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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a fundus examination device, and particularly to
an improvement of an electronic ophthalmoscope which uses laser light as
its light source, in which the laser beam is deflected for scanning in two
dimensions to illuminate the fundus of the eye being examined, and light
reflected from the eye fundus is photoelectrically detected and processed
to obtain fundus information.
2. Description of the Prior Art
Conventionally, examination of the fundus of the eye is widely carried out
by either the method of the physician directly examining the fundus of the
patient's eye using a device called a funduscope or the method of
photography using a special camera called a fundus camera. Furthermore,
with advances in electronic technology in recent years, the photographic
film of a conventional fundus camera is giving way to methods using a
camera tube or other photoelectric transducer to directly obtain fundus
information as an electrical signal, which may be then processed, stored
or displayed on a TV monitor.
Against this background, the first laser-scanning electronic ophthalmoscope
was developed in the United States (see U.S. Pat. No. 4,213,678 and
Applied Optics, Vol. 19 (1980), p. 2,991) and had many benefits,
attracting much attention.
Namely, a laser is used as a light source of a previously studied
cathode-ray-tube flying-spot scanning fundus imaging device. Furthermore,
light is permitted to go through only a small region in the center of the
pupil, and light reflected from the eye fundus is obtained from a wide
region around the pupil, photoelectrically converted and amplified to give
low-illuminance yet high signal-to-noise ratio images of the eye fundus
which can be projected onto TV monitors in real time. Moreover, the amount
of fluorescent dye which must be intravenously injected during fluorescent
fundus imaging may be greatly reduced. In addition, by modulating the
incident scanning light, retinal function may be examined while observing
images of the eye fundus, thus serving as a so-called fundus perimeter or
fundus analyzer. Furthermore, that invention provides the potential of
obtaining a superior diagnostic device from the standpoint of making the
striking depth of field with the laser beam, eliminating corneal
reflections due to polarization, employing its monochromaticity and
providing expansion into therapeutic machines (coagulators).
This new type of ophthalmoscope has since been subject to many further
experiments and improvements by research groups around the world. Among
them, the present inventors have invented, developed and applied for a
patent on an extremely innovative stereoscopic shape-measuring device
based on a completely new principle which allows three-dimensional
measurement of an eye fundus in vivo (see Japanese Patent Application
Public Disclosure No. 1(1989)-113605, corresponding to U.S. Pat. No.
4,900,144). That invention employs a means of detecting displacement of
the position of the focus of light reflected from the subject and a signal
processing means which eliminates the effects of the optical reflection
characteristics of the subject, thereby extracting three-dimensional
stereoscopic information from the subject. The invention enables an
extremely short measurement and processing time, high accuracy and
reproducibility of measurement, and moreover, information regarding the
normal two-dimensional reflection characteristics can be obtained at the
same time as the three-dimensional information. Thus the invention
provides superior technology for stereoscopic shape measurement of the
in-vivo eye fundus in general and measurement of the optic disk for early
diagnosis of glaucoma in particular.
However, when applying this stereoscopic shape-measuring device to an
actual in-vivo eye fundus, as is apparent from considering the principle
of measurement, the accuracy of measurement is degraded if the state of
dilation of the pupil is poor. Namely, in the device disclosed in Japanese
Patent Application Public Disclosure No. 1(1989)-113605), changes in the
contour of the fundus subject to examination are detected as changes in
the flux of light passing through detection slits, but if the state of
dilation of the pupil is poor, then the changes in light flux are reduced.
Therefore, determination of whether the three-dimensional shape data
obtained with this device is valid or not necessitates a judgmental check
of the state of dilation of the pupil. However, the device disclosed in
Japanese Patent Application Public Disclosure No. 1(1989)-113605) does not
provide a function for measuring the diameter of the pupil in the anterior
portion of the eye, so the examiner must check the state of dilation of
the patient's pupil either visually or using a separate method or device.
In addition, while unrelated to three-dimensional measurement, in normal
two-dimensional fundus imaging methods typically using laser scanning, it
is possible to display an image of the fundus on a monitor screen in real
time using low-intensity illumination which causes little discomfort to
the patient. However, when imaging the fundus using visible,
short-wavelength laser light in particular, after positioning the device
with respect to the eyeball as in conventional non-pupil-dilating fundus
cameras, the eye is flash-illuminated with laser light for only the time
required for one frame or several frames at a time to record an image of
the fundus in memory. This imaging method has been found to be effective
from the standpoint of reducing discomfort in the patient. In this case,
if the positioning of the device to the eyeball is insufficient, flare due
to light reflected from the anterior portion of the eye will impinge on
the image of the fundus and result in a loss of picture quality. Although
the process of observing the anterior portion of the eye is important for
positioning the device to the eyeball, conventional laser-scanning fundus
examination devices are not provided with functions for observing the
anterior portion of the eye, making the adjustment of the device position
with respect to the eyeball difficult.
Naturally, techniques for observing the anterior portion of the eye are
employed in the traditional non-pupil-dilating fundus cameras of the past,
namely by redirecting the light path somewhere in the midst of the
camera's optical system or by inserting an auxiliary lens into the optical
system and thereby observing the anterior portion of the eye. Naturally,
these sorts of techniques can also be employed in the laser-scanning type
of fundus imaging system. However, these traditional techniques involve
the bother of mechanically redirecting the light path or inserting and
removing lenses and moreover the fundus and the anterior portion of the
eye cannot be observed at the same time, so they are not necessarily the
ideal solution.
SUMMARY OF THE INVENTION
Principle objects of this invention are to solve the aforementioned
problems by providing a new laser-scanning fundus examination device which
simplifies the positioning of the optical system of the device in relation
to the eyeball of the eye being examined when imaging an eye fundus using
flash illumination, and is provided with a function for examining the
anterior portion of the eye together with a function for imaging the
fundus so that the state of dilation of the pupil can be reliably examined
to check the validity of data during three-dimensional measurements.
According to the invention, a fundus examining device is provided in which
laser light from a laser light source is deflected for scanning in two
dimensions over an eye fundus of an eye being examined, and light
reflected from the eye fundus is photoelectrically detected with a
photodetector to obtain fundus information. The fundus examining device
comprises a laser light source for producing a laser beam; means for
deflecting the laser beam to scan across the eye fundus; an illumination
light source for producing illuminating light which illuminates an
anterior portion of the eye being examined; an optical system for
observing the illuminated anterior portion of the eye being examined; and
an optical element which reflects light of the wavelength of the laser
beam but is transparent to the illuminating light; wherein the optical
element reflects the deflected laser beam to illuminate the fundus of the
eye being examined, while guiding light from the illuminated anterior
portion of the eye to the observing optical system.
The fundus examining device further comprises a laser light source for
producing a laser beam; means for deflecting the laser beam to scan across
the eye fundus; a first optical system for illuminating the fundus of the
eye being examined with the laser beam which is deflected by the
deflecting means; means for detecting displacement of the position of the
focus of light reflected from the eye fundus to derive therefrom eye
fundus shape-related information in the direction of the optic axis which
is perpendicular to the scanning direction of the deflecting means; signal
processing means for removing the effects of the optical reflection
characteristics of the fundus from the output signal of the detecting
means; an illumination light source for producing illuminating light
having a wavelength different than the wavelength of the laser beam to
illuminate an anterior portion of the eye being examined; and a second
optical system for observing the illuminated anterior portion of the eye
being examined.
Furthermore, the fundus examining device comprises a laser light source for
producing a laser beam; means for deflecting the laser beam to scan across
the eye fundus; an objective mirror which reflects the laser beam
deflected by the deflecting means but is transparent to light of a
wavelength different than the wavelength of the laser beam; means for
detecting displacement of the position of the focus of light reflected
from the eye fundus to derive therefrom eye fundus shape-related
information in the direction of the optic axis which is perpendicular to
the scanning direction of the deflecting means; signal processing means
for removing the effects of the optical reflection characteristics of the
fundus from the output signal of the detecting means; an illumination
light source for producing illuminating light having a wavelength
different than the wavelength of the laser beam to illuminate an anterior
portion of the eye being examined; and an optical system for observing the
illuminated anterior portion of the eye being examined; wherein light from
the illuminated anterior portion of the eye being examined is guided to
the observing optical system via the objective mirror.
With the above structure, the optical element which reflects light of the
wavelength of the laser beam but is transparent to the illuminating light
is provided, and this optical element comprises an objective mirror in one
preferred embodiment of the invention. An image of the fundus is obtained
from the laser light reflected from this objective mirror, while the light
which passes through the objective mirror is used to obtain an image of
the anterior portion of the eye, so the bothersome manipulation of the
optical system required by traditional non-pupil-dilating fundus cameras
of the past is eliminated and the anterior portion of the eye being
examined can be continuously observed. Therefore, positioning of the
optical system of the device in relation to the eyeball of the eye being
examined is simplified when imaging a fundus using flash illumination, and
the state of dilation of the pupil can be reliably examined during
three-dimensional measurement of the fundus.
Furthermore, by using an objective mirror when observing the fundus, the
problem of surface reflections which appear when using an objective lens
is eliminated. In addition, since an image of the anterior portion of the
eye is always visible, the device may be employed as an electronic
pupillometer. For these and other reasons, the present invention may be
embodied as an extremely practical laser-scanning fundus examination
device.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and features of the present invention will become more apparent
from the accompanying drawings and the following detailed description of
the invention.
FIG. 1 is a structural diagram showing the overall, structure of the
optical system of a fundus examination device according to the present
invention;
FIG. 2 is a graph of the spectral response of the optical element; and
FIG. 3 is a structural diagram of the optical system which illuminates the
anterior portion of the eye being examined.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The above and other features of the present invention will become apparent
from the following description made with reference to FIGS. 1 through 3.
FIG. 1 illustrates the overall structure of the optical system and
electrical system of a fundus examination device according to the
invention. In FIG. 1, 1 is an argon (Ar.sup.+), helium-neon (He-Ne) or
other laser which acts as a source of visible laser light. The laser beam
2 from the laser light source is expanded to a certain size with a beam
expander 3 and reflected by a mirror 4 into a lens 5. The lens 5 is a
combination of a plurality of cylindrical lenses used to shape the laser
beam and direct it into a rectangular aperture in an acoustooptic
deflector (hereinafter AOD) 6.
In order to correct the wavelength dependence of the angles for the laser
beam incidence and emergence with respect to the AOD, prisms 7 and 8 are
disposed at the front and rear of the AOD 6. Note that these prisms are
not necessarily required if a monochromatic laser beam is used. The AOD 6
is actuated by a signal source 6', whereby the laser beam is deflected to
scan at a frequency of 15.75 kHz, for example, corresponding to the
horizontal scanning of a normal television system. The laser light
deflected by the AOD 6 in one direction (the horizontal direction) is then
passes through a lens 9 of a construction similar to that of lens 5 and
reshaped into its original round beam from the rectangular beam matching
the aperture of the AOD 6.
The scanning light exiting from the lens 9 passes through lenses 10 and 11
and reaches a beam splitter 12 which reflects part of the light and allows
the remainder to pass through. The beam splitter 12 may be, for example, a
polarizing beam splitter or a non-polarizing beam splitter of roughly 25%
reflectance and 75% transmittance. The laser light which passes through
the beam splitter 12 enters a photodetector 13 comprising a photodiode or
the like, the output signal of which is used to monitor the power of the
laser light.
On the other hand, the laser light reflected by the beam splitter 12 is
guided to a mirror 15 mounted on a galvanometer 14. The
galvanometer-mirror 15 is actuated by a signal source 14', whereby the
laser beam is deflected to scan at a frequency of 60 Hz, for example,
corresponding to the vertical scanning of a television system, and its
scanning direction is perpendicular to the direction of scanning
controlled by the AOD 6. The thus-formed two-dimensional laser rasters
corresponding to television scanning lines is then reflected by a mirror
16 and an objective mirror 17 and projected through the center of the
pupil of the eye 18 being examined.
As is evident in the spectral response of the objective mirror 17 used
here, illustrated in FIG. 2, the objective mirror 17 reflects 99% of the
incident light at visible wavelengths, but is almost transparent to
infrared light, reflecting almost no infrared component. Such an optical
characteristic may be obtained by means of vacuum-deposition technology
used to create coatings of dielectric multilayers. The eye 18 being
examined is illuminated by an infrared illumination light source 20
comprising an infrared lamp, lenses, filters and the like. Behind the
objective mirror 17 is disposed a lens 19. This lens 19 is used to monitor
the anterior portion of the eyeball by forming an image of the anterior
portion of the eye 18 being examined on an imaging plane of CCD or other
infrared imaging elements 21.
At this time, as shown in FIG. 3, if an additional optical system 20'
comprising, for example, a lens, ND filter and the like is used to
illuminate the eye 18 being examined with the purpose of projecting an
index image, the distance between the eyeball of the eye being examined
and the device (the working distance) may be determined from the image
output from the infrared imaging elements 21. Namely, the principle of
triangulation can be used to check the working distance by seeing, for
example, if a dark portion due to the additional optical system 20'
appears on the center of the cornea in the image of the anterior portion
of the eye.
On the other hand, the light reflected from the fundus illuminated by the
visible laser scanning light (shown by dotted lines in FIG. 1) is
reflected and guided back by the objective mirror 17, mirror 16, and
galvanometer-mirror 15 to pass through the beam splitter 12 and a lens 22,
after which it is divided in half by a half mirror 23, each half being
detected by one of photodetectors 24 or 25.
Detection slits 26 and 27 are disposed near focusing planes (conjugate
planes of the fundus of the eye being examined) 26a and 27a between the
half mirror 23 and photodetectors 24 and 25. Disposed on the front
surfaces of photodetectors 24 and 25 are filters 28 and 29 which are
transparent to only light of the same wavelength as the laser light being
used. Note that in order to prevent photodetectors 24 and 25 from
detecting light reflected directly from the anterior portion of the eye 18
being examined, the center portions 28a and 29a of filters 28 and 29 are
marked. Furthermore, in this sort of optical system using an objective
mirror 17, there is no need for the black spots (light stops ) used to
eliminate reflections from the surface of the objective lens, as was
required in the optical system the present inventor had disclosed in the
Patent Application Public Disclosure as earlier described, so the loss of
light flux and problems with positioning of the black spots in the optical
system are completely eliminated.
Detection slits 26 and 27 are used for making three-dimensional
measurements of the fundus based on the principles described in the
above-mentioned Patent Application Public Disclosure, and U.S. Pat. No.
4,900,144 and these two are disposed near but slightly in front and behind
their respective positions 26a and 27a conjugate to the fundus. Namely,
when contours in the fundus cause the fundus-conjugate position near the
slit to vary slightly, the focal point of light reflected from the fondus
will vary and a change will appear in the output signal from the
photodetectors which detect light from the fundus after passing through
the slit. For example, if there is a depression on the fundus, a
difference in intensity of the output signals from the two photodetectors
will appear, so the depth of the depression in the fundus can be
determined by eliminating information on the reflectance of the fundus
from the output signals of the two. Now, assuming that the output signal
intensity of the two photodetectors are I.sub.1 and I.sub.2 and I.sub.0
(x,y) is the reflection light intensity at location x,y on the fundus,
then these variables related to the reflectance of the fundus can be
expressed mathematically as
I.sub.1 =f.sub.1 (z).times.I.sub.0 (x,y)
I.sub.2 =f.sub.2 (z).times.I.sub.0 (x,y)
where f.sub.1 (z) and f.sub.2 (z) are functions of the distance z in the
direction of the optic axis of the optical system, appearing due to the
presence of the slits. Therefore, since I.sub.1 /I.sub.2 =f.sub.1
(z)/f.sub.2 (z), if a division operation is performed on the output
signals from the two photodetectors, information related to the distance z
in the direction of the optic axis, namely the degree of changes in the
contour of the fundus can be extracted and determined regardless of the
intensity of reflection from the fundus.
Or since
(I.sub.1 -I.sub.2)/(I.sub.1 +I.sub.2)=(f.sub.1 (z)-f.sub.2 (z))/(f.sub.1
(z)+f.sub.2 (z))
information related to z can be detected by a combination of the arithmetic
operations of addition, subtraction and division.
The output signals from photodetectors 24 and 25, after being amplified to
a specified level by amplifiers 30 and 31, are provided as inputs to a
signal processing device 32 which carries out the aforementioned division
operation and other arithmetic operations to extract information on the
contour of the fundus (stereoscopic shape data). The signal processing
device 32 employs a built-in microprocessor and software processing to
generate three-dimensional graphic patterns or the like from the
stereoscopic shape data thus obtained, and the final results are displayed
on a TV monitor or other output device 33.
On the other hand, the infrared imaging elements 21 onto which an image of
the anterior portion of the eye being examined is formed are controlled by
a dedicated driving circuit 34 for the CCDs or the like. Its output
signal, after being amplified to a specified level by amplifier 35, is
displayed through the signal processing device 32 on the output device 33
as an image of the anterior portion of the eye being examined. For
example, if two TV monitors are used as the output device 33, both an
image of the anterior portion of the eye being examined and an image of
the fundus can be displayed and observed simultaneously, or if a function
for creating multiple windows is added to the signal processing device 32,
the same effect can be attained using one TV monitor.
Since an image of the anterior portion of the eye being examined can always
be obtained from the output signal from the imaging elements 21, the state
of dilation of the pupil can be conveniently checked, and when carrying
out three-dimensional measurements of the fundus, the validity of the
measurements can be reliably checked while observing the data. In
addition, positioning the device to the eyeball together with adjustment
of the optic axis and the working distance can be carried out conveniently
using the image of the anterior portion of the eye, so a device having
such an optical system can be also employed as an electronic pupillometer
to measure pupillary reflex and the like.
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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