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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ophthalmic examination apparatus, and more
particularly to an electronic type ophthalmic examination apparatus which
uses a laser beam for two-dimensional scanning of the eye fundus, collects
the light reflected back from the eye fundus and subjects the light to
photoelectric conversion to obtain information about the eye fundus.
2. Description of the Prior Art
Conventionally, in order to examine the eye fundus there are in wide use
the method whereby the physician examines the patient's eye directly by
means of an opthalmoscope, and the method whereby a special fundus camera
is used to take photographs of the eye fundus. Also, with the advance in
recent years of electronic technology, use is also being made of
optoelectronic transducers such as imaging tubes and the like in place of
the photographic film of the conventional fundus camera, eye fundus
information is read out directly in the form of electric signals which are
processed and stored in a memory or displayed on a monitor television or
the like.
Of these conventional electronic examination apparatuses, one that employs
laser scanning and which has developed by the Retina Foundation of the
U.S. (see Applied Optics, vol. 19 (1980) page 2991) has attracted
attention for the many features it possesses. Specifically, by replacing
the light source conventionally used in the flying spot scanning type
video image input system by a laser beam for eye fundus applications,
restricting the incident light beam to a small zone in the center of the
pupil and receiving, photoelectrically converting and amplifying the light
reflected by the eye fundus from a larger area around the periphery of the
pupil, it becomes possible to display on a monitor television a real-time
video image of the eye fundus with a low brightness and a high S/N ratio.
In addition, it becomes possible to decrease greatly the amount of
fluorescent agent that is administered when fluorescent image photography
of the eye fundus is to be performed. Also, by modulating the scanning
laser beam it becomes possible to examine retina function in the course of
observing the eye fundus image, and by utilizing the advantages of the
laser beam's depth of focus, the elimination of corneal reflection due to
polarization and the monochromatic nature of the light, it becomes
possible to provide an excellent diagnostic apparatus.
The drawback with this type of apparatus is that the system for controlling
the laser beam deflection is difficult. In the reference material cited in
the above, two mechanical laser beam deflection systems are employed which
are operated at scanning frequencies of 7.8 kHz for the horizontal
scanning and 60 Hz for the vertical scanning. But, in order to obtain
high-definition video images it is necessary to use higher laser beam
scanning frequencies. In the case of laser beam deflection corresponding
to standard NTSC raster scanning, the horizontal scanning frequency is
15.75 KHz, which is basically impossible to realize with a mechanical type
of optical deflector from the standpoint of service life and durability.
The practical application was then tried of an idea that was announced
which involved the use for the horizontal deflector of a non-mechanical
acousto-optical device having no moving parts. If, however, the apparatus
is to be used to obtain color information, because the acousto-optical
device utilizes diffraction, the angle of deflection inherently differs in
accordance with the color, and compensating for this requires an extremely
complex optical system. In addition, each such type of optical system is
effective only with respect to a set laser beam wavelength. Thus, as the
eye fundus is comprised of a plurality of layers each of which has
reflection characteristics that differ from those of the other layers with
respect to a set wavelength, in order to accurately diagnose morbid
portions it is necessary to observe the eye fundus with light of each of
the necessary wavelengths. However, in the case where an acousto-optical
device was employed for the laser beam deflection, it was impossible to
change to a laser beam having any desired wavelength among an arbitrary
plurality of wavelengths.
It is therefore an object of this invention to solve the aforementioned
problems by providing an ophthalmic examination apparatus that enables the
realization of a laser beam scanning system that makes it possible to
realize high-frequency laser beam scanning, can be freely adapted to any
arbitrary wavelength, allows color information to be obtained and has
outstanding reliability and operability.
SUMMARY OF THE INVENTION
In order to solve the aforementioned problems, the present invention
comprises an ophthalmic examination apparatus which directs a laser beam,
generated by a laser light source, at a subject eye fundus to scan the
fundus two-dimensionally, picks up the light reflected back from the eye
fundus and by means of a photoelectric transducer subjects the light to
photoelectric conversion to obtain information about the eye fundus. The
apparatus comprises a laser light source that generates laser beams of a
plurality of wavelengths;
a first acousto-optical device which selects one wavelength from among the
plurality of laser beam wavelengths;
a first optical deflector for scanning a laser beam in one direction at a
predetermined frequency; and
a second optical deflector for scanning the laser beam in a direction that
is at right-angles to the above said direction at a frequency that is
lower than the above said frequency. The first optical deflector is
constructed as the second acousto-optical device driven at an ultrasonic
frequency, and by changing the range of variability of the ultrasonic
frequency of the second acousto-optical device in accordance with the
laser beam wavelength selected by the first acousto-optical device, the
angle of deflection of the first optical deflector becomes the same
irrespective of the wavelength of the laser beam. In the present invention
for each image frame as determined by the scanning frequency of the second
optical deflector the laser beam wavelength selected by the first
acousto-optical device is also changed to obtain color information.
With the aforementioned construction, first, one out of a plurality of
laser beam wavelengths is selected by means of an acousto-optical device
and the range of variability of the ultrasonic driving frequency of
another acousto-optical device is changed in accordance with the selected
laser beam wavelength, so that the angle of deflection of the first
optical deflector scanning the laser beam in one direction at a
predetermined frequency can be made the same irrespective of the
wavelength of the laser beam, thereby making high-speed laser beam
scanning possible. In addition, the arrangement is such that the laser
beam wavelength selected for each video image frame as determined by the
scanning frequency of the second optical deflector which scans in a
direction that is at right-angles to the scanning direction of the first
deflector can be changed, so that by obtaining color information for each
frame, such as for example red, yellow, green and blue color image
information and storing this information in a memory or the like and
synthesizing it afterward, it becomes possible to obtain color video
images.
Thus, in accordance with the present invention, highly reliable control of
the laser beam deflection at a high scanning frequency that corresponds to
the raster scan of an ordinary television becomes possible, and with the
scanning range it is possible to make the deflection constant regardless
of the laser beam wavelength. As it is possible to freely select a laser
beam of any desired wavelength from the laser light source, it also is
possible to display images of a different layer of the eye fundus in a
desired hue. Furthermore, when color images are to be obtained, unlike in
the case of a system where laser beams of a plurality of wavelengths are
beamed simultaneously, with the present invention, for each frame
monochrome images at the selected single wavelength is obtained, and by
afterward sythesizing these color images can be obtained. Thus, an
excellent ophthalmic examination apparatus can be obtained that reduces
the burden on the patients eye and decreases the cost because a single
photosensor suffices.
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 a schematic diagram showing the overall structure of an
ophthalmic examination apparatus according to the present invention;
FIG. 2 is an explanatory diagram showing the operating principle of
wavelength selection by means of color dispersion produced by the
acousto-optical deflector; and
FIG. 3 is a diagram of a signal waveform to explain the operation of the
apparatus according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to the schematic diagram of FIG. 1 showing the general
construction of an ophthalmic examination apparatus according to the
present invention, the numeral 1 denotes a laser beam which includes a
plurality of wavelengths such as, for example, that of red, yellow, green
and blue light synthesized from a laser light source (not shown) such as
helium-neon (He-Ne), argon (Ar.sup.+), krypton (Kr.sup.+) or the like. The
laser beams 1 impinge via a lens 4 on a wavelengthselection first
acousto-optical device 2. As shown in FIG. 2, when the first
acousto-optical device 2 is driven at an ultrasonic frequency f by means
of a signal source 2a, when the laser beam 1 having a plurality of
wavelengths impinges on the first acousto-optical device 2 and wavelength
selection is performed through the color dispersion, if the laser beam
wavelength is .lambda., the ultrasonic frequency f and the ultrasonic
velocity is v, then the angle of deflection .theta. of the laser beam can
be expressed as .theta.=.lambda.f/v, so that by varying f it is possible
to select only light which is of a predetermined wavelength. In order to
effectively select this specific wavelength, the laser beam is deflected
by the first acousto-optical device 2 onto a slit 3 via a lens 5, cutting
off light of other wavelengths. Also, by varying the electrical power of
the signal source 2a, the first acousto-optical device 2 can be used to
intensity-modulate the laser beam to project a fixation target or desired
index for examination of retinal functions onto the eye fundus.
What has become a single-wavelength laser beam by passage through the slit
3 impinges on a second acousto-optical device 6 of the same construction
as the first acousto-optical device 2 and which is driven by a signal
source 6a. Here, the frequency which changes the ultrasonic frequency f is
selected as, for example, the 15.75 kHz of the ordinary television
horizontal scan, so that the laser beam is thus deflected at this
frequency and (horizontally) scans the eye fundus at this frequency.
Lenses 7 and 8 are disposed one at each end of the second acousto-optical
device 6 and the laser beam is thereby shaped for impingement on the
rectangular opening of the second acousto-optical device 6, to obtain a
shaped laser beam exiting therefrom. The laser beam deflected for
horizontal scanning by means of the second acousto-optical device 6 is
guided via relay lenses 9 and 10 to a mirror 12 (a galvanomirror) mounted
on a galvanometer 11. Provided between the lenses 9 and 10 is a slit 13
which cuts off zero-order light coming from the second acousto-optical
device 6 and allows only first-order diffraction light to pass. The
galvanomirror is driven at, for example, 60 Hz in synchronization with the
vertical scan rate of an ordinary television, to deflect the laser beam
vertically for scanning.
After the laser beam that thus scans in the two dimensions, horizontally
and vertically, has passed through a lens 14 it is reflected by a mirror
15 and directed at an object lens 16 by means of which it is directed
through the center portion of the pupil of the eye 17 being examined and
impinges on the eye fundus. The light reflected from the eye fundus passes
back though the object lens 16, and by means of a condenser lens 18 is
condensed onto the photosensitive surface of a photosensor 19. Disposed at
the front face of the photosensor 19 is a filter 20 corresponding to the
wavelength of the laser beam selected by means of the first
acousto-optical device 2. The filter 20 is arranged so that it can be
selectively interposed by the rotation of a motor 21. The first
acousto-optical device 2 can for example change laser beam wavelengths for
each frame, and the filter is selected to allow passage of the selected
light by the synchronized rotation of the motor 21.
The electrical signals which have been produced by the photoelectrical
conversion by the photosensor 19 carry eye fundus information which is
stored in a frame memory 23 by means of a signal processing circuit 22.
This frame memory is comprised of, for example, frame memories 23a to 23d
for storing red, yellow, green and blue image information. The images
processed by the signal processing circuit 22 can be displayed on a
monitor television 24.
The operation of the apparatus according to the present invention
constructed as described in the foregoing will now be explained.
First, the laser light source is activated to generate the laser beam 1
possessing a number of wavelengths. The laser beam 1 is made to impinge on
the first acousto-optical device 2 via the lens 4. At this point, by
activating the signal source 2a, as shown in FIG. 2, only the light of the
prescribed wavelength as selected by the slit 3 is allowed to pass. The
laser beam passing through the slit 3 is deflected by the second
acousto-optical device 6 for horizontal scanning. The scanning frequency
is related to changes in the ultrasonic frequency of the signal source 6a,
but because in this case the angle of deflection .theta. depends on the
wavelength of the laser beam (.theta.=80 f/v), each time the laser beam
wavelength selected by the first acousto-optical device 2 changes, the
scanning range is changed.
Therefore, with this invention, in order to compensate for this the range
of variation of the ultrasonic frequency that drives the second
acousto-optical device 6 is changed to f.sub.1R -f.sub.2R in the case of a
red laser beam wavelength, f.sub.1Y -f.sub.2Y in the case of yellow,
f.sub.1G -f.sub.2G in the case green and f.sub.1B -f.sub.2B in the case of
blue. As a result, when the eye fundus of an eye 17 that is being examined
is scanned via the lenses 8 and 9, slit 13, lens 10, mirror 12, lens 14,
mirror 15 and object lens 16, it becomes possible to control the range of
the horizontal scanning so that it becomes the same.
In cases where color images are to be obtained, as the laser light source,
when using for example the four colors, red, yellow, green and blue, as
shown by FIG. 3, the first acousto-optical device 2 is used for the
sequential selection of a different laser beam wavelength every other
frame. When the selected laser beam is projected onto the fundus of the
eye 17 and information therefrom is extracted, the signals coming from the
photosensor 19 are processed by the signal processing circuit 22 and,
assuming for example that it is the red light laser beam that has been
selected, the red image information will be stored in the frame memory
23a. Likewise, if yellow has been selected it will be stored in the frame
memory 23b; if green, in the frame memory 23c; and if blue, in the frame
memory 23d. At this time, the filter 20 interposed in correspondence to
the selected laser beam wavelength by the synchronized rotation of the
motor 21. Finally, a four-frame time period is applied and four frames of
monochromatic eye fundus images are obtained, and by selecting the desired
images from among them and displaying said images on the monitor 24, it
becomes possible to display images of different layers of the eye fundus
in a desired hue.
In addition, because in the present invention control for selection of the
laser beam wavelength is entirely electronic, even if the laser beams of a
wavelength not used previously appear and is to be used in the near
future, there is no need to make any special changes to the optical system
other than the filters with the result that adaption thereto is extremely
simple.
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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