Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Patent
1996-05-09
1998-09-29
Dang, Hung X.
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
351205, 351246, A61B 310
Patent
active
058152423
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a scanning ophthalmoscope for scanning the retina of the eye.
The need for a wide field retinal imaging ophthalmoscope is based on the fact that current fundus camera designs can produce high quality film based images in colour, but are limited in their field of view of the object plane to a maximum of 60 degrees from the pupillary point. Many manufacturers in fact produce less than the 60 degree field of view. Scanning laser ophthalmoscopes currently produce a 40degree field of view in true monochrome or synthesised (not true) colour images. The resolution of the scanned laser-based images may be two to three orders of magnitude less than the film based images, but the advantage of the scanning laser ophthalmoscope is that dynamic images are available for display on standard television monitors. These images may be recorded on standard video tape.
The field of view of both film based cameras and scanning laser ophthalmoscopes may be artificially increased by rotating the camera about a specific centre of rotation, lying between the rear surface of the cornea and the front surface of the lens, the so-called "pupillary point". However, an instrument which produces complete wide field images in one scan, without mechanical machine movement, is valuable in producing more efficient diagnostic information for the clinician. High resolution of the scanning laser ophthalmoscope image enhances interpretation of fine detail, while the production of the image in colour in such an instrument would assist in the clinical diagnostic process particularly for certain diseases such as proliferative diabetic retinopathy.
A limitation exists with the optics of the human eye in that the resolution of an incident-laser beam at the retina is restricted by diffraction to about 10 mms in diameter. This is due to the cornea, lens, and the aqueous and vitreous humours of optical path of the eye. Thus, no image may contain higher resolution than this however the image is produced.
It is the objective of this invention to produce high resolution true colour images in a single scan.
Colour imaging from scanning laser based instruments has been proposed from a variety of sources, but none of these embraces a wide field of view at the retina, in colour and at acceptably high resolution. In order to achieve this, the characteristics of the structure of the optical system of the eye must be examined in more detail than has been carried out to date.
PRIOR ART
In this field it is already known that three generic types of scanning laser ophthalmoscope exist for use in clinical environments. The function and use of the three types are fundamentally different.
The three types may be classified by the manufacturers' names viz;
The functions of the three types are summarised as follows:
This system scans the retina of the eye through the use of a tilted spherical mirror. X and Y scans are produced by means of a high speed rotating polygon, and an oscillating galvanometer mirror. Laser beams of wave lengths corresponding to Argon ion (488 nm), Helium Neon (566 nm) and Infra-red (790 nm) are focused through a manually variable focusing system to produce a best image. The scanning system produces pixel image data, by means of an electronic detector system, synchronised with the scanning mirrors to produce data at video rate output. The signal is electronically processed (not through a framestore) to produce an image which is capable of being displayed on an ordinary television monitor. They system is therefore capable of real time dynamic imaging.
The deficiencies of this system are that the image is currently only obtainable in monochrome and has a maximum resolution of approximately 400,000 pixels. The field of view is also restricted to about 32 degrees maximum at this resolution. The mirror system is acknowledged to contain abberations, but no corrections are effected on the instruments, although some cross-scan errors are highlighted for correction through the use of toroidal lenses in later patents.
The Heidelburg
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Anderson Douglas Crombie
Henderson Robert
Lucas Roger Albert
Dang Hung X.
OPTOS Public Limited Company
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