Scanning laser ophthalmoscope - Patent Review 5430509

Claims

What is claimed is:

1. A scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected and photoelectrically converted by light receiving means to thereby obtain image information on the eye, comprising:

three or more optical scanning means with different scanning frequencies for two-dimensionally scanning a laser beam from a laser light source;

light receiving means for detecting and photoelectrically converting light from one or more of the optical scanning means reflected by the eye, via a prescribed detection aperture; and

signal processing means for converting a detection signal obtained from the light receiving means to a standard television vertical and horizontal scanning line system corresponding to a laser beam scanning pattern obtained with the three or more optical scanning means.

2. A scanning laser ophthalmoscope according to claim 1, in which the detection aperture is located at a position that is optically conjugate with the laser beam focal point in the eye, and is formed in a prescribed shape around a region in which the laser beam focal point moves in accordance with the laser beam scanning pattern of the optical scanning means.

3. A scanning laser ophthalmoscope according to claim 1, in which the laser beam scanning pattern formed by all of the optical scanning means is a two-dimensional zig-zag pattern formed by scanning the laser beam in a second direction perpendicular to a first scanning direction while the beam makes small oscillations along the first scanning direction, and scanning the laser beam in a third direction parallel to the first scanning direction.

4. A scanning laser ophthalmoscope according to claim 1, in which the signal processing means for converting a detection signal obtained from the light receiving means to a standard television scanning line system consists of an A/D converter for performing analogue-to-digital conversion of the detection signal, a memory for storing digital data output by the A/D converter, control means for controlling the writing of data to, and readout of data from, the memory, and a D/A converter for performing digital-to-analogue conversion of digital data read out of the memory.

5. A scanning laser ophthalmoscope according to claim 1 further comprising optical focal adjustment means for moving the laser beam focal point in the eye.

6. A scanning laser ophthalmoscope according to claim 5, in which the focus adjustment means is a slanted lens that can be moved along the path of the laser beam.

7. A scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected and photoelectrically converted by light receiving means to thereby obtain image information on the eye, comprising:

first optical scanning means for scanning a laser beam from a laser light source in one direction at a prescribed frequency;

second optical scanning means for scanning the laser beam in a direction that is perpendicular to the scanning direction of the first optical scanning means at a frequency that is lower than above said frequency;

third optical scanning means for scanning the laser beam in a direction that is perpendicular to the scanning direction of the first or second optical scanning means at a frequency that is lower than either of above said frequencies;

light receiving means for detecting and photoelectrically converting light from one or more of the first, second, and third optical scanning means reflected by the eye, via a prescribed detection aperture; and

signal processing means for converting a detection signal obtained from the light receiving means to a standard television vertical and horizontal scanning line system corresponding to a laser beam scanning pattern obtained with the first, second, and third optical scanning means.

8. A scanning laser ophthalmoscope according to claim 7, in which the detection aperture is located at a position that is optically conjugate with the laser beam focal point in the eye, and is formed in a prescribed shape around a region in which the laser beam focal point moves in accordance with the laser beam scanning pattern of the optical scanning means.

9. A scanning laser ophthalmoscope according to claim 7, in which the laser beam scanning pattern formed by all of the optical scanning means is a two-dimensional zig-zag pattern formed by scanning the laser beam in a second direction perpendicular to a first scanning direction while the beam makes small oscillations along the first scanning direction, and scanning the laser beam in a third direction parallel to the first scanning direction.

10. A scanning laser ophthalmoscope according to claim 7 in which the signal processing means for converting a detection signal obtained from the light receiving means to a standard television scanning line system consists of an A/D converter for performing analogue-to-digital conversion of the detection signal, a memory for storing digital data output by the A/D converter, control means for controlling the writing of data to, and readout of data from, the memory, and a D/A converter for performing digital-to-analogue conversion of digital data read out of the memory.

11. A scanning laser ophthalmoscope according to claim 7, further comprising optical focal adjustment means for moving the laser beam focal point in the eye.

12. A scanning laser ophthalmoscope according to claim 11, in which the focus adjustment means is a slanted lens that can be moved along the path of the laser beam.

13. A scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected and photoelectrically converted by light receiving means to thereby obtain image information on the eye, comprising:

an acousto-optical deflector for scanning a laser beam from a laser light source in one direction at a prescribed frequency;

a vibration mirror galvanometer for scanning the laser beam in a direction that is perpendicular to the scanning direction of the acousto-optical deflector at a frequency that is lower than above said frequency;

a mirror galvanometer for scanning the laser beam in a direction that is perpendicular to the scanning direction of the vibration mirror galvanometer at a frequency that is lower than either of above said frequencies;

light beam separating means disposed on the light path between the acousto-optical deflector and the vibration mirror galvanometer for separating an illuminating beam projected at the eye from a beam reflected from the eye;

light receiving means for detecting and photoelectrically converting light from the two mirror galvanometers and the light beam separating means reflected by the eye, via a prescribed detection aperture; and

signal processing means for converting a detection signal obtained from the light receiving means to a standard television vertical and horizontal scanning line system corresponding to a laser beam scanning pattern obtained with the three optical scanning means.

14. A scanning laser ophthalmoscope according to claim 13, in which the detection aperture is located at a position that is optically conjugate with the laser beam focal point in the eye, and is formed in a prescribed shape around a region in which the laser beam focal point moves in accordance with the laser beam scanning pattern of the optical scanning means.

15. A scanning laser ophthalmoscope according to claim 13, in which the laser beam scanning pattern formed by all of the optical scanning means is a two-dimensional zig-zag pattern formed by scanning the laser beam in a second direction perpendicular to a first scanning direction while the beam makes small oscillations along the first scanning direction, and scanning the laser beam in a third direction parallel to the first scanning direction.

16. A scanning laser ophthalmoscope according to claim 13, in which the signal processing means for converting a detection signal obtained from the light receiving means to a standard television scanning line system consists of an A/D converter for performing analogue-to-digital conversion of the detection signal, a memory for storing digital data output by the A/D converter, control means for controlling the writing of data to, and readout of data from, the memory, and a D/A converter for performing digital-to-analogue conversion of digital data read out of the memory.

17. A scanning laser ophthalmoscope according to claim 13, further comprising optical focal adjustment means for moving the laser beam focal point in the eye.

18. A scanning laser ophthalmoscope according to claim 17, in which the focus adjustment means is a slanted lens that can be moved along the path of the laser beam.

19. A scanning laser ophthalmoscope according to claim 13, in which a prism is provided in front of and behind the acousto-optical deflector to correct the wavelength dependence of the angle of incidence and angle of emergence of the beam with respect to the deflector.

20. A scanning laser ophthalmoscope according to claim 13, in which the light beam separation means separates zero-order and first-order diffracted light from the acousto-optical deflector.

21. A scanning laser ophthalmoscope according to claim 20, in which the zero-order light separated by the light beam separation means is used to monitor the power of the laser beam.

22. A scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected and photoelectrically converted by light receiving means to thereby obtain image information on the eye, comprising:

non-mechanical optical scanning means for scanning a laser beam from a laser light source in one direction at a prescribed frequency;

a vibration mirror galvanometer for scanning the laser beam in a direction that is perpendicular to the scanning direction of the non-mechanical optical scanning means at a frequency that is lower than above said frequency;

synchronous detection means for detecting synchronously with the scanning of the vibration mirror galvanometer;

a mirror galvanometer for scanning the laser beam in a direction that is perpendicular to the scanning direction of the vibration mirror galvanometer at a frequency that is lower than either of above said frequencies;

light receiving means for detecting and photoelectrically converting light from the two mirror galvanometers reflected by the eye, via a prescribed detection aperture; and

signal processing means for converting a detection signal obtained from the light receiving means to a standard television vertical and horizontal scanning line system corresponding to a laser beam scanning pattern obtained with the three optical scanning means, and in accordance with a control signal from the synchronous detection means.

23. A scanning laser ophthalmoscope according to claim 22, in which the detection aperture is located at a position that is optically conjugate with the laser beam focal point in the eye, and is formed in a prescribed shape around a region in which the laser beam focal point moves in accordance with the laser beam scanning pattern of the optical scanning means.

24. A scanning laser ophthalmoscope according to claim 22, in which the laser beam scanning pattern formed by all of the optical scanning means is a two-dimensional zig-zag pattern formed by scanning the laser beam in a second direction perpendicular to a first scanning direction while the beam makes small oscillations along the first scanning direction, and scanning the laser beam in a third direction parallel to the first scanning direction.

25. A scanning laser ophthalmoscope according to claim 22, in which the signal processing means for converting a detection signal obtained from the light receiving means to a standard television scanning line system consists of an A/D converter for performing analogue-to-digital conversion of the detection signal, a memory for storing digital data output by the A/D converter, control means for controlling the writing of data to, and readout of data from, the memory, and a D/A converter for performing digital-to-analogue conversion of digital data read out of the memory.

26. A scanning laser ophthalmoscope according to claim 22, further comprising optical focal adjustment means for moving the laser beam focal point in the eye.

27. A scanning laser ophthalmoscope according to claim 26, in which the focus adjustment means is a slanted lens that can be moved along the path of the laser beam.

28. A scanning laser ophthalmoscope according to claim 22, in which the synchronous detection means uses light reflected by the reverse side of the vibration mirror galvanometer to perform synchronous detection.

29. A scanning laser ophthalmoscope according to claim 22, in which the signal output by the synchronous detection means is used to correct hysteresis accompanying the sine wave scanning of the vibration mirror galvanometer.

30. A scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected and photoelectrically converted by light receiving means to thereby obtain image information on the eye, comprising:

light intensity modulating means for modulating the intensity of a laser beam from a laser light source;

three or more optical scanning means with different scanning frequencies for two-dimensionally scanning a laser beam from a laser light source;

light receiving means for detecting and photoelectrically converting light from one or more of the optical scanning means reflected by the eye, via a prescribed detection aperture;

signal processing means for converting a detection signal obtained from the light receiving means to a standard television vertical and horizontal scanning line system corresponding to a laser beam scanning pattern obtained with the three or more optical scanning means;

a video signal source that generates a prescribed video pattern for visual examination purposes; and

signal processing means that in accordance with a laser beam scanning pattern obtained with the optical scanning means converts a standard video signal output from the video signal source to a control signal that can be supplied to the light intensity modulating means.

31. A scanning laser ophthalmoscope according to claim 30, in which the detection aperture is located at a position that is optically conjugate with the laser beam focal point in the eye, and is formed in a prescribed shape around a region in which the laser beam focal point moves in accordance with the laser beam scanning pattern of the optical scanning means.

32. A scanning laser ophthalmoscope according to claim 30, in which the laser beam scanning pattern formed by all of the optical scanning means is a two-dimensional zig-zag pattern formed by scanning the laser beam in a second direction perpendicular to a first scanning direction while the beam makes small oscillations along the first scanning direction, and scanning the laser beam in a third direction parallel to the first scanning direction.

33. A scanning laser ophthalmoscope according to claim 30, in which the signal processing means for converting a detection signal obtained from the light receiving means to a standard television scanning line system consists of an A/D converter for performing analogue-to-digital conversion of the detection signal, a memory for storing digital data output by the A/D converter, control means for controlling the writing of data to, and readout of data from, the memory, and a D/A converter for performing digital-to-analogue conversion of digital data read out of the memory.

34. A scanning laser ophthalmoscope according to claim 30, further comprising optical focal adjustment means for moving the laser beam focal point in the eye.

35. A scanning laser ophthalmoscope according to claim 34, in which the focus adjustment means is a slanted lens that can be moved along the path of the laser beam.

36. A scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected and photoelectrically converted by light receiving means to thereby obtain image information on the eye, comprising:

three or more optical scanning means with different scanning frequencies for two-dimensionally scanning a laser beam from a laser light source;

a tilted objective mirror for projecting a scanning laser beam obtained with the optical scanning means at an eye;

control means that controls the deflection angle of one or more of the optical scanning means to correct optical aberration produced by the tilted objective mirror;

control means that corrects raster distortion arising from sine wave oscillation in one of the optical scanning means by adjusting the deflection angle of an optical scanning means having a scanning frequency higher than the sine wave oscillation frequency;

light receiving means for detecting and photoelectrically converting light from one or more of the optical scanning means reflected by the eye, via a prescribed detection aperture; and

signal processing means for converting a detection signal obtained from the light receiving means to a standard television vertical and horizontal scanning line system corresponding to a laser beam scanning pattern obtained with the three or more optical scanning means.

37. A scanning laser ophthalmoscope according to claim 36, in which the detection aperture is located at a position that is optically conjugate with the laser beam focal point in the eye, and is formed in a prescribed shape around a region in which the laser beam focal point moves in accordance with the laser beam scanning pattern of the optical scanning means.

38. A scanning laser ophthalmoscope according to claim 36, in which the laser beam scanning pattern formed by all of the optical scanning means is a two-dimensional zig-zag pattern formed by scanning the laser beam in a second direction perpendicular to a first scanning direction while the beam makes small oscillations along the first scanning direction, and scanning the laser beam in a third direction parallel to the first scanning direction.

39. A scanning laser ophthalmoscope according to claim 36, in which the signal processing means for converting a detection signal obtained from the light receiving means to a standard television scanning line system consists of an A/D converter for performing analogue-to-digital conversion of the detection signal, a memory for storing digital data output by the A/D converter, control means for controlling the writing of data to, and readout of data from, the memory, and a D/A converter for performing digital-to-analogue conversion of digital data read out of the memory.

40. A scanning laser ophthalmoscope according to claim 36, further comprising optical focal adjustment means for moving the laser beam focal point in the eye.

41. A scanning laser ophthalmoscope according to claim 40, in which the focus adjustment means is a slanted lens that can be moved along the path of the laser beam.

42. A scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected and photoelectrically converted by light receiving means to thereby obtain image information on the eye, comprising:

non-mechanical optical scanning means for scanning a laser beam from a laser light source in one direction at a prescribed frequency;

two mirror galvanometers for scanning the laser beam perpendicularly and parallel to the scanning direction of the non-mechanical optical scanning means at two frequencies that are lower than above said frequency;

light receiving means for detecting and photoelectrically converting light from the two mirror galvanometers reflected by the eye, via a prescribed detection aperture;

control means that performs angle of view conversion by changing the laser beam deflection angle of the three optical scanning means; and

signal processing means for converting a detection signal obtained from the light receiving means to a standard television vertical and horizontal scanning line system corresponding to a laser beam scanning pattern obtained with the three optical scanning means.

43. A scanning laser ophthalmoscope according to claim 42, in which the detection aperture is located at a position that is optically conjugate with the laser beam focal point in the eye, and is formed in a prescribed shape around a region in which the laser beam focal point moves in accordance with the laser beam scanning pattern of the optical scanning means.

44. A scanning laser ophthalmoscope according to claim 42 in which the laser beam scanning pattern formed by all of the optical scanning means is a two-dimensional zig-zag pattern formed by scanning the laser beam in a second direction perpendicular to a first scanning direction while the beam makes small oscillations along the first scanning direction, and scanning the laser beam in a third direction parallel to the first scanning direction.

45. A scanning laser ophthalmoscope according to claim 42, in which the signal processing means for converting a detection signal obtained from the light receiving means to a standard television scanning line system consists of an A/D converter for performing analogue-to-digital conversion of the detection signal, a memory for storing digital data output by the A/D converter, control means for controlling the writing of data to, and readout of data from, the memory, and a D/A converter for performing digital-to-analogue conversion of digital data read out of the memory.

46. A scanning laser ophthalmoscope according to claim 42, further comprising optical focal adjustment means for moving the laser beam focal point in the eye.

47. A scanning laser ophthalmoscope according to claim 46, in which the focus adjustment means is a slanted lens that can be moved along the path of the laser beam.

48. A scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected and photoelectrically converted by light receiving means to thereby obtain image information on the eye, comprising:

a laser diode array with optical scanning capability as a laser light source for generating laser beams of one or multiple frequencies and scanning the emitted laser beams in one direction at a prescribed frequency;

two mechanical optical scanning means for scanning the laser beam perpendicularly and parallel to the scanning direction of the laser diode array at two frequencies that are lower than above said frequency;

light receiving means constituted by a photodiode array for detecting and photoelectrically converting light from the mechanical optical scanning means reflected by the eye; and

signal processing means for converting a detection signal obtained from the light receiving means to a standard television vertical and horizontal scanning line system corresponding to a laser beam scanning pattern obtained with the laser diode array and two mechanical optical scanning means.

49. A scanning laser ophthalmoscope according to claim 48, in which the laser beam scanning pattern formed by all of the optical scanning means is a two-dimensional zig-zag pattern formed by scanning the laser beam in a second direction perpendicular to a first scanning direction while the beam makes small oscillations along the first scanning direction, and scanning the laser beam in a third direction parallel to the first scanning direction.

50. A scanning laser ophthalmoscope according to claim 48, in which the signal processing means for converting a detection signal obtained from the light receiving means to a standard television scanning line system consists of an A/D converter for performing analogue-to-digital conversion of the detection signal, a memory for storing digital data output by the A/D converter, control means for controlling the writing of data to, and readout of data from, the memory, and a D/A converter for performing digital-to-analogue conversion of digital data read out of the memory.

51. A scanning laser ophthalmoscope according to claim 48, further comprising optical focal adjustment means for moving the laser beam focal point in the eye.

52. A scanning laser ophthalmoscope according to claim 51, in which the focus adjustment means is a slanted lens that can be moved along the path of the laser beam.

FIELD OF THE INVENTION

The present invention relates to an ophthalmoscope, and more particularly to a scanning laser ophthalmoscope in which a laser beam from a laser light source is projected onto a prescribed part of an eye and scanned in two dimensions, and light reflected from the eye is detected by light receiving means and photoelectrically converted to thereby obtain image information about the eye.

BACKGROUND OF THE INVENTION

In vivo examination of the eye fundus is used not only for ophthalmological purposes but also for diagnosing other disorders that include hypertension, diabetes, and diseases of the cerebral nerves. One technique used for such examinations involves the physician using a device known as the ophthalmoscope to directly observe the eye fundus. In another method that is extensively applied, a special fundus camera is used to record photographs of the fundus on conventional film. The advances made in recent years by electronics have led to the use of optoelectronic transducers such as imaging tubes and the like in place of the photographic film of the conventional fundus camera, whereby eye fundus information is directly obtained in the form of electric signals which can be processed, stored and displayed on a television monitor or the like.

One innovative development in ophthalmology that has attracted attention is that of an electronic ophthalmoscope that utilizes laser scanning techniques. Such a device is known as a scanning laser ophthalmoscope (hereinafter also abbreviated to "SLO"), and is being developed and improved mainly in the U.S., Germany, France and Japan.

With the first SLOs, a laser beam was passed through the center of the pupil and used to scan the eye fundus two-dimensionally, and the light reflected from the fundus through a larger area around the periphery of the pupil was picked up, photoelectrically converted and amplified, whereby with the fundus illuminated at a low brightness it was possible to display a video image of the fundus on a monitor television in real-time with a high S/N ratio (see Reference (1): U.S. Pat. No. 4,213,678 and Applied Optics Vol. 19 (1980) pp 2991 to 2997).

The feasibility of using an active optical element to compensate for the effect of aberration in the optical system of the eye was studied as a way of achieving a major improvement in fundus image resolution, compared to the conventional fundus camera (see Reference (2): DP 3245939, U.S. Pat. No. 4579430, JP-A-59-115024, SPIE Proceedings Vol. 498 (1984) pp 76 to 82).

The adoption of a confocal optical system in the device arrangement was particularly effective for improving picture quality. This eliminated the effect of stray and scattered light and produced a marked improvement in fundus image contrast and resolution (see Reference (3): FP 2555039, JP-A-60-132536, Journal of Optics (Paris) Vol. 15 (1984) pp 425 to 430; Reference (4): U.S. Pat. No. 4764005, JP-A-62-117524, Applied Optics Vol. 26 (1987) pp 1492 to 1499). In such an arrangement, by simultaneously scanning the incident and reflected light beams (double-scanning) and using an optoelectronic detector to acquire and fix reflected light scanning, just the reflected light from a point that is optically conjugate with the fundus of the eye being examined can be detected via a confocal aperture such as a pinhole, enabling the effect of unrequired light scattered by the optical system of the eye to be totally excluded.

The confocal optical system has also been used in attempts to detect three-dimensional sectional shapes of the fundus and anterior chamber by two-dimensional laser beam scanning (X and Y scanning) combined with scanning in the direction of the optical axis (Z scanning) (sec Reference (5): SPIE Proceedings Vol. 1028 (1988) pp 127 to 132).

It has been confirmed that a confocal optical system that uses a slit instead of a pinhole is also a highly effective way of improving the quality of fundus images (see Reference (6): JP-A-64-58237, U.S. Pat. No. 4854692, Measurement Science and Technology Vol. 2 (1991) pp 287 to 292).

More recently still, an apparatus has been developed which represents a major advancement, in that it can provide completely real-time detection and display of uneven configurations in the fundus (see Reference (7): JP-A-1-113605, U.S. Pat. No. 4,900,144, Optics Communications Vol. 87 (1992) pp 9 to 14).

Each of these new ophthalmological devices are highly practical because they enable the fundus image to be observed without using a mydriatic to dilate the pupil, despite the relatively small diameter of the pupil. With these devices, the crystalline lens, the anterior chamber and other such regions can be observed by shifting the focal point of the laser beam from the focal plane, the amount of fluorescent agent that needs to be administered when carrying out fluorography of the eye fundus can be considerably decreased, visual function can be examined during observation of the fundus by modulating the scanning laser beam, and a wide range of precise, microscopic examinations of the fundus are possible, using the monochromatic properties of lasers. SLOs are bringing about major innovations in ophthalmology.

However, a major drawback with such devices is the difficulty of the laser beam deflection control system. In References (1) and (3), for example, two mechanical deflectors (swinging mirrors) arc used to scan the laser beam at a horizontal frequency of some 8 kHz and a vertical frequency of 60 Hz (or 50 Hz). Reference (3) also uses a modified system configuration in which the 8 kHz oscillation rate of the horizontal mirror is doubled to enable tracking at a standard TV horizontal scanning frequency of about 16 kHz.

The rapid wear of the mirror suspension bearings caused by this high mirror oscillation frequency of 8 kHz used in those systems to effect the horizontal scanning has an adverse affect on system durability. Over time, shaft wear and fatigue can result in shaft run-out, deviation, hysteresis and other such variations, which, in the case of a SLO in which image quality depends on beam scanning precision, degrades the reliability of the apparatus itself. Another problem with a mirror oscillating at a scanning frequency above about 8 kHz is that in order to achieve the increased oscillation frequency the mirror has to be no more than 5 mm in diameter, and the deflection angle must not exceed 10 degrees or so, which in the case of an eye fundus image system make it impossible to provide high resolution with a wide field of view.

References (2) and (4) use a mirror galvanometer for the low frequency vertical scanning and a rotating polygonal mirror as the deflector for the horizontal scanning. Systems using a polygonal mirror have good beam scanning high-speed characteristics and linearity, and as they are capable of a deflection angle of 20 degrees or more they are better able to provide high image quality with a wide viewing angle than systems that use high frequency vibration mirrors. For full synchronization with the standard NTSC television scanning system, Reference (4) uses a horizontal scanning frequency of 15.75 kHz and a vertical scanning frequency of 60 Hz. In view of current state of the technology, these scanning frequencies are an eminently good choice, and are also practical with reference to interfacing with peripheral equipment.

However, a problem with achieving a scanning frequency of 15.75 kHz is that of the high speed at which the polygonal mirror has to spin, 37,800 rpm in the case of a mirror with 25 facets. In other words, there is still the problem of durability that arises in the case of a mechanical deflector operating at high frequencies, with parts affected by wear and metal fatigue degrading the precision and shortening the service life of the system. With a polygonal mirror, also, image quality can be degraded by unevenness in the laser beam raster caused by shaft play and slight differences in facet trueness and facet division tolerance, and the mirror is also prone to external vibration. A system that uses high-speed rotation needs large bearings and rotation is restricted to a predetermined direction, which make it difficult to reduce the size of the system. A further drawback is the small size that each facet of the mirror is restricted to, so that the scanning is accompanied by an optical shift of the pupil which, in a confocal optical system, results in a reduction of detection efficiency at the end of scan lines and shading of the images.

To avoid the problems relating to durability, shaft run-out and the like that are inherent to mechanical deflectors, for the horizontal scanning the systems of References (6) and (7) each use a non-mechanical acousto-optical deflector (AOD) having no moving parts. An AOD ensures a long service life and highly stable and precise scanning, and also makes it easier to reduce system size.

However, with an AOD the size of the crystal aperture is limited, and generally there are also limitations on the polarization direction that transmits the light, which make it difficult to configure a perfectly confocal optical system using simultaneous double scanning of the incident beam and light reflected by the fundus. Reference (6) therefore describes use of a modified SLO optical system in which the confocal aperture is a slit. Compared to a non-confocal optical system, one that uses a slit aperture offers a marked improvement in image contrast, and is also advantageous in terms of the design of the system apparatus. Even compared to a pinhole (i.e. round aperture) confocal optical system, a slit does not produce much of a difference when the fundus image is observed using short-wavelength visible light (such as blue, green, yellow). However, when long-wavelength light (such as red and infrared) is used, an image obtained with a SLO that uses a slit aperture is closer to what is obtained with a non-confocal optical arrangement, with the contrast of the retinal vessels in the fundus image slightly lower than that obtained with pinhole aperture.

Thus, a problem of an AOD with a slit aperture confocal optical system, especially when using infrared light, is that to some extent it has limited the contrast in images of retinal vessels. The biggest drawback of an AOD deflector is that when high image resolution is required, the AOD has to be constituted of a special substance such as TeO.sub.2 or PbMoO.sub.4, formed into a flat, uniform optical medium with a large-diameter aperture. This usually requires that an anamorphic lens, which is complex to adjust, be disposed to the front and rear of the AOD along the optical axis, and the crystal itself is far more costly than small-aperture media.

With reference to the cost aspect, a high-precision, high-speed swinging mirror or polygonal mirror with pneumatic bearings is also very costly. Thus, the matter of cost and problems concerning deflector scanning performance and reliability are probably what has hindered the practical use and spread of SLOs.

Recent years have also seen the realization of high definition television (HDTV), the aim of which is to provide improved resolution and picture quality, and the feasibility of HDTV compatible SLO systems is being studied. However, with an HDTV system having a horizontal scanning frequency of 30 kHz or more, the above-described problems of each type of deflector arrangement would be correspondingly magnified if attempts were made to operate them at such a high frequency. For this reason, despite the interest in the potential of a HDTV compatible SLO, specific working principles or methods for a commercially practicable system have not yet emerged.

The object of this invention is to provide a scanning laser ophthalmoscope having optical scanning means which combines stable, high frequency operation with long service life, high scanning precision and low cost, thereby reducing the overall cost of the system apparatus, a compact system configuration, a confocal optical system that enables high contrast images to be obtained whatever the wavelength of the light used, and which is also fully HDTV adaptable.

SUMMARY OF THE INVENTION

In accordance with the present invention, a laser beam is scanned two-dimensionally using three or more optical scanning means each having a different scanning frequency, which enables the performance conditions required of each scanning means to be reduced, thus providing a durable, low cost deflection control system that is highly stable and precise even at high scanning frequencies. Moreover, the ability to use either a slit or pinhole detection aperture for the confocal optical system provides a large degree of configurability and is also an advantage with respect to improving the contrast and resolution of eye fundus images. Furthermore, although a special scanning pattern is used, as the system is equipped with signal processing means for converting the output of the photosensors to a standard television line scanning system, eye fundus images can be displayed on a standard television and, as such, can be readily adapted for high-definition television.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the general arrangement of the optical system of the scanning laser ophthalmoscope apparatus according to the present invention.

FIG. 2 shows the beam scanning pattern produced by the three scanning means of the apparatus.

FIG. 3 shows details of the optical system of the apparatus.

FIGS. 4 (a-c) show the shape and size of the confocal optical aperture in the light receiving system of the apparatus.

FIG. 5 is a block diagram of the electrical configuration of the apparatus.

FIG. 6 is a block diagram of the signal processing arrangement used to convert the scanning system in the apparatus.

FIG. 7 illustrates the system's line memory based scanning conversion method.

FIGS. 8 (a-e) illustrate the method of correcting image plane distortion in the apparatus.

FIGS. 9 (a-b) illustrate the angle of view conversion principle used in the apparatus.

FIG. 10 shows the arrangement of the main components of an optical system according to another embodiment of the invention.

FIG. 11 shows a laser beam scanning pattern produced by the scanning means of the apparatus of the other embodiment.

FIG. 12 is a perspective view of the configuration of laser diode array used in the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the general arrangement of mainly the optical system of the scanning laser ophthalmoscope according to the present invention. In FIG. 1, reference numeral 1 denotes a laser diode (LD), helium-neon (He--Ne), argon (Ar.sup.+) or other such laser light source which produces light at any of the wavelengths 780 nm (infrared) or 670 nm (red) in the case of an LD, 632.8 nm (red), 611.9 nm (orange), 594.1 nm (yellow) or 543.5 nm (green) in the case of an He--Ne laser, or 514.5 nm (blue-green) or 488 nm (blue) in the case of an Ar.sup.+ laser. Although usually a plurality of laser sources will be used, with beams being combined by a dichroic mirror or the like and a shutter or other such means to enable particular wavelengths to be selected as required, for the purpose 6f this example, one laser source is assumed and illustrated.

A laser beam 1 a emitted by the laser light source 1 is collimated by a lens 2 and reflected by a mirror 3 onto a first scanning means 4. This first scanning means 4 is a non-mechanical optical deflector that can deflect the laser beam at or above a scanning frequency of 5 MHz, such as an AOD or electro-optic deflector (EOD), for example, and is controlled by drive signals from a signal source 4s. With the first scanning means 4, scanning frequency of 7.16 MHz, for example, was selected for overall synchronization of the system with the standard NTSC television scanning system.

Part of the laser beam 4a scanned at high speed in one direction by the first scanning means 4 is reflected by a beam-splitter 5 and is guided via lenses 6 and 7 to a vibration mirror (resonant swinging mirror) 8M provided on a vibration galvanometer 8. Vibration galvanometer 8 is driven from a signal source 8s to use mirror 8M to effect sine wave deflection of the beam. Galvanometer 8 and mirror 8M form a second scanning means that operates at a frequency such as 3.94 kHz, for example, selected based on an overall system consideration of the standard TV scanning system. Scanning by the second scanning means is perpendicular to the scanning direction of the first scanning means.

The raster of the laser beam scanned two-dimensionally by the first scanning means 4 and second scanning means (8 and 8M) is guided by spherical relay mirrors 9 and 10 and further deflected by a mirror galvanometer 11M provided on galvanometer 11. Galvanometer 11 is driven from a signal source 11s to deflect the laser beam in a sawtooth scanning pattern, forming a third scanning means. A scanning frequency of 60 Hz, for example, is selected for the mirror galvanometer 11M, based on a consideration of the standard TV scanning system. Scanning by the third scanning means is perpendicular to the scanning direction of the second scanning means, and therefore parallel to the scanning of the first scanning means.

The raster scan produced by the laser beam being reflected by the mirror galvanometer 11M (i.e. the third scanning means) is reflected by mirror 12 and tilted objective mirror 13 and directed through the pupil 14a of the eye 14 and onto the eye fundus 14b.

Reflected light (shown in the drawing by dotted lines) from the fundus 14b travels back along the same path, going via optical system elements 13, 12, 11, 10, 9, 8, 7 and 6, and is then guided by beam-splitter 5, lens 15 and confocal aperture (detection diaphragm) 16 to photosensor 18. The confocal aperture 16 is located at a point that is an optical conjugate of the fundus focal plane to block unrequired scattered light from within the eye and glare components produced in the optical system, and thereby serves to provide a major improvement in the contrast of the fundus image. Eye fundus reflected light signal components from which stray light components have been eliminated by the confocal aperture 16 are photoelectrically converted to detection signals by the photosensor 18. After being processed by a signal processor 19, the detection signals are input to an image output device 20 such as a TV monitor to display the images of the eye fundus 14b.

The lenses 6 and 7 provided in the light projection/receiving path are used for focal point adjustment (focus or diopter correction) to correct for nearsightedness, farsightedness, astigmatism or other such vision conditions of the eye being examined. For example, misalignment of the laser beam forcal point caused by refractive differences in the eye can be adjusted for by moving the lens 7 along the optical axis, as indicated by the arrow 7a. To prevent light components reflected by the front and rear surfaces of the lenses 6 and 7 from finding its way to the photosensor 18 and thereby reducing the contrast of fundus images, it is preferable for each of the lenses to be tilted at a slight angle to the optical axis, as shown in FIG. 1. Lenses 6 and 7 can also be used for a fourth scanning means for scanning the beam in a fourth direction. The component of the projection laser beam emitted by laser light source 1 that is transmitted by the beam-splitter 5 goes via lens 21 to a second photosensor 22 where it is photoelectrically converted and used to monitor laser beam intensity.

The back of the vibration mirror 8M constituting the second scanning means is illuminated by a light beam from a light-emitting diode 23 via lens 24, which is reflected onto a third photosensor 25. The third photosensor 25 puts out a signal that is used as a reference signal to control synchronization timing and correct hysteresis accompanying sine wave scanning by the second scanning mea