Optics: eye examining – vision testing and correcting – Spectacles and eyeglasses – Ophthalmic lenses or blanks
Patent
1994-03-18
1996-10-29
Nguyen, Viet Q.
Optics: eye examining, vision testing and correcting
Spectacles and eyeglasses
Ophthalmic lenses or blanks
351160R, 351160H, 351159, 359711, G02C 702, G02C 704
Patent
active
055701434
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT/AU92/00418 filed Aug. 7, 1992.
The present invention relates to toric lens technology and more particularly relates to a toric lens which employs non-spherical surfaces including aspheric topography which produces a depth of field effect which reduces or neutralises axis mislocation of the lens and improves the astigmatic lens axis stability latitudes.
For some years now soft and/or hard toric lenses have been manufactured to correct an ocular phenomenon known as astigmatism. In the case of this particular ocular problem the eye has more than one correctable plane of focus.
Whereas a simple ocular problem may be of a short or long sighted nature, an astigmatic eye can incorporate infinite variations of either type of defect or both. This, therefore requires a highly specialized design of lens to be fitted in order to correct this visual defect.
The major criteria involved in correcting astigmatism is accurately aligning the principle power meridians of the lens with the principle refractive meridians meridians in the eye. If this is not done accurately, a resultant misalignment error occurs (i.e. the incorrectly aligned lens powers and the refractive powers of the eye combine to create totally different powers from which the eye needs to attain correct vision) preventing proper correction of astigmatism. This accuracy is determined by how closely the axis of powers in the lens aligns with those of the eye.
As a rule the contact lens Practitioner can refract the astigmatic eye to within one degree of accuracy. The manufacturer should be able to control his lens accuracy to within 5 degrees. This is generally accepted as an industry standard. If however the lens does not position exactly on the axis when in vive then a misalignment error occurs. Even with the 5 degrees manufacturing tolerance, a misalignment error can occur but will generally be accepted by the patient if the cylindrical component of their astigmatism is relatively low (eg. around -2.00 diopters or lower).
If the lens swings 10 degrees or more in vive due to physiological forces exerted during blinking, eye turning, retropulsion of the eye during blinking or similar, then the patients vision will be compromised.
Within the industry, most manufacturers design their lenses specifically to overcome this problem. This can sometimes be at a cost to other very important criteria as for example by making the lens very thick in order to help location, but at a cost of oxygen deprivation to the cornea and a loss of comfort.
It is well known that spherical surfaced optical elements applied to the eye (both spectacle and contact lenses) will induce spherical aberration effects on the retina. Spherical aberration is a common optical phenomenon associated with spherical surfaces. Spherical aberration occurs when rays passing through the periphery of the lens are deviated more than those passing through the paraxial zone. The prismatic effect of a spherical lens is least in the paraxial zone and increases towards the periphery of the lens. It reduces the clarity and luminence of objects focussed by optical elements which are of a spherically surfaced nature. Most cameras and telescopes etc. utilise aspheric or doublet lens systems to reduce or eliminate this spherical aberration. A doublet consists of a principal lens and a somewhat weaker lens of different refractive index cemented together. The weaker lens must be of opposite power, and because it too has spherical aberration, it will reduce the power of the periphery of the principal lens more than the central zone.
Up until recently most contact lenses were manufactured with spherical surfaces due to simplicity of manufacture and as that was the only technology available. The resultant aberrations manifested in the eye were usually less important to the wearer than the improved comfort, cosmesis etc, afforded over the wearing of spectacles. Recently however, aspheric lens forms have been used more frequently to improve visual performance.
This has been particularly so
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Capricornia Contact Lens Pty. Ltd.
Nguyen Viet Q.
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