How the Eye Works
The eye is a complex organ that fulfils an even more complex process. Located within the orbit of the skull, it is responsible for detecting light from the environment and changing it into signals that can be understood by the brain. This process is known as the visual pathway and allows us to see what is in our environment.
The eye is roughly spherical, with the front part being split into two segments, also known as chambers. The anterior or front chamber contains a liquid called the aqueous humour, which is clear and watery. This fluid occupies the space in between the iris (the coloured part of the eye) and the very front part of the eye the cornea. The posterior chamber is the space behind the iris, but in front of the lens.
The part of the eye responsible for detecting light is the retina, which is located at the back. This area is made up almost entirely of photoreceptors (things that detect light), known as rods and cones. The rods are found in very high concentrations around the edge of the retina, allowing us to distinguish between objects in very low light conditions.
Cones on the other hand allow us to see in colour, providing us with very accurate pictures, but only when there is lots of light. They are found in very high concentrations in an area called the fovea which is part of the macula on the retina. The macula is the area that is responsible for central vision and when damaged, such as in Age Related Macular Degeneration (ARMD), leads to the loss of vision in the central field.
For the rods and cones to function correctly, the light that enters the front of the eye must be focussed onto the retina, to give an accurate image of the object. There are two main parts of the eye that are crucial in this process, the cornea and the lens.
The cornea is the clear covering over the front of the eye. It has 5 layers, however the 3 most important in laser surgery are (from out to in):
Each of the three layers can grow at different rates. The corneal epithelium is able to grow very quickly over a number of days, whereas both the Bowman’s and stromal layers cannot grow if damaged. This is very important in laser surgery, as remodelling these layers can permanently correct problems with vision.
The cornea is responsible for nearly 80% of the focussing ability of the eye and is the main target for laser eye treatments. The lens on the other hand, does the other 20% worth of the focussing. Together they act like the lens in a camera.
For us to see an object clearly, light rays must enter our eye and pass through the cornea and the lens. At this point, they are bent inwards in a process called refraction. This causes the image to become focussed on a small point of the retina. How well the eye can bend the light is called the refractive or optical power and is measured in dioptres. The subsequent activation of the rods and cones leads to an electrical impulse being made in the optic nerve, which travels to the brain. From here, the brain assembles the image, much like a jigsaw, enabling us to see the environment around us.
Problems can arise when the focussing ability, sometimes known as refractive or optical power, of our eye changes. In some cases the refractive power of the eye can be too great, causing the image to be focussed in front of the retina. This is a condition known as short sightedness or myopia, where the affected individual cannot see objects that are far away clearly. Sometimes, children can be born with eyeballs that are too long, which gives exactly the same symptoms. This is known as congenital myopia.
On the other hand, if the refractive power is too small, or the eyeball is too short, the image is focussed behind the retina. This condition is known as being ‘long sighted’ or hyperopic and prevents people from seeing objects that are close clearly.
Laser Eye Surgery for Vision Problems
Both these conditions are amenable to laser eye surgery/laser vision correction, as are a number of other visual defects such as astigmatism.
The procedure itself is relatively straight forward. Depending on the type of visual defect you have, the ophthalmologist will decide how best to correct your vision. If you are short sighted, the refractive power of your eye will be too great, causing images from objects that are far away to seem blurred. To correct this, your ophthalmologist will flatten your cornea using a laser, helping distant images to become focussed.
On the other hand, if you are long sighted, the refractive power of your eye will be too small. To correct this, your ophthalmologist will make the sides of your cornea steeper, allowing your eye to focus images that are close to you. If you have a condition known as astigmatism, this can be corrected by smoothing the surface of your cornea with a laser to restore your vision.
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