 |
Claims  |
|
|
I claim:
1. An apparatus for performing photorefractive keratectomy on an anterior
surface of a cornea comprising:
a laser means for producing a collimated beam of photoablative radiation
said beam having a cross sectional area;
a modulating device having dividing means for subdividing said beam's cross
sectional area into sub beams, said sub beams comprised of laser radiation
covering an optically useful area of the cornea, said modulating device
further comprises blocking means for either blocking or transmitting said
radiation comprising said sub beams; an optical projection system
comprised of a light source emitting visible light, said light being
collimated and directed through a reticle, said reticle producing a grid
of collimated rays equal in number to said sub beams;
a dielectric mirror transparent to the light of said optical projection
system, said mirror fully reflecting said radiation of said sub beams,
said mirror collinearly aligning the collimated rays of said optical
projection system with said sub beams to produce a combined beam, said
combined beam being projected upon said cornea;
a video camera for imaging said grid projected on said cornea onto a raster
of pixels;
an analog to digital converter for converting said light projected image of
each said pixel into a binary word and transmitting said word through a
direct memory access channel;
a preprocessor containing a high speed microprocessor and memory, said
preprocessor receiving and translating said words into three-dimensional
corneal coordinates;
a computer means receiving said corneal coordinates and generating an array
of errors between said corneal coordinates and desired coordinates; a
digital to analog converter generating said external control signals in
response to said errors, said signals in turn controlling said modulating
device.
2. An apparatus according to claim 1 wherein said modulating device is a
micromechanical structure comprising a matrix of windows whereby each said
sub beam is produced by a portion of said beam passing through said window
on said micromechanical structure, said window comprising a movable mask
allowing said sub beam to be blocked or unblocked, said mask attached to a
spring structure, said structure comprising two tent shaped leaf springs
having opposite sides, joined together through a flexible movable joint,
said opposite sides of said leaf springs being hinged to said
micromechanical structure through flexible stationary joints, said movable
joint able to be translated spatially from a first stable position to a
second stable position by applying an external force to said movable
joint, said force being greater than a resisting force created by rotation
of said springs about said stationary joints, two stationary plates extend
from said stationary joints, one of said plates being inclined so that it
is parallel and in close proximity with a side of said leaf spring when
the flexible joint is in the first stable position, another of said plates
being parallel and in close proximity to corresponding sides of said leaf
springs when said flexible joint in the second stable position, where
movement; of said movable joint causes said mask to be cantilevered
thereby blocking or unblocking said window, said external force to
cantilever said mask being supplied by electrostatically statically
charging said plates and said springs resulting in a repulsive force when
said plates and springs are in close proximity but not directly contacting
one another.
3. The micromechanical modulating device of claim 2 wherein all said
springs in a given column constituting said matrix of windows are
electrically interconnected by wires, wherein said stationary plates in a
given row are electrically interconnected by wires, where in order to
block or unblock said radiation through a given window, said plates and
said springs are electrically charged by application of a voltage pulse
sent simultaneously to corresponding column and row wires;
4. The micromechanical modulating device of claim 3 in which blocking or
unblocking of a window depends on voltage polarity of said voltage pulse
applied to said row and column wires wherein polarity is changed via
electrical interconnection of said wires.
5. The micromechanical modulating device of claim 2 where all portions of
said structure exposed to said radiation are coated with aluminum
magnesium fluoride to reflect away said radiation for protection of said
device.
6. The apparatus according to claim 1 further comprising optical means for
facilitating greater surface area of the micromechanical modulating device
wherein said beam from said laser is diverged and recollimated by said
lens means to produce reduced intensity radiation, said reduced intensity
radiation being applied to said micromechanical modulating device
producing reduced intensity sub beams, said reduced intensity sub beams
are then converged and recollimated by further lens means to return the
sub beams to desired intensity.
7. The apparatus according to claim 1 wherein the laser radiation beam is
directed to a contiguously spaced bundle of optical fibers, said fibers
being spread apart where each input fiber is aligned with a window of the
micromechanical modulating device, out of said window said sub beam enters
one of a second grouping of output fibers, said output fibers being
brought together substantially contiguously to form said desired sub beam.
8. The apparatus of claim 7 wherein the laser radiation out of the input
fibers is intercepted by an optical plate having micro fresnel-like lens
structures resulting in the radiation from each fiber being diverged and
recollimated into a larger cross sectional area of lower intensity
radiation falling on the windows of the micromechanical modulating device,
said reduced intensity radiation upon exiting said modulating device,
enters a similar optical plate that converges and recollimates said
radiation into said output fibers.
9. The apparatus according to claim 1 further including a positional
detecting device for maintaining corneal alignment, said detecting device
consisting of three emitters of soft ultraviolet light and three detectors
of said light, said emitters directed orthogonally towards three small
respective areas of fluorescent material embedded in the cornea, said
three detectors focussing respectively on said areas and converting
intensity therefrom to voltages, said voltages being fed to a thresholding
AND circuit, said circuit either enabling or inhibiting said laser from
firing.
10. The apparatus according to claim 1 where said dielectric mirror is
removed and said sub beams directed axially towards the cornea.
11. The apparatus of claim 10 where an optical device intercepts said sub
beams, said optical device either reflects of refracts said sub beams
obliquely to said cornea.
12. The apparatus according to claim 1 where said computer means includes
parallel processing digital hardware.
13. A method for performing photorefractive keratectomy on an eye
comprising the steps of:
removing an epithelium of a cornea over an optically useful portion of the
cornea and holding the eye immobile and providing detectors to inhibit
laser pulsing if cornea is not in position;
projecting a collimated beam of visible light through a measurement grid of
dot-marks upon said cornea, said grid projection on said cornea being
imaged on a pixel defined raster of a video camera, said pixels being
digitized and stereographically analyzed to produce a number of
three-dimensional coordinates, said coordinates being equal to in number
to said grid dot-marks and from said coordinates curve fitting a conic
surface of revolution, generating a series of intermediate target
surfaces, said intermediate surfaces starting with a curve fitted
preablated cornea and gradually changing into a desired corneal surface;
subdividing a beam of photoablative radiation into a number of sub beams,
aligning each of said sub beams with each said dot-mark projected through
said collimated beam of visible light forming a combined beam, projecting
said combined beam over said cornea;
controlling corneal ablation by: an iterative step comprising comparing
cornea surface with one of saint intermediate target surfaces from which
elevation differences or errors are generated, using these errors to
direct said sub beams to be turned either on or off depending on whether
said errors are positive or negative, ablating cornea with these
controlled sub beams, measuring cornea and comparing with said
intermediate target surface.
14. A method according to claim 13 where the step of controlling the
ablation is further defined by the steps:
introducing a new set of intermediate offset target coordinates,
determining a minimum difference between said target coordinates and said
cornea coordinates, subtracting said minimum difference from said offset
target; coordinates to get reference target coordinates, subtracting each
said reference target coordinate from each corresponding said cornea
coordinate giving a first error, summing said first errors giving a first
error sum, determining largest first error;
selecting an incremental ablation depth, subtracting said depth from said
reference target coordinates giving a set of ablation target coordinates,
subtracting said ablation target coordinates from said cornea coordinates
yielding a set of positive errors and negative errors, turning on said sub
beams corresponding to said positive errors, turning off said sub beams
corresponding to negative errors thereby subjecting the cornea to a short
period said selective ablation;
measuring cornea to produce a new set of corneal coordinates, subtracting
said reference target coordinates from said new cornea coordinates giving
a second set of errors, summing absolute values of these errors giving a
second error sum and getting a second error maximum, determining whether
the said second error sum is less than said first error sum and whether
said second error maximum is less than said first error maximum and if so,
checking to see if second error is below a threshold error and if so,
introducing next consecutive said offset target coordinates and iterating,
and if not, iterating said steps with same said reference target
coordinates; halting execution if said second error sum is larger than
said first error sum or said second error maximum is larger than said
first error maximum,
15. A method according to claim 13 where the iterative step is performed in
less than 0.2 seconds.
16. A method according to claim 13 wherein the step of aligning each said
sub beam comprises non-collinearly aligning each said dot-mark, further
including the step of correlating nasal-temporal positions of said sub
beam projections on the cornea with said grid dot-mark projections on the
cornea, said correlating step consisting of connecting straight lines
between coordinates defined by said positive and negative errors versus
nasal-temporal coordinates corresponding to said grid of dot-marks,
translating said nasal-temporal coordinates of the sub beams into
equations formed by the straight line segments, turning said sub beams on
if said errors are positive, turning said sub beams off if said errors are
negative.
17. A method for rapidly converting a raster of pixel intensity data
produced by repeated stereographic imaging of a corneal surface undergoing
photoablation into three-dimensional coordinates defining corneal surface
topography, said method being contingent on an initial set of coordinates
defining said cornea, said method requiring that changes in topography of
successive measurements result in a shifting of projected grid dot-marks
projected on the corneal surface by less than one half of adjacent
dot-marks separation, said method comprising the steps:
specifying previously determined raster coordinates corresponding to pixel
intensity minimums, measuring a new raster of pixel intensity data,
determining directions on the caster that reduce pixel intensity and then
iteratively moving in those directions until new raster coordinates
corresponding to the pixels of lowest intensity are found, determining via
computer means from these new raster coordinates a new three dimensional
topography of the cornea. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
This invention relates to an apparatus and method, for performing either
photorefractive keratectomy (PRK) or phototherapeutic keratectomy (PTK) on
the anterior surface of the cornea of the eye using a feedback-controlled
segmented laser beam.
The now widely recognized property of the excimer laser operating at a
wavelength of 193 nm to ablate corneal stromal tissue has given rise to a
number of inventions for controlling this radiation in a manner to reshape
the full optically effective anterior surface of the cornea. These
inventions include: en face methods where the depth of ablation is
proportional to the intensity of radiation and duration of application;
and tangential methods where ablation depth is a primarily a function of
the axial position of the cornea relative to the region of laser
radiation. Despite significant differences among the various methods and
their apparatuses, they all lack the capability of being able to rapidly
measure the topography of the cornea and based on these measurements
modify the laser radiation pattern continually throughout the
photoablation process. Contained in prior art are many claims for
apparatuses and methods for controlling the ablation process through
computer conducted feedback control--e.g. U.S. Pat. Nos. 4,941,093,
4,724,522, 4,729,372. Primarily two problems militate against the
effectiveness of these techniques. The first is in the limitation of the
keratometry techniques used to measure surface topography of the cornea
because of the diffuse surface of the cornea undergoing ablation; the
second limitation lies in the inability of these techniques to control the
laser beam cross sectional intensity profile in response to instant by
instant changes in the corneal topography. Means for surmounting these
limitations are envisioned by the present invention.
Present state-of-the-art excimer lasers achieve beam cross sectional
intensity uniformity to within about 5%; additionally, en face apparatuses
deliver the radiation in the form of expanding or contracting circular or
elliptical projections that are very accurately calculated and controlled
to achieve desired ablation depths. Notwithstanding these controls, the
standard deviation from desired for most PRK test groups with less than 5
D (diopters) of preoperative myopia is 1 D, implying a deviation of some
1/5=20% from the desired result which is some 4 times higher than the
inherent accuracy (5%) of the procedure. Further, scanning electron
microscope photographs of ablated corneas evidence surface irregularities
some 8 to 10 times larger than preoperative corneas. These results suggest
an inherent nonlinearity in the ablation of the corneal tissue and that
regardless of how uniform the radiation reaching the cornea is, the
ablated cornea will always depart from the nominal desired post operative
goal.
Existing techniques for topographical measurement (keratometry) of the full
anterior surface of the cornea are based upon either reflection or
projection of reference light patterns. Because the removal of the
epithelium from the cornea prior to PRK results in exposing the stromal
corneal surface which is not optically reflective, keratometry techniques
based upon reflection are unsuitable. However, research has found that
projective techniques utilizing rasterstereographic imaging such as the
PAR Technologies Corneal Topography System (U.S. Pat. No. 4,995,716) has
successfully been used on deepithelialized and freshly keratectomized
corneas. The PAR CTS projects a grid on the cornea which is then imaged by
a video camera, digitized and analyzed to produce a tabulation of corneal
elevations versus corneal diameters by means of algorithms that require
about a half minute to compute by a 486 50 Mhz equivalent computer.
Although the PAR CTS can measure corneal surfaces to within an accuracy of
about 0.1 diopter which is within the limit for achieving complete
refractive correction of the human eye, it can be useful in terms of the
present invention only if it can be made to operate in conjunction with an
apparatus that modulates the cross sectional intensity of the laser beam
so that once a measurement is made on a region of the cornea, it can be
compared with a desired calculated value and the intensity of the laser
corresponding to the said region be changed to achieve the desired value.
Such a procedure, to be successful, must make measurements and supply the
corrective action to a modulated laser beam many times over the duration
of the ablation process--perhaps as often as every laser pulse.
Recent developments in the field of three-dimensional integrated
micromechanical structures (U.S. Pat. No. 4,918,032) and in constructing
leaf spring switches (U.S. Pat. No. 4,681,403), and in fabricating bending
joints (U.S. Pat. No. 4,953,834) enable a means for modulating a light
beam. Existing modulators are limited in regard to application to the
present invention in that they are either based upon reflection (rather
than transmission) or are limited in efficiency. It is therefore one
object of this invention to provide a micromechanical structure that
functions by modulating transmitted laser radiation and doing so
efficiently.
The recent advent of high speed low cost computers along with high speed
analog to digital conversion devices afford a means for implementing a
system for photoablative topographical control that until recently, would
have been either too expensive and/or physically impossible. Accordingly,
it an object of this invention to implement such a system using the
available technology.
SUMMARY OF THE INVENTION
It is the overall object of this invention to provide apparatus and method
to photoablate the cornea by subdividing the cross section of a laser beam
into a multiplicity of beams for each of which there shall be a
corresponding cross sectional area on the cornea, said beams shall each
correspond to a unique intersection of a grid of dot marks projected onto
the stroma of the cornea and cover the optically useful surface of the
cornea, the positions of these dot marks being imaged by a video camera,
digitized, sent to a computer where the instantaneous corneal topography
is computed and compared to a desired topography by which intensity
modulation of the subdivided laser beams controls the photoablation to
force the measured topography to conform with the desired topography. The
apparatus of the invention shall consist of: A micromechanical modulating
device consisting of a square or circular array of tiny movable masks that
block or transmit the sub beams; an optical device for projecting a grid
image onto the cornea; a video camera focussing on the cornea to produce a
raster image; an analog-to-digital converter which digitizes each pixel of
the raster image; a dedicated digital preprocessor which analyzes each
pixel to determine the three dimensional coordinates (topography) of the
cornea; a computer which receives the coordinates and calculates their
variance from a desired set of coordinates; which computer also performs
trending of the coordinates and timing of the laser pulses to insure
correct laser-cornea alignment and control the overall photoablation
process to minimize traumatizing of unablated eye tissues.
The essence and novelty of the invention is the real-time control of the
intensity of the subdivided beams of the laser radiation, each subdivided
or sub beam being collinear/coincident with an associated dot mark
projected onto the cornea so that regardless of the shape of the cornea
the dot mark always remains in the center of the sub beam, the totality of
cross sectional areas of the laser sub beams completely covering the
cornea.
A further novelty of the invention is the ability to change the desired
corneal topography coordinates in a series of small steps so that after
only a few laser pulses the actual cornea is ablated to a new,
intermediate corneal shape. Such a gradual process avoids any excess
errors from occurring in the ablated surface in the progression towards
the ultimate target surface,
Within the essential structure of the invention, many different embodiments
are possible, The preferred embodiment is constrained to present
technology and, accordingly, comprises: 1) a projection grid comprising
about 2000 dot marks, 2) a 193 nm argon fluoride excimer laser pulsing at
a nominal 10 Hz rate, 3) a micromechanical modulating device to segment
the laser beam into the same number of sub beams as there are grid
dot-marks and provide a means for blocking and unblocking each sub beam,
4) a dielectric mirror transparent to the light of the projection system
and fully reflective of the laser radiation, said mirror to enable the
collinear alignment of the collimated rays of the projection system with
the collimated sub beams of the laser, 5) a video camera imaging the
cornea onto a one million pixel charge coupled device or vidicon tube, 6)
an analog to digital converter converting the intensity of each pixel into
an eight bit binary number, 7) a dedicated digital preprocessor, 8) a 486
66 Mhz or 586 equivalent computer for performing analysis and controlling
the micromechanical modulating device through a digital to analog
converter.
Alternate embodiments or modifications to the preferred embodiment include:
1) an optical means for diverging the laser beam in order to reduce its
intensity before it, falls on the micromechanical modulating device and
thereby avoid any possible damage to the device, followed by a converging
and recollimating means that permits optimizing the cross section and
spacing of the sub beams, 2) incorporating 193 nm transparent optical
fibers into the micromechanical modulating device for the purpose of more
efficient laser beam utilization, 3) using a micro binary/fresnel lens
array in conjunction with optical fibers to obtain both efficient laser
beam utilization and reduced intensity exposure, 4) a corneal positional
detecting adjunct means for instantaneously detecting any corneal movement
that could impact negatively on the functioning of the invention, 5)
dispensing with the dielectric mirror in the preferred embodiment and
directing the modulated laser beam collinear with the optical axis of the
eye for performing either en face ablation or tangential ablation of the
corneal surface, 6) combining the functions of the preprocessor and
computer into a single parallel processing computer and/or utilizing
digital signal processing hardware to further speed the topographic
conversion of pixel intensity.
The basic procedure method of the invention involves the steps of: 1) curve
fitting the topographically-measured coordinates of the preoperative
cornea to produce a starting surface of revolution from which are
generated a number of intermediate desired target surfaces, 2) comparing
each intermediate surface with the corneal surface resulting in an error
function that is used to set the modulation control of the laser beam, 3)
pulsing laser, 4) comparing corneal surface with target surface again and
if the error is reduced, introducing a new target and repeat through the
final target surface.
Within the overall method of the invention are the algorithmic functions
for 1) rapidly determining subsequent corneal topographies once the
initial topography has been obtained, 2) relating the windows of the
micromechanical modulating device to the projection system grid dot marks
when the two beams are not collinear.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a plan view of the primary components comprising a preferred
embodiment of the invention.
FIG. 2 shows the elements and function of the micromechanical modulating
device in side and perspective views.
FIG. 3 is a side view of a lens based means for reducing radiation
intensity exposure on the micromechanical modulating device.
FIG. 4 shows a fiber optic means for maintaining high radiation efficiency
and optimizing the sizing of the micromechanical modulating device; also a
microlens means for reducing intensity.
FIG. 5 shows an adjunct corneal positional detector device for inhibiting
laser firing when the cornea is mis-aligned.
FIG. 6 illustrates the ablation action as it is controlled by the control
methodology of the invention.
FIG. 7 is a flow diagram describing the real-time control algorithm of the
invention.
FIG. 8 illustrates a method for correlating non-coincident modulated laser
radiation and measurement beams.
FIG. 9 illustrates a method for rapid stereographic conversion of pixel
intensity data.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows the components of a preferred embodiment apparatus. The
projection system unit 1 incorporates a pulsed light source (e.g. xenon
bulb) and optics which collimate the light and direct it at an angle of
about 10.degree. to 15.degree. with respect to the optical axis of the
cornea. The collimated light then passes through a reticle having a grid
of lines or of circular dots (spaced about 0.2 mm apart), which produces
the desired projected image through the beam 4 and covers the useful
optical area of the cornea. By pulsing the light source, optimum
brightness/contrast can be achieved in the projected image only in the
time interval it is desired thereby avoiding excessive heating and energy
consumption. Although the reticle is intended to produce a pattern of dark
(low light intensity) circular dot marks equidistant from one another in a
square pattern, other regular patterns of either dark or bright lines
could be applicable.
The source of photoablative radiation 2, an excimer laser or equivalent, is
assumed to produce a collimated beam of radiation which is intercepted by
a micromechanical modulating device 3. As seen in FIG. 2a, the device is
composed of a square array of windows 20 each of which has a means for
blocking or unblocking (masking or unmasking) the radiation 15, the
windows forming collimated sub beams of radiation 14. These some 2000
windows, covering a total area of about 4 mm.times.4 mm, are capable of
moving very fast to block or unblock the laser radiation because of their
small size. Then by means of a dielectric mirror 4, which is transparent
to the light in beam 5 but totally reflective (99%) to the laser
radiation, beam 5 and sub beams 14 are merged into a combined beam 6 so
that each projection beam dot mark passes through the center of each sub
beam 14 cross section. This identical alignment of beams means that
regardless of the shape of the cornea 7, wherever a dot mark is projected
onto the corneal surface, it will always be surrounded by the laser sub
beam. It is this characteristic that permits: 1) measurement of a surface
spatial coordinate, 2) subtraction of this coordinate from a desired
coordinate value to generate an error, 3) using this error to turn the sub
beam either on or off. This negative feedback property has the theoretical
capability of controlling the ablated shape of the cornea to within an
error limited by the accuracy of the measurement process and the ablation
depth caused by a single laser pulse. More practically, the ultimate
refractive accuracy imparted to the cornea will be limited by unavoidable
vibrations/motions, and the healing response of individual corneas.
When the visible (or wavelength greater than 193 nm) rays of the combined
beam 6 fall upon the deepithelialized/keratectomized cornea 7, the video
camera 9 detects the projected image on a charge coupled device or vidicon
tube or similar image raster-generating device. There are one million
picture elements (pixels) comprising the scanned image (raster frame).
Each pixel is digitized by the high speed analog to digital converter 10
into an eight bit binary word and transmitted to the high speed digital
preprocessor 11 by a direct memory access channel 16 in about 30
nanoseconds. So, from the time that the projection system 1 light source
is strobed, one million bytes of sequential pixel data is stored in the
memory of the preprocessor in about 30 milliseconds. Upon receiving the
initial pixel array, the high speed digital preprocessor 11 (e.g. a 586
microprocessor having at least a two megabyte high speed cache memory)
requires three or four seconds to completely process all one million pixel
elements including the tasks of: precisely locating the video raster
position of pixel intensity maxima/minima points, associating each of
these points with the projection system dot mark that produced it,
compensating for distortions in the video camera imaging, and finally
calculating all of the fifteen hundred to two thousand coordinates
comprising the topography of the cornea. Algorithms for accomplishing this
have either already been patented (e.g. U.S. Pat. Nos. 4,995,716,
4,761,071) and/or are in the public domain. If this process of complete
pixel processing had to be repeated for each pass of the feedback loop
control cycle, the resulting delay could compromise the performance of the
invention. To avert this problem, a simplified algorithm has been
developed specifically for this invention that can reduce each succeeding
topography calculation to less than 0.01 seconds once the initial
topography calculation has been completed. The details of this algorithm
appear later on in the methods description of the invention.
After the corneal coordinates are calculated in the preprocessor 11 they
are transmitted to the computer 12 which has a 486 or 586 (or more
powerful) equivalent engine. The highest priority tasks of the computer
are: 1) measurement of the departure (error) of the corneal coordinates
from one of a progression of predetermined conical surfaces of revolution
and performing all of the translating and summing calculations, 2) sending
the resulting control commands to the micromechanical modulating device 3
through the digital to analog converter 13. This task, requiring some
500,000 floating point operations is accomplished in less than 0.05
seconds, after which the computer is free to perform lower priority tasks
such as trending, monitoring, display, etc.
Thus, in this preferred embodiment of the invention, the speed of the
various components around the complete control loop permits a complete
cycle to be completed in less than 0.2 second.
The size of each laser sub beam projection on the cornea (i.e 0.2.times.0.2
mm) was selected on basis of existing corneal stereographic systems. To
answer the question as to whether this projection size is small enough to
obtain adequate corneal optical smoothness, the following arguments are
presented: If the sub beam intensity is so distributed so that the
overlapping of adjacent beams results in uniform intensity, then no
surface perturbations having the 0.2 mm period will occur; even if
intensity and overlap is not perfect, the residual 0.2 mm period
perturbations on the ablated cornea will be small enough so that when the
epithelium reforms on the postoperative corneal stroma, the resulting
projection of these perturbations through the epithelium will be
negligible.
The principles of operation of electrostatically powered micromechanical
structures and how they might relate to the present device 3 are discussed
prior art. However, some customization is required for application to the
present invention. In particular, among other characteristics, it is
desired that: 1) the size of the individual windows in the micromechanical
modulating device occupy as large a fraction of the area of the total
array as possible in order to avoid excessive laser inefficiency, 2) that
the materials comprising the array minimize ablation and heat buildup in
the micromechanical modulating device, and 3) that the speed of
masking/unmasking the windows be as high as possible.
FIG. 2b shows an implementation of a micromechanical concept for achieving
these characteristics. Because space must be provided for the mask
actuating mechanisms and to insure sufficient strength of the supporting
structure, the window areas 28 comprise only about 50% of the total cross
sectional area of the device. The principle of operation of the actuating
device is as follows: Two electrically conductive leaf spring
microstructures 21 and 22 having a "V" or tent shape are joined together
by a flexible movable joint 24. The opposite ends of the leaf springs are
hinged to the stationary structure 29 of the array through flexible
stationary joints 241 and 242. As shown in FIG. 2b, applying an upward
force to move the joint 24 vertically results in a compression of the
springs which is resisted by a downward force at joint 24. As vertical
motion continues, at some point this resisting force will reverse, rapidly
driving joint 24 upward until leaf spring equilibrium is reached as shown
in FIG. 2c. Thus the mask 23 attached in cantilever fashion to leaf spring
22 will be tilted downward to block the radiation 15 through the window
28. Conversely if, in FIG. 2c, joint 24 is forced downward, the oppositely
directed resisting force will reverse at the same vertical position as
before, and the joint 24 will be driven rapidly downward resulting in mask
23 being tilted upward thereby unmasking the window 28 and permitting the
passage of radiation 15. The desired initiating force at joint 24 is
obtained by simultaneously electrically charging the leaf springs 21, 22
and the conducting stationary plates 25 and 26. In order to maximize
electrostatic force effectively driving joint 24, an insulating layer of
only about 1 micrometer separates leaf springs and plates when they are in
close proximity. Applying voltages of opposite polarities to charge the
plates 25 and 26, respectively, and changing the polarity of the voltage
to charge the leaf springs, permits a means to force the mask into the
desired position. For example, in FIG. 2b, applying a negative voltage to
the springs, a negative voltage to plates 26 and a positive voltage to
plates 25 will result in a large repulsive force between the springs and
plates 26 causing mask 23 to be tilted to block or close window 28 as
shown in FIG. 2c. In FIG. 2c, the application of a positive voltage to the
springs causes the repulsion to occur between the springs and plates 25
tilting mask 23 to unblock or open window 28. In effect, a bistable or
flip-flop mechanism has been devised permitting two stable states for the
position of mask 23. FIG. 2d gives a perspective view of the
micromechanical modulating device. The width of leaf springs 21, 22 and
conducting plates 25,26 are as wide as is allowed by fabrication
limitations. The reason for this is to maximize the spring surface
conducting plate area in order to maximize the electrostatic repulsion
force. Although the window has been designated as being square, it could
be made rectangular to maximize efficiency. However, this would result in
a zonal rectangular projection of laser radiation on the cornea which
would be incompatible with the square grid of dot marks from the
projection system. If the grid of dot marks were made similarly
rectangular, then the rectangular masks would be suitable. Another factor
in sizing the windows of the micromechanical modulating device involves
how the laser radiation sub beams will project on the cornea. If the sub
beams were perfectly collimated and diffraction did not exist, then the
ablated cornea surface would resemble a honeycomb because of the gaps
between zonal radiation patterns. Fortunately, the effect of diffraction
is such that there is a spreading of the 0.14.times.0.14 mm sub beam
emerging from a window of the micromechanical modulating device so that
after traveling about three inches, the sub beam cross section is spread
out to an area somewhat greater than 0.25.times.0.25 mm. Because the
intensity of the sub beam is reduced around its peripheral projection on
the cornea, adjacent sub beam overlap results in a more or lees uniform
intensity over the corneal surface.
The dielectric strength of available insulating materials (e.g. silicon
dioxide) permits the application of about 5 volts to charge the spring and
plate structures of the micromechanical modulating device. Because of the
small mass of the springs and mask, the window close to open or open to
close transition can be accomplished in some 10 microseconds, so the
applied voltage pulses would be of this same duration. To avoid having one
wire to control each mask, i.e. 2000, a matrix technique known in the art
is resorted to, requiring only two times the square root of the number of
windows, i.e. 90 wires, whereby all leaf springs in a given column of
windows of the micromechanical modulating device are permanently
electrically connected together giving X position selection, and all the
plates in a given row of windows are electrically connected together
giving Y position selection. Then by applying a voltage pulse of the same
polarity simultaneously to the wires of a given column and row, the window
always changes state. A problem with this arrangement is that the state of
a window cannot be set without preknowledge of the existing state of each
window. This limitation can be precluded by the use of an additional wire
for each row. Here, by connecting all stationary plates to be positively
charged (plates 25) in a given row, and by a separate common electrical
connection all stationary plates to be negatively charged (plates 26) in a
given row, the desired state of the window may be selected. So, to cause a
selected mask transition, three simultaneous voltage pulses are applied:
one pulse to a selected column wire with a positive polarity to open a
window or negative polarity to close a window, a second pulse of positive
polarity to the positive designated wire (for plates 25) of a selected
row, and a third pulse of negative polarity to the negative designated
wire (for plates 26) of the same selected row. This technique requires
that only one such triplet of pulses at a time be transmitted by the
digital to analog converter 13, If every window in the micromechanical
modulating device 3 were to be pulsed, the time required would be about
2000.times.0.000010=0.02 second. This time can be greatly reduced by
pulsing only those windows that need to change state.
The fabrication of the micromechanical modulating device 3 involves
three-dimensional silicon photolithography; such techniques are covered in
prior art. Also in prior art is covered the methods whereby aluminized
magnesium fluoride can be deposited on the structures of the
micromechanical modulating device exposed to laser radiation resulting in
a reflection of some 95% of the energy away from the device.
An alternate implementation for the micromechanical modulating device is
shown in FIG. 3. Here, the laser beam 15 is first passed through a
diverging lens 31 producing a divergent beam 35 which is intercepted by
converging lens 32 resulting in a recollimated beam 36 of reduced
intensity which in turn is modulated by the micromechanical modulating
device 3 where the resulting beam 37 is converged by lens 33 and then
recollimated by diverging lens 34. This scheme has the advantage of being
able to reduce the intensity falling on the micromechanical modulating
device 3 to below the threshold of ablation and/or heat damage. Also the
positioning of the lenses permits a precise adjustment of the cross
sectional dimensions of the beam 14. Further, the scheme permits
optimizing the size of the micromechanical modulating device 3.
A means for avoiding losses in laser efficiency is shown in FIG. 4a. Here,
the laser beam falls axially upon the ends of transparent optical fibers
41 which are bundled together at 43 and then spread apart before being
inserted into the axially-directed holes of an alignment plate 44. Thus
the radiation leaving each fiber falls only on the window (movable mask)
areas to avoid potential damage to other parts of the micromechanical
modulating device 3. Alignment plate 45 aligns fibers 42 collinearly with
the fibers 41 after which the fibers 42 are inserted into a third
alignment plate 46 that maintains positional correspondence of the fibers
42 with the array of windows in the micromechanical modulating device 3
and hence the emerging sub beams 14. A refinement detail regarding the
fibers 42 is to use graded-index fabrication which can minimize the
divergence of the sub beams 14 to insure that the cross sectional area of
impingement on the cornea provides just the right amount of overlap among
adjacent sub beam projections.
While the fiber optic scheme of FIG. 4a solves the problems of radiation
inefficiency and exposure of the entire micromechanical modulating device
to radiation, the problem of the full laser intensity (i.e, energy per
unit area) falling on the masks of the micromechanical modulating device
still exists. One remedy for this is found in the application of the newly
emerging technology of binary optics as illustrated in FIG. 4b. Here
silicon dioxide (or optical equivalent) substrates 47 are sandwiched
between the alignment plates 44 and micromechanical modulating device 3.
Then a matrix of fresnel-like micro lenses 48 is deposited on each side of
the substrate 47 where the micro lens in proximity with the optic fiber 42
causes the radiation to be diverged to the other side of the substrate
where a converging fresnel-like lens collimates the reduced intensity
radiation and sizes the cross section of this collimated reduced intensity
radiation to either completely pass through each window 28 or be blocked
by the mask of the micromechanical modulating device 3. The radiation sub
beams emerging from the micromechanical modulating device 3 are
reconverged, recollimated (and re-intensified) by the corresponding
structure on the other side of the micromechanical modulating device.
In the preferred embodiment of FIG. 1, the maintenance of alignment of the
measurement beam 5 with the laser sub beams 14, greatly facilitates the
speed and accuracy objectives of the invention. A possible disadvantage of
this innovation is that because of the requirement that the measurement
beam 5 be inclined (in the horizontal plane) to the axis of the cornea,
the size of the incremental areas and hence the intensity that each of the
laser sub beams project on the cornea will vary from one portion of the
cornea to another. However, because of ability of the invention to
modulate (switch on or off) each of the laser sub beams many times a
second, changes in zonal intensity over the surface of the cornea can be
compensated for, and thus this intensity variation is not fatal to the
objects of the invention. Further, it is a simple matter to selectively
attenuate those sub beams more directly falling upon the cornea so that
the intensities of all incremental areas are held substantially constant.
Nonetheless, because of the high versatility permitted by the computing
elements of the invention, the collinearity requirement of laser sub beams
14 and the measurement beam 5, is not essential. For example, if the laser
sub beams 14 were to be aligned with the axis of the cornea, the pattern
of measurement beam dot marks would not correspond to the same pattern of
incremental areas caused by the laser sub beams, but will vary depending
on the position and shape of the cornea, However, once a coordinate on the
cornea is determined, then it is a relatively simple computational
procedure to determine which laser sub beam or combination of sub beams
fall in this region of the cornea. Although the burden on the digital
hardware (e.g. preprocessor and computer) is increased, it is manageable
within the scope of the hardware discussed herein.
This computational capability also permits the insertion of an optical
device into the modulated beam 14 for the purpose of performing tangential
photoablation, In this case each sub beam will be deflected, by refracting
and/or reflecting means, obliquely upon the cornea. Although correlating
the projection of the laser sub beams on the cornea with the measurement
grid of dot marks will be more computationally complex than for en face
ablation, it is still feasible.
To this point in the description of the invention, it has been implicitly
assumed that no corneal motion will occur between the instant a cornea 1
topography measurement is made and the pulsing of the modulated laser
beam. Such a condition can be largely achieved by placing a vacuum
(Thornton) ring around the in vivo cornea and mechanically fixing the ring
to the frame of reference of the apparatus of the invention. Even by such
means, small motions of the cornea on the order of a tenth of a millimeter
could deteriorate performance. Although the three-dimensional measurement
capability can readily detect corneal motion much smaller than this and
automatically compensate for this by means of the software every
computational cycle, the problem is that between the time that a
topography measurement is made, the required correction calculated and the
laser pulsed, a transient displacement of the cornea could occur that in
turn would cause a displacement of the ablation pattern. A continuation of
such corneal motions on successive ablation pulses could give rise to an
instability in the control process. In order to deal with this
contingency, the following presents a technique which can determine
whether the position of the cornea is within an acceptable band
immediately before the laser is pulsed--i.e. within about a millisecond.
FIG. 5 shows a possible implementation of one such positional indicating
device. It consists of three low intensity soft ultraviolet
emitters/detectors 51 arranged orthogonally about the cornea 7 and rigidly
interconnected by a frame 52 which in turn is rigidly connected to the
frame of the apparatus of the preferred embodiment. Small quantities of
fluorescein dye are injected into the periphery (non ablated region) of
the cornea to form three separate dots 56, Each emitter/detector 51 emits
a thin cone of ultraviolet light 53 whose cross sectional area at the
cornea is about the same size of the fluorescein dot so that the intensity
of the fluoresced light will drop rapidly as the uv light 53 is moved off
the center of the dot 56. The emitter/detectors 51 each contain a small
telescope that focusses the fluorescent light emitted by of the associated
fluorescein dot onto a photovoltaic cell. Once all three emitter/detectors
are aimed to bring the voltage outputs above a preset, near maximum
threshold value, the voltage output of the AND cir | | |
