the lengths of the straight line marks may be designed to assist the surgeon in addition to providing the reference pattern. In particular, the shorter lines end at an outer diameter of 12 mm, while the longer lines end at an outer diameter of 14 mm. The pattern thus provides size cues for the surgeon to check the actual diameter of the patient's limbus prior to initiating the surgical procedure. The diameters of ablated areas also may be judged by comparison to these markings during or after completion of the procedure. Of course, the radii dimensions and lengths of the straight line marks may be designed as desired.

Referring to Figure 3, a laser 1 emits a laser beam 2a that is directed towards a beam combiner 15. The precise nature of laser beam 2a is not important to this invention. Preferably, however, the laser beam 2a is a mid-infrared treatment laser beam having a wavelength of about 2.94 μm. In one embodiment, the beam combiner 15 is a plane-parallel plate of material, such as calcium fluoride, that efficiently transmits the laser radiation. The beam combiner plate is tilted at an angle of incidence to the treatment laser beam. Typically, this angle may be 45 degrees. A thin film coating on one face of the beam combiner plate (typically the exit face) transmits the laser radiation efficiently while reflecting visible and/or near-infrared radiation efficiently. Such a coating is referred to as dichroic, i.e., having different reflection transmission characteristics at different wavelengths. In another embodiment, the beam combiner may be a glass cube. The cube may compπse two right-angle prisms cemented at their hypotenuse surfaces. Similar to the beam combiner plate mentioned above, one of the surfaces of the prisms may also bear a dichroic coating.

The entrance face of the beam combiner plate 1 may be coated with an anti-reflection thin film coating optimized for the selected laser wavelength and the selected angle of incidence in order to maximize transmission of the laser beam 2a. If the laser beam is linearly polarized, it may be possible to orient the beam combiner plate 15 at Brewster's angle with respect to the input laser beam so as not to need an anti-reflection film on the entrance face. In such a case, the polarized beam is transmitted without loss due to reflection. As is well known, Brewster's angle is defined as the angle of incidence equal to arctangent n, wherein n is the refractive index for the plate material at the wavelength of interest. In the case of calcium fluoride, the index is approximately 1.418 at a wavelength of 2.94 μm so Brewster's angle is approximately 54.8 degrees. The latter case is a preferred embodiment since the cost of an additional coating is eliminated.

An optical beam 11 a carrying an image of the reference pattern 20b is generated by projection optics 6. In one embodiment, projection optics 6 includes an opaque mask 9 with transparent lines that is back-illuminated by a light source 7 and imaged by lens 10. The mask may be made of high-contrast photographic film of a type generally used for lithographic purposes or the mask may be made of glass. In the former case, the reference pattern 20b is reproduced on the film by photographing a large hand- or computer-drawn master drawing as a reduced-size negative image at the proper scale for use in the surgical application. In the latter case, the pattern may be reproduced on a photoresist-coated glass substrate that has previously been coated with a metallic film. The photographic exposure records the image as a reduced-size negative pattern due to interaction of light with the photoresist. After exposure, the photoresist is removed chemically from the exposed regions and the metallic film etched away leaving transparent lines in the otherwise opaque metallic coating. This pattern would be a reduced size copy of the master drawing at the proper scale for use in the surgical application.

In one embodiment, the light source 7 may be a tungsten halogen lamp operating as a black body at a temperature of about 3500 K. This lamp may be located in relation to a condensing lens system 8 that re-images the lamp filament at lens 3. Preferably, the light source 7 may be located at a distance from the lens 8 so as to reduce heating of the optics and other components of the apparatus. Light from this lamp then may be transferred through a plastic or glass fiber bundle, such as Model BGT1826 manufactured by Dolan-Jenner Industries (Lawrence, MA) to the same location as occupied by the lamp filament in Figure 3.

As indicated in Figure 3, the optical beam 1 la is injected into the path of the laser beam

2a by reflection from the dichroic coating on beam combiner plate 15 described above. The combined and deflected beams 2c and 1 lc are reimaged by lens 3 towards beam combiner 4. In this embodiment, beam combiner 4 is also a plane parallel plate of glass. The entrance face thereof is coated with a thin film dichroic coating that reflects essentially all of the 2.94 μm wavelength laser radiation of beam 2c towards the patient's comea 5 as beam 2d. Approximately equal portions of the visible radiation of beam 1 lc are transmitted through beam combiner plate 4 as beam I ldl and reflected by combiner 4 as beam 1 ld2. The reflected portion 1 ld2 is directed harmlessly towards the patient's eye 19 while the transmitted portion 1 ldl is directed towards a beam reverser mirror 17. The beam reverser mirror 17 is a glass or metallic mirror with circular aperture that intercepts the aforementioned transmitted beam portion 1 l dl and reflects it back as beam 1 le towards beam combiner 4. This reflected beam 1 le transmits through the substrate of the combiner 4 and partially reflects from the dichroic coating on the first surface thereof as beam 1 I f in the direction of the microscope objective 12. This beam 1 I f carries an image of the reference pattern 20b. which is observed by the surgeon through microscope 13. It is preferred that the beam reverser mirror 17 be located substantially at the same optical distance from the microscope 13 as the co ea 5, so that the reference pattern seen by the surgeon will be in focus and will appear superimposed on the image of the eye. A portion of beam 1 le is transmitted by beam combiner 4 and passes harmlessly back into the projection optics 6.

In a preferred embodiment, the beam reverser mirror 17 is spherically curved so as to retroreflect all incident rays substantially back onto themselves thereby ensuring that the paths of the rays reflected upwards to the microscope 13 can be made to coincide with the extended paths of the same rays of beam 1 ld2 reflected towards the eye. This prevents instrumental error in which the projected image would appear offset laterally with respect to the image of the eye. In the embodiment shown in Figure 3, the beam reverser mirror 17 is concave. In other designs, beam reverser mirror 17 may be flat, while in still others, it may be convex. In yet other embodiments, beam reverser minor 17 may be aspheric, to correct aberration of the microscope exit pupil. The shape of beam reverser mirror 17 is such that the exit pupil of the optical system comprising projection optics 6 and lens 3 which project the reference pattern 20b into the beam combiner plate 4 is reimaged by mirror 17 back onto itself. The shape of beam reverser mirror 17 does not affect the focus of the reference pattern image 20b seen through the microscope, since beam reverser mirror 17 is located at the object plane for the microscope objective lens 12. The axial location of the beam reverser mirror 17 does affect focus of the projected image, so appropriate means for adjusting this mirror's axial location should be provided. One or more flat mirrors can be used to fold the light path between the beam combiner plate 4 and the beam reverser mirror 17 for convenient packaging of the optical system. The second surface of combiner 4 typically is coated with a thin film anti-reflection coating to reduce light losses in beams 1 ldl and 1 le.

Unlike the prior art system shown in Figure 2 in which the laser beam and optical beam propagate from opposite sides of beam combiner 4, the laser beam 2a and optical reference beam 1 la of the instant application propagate from the same direction. This enables an eye tracking system such as the one disclosed in Patent Application Serial No. 08/969, 128 to be used, in order for both the reference pattern and the laser beam to remain centered with respect to the eye.

Figure 5 shows one way in which the instant system can be used with an eye tracking apparatus. As shown in Figure 5, a laser 1 emits a laser beam 2a that is directed towards a first beam combiner plate 15a. An optical beam 1 la carrying an image of the reference pattern 20b is generated by projection optics 6 and is directed by way of a second beam combiner plate 15b towards first beam combiner plate 15a that injects the optical beam 1 la into the path of the laser beam 2a by reflection from the dichroic coating on first beam combiner plate 15a described above. Beam combiner plate 15b serves to transmit optical beam 1 la while reflecting an image of the comea 5 of the eye 19 as beam 26d into an eye tracker 26. The image of the comea is created by illumination of the eye by a tracking light source 1005 which creates light rays 26a which are focused by lens 3 onto the detector elements of the detector 1600 which is part of the eye tracker 26. A portion 1 lg of optical beam 1 la is reflected harmlessly by beam combiner plate 15b.

Following beam combiner plate 15a, the combined laser beam 2b and beam 1 lb are directed towards a pair of motor-driven tracking mirrors 16a and 16b. These mirrors typically are somewhat larger than either beam 2a or 1 1 b and are glass or metal substrates coated with aluminum or silver thin films for high reflectivity. In accordance with the teachings of Parent Application Serial No. 08/969, 128, the mirrors are attached to the shafts of servo-controlled motors that can be driven in closed-loop fashion so as to rotate the mirrors 16a and 16b through small angles thereby causing both beams 2b and 1 l b to angularly deflect in orthogonal directions in consonance with signals 27a and 27b from the aforementioned eye tracker 26 in order that those beams may remain aligned to the patient's eye 19 in spite of lateral or rotational movements of the eye.

After reflecting from mirrors 16a and 16b, the combined and deflected beams 2c and He are reimaged by lens 3 towards beam combiner 4. In this embodiment, beam combiner 4 is also a plane parallel plate of glass. The entrance face thereof is coated with a thin film dichroic coating that reflects essentially all of the 2.94 μm wavelength laser radiation of beam 2c towards the patient's comea 5 as beam 2d. Approximately equal portions of the visible radiation of beam 1 lc are transmitted through beam combiner 4 as beam 1 ldl and reflected by combiner 4 as beam 1 ld2. The reflected portion 1 ld2 is directed harmlessly towards the patient's eye 19 while the transmitted portion 1 ldl is directed towards a beam reverser mirror 17.

The beam reverser mirror 17 is a glass or metallic mirror with circular aperture that intercepts the aforementioned transmitted beam portion 1 ldl and reflects it back as beam l ie towards beam combiner 4. This reflected beam l i e transmits through the substrate of the combiner 4 and partially reflects from the dichroic coating on the first surface thereof as beam 1 If in the direction of the microscope objective 12. This beam 1 I f carries an image of the reference pattern 20b, which is observed by the surgeon through microscope 13.

As shown in Figure 5, the instant system may include an eye tracker 26 for facilitating tracking of the eye 19 as described in detail in the Parent Application Serial No. 08/969,128. In particular, an apparatus is disclosed in this parent application for performing a surgical procedure by directing a laser beam upon an eye of a patient using a mirror, wherein the eye has a feature and a visual axis associated therewith, wherein the feature is illuminated with ambient light, wherein the laser beam has an optical axis associated therewith, and wherein a surgeon uses a microscope having a field of view associated therewith to view the eye during the surgical procedure. The apparatus includes a laser for generating the laser beam: projection optics for generating an optical beam carrying a reference pattern, wherein the optical beam is associated with the optical axis; injection optics for combining the optical beam with the laser beam so that both beams propagate in the same direction; reflection optics for receiving the optical beam and the laser beam and for reflecting the optical beam towards the microscope in order to cause the reference pattern to be visible in the field of view of the microscope; and means for compensating for movement of the eye during the surgical procedure. Means for compensating for movement of the eye includes illumination means for illuminating at least a feature of the eye with a tracking light 1005; detection means for detecting an image of the feature and for outputting signals corresponding to movement of the eye, wherein the signals have a first component due to the tracking light and a second component due to the ambient light; filter means for filtering the second component from the signals and for outputting the first component of the signals so that the tracking light is discriminated from the ambient light; logic means for receiving the filtered signals and for generating tracking signals based thereon; and means for directing the laser beam and optical beam upon the eye based on the tracking signals to maintain a substantially centered condition between the optical axis of the laser beam and the optical beam, and the visual axis of the eye.

Figure 6 shows in detail one embodiment of a detection means including a detector array 1600 comprising a plurality of detector elements 1620A - 1620D. The detector elements 1620A and 1620C are positioned opposite each other on the Y-axis 164. The detector elements 1620B and 1620D are positioned opposite each other on the X-axis 163. The opposing pairs of detector elements 1620A/1620C and 1620B/1620D produce varying electrical output signals as the image of the tracking feature, for example, the limbus 22. moves with respect to the X and Y axes. The electrical output signals are fed into the eye tracker 26. Reference numbers 28 and 29 indicate the upper eye lid and the lower eye lid, respectively. These detector elements can, of course, be oriented in different axes (e.g., along the 45 degree axes) or comprise a number of elements (e.g., 6 or 8 elements instead of 4) to better or more fully detect the position of the tracking feature. The translation of the detector element signals into X-axis or Y-axis components is a simple mathematical function based upon the geometry of the detector element positions.

In yet another embodiment of the instant apparatus, a shutter 18 is present as indicated in Figure 5. This shutter can be activated manually or electrically (if motor driven) so as to open the optical path to the projected beam 1 ldl or to close that path so no light from beam l i e reaches the microscope 13. This ability to positively shut off the beam path is important from the eye safety viewpoint, since a beam from a visible light laser (not shown) may be projected through the optical system during alignment checking. Such a laser beam may be injurious to the eye of any person accidentally looking through the microscope while the alignment laser is turned on. The shutter 18 also serves to block light from an optional fixation target 32 that may be located in the optical path near the microscope objective 12 for purposes of patient cooperative eye immobilization. The portion of the latter light reflected via the beam combiner plate 4 into the beam reverser mirror 17 and thence back by way of beam combiner 4 to the microscope 13 may be detrimental to the surgeon's ability to examine the patient's eye closely as mentioned above.

The apparatus of Figures 3 and 5 may also include a spectral filter 14 located near the mask 9. This filter limits the color (transmitted wavelength) of the projected light beam to the particular transmission characteristic of the filter. Typically an interference filter with a bandpass of <100 nm width at 50% intensity may be used. Functionally similar filters effective at other spectral regions may then be used in the optical trains of sensors such as the eye tracker within the surgical apparatus so as to optically isolate those systems from stray light originating in the projection system. As shown in Figure 5, inclusion of a spectral filter 30 may also be desirable in the light path of the projector between the beam combiner 15c and the beam reverser mirror 17 to eliminate light from illumination sources not shown or fixation targets (such as 32₎ located near the microscope from reflecting back into the field of view of the microscope 13.

The instant apparatus and method may track the patient's eye and compensate for involuntary motions of the comea by deflecting the treatment laser beam so the center of the pattern that the laser beam ablates into the comeal surface is properly centered on the comea. Because the reference pattern beam and laser beam are linked and are affected by the tracking means identically, the instant apparatus and method provide a bright-line reference pattern that appears centered to the eye, even when that eye moves within the microscope field of view. Thus, inadvertent misalignment of the microscope with respect to the laser or to the patient's eye is not likely to introduce errors in location of the treatment pattern on the comea. The surgeon knows that the eye tracker is functioning properly as long as coincidence of eye image and pattern image is observed.

Although the particular embodiments shown and described above will prove to be useful in many applications relating to the arts to which the present invention pertains, further modifications of the present invention herein disclosed will occur to persons skilled in the art.

All such modifications are deemed to be within the scope and spirit of the present invention as defined by the appended claims.

Claims

CLAIMSWhat is claimed is:

1. .An apparatus for projecting a reference pattern into a field of view of an optical system, comprising:

(a) a laser for generating a laser beam;

(b) projection optics for generating an optical beam carrying the reference pattern;

(c) injection optics for combining the optical beam with the laser beam so that both beams propagate in the same direction; and

(d) reflection optics for receiving the optical beam and the laser beam and for reflecting the optical beam towards the optical system in order to cause the reference pattern to be visible in the field of view of the optical system.

.An apparatus for performing a surgical procedure by directing a laser beam upon an eye of a patient using a mirror, wherein the eye has a feature and a visual axis associated therewith, wherein the feature is illuminated with ambient light, wherein the laser beam has an optical axis associated therewith, wherein a surgeon uses a microscope having a field of view associated therewith to view the eye during the surgical procedure, the apparatus comprising:

(a) a laser for generating the laser beam;

(b) projection optics for generating an optical beam carrying a reference pattem, wherein the optical beam is associated with the optical axis;

(c) injection optics for combining the optical beam with the laser beam so that both beams propagate in the same direction;

(d) reflection optics for receiving the optical beam and the laser beam and for reflecting the optical beam towards the microscope in order to cause the reference pattem to be visible in the field of view of the microscope;

(e) means for compensating for movement of the eye during the surgical procedure, comprising:

(i) illumination means for illuminating at least the feature of the eye with a tracking light;

(ii) detection means for detecting an image of the feature and for outputting signals corresponding to movement of the eye, wherein the signals have a first component due to the tracking light and a second component due to the ambient light; (iii) filter means for filtering the second component from the signals and for outputting the first component of the signals so that the tracking light is discriminated from the ambient light;

(iv) logic means for receiving the filtered signals and for generating tracking signals based thereon; and

(v) means for directing the laser beam and optical beam upon the eye based on the tracking signals to maintain a substantially centered condition between the optical axis of the laser beam and the optical beam, and the visual axis of the eye.

3. The apparatus of Claim 2, wherein the filter means comprises means for modulating the illumination means at a predefined frequency and means for demodulating the signals at the predefined frequency.

4. The apparatus according to Claim 1 , wherein the laser beam is a mid-infrared laser beam.

5. The apparatus according to Claim 1, wherein the projection optics comprises

(a) a light source which emits light;

(b) a spectral filter for receiving the light and for permitting light of a selected wavelength to pass; and

(c) a mask having a shape of the desired reference pattern to receive the passed light so as to generate an optical beam carrying the reference pattern.

6. The apparatus according to Claim 1, wherein the projection optics comprises an interference filter having a bandpass of less than approximately 100 nm width at approximately 50% transmitted beam light intensity.

7. The apparatus according to Claim 1 , wherein the injection optics comprises a beam combiner.

8. The apparatus according to Claim 7, wherein the beam combiner comprises a plane parallel glass plate.

9. The apparatus according to Claim 7, wherein the beam combiner comprises a prism.

10. The apparatus according to Claim 1 , wherein the reflection optics comprises a beam combiner.

1 1. The apparatus according to Claim 1 , wherein the reflection optics comprises

(a) a first set of optics for reflecting the laser beam and a first portion of the optical beam toward a comea of a patient's eye while allowing a second portion of the optical beam to pass through the first set of optics, and

(b) a second set of optics for projecting a second portion of the optical beam into the field of view of the optical system.

12. The apparatus according to Claim 1 1 , wherein the first set of optics comprises a beam combiner.

13. The apparatus according to Claim 12, wherein the beam combiner comprises a plane parallel glass plate.

14. The apparatus according to Claim 12, wherein the beam combiner comprises a prism.

15. The apparatus according to Claim 1 1 , wherein the second set of optics comprises a beam reverser mirror which reflects the second portion of the optical beam back towards the first set of optics for projection into the field of view of the optical system.

16. The apparatus according to Claim 15. wherein the beam reverser mirror is located at substantially the same optical distance from the optical system as the patient's eye.

17. The apparatus according to Claim 15, wherein the beam reverser mirror is flat.

18. The apparatus according to Claim 15, wherein the beam reverser mirror is spherical.

19. The apparatus according to Claim 15, wherein the beam reverser mirror is concave.

20. The apparatus according to Claim 15, wherein the beam reverser mirror is convex.

21. The apparatus according to Claim 15, wherein the beam reverser mirror is aspheric.

22. The apparatus according to Claim 1 , wherein the reference pattem comprises a circular pattem.

23. The apparatus according to Claim 1, wherein the reference pattem comprises radially oriented lines that provide size cues for observing the eye.

24. The apparatus according to Claim 1, wherein the optical system comprises a microscope.

25. The apparatus according to Claim 1, further comprising a shutter for selectively blocking the optical beam carrying the reference pattern.

26. The apparatus according to Claim 1, further comprising a shutter which may be used to block an alignment laser beam.

27. A method for projecting a reference pattem into the field of view of an optical system, comprising:

(a) combining an optical beam carrying the reference pattern with a laser beam; and

(b) reflecting the optical beam towards the optical system to cause the reference pattem to be visible in the field of view of the optical system.

28. A method for projecting a reference pattern into the field of view of an optical system, comprising:

(a) injecting an optical beam carrying the reference pattem into the path of a laser beam;

(b) deflecting the optical beam carrying the reference pattern and the laser beam to maintain alignment of the beams with a patient's eye that is moving; and

(c) reflecting the laser beam and a first portion of the optical beam carrying the reference pattem towards the patient's eye.

29. The method according to claim 28, further comprising the step of reflecting a second portion of the optical beam carrying a reference pattem by means of a beam combiner towards the optical system to cause the reference pattern to be visible in the field of view of the optical system.

30. The method according to claim 28, further comprising the step of providing commands to servo-control a motor-driven mirror in closed-loop fashion in order to maintain alignment of the optical and laser beams with the patient's eye.

31. The method according to claim 28, further comprising the step of positioning a beam reverser mirror at substantially the same optical distance from the optical system as the patient's eye.