Physics of the Eye and Intralase Surgery. Sight and lenses. The eye is a series of lenses that refract and focus light. These lenses follow the same laws of physics that artificial lenses would. Lenses and focal points.
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To find the focal point, distance of an image, or distance to the object you can use
the thin lens equation.
Light bounces off an object
from a distance s1. This light
passes through the lens to be
focused on the other side. This
image is real, inverted, and
smaller, and is located at a
distance of s2 from the lens.
The thin lens equation
A convex lens
Speed of light in vacuum
Speed of light in substance
This also changes the bulge of the
cornea somewhat. This also aids to
adapt vision into different situations.
The eye can change the shape of the lens
using muscles called ciliary muscles that
stretch and flatten the lens, or relax and
allow it to return to normal shape. This is
Sometimes this bulging of the cornea is an appreciable change that can readily be seen
by the naked eye.
Normal eye. Focus hits directly
on fovea to produce sharp image.
This is known as nearsightedness, or myopia. The image comes into focus before
the retina. Things that are near to the eye can more easily be seen. This
condition can result from the eye being too long, the cornea being too round,
or the lens having too much focus power.
This condition is known as farsightedness, or hyperopia. The image produced by the
lens and cornea comes into a focal point behind where the retina is located. Things
that are farther away can be more easily seen. This results from the eye being too
short, the cornea being too flat, or the focus power of the lens being too weak.
This is a condition known as astigmatism. The shape of the lens and/or cornea is
irregular leading to a loss of focal points or, more commonly, two focal points on
This condition is known as presbyopia. The lens becomes rigid and fails to bend
or accommodate to changing conditions.
A diverging lens will correct
A converging lens corrects for
Bifocal lenses will compensate for
A custom ground lens made specifically
for each eye will correct astigmatism.
Photorefractive keratotomy is the precursor to LASIK surgery.
This type of surgery was introduced in 1987. It uses a Eximer laser
to reshape the outside of the cornea and thus correct vision. PRK can correct
myopia, astigmatism, and some hyperopia.
In LASIK surgery a mechanical
cut is made across the cornea to
create a flap of tissue and exposing
the stroma of the cornea.
This flap is moved aside and a “cool ultraviolet” laser
sculpts and re-shapes the cornea, correcting acuity
problems. This laser allowed for greater precision and
thus greater flexibility for treatment of a broader
range of symptoms.
In 2001, the FDA approved a laser to be used in a surgery called Intralase.
Before surgery, exhaustive measurements are taken of each eye that will be operated on. Measurements of refractivity of the eye tell the prescription that each cornea and lens is contributing.
The topography of the cornea and the measurements of the pupil are also taken.
The eye is anesthetized with drops and a suction ring is placed on the cornea to immobilize the eye. A guide cone is then placed on the ring to direct the eye appropriately. The laser will track the eye in case of accidental eye movement during the operation.
A femosecond laser then makes a very precise
(+/- 10 microns) flap across the surface of the cornea,
instead of a microkeratome. (A human hair is 50
microns in diameter). This reduces rare complications
that LASIK presented, and also opened corrective
eye surgery to previously ineligible patients.
This laser produces carbon dioxide and water bubbles in the cornea. These bubbles
are what actually produce the “incision” to create the flap. This can also be done vertically to produce a deeper incision.