Ashkan Azarsepandan



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Ashkan Azarsepandan

PH 464


March 24, 2006

Larosa


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The topic I have chosen is Astigmatism. Astigmatism means that the cornea of the eye is oval like a football instead of spherical like a basketball. Most astigmatic corneas have two curves a steeper curve and a flatter curve. This causes light to focus on more than one point in the eye, resulting in blurred vision at distance or near. Astigmatism often occurs along with nearsightedness or farsightedness.

It is very important to learn about Astigmatism because millions of people are affected by it and in the past people who had Astigmatism only had two alternatives for correcting Astigmatism which was eyeglasses and contact lenses but just recently within the last five years Astigmatism can be corrected surgically which allows the person to live a life style free from eyeglasses and contact lenses.



I will define what Astigmatism is, its symptoms, causes, detection and diagnosis and treatment. I will also include the surgical procedure that one must undergo to correct astigmatism. It is interesting to me because I find it amazing that most people that have astigmatism have to wear some form of corrective lenses such as eyeglasses or contact lenses to correct their vision however through surgery such as astigmatic keratotomy and Lasik the person’s vision can be corrected and they no longer will need to depend on corrective lenses.

What is Astigmatism?

Astigmatism usually occurs when the front surface of the eye, the cornea, has an irregular curvature. Astigmatism is one of a growing eye conditions known as refractive errors. Refractive errors cause disturbance in the way that light rays are focused within the eye. Astigmatism often occurs with nearsightedness and farsightedness conditions also resulting from refractive errors. Astigmatism is not a disease nor does it mean that you have “bad eyes.” It simply means that you have a variation or disturbance in the shape of your cornea. (Kellog Eye Center)



 Astigmatism Symptoms and Signs

-Distortion or blurring of images at all distances

-Headache and fatigue

-Squinting and eye discomfort or irritation

If you have a small amount of astigmatism, you may not notice it or have just slightly blurred vision. But sometimes uncorrected astigmatism can give you headaches or eyestrain, and distort or blur your vision at all distances.

Adults are not the only ones who can be astigmatic. A recent study from Ohio State University School of Optometry, found that of 2,523 children that more than 28 percent of them had astigmatism. Children may be even more unaware of the condition than adults, and they are unlikely to complain of the blurred or distorted vision. It can affect their ability to see well in school and during sports, so its important to get their eyes examined at regular intervals in order to detect any astigmatism early on. (All About Vision)

What Causes Astigmatism?

Astigmatism occurs when the cornea is shaped more like an oblong football than a spherical baseball, which is the normal shape. In most astigmatic eyes, the oblong or oval shape causes light rays to focus on two points in the back of your eye, rather than just one. This is because, like a football, an astigmatic cornea has a steeper curve and a flatter one.

Usually astigmatism is hereditary many people are born with an oblong cornea, and the vision problem can get worse over time. But astigmatism can also result from an eye injury that has caused scarring on the cornea, from certain types of eye surgery, or from Keratoconus, a disease that causes a gradual thinning of the cornea. (All About Vision)

Detection and Diagnosis

Astigmatism can be detected and measured with corneal topography, Keratometry, vision testing and refraction. (Stlukes Eye)



Treatment

Astigmatism can be corrected with:



-Eye glasses

-Contact lenses

-Surgically

If the degree of astigmatism is slight and there is no other refraction problem such as nearsightedness or farsightedness, than corrective lenses may not be needed. If the degree of astigmatism is great enough to cause eyestrain, headache, or distortion of vision then prescription lenses will be needed for clear and comfortable vision. The corrective lenses needed when astigmatism is present are called Toric lenses and have an additional power element called a cylinder. They have greater light- bending power in one axis than in others. Your eyecare professional will perform precise tests during your examination to determine the ideal lens prescription. (Kellog)

Many people with astigmatism believe that they can’t wear contact lenses, or that only rigid contact lenses (RGPs, also called GP lenses) can correct astigmatism. However, this was true many years ago, but now there are soft contact lenses that correct astigmatism; they are called toric contact lenses. Toric lenses have a special correction built into them and may also contain a prescription for nearsightedness or farsightedness if you need it. Depending on the type and severity of your astigmatism, you may be able to have it corrected with refractive (Lasik) surgery. (All About Vision)

This section I will elaborate on the analytical and important aspect part of the Astigmatism which will be related to the course text book. These figures, 1, 2, and 3 will illustrate the analytical and important aspects of the Astigmatism.



Figure 1 (above) illustrastes an Astigmatism lens which begins with the “Object Point” which divides up into the horizontal and vertical axis of “Meridional Ray” which is the ray that passes through the optical axis of an optical fiber and it is in the “Meridional Plane” which is the ray that is confined to the y-z plane. The z plane is along the optical axis of the system which is perpendicular to the y plane. Once it passes through the lens, it is identified as the “Sagittal Ray Plane” which changes slope as the “Chief Ray” (a ray from an off-axis object point that passes through the center of the “Aperture Stop” is deviated by the elements composing the lens system.



The above figure also illustrates the “Astigmatic Difference”, the discrepancy in focal length that measures the Astigmatism which is dependent of the inclination angle of the light ray and increases rapidly as the rays become more oblique. (Microscopyu)

Figure 2 (above) is the “Astigmatism Aberration” illustrates the “Circle of Least Confusion” which is a zone that between the “Sagittal Ray Plane” when is refracted differently and both sets of the rays intersect the “Chief Ray” at different image points, and that is termed as the “Tangential Line Image” which in Astigmatism the rays fail to produce a focused image point; however, it produces a series of elongated images that is ranging from linear to elliptical that can depend on the position within the optical train. It is the point after the intersection of “Sagittal Ray Plane” Tangential Line Image” which is the major and minor axes of the ellipse are equal and the image approaches a circular geometry.



The “Circle of Least Confusion also illustrates the approximation of the “Airy Diffraction Pattern” of the implementation approach locations of the pathway. The “Airy Diffraction Pattern” which is a radial intensity distribution functions that are sensitive to the combination of objective and condenser numerical apertures and also the wavelength of the illuminating light when it is monochromatic light is used to illuminate the specimen. A Well-accurate concept of a uniform circular aperture; for example, two adjacent point that are only just to be resolved when the center of its “Air Diffracted Patterns” are separated by a minimum distance that equals to the radius of the central disk in the “Airy Diffraction Pattern”. (Microscopyu)

The above figure illustrates the analytical and important aspects of the Astigmatism that is the process of correction Astigmatism. In every lens system, it has an inherent curvature, “Petzval Curvature” which is the refractive index of the component lens which consists of elements and surface curvatures. The systems are lacking Astigmatic Abberation that contains Sagittal and Tangential image surfaces that occupies each other that lie on the Petzval surface.

As mentioned earlier that Astigmatism dependence of the angle of between “hief Ray” and the optical axis of the lens system which is termed “Field Angle” which the lens correction is generally represented by a plot of parabolic curves which is represented by “T” (“Tangential”), and “S”(Sagittal”) image points as a function of the field image. The uncorrected lens illustrated above in part a, the Astigmatism is negative, undercorrected, or inward-curving and in part b and part c illustrates many corrected lenses needs a necessity to reduce the value of the “Astigmatic Difference” or the distance between “Sagittal” and “Tangential” line images. It merely impossible to have perfectly “S” and “T” curves to become really flat but it can become really closed to be flat and coincide the image which is formed in a region near the “P” (“Petzval Surface”). (microscopyu)

Poor lens centration in the objective or poor alignment between the objective, the intermediate optics, and the eyepiece increases astigmatism as it does coma. When a lens system is mis-assembled, astigmatism is severely affected, usually producing asymmetrical performance over the entire image area. Small tilt angles, even as low as 5 minutes of arc, are serious and lead to image degradation. Astigmatism errors are usually corrected by design of the objectives to provide precise spacing of individual lens elements as well as appropriate lens shapes, aperture sizes, and indices of refraction. The correction of astigmatism is often accomplished in conjunction with the correction of field curvature aberrations. (Microscopyu)

   I didn’t know that astigmatism is very common and that it is usually hereditary. Many people are born with astigmatism, and it will remain the same throughout life until it is diagnosed and treated. And many experts believe that almost everyone has a degree of astigmatism.

I have learned that adults are not the only ones with astigmatism children can have it as well. However, it is harder to detect in children than adults because they are less likely to complain about the symptoms that are associated with astigmatism such as blurred vision or headaches and in this case Astigmatism in children may go undetected unless proper screening is done on a regular basis to detect astigmatism early on in the child’s life.



References

Hecht, E. (2002). Optics (4th ed.). San Francisco: Pearson Education, Inc. http://www.allaboutvision.com/conditions/astigmatism.htm



http://www.kellogg.umich.edu/patientcare/conditions/astigmatism.html

http://www.microscopyu.com/tutorials/java/aberrations/astigmatism

http://www.stlukesye.com/Conditions/Astigmatism.asp


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