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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ophthalmic diagnostic method and apparatus,
and more particularly to a method and apparatus for measuring the length
of the optical axis of an eye, i.e., the distance between the corneal
surface and the eye fundus of an eye undergoing ophthalmic examination.
2. Description of the Prior Art
The use of ultrasound in instruments for measuring the length of the
optical axis of the eye is known in the prior art, and such instruments
have come into general use as commercial products. Another measurement
method that can be employed for such an application is one based on
optical interference.
The ultrasonic method consists of transmitting a beam of ultrasonic waves
into the eye and receiving the reflections of the ultrasonic waves from
the eye, and obtaining the distance of boundary surfaces in the eye on the
basis of delays in the reflected waves. There are a number of problems
with this method, which are described below.
1. The precision of the measurement of the length of the optical axis of
the eye is low, being in the order of 0.1 mm.
2. To carry out the measurement, a probe that includes the ultrasonic
oscillator has to be brought into contact with the eye. Although either of
two variations may be used, the contact method or the immersion method,
both impose a fairly considerable strain on the person being examined.
3. There is a difference between the length of the optical axis of the eye
as measured by ultrasonic waves, and the length of the optical path of the
eye axis.
The use of the optical interference method to measure the length of the
optical axis of the eye is one way of making up for the drawbacks of the
ultrasonic method.
One such method is that disclosed in German Patent Publication No.
3,201,801. It comprises directing a beam of partially coherent light into
the eye and extracting two beams of light from the light reflecting from
the various boundaries in the eye, the two beams of reflected light used
usually being light reflected by the corneal surface and light reflected
by the retina. The two beams are guided into a Michelson interferometer.
On one arm of the interferometer is a fixed mirror that only reflects
light reflected by the corneal surface, and on the other arm is a movable
mirror that reflects both of the beams of reflected light. When the beams
of reflected light from the two arms are combined for observation while
the movable mirror is moved, interference fringes will appear twice. By
reading off the positions of the movable mirror at the points at which the
fringes appear, the length of the optical axis of the eye can be
determined from the difference between the readings.
Thus, in accordance with this method the length of the optical axis of the
eye is measured on the basis of interference between beams of partially
coherent light, and as such the length of the optical path of the eye axis
can be determined, while another merit of the method is that as the
measurement is accomplished without physical contact with the eye, it does
not impose any burden on the patient. However, there are the following
problems.
1. The measurement procedure requires that the movable mirror be moved by
mechanical means, which increases the structural complexity of the
apparatus and adversely affects the stability, rendering it unsuitable for
clinical applications. In addition, it is difficult to maintain a
satisfactory level of precision in the movement of the movable mirror that
determines the measurement precision. Also, during the measurement
procedure the examiner has to confirm the interference fringes visually,
which introduces a further element of imprecision into the measurement
results.
2. A visible light has to be used because an observer has to view the
interference fringes directly by eye, which dazzles the patient.
3. The patient has virtually no sensation of light if a semiconductor laser
is used that operates in the near-infrared region, but interference
fringes have to be viewed via an infrared scope or the like, which makes
the apparatus complex and costly.
4. It takes at least two or three seconds to carry out the measurement, and
any movement of the patient's eye during that time results in measurement
errors.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an ophthalmic measuring
method and apparatus whereby accurate, objective measurement results can
be obtained using a configuration that is simple and low-cost.
According to the invention, an ophthalmic diagnostic method and apparatus
for measuring a distance between the corneal surface and the eye fundus of
an eye undergoing ophthalmic examination comprises projecting a beam of
monochromatic coherent light at the eye, measuring a phase difference
between two light waves reflected by the corneal surface and the eye
fundus which is produced depending upon the distance therebetween along
which the two light waves travel, varying the wavelength of the coherent
light within a predetermined range, and measuring an amount of change in
phase difference caused by the variation in wavelength to determine the
distance between the corneal surface and the eye fundus.
The above configuration enables the length of the optical axis of the eye
to be measured on the basis of changes in the phase difference of two
light waves reflected by the cornea and the retina, without any need for
mechanical control.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and features of the present invention will become more apparent
from the following detailed description taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is an explanatory drawing illustrating the construction of the
apparatus for measuring the length of the optical axis of the eye in
accordance with the present invention;
FIGS. 2 and 3 are graphs illustrating the characteristics of interference
fringes obtained using the apparatus of FIG. 1;
FIG. 4 is a graph illustrating the oscillation wavelength characteristics
of a semiconductor laser plotted against the injection current;
FIG. 5 is a graph illustrating the drive current control characteristics of
a semiconductor laser; and
FIG. 6 is a graph illustrating the output waveform of the photosensor shown
in FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a general view of the configuration of the apparatus for
measuring the length of the optical axis of the eye in accordance with the
present invention, which will now be described, starting with the optical
projection system.
A laser light source is comprised of a semiconductor laser 3 which operates
in a single longitudinal mode, and comprised of an
Automatic-Temperature-controlled Module (ATM) 2 for controlling the
temperature of the semiconductor laser 3. The semiconductor laser 3 emits
light having a wavelength in the near-infrared region and can be
controllably changed within that range by means of a drive control circuit
1 that is used to change the drive current of the semiconductor laser 3
and thereby change the index of refractivity of the waveguide path of the
semiconductor laser 3 in order to make its wavelength variable.
The light beam emitted by the semiconductor laser 3 is collimated by a
collimating lens 4, and is then passed through a diaphragm 5 to reduce the
beam to a diameter that is no larger than the diameter of the pupil of the
eye being examined. A light amount adjustment filter 6 is used to reduce
the amount of light to a permissible safety level, and the direction of
the light beam is then altered by a beam splitter 7 so that it impinges on
the eye 8 being examined. With this arrangement, there is no need to use a
mydriatic or a protective contact lens.
The optical measurement system is comprised of an interferometer, and is
configured here as a Fizeau interferometer type optical system for
measuring the distance d between the corneal surface and the retina of the
eye 8.
The collimated laser beam impinging upon the eye 8 is first reflected by
the surface of the cornea; the light reflected at the cornea is divergent.
The laser beam passing through the crystalline lens is refracted by the
lens effect of the crystalline lens and cornea and focuses at about the
position of the retina located at the focal point, where it is reflected.
The reflected light passing back through the crystalline lens, cornea and
so forth is again formed into a parallel beam by refraction, and emerges
in that form.
Circular concentric interference fringes formed by the interference of the
two beams of corneal reflected light 9 and retinal reflected light 10,
produced when the two beams are converged once by a lens 11, reach an
optimum contrast level at a position at which the diameters of the two
beams are about the same, so a pinhole or ring slit 12 is disposed at that
position to form interference fringes. Changes in the contrast of the
interference fringes thus formed are detected by a photosensor 13 that
measures the amount of light at a prescribed point on the ring slit plane
or pinhole. The electrical signal obtained by the photoelectronic
conversion effected by the photosensor 13 shows changes in the amount of
light at a prescribed point on the interference fringe, and the length of
the optical axis of the eye can be measured by varying the wavelength of
the semiconductor laser 3 while the changes in the output signals of the
photosensor 13 are analyzed by a processor 14.
By changing the direction of the two beams of reflected light by inserting
a swingable mirror 15 between the lens 11 and the pinhole or ring slit 12
and picking up the light by means of a CCD camera 16 located at a position
at which the length of the optical path is the same as the length of the
optical path to the photosensor 13, it is possible for the examiner to
observe the interference fringes on a monitor 17. If required, still
pictures can be recorded for image processing by computer, or a video
recording means can be used for full motion recording.
The principle behind the measurement of the length of the optical axis of
the eye using the above configuration will now be described.
If d is taken as the length of the optical axis of the eye, corneal
reflected light 9 and retinal reflected light 10 obtained from a beam of
light of wavelength.sub..lambda.0 from the semiconductor laser 3 is
projected into the eye may be shown by the following equations.
a=Aexpj((2.pi./.sub..lambda.0)x+.sub..phi.0) (1)
b=Bexpj((2.pi./.sub..lambda.0)(x+2d)+.sub..phi.1) (2)
Here, A and B are constants and .sub..phi.0 is the initial phase.
The interference fringe obtained from the interference of the two beams of
reflected light may be shown by the following equation.
I=.vertline.A.vertline..sup.2 +.vertline.B.vertline..sup.2
+2ABcos((2.pi./.sub..lambda.0)2d) (3)
In the graph of FIG. 2, the vertical axis is the amount of interference
fringe light I and the horizontal axis is 2d. If, for example, 2d is
changed from 0 to 4.sub..lambda.0, four cycles worth of interference
fringes are obtained. This can be shown by N.sub.0 =2d/.sub..lambda.0
=4.sub..lambda.0 /.sub..lambda.0 =4.
The graph of FIG. 3 shows when the operating wavelength.sub..lambda.1 of
the semiconductor laser 3 is controlled to 2.sub..lambda.0. In this case,
even if 2d is changed from 0 to 4.sub..lambda.0, only two cycles worth of
interference fringe change will be obtained. This can be shown by N.sub.1
=2d/.sub..lambda.1 =2.sub..lambda.1 /.sub..lambda.0 =2.
From these two examples, if 2d is now maintained at a constant 2d
=4.sub..lambda.0 and the drive control circuit 1 is used to change the
wavelength of the laser beam generated by the semiconductor laser 3 from
.sub..lambda.0 to .sub..lambda.1, the interference fringe will two cycles
worth of change, i.e. n=N.sub.0 -N.sub.1 =2. The amount of change n in the
interference fringe depends on the amount of change in the wavelength and
the length of the optical axis of the eye, so the length of the optical
axis of the eye can be determined by obtaining the amount of change in the
interference fringe and the amount of change in the wavelength. The
relationships involved are shown by:
##EQU1##
In this embodiment, the single longitudinal mode laser 3 is used as the
variable-wavelength coherent light source. FIG. 4 shows the
characteristics of the generated wavelength .lambda. plotted against
injection current I. As can be seen, mode popping gives rise to a
staircase-shaped characteristic curve, and the range of variation is
limited. However, when a region is used in which the injection current and
the generated wavelength are linear, the applied current value can be
regarded as corresponding to the generated wavelength.
FIG. 5 shows the waveform of the semiconductor laser injection current. The
injection current is changed at a fixed rate and the wavelength is made
variable at a fixed rate. If the rate of change in the wavelength of the
semiconductor laser is K (nm/mA) and the generated wavelength is
.sub..lambda.0 when the injection current is io, then
.sub..lambda.0.fwdarw..lambda.0 +K.multidot..DELTA.i when
io.fwdarw.io+.DELTA.i.
Substituting these in equation (4) gives equation (5).
##EQU2##
Also, as .sub..lambda.0 >>K.multidot..DELTA.i, use of approximation gives:
##EQU3##
Therefore, the axial length d of the eye can be obtained by using the
processor 14 to perform calculations based on equations (5) and (6) using
the n, K,.DELTA.i,.sub..lambda.0 of the drive control characteristics of
the semiconductor laser 3. The processor 14 can be constituted of a
microcomputer-based control system, for example, that processes the output
of the photosensor 13 in synchronization with the drive control by the
drive control circuit 1. FIG. 6 shows a sample waveform of an interference
fringe change signal obtained from the photosensor 13 with the apparatus
of this invention.
As described above, in the embodiment monochromatic light with a wavelength
in the near-infrared region is generated by a semiconductor laser and
projected into the eye, and the wavelength of the coherent light beam is
varied while changes in the interference fringe corresponding to optical
path differences between the two light waves reflected by the cornea and
the eye fundus are measured using a Fizeau interferometer to thereby
measure the length of the optical axis of the eye, so that the length of
the optical axis of the eye can be measured from changes in the
interference fringe, which thereby enables measurements to be made that
are speedy and precise by non-contact, high-speed wavelength scanning.
Moreover, the use of an interferometer to perform the measurements means
that there is no naked-eye evaluation of interference fringes, so the
measurements obtained are objective and accurate. In addition, the use of
a semiconductor laser enables the apparatus to be simple, low-cost, light
and compact.
Further clinical advantages are that as infrared light is used, the patient
is not dazzled, and the noncontact, speedy nature of the measurement
decreases the strain on the patient. Also, because the beam of
near-infrared light is collimated after it passes through the pupil, there
is no need to use a mydriatic or anesthetic, and the ability to regulate
the light that impinges on the eye eliminates any need for protective
contact lenses.
In FIG. 1 the beam of coherent light impinging on the cornea is shown as
collimated, but the beam can be divergent or convergent for eyes that are
near-sighted or long-sighted.
A semi-transparent mirror may be used instead of the beam splitter 7. In
such a case it is preferable to use a wedge shaped mirror to avoid the
detection of interference fringes formed between the top and bottom
surfaces.
If a polarizing beam splitter is used in place of the beam splitter 7 shown
in FIG. 1, by placing a .lambda./4 plate between the polarizing beam
splitter and the cornea, the beam of light directed at the eye can be made
orthogonal to the plane of polarization of the reflected light, enabling
the full amount of light coming from the light source to reach the cornea,
and the full amount of light coming from the eye to reach the detection
part of the system. In addition this arrangement prevents light being
returned to the light source side, so it is possible to avoid unstable
laser oscillation such returning light can cause.
When the light is detected via a ring slit, it is preferable that the
arrangement be such that the center of the ring slit coincides with the
center of the circular concentric interference fringes and the ring slit
gap is substantially the same as the interference fringe gap.
Thus, in accordance with the present invention, the configuration of an
ophthalmic measuring method and apparatus for measuring the distance from
the corneal surface to the eye fundus in an eye being examined comprises
projecting a beam of monochromatic coherent light at the eye, varying the
wavelength of the coherent light beam within a prescribed range while
observing differences between the optical paths of two light waves
reflected by the cornea and the eye fundus, the differences being based on
the length of the optical axis of the eye and observed as a phase
difference between the two light waves, and obtaining a measurement of the
length of the optical axis of the eye being examined from the amount of
change in the measured phase difference corresponding to the variation in
wavelength. This configuration enables the length of the optical axis of
the eye to be measured on the basis of changes in the phase difference of
two light waves reflected by the cornea and the retina, without any need
for mechanical control. In addition, as the measurement procedure is quick
and non-contact, there is less strain on the patient.
While the invention has been described with reference to a preferred
embodiment, it will be understood by those skilled in the art that various
changes may be made and equivalents may be substituted for elements
thereof without departing from the scope of the invention. In addition,
many modifications may be made to adapt a particular situation or material
to the teachings of the invention without departing from the essential
scope thereof. Therefore, it is intended that the invention should not be
limited to the particular embodiment disclosed as the best mode
contemplated for carrying out the invention, but that the invention will
include all embodiments falling within the scope of the appended claims.
* * * * *
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