Association of Technical Opticians

RA=radial astigmatism
Pt=tangential power
Ps=sagittal power
ang=angle off optical axis

Radial Astigmatism
RA=Pt-Ps

-or-

RA=P*tan^2(ang)

small order aproximation*
RA=P(ang^2)

*small order aproximation measured in radians and below 20 degrees

Pt=power in the tangential meridian
Ps=power in the sagittal meridian
P=power at optical axis
n=index of material
ang=degrees off axis

Tangential Meridian
Pt=P[(2n+sin^2(ang))/(2n*cos^2(ang)]
-or-
Pt=Ps/cos^2(ang)

small angle approxiamation*
Pt=P[1+(4*(ang^2)/3]

Sagittal Meridian
Ps=P[1+(sin^2(ang)/2n)]

small angle aproximation*
Ps=P[1+(ang^2)/3)]

*small angle aproximations use off axis measure in radians less than 20 degrees

Tavg=Average light transmission
n=index of material

Average Transmission
Tavg = [(4*n)/(1+n)^2]^2/[1-(((1-n)/(1+n))^2)^2]

by: Alvaro Cordova

Introduction:
The night sky has been a source of wonder since time immemorial. Many cultures through out the world have used the stars as a means of navigation or by the farmer as a window of time when to plant crops (Bennett, 1999, pp. 97, 104). However, the night sky would also be a source of fear. The occasional comet was almost always thought to precede a bad omen. Comets were described as a Harbinger of Doom or Menace of the Universe. (Goldman, 2005) The regularity in which the planets moved and the perfect circle shapes the moon and other night sky objects appeared to have, brought about one of the first models of the universe. Ptolemy put the earth at the center of the universe and the planets and sun would orbit the earth. This model was perhaps the first attempt to explain the apparent retrograde motion exhibited by the planets. The early Catholic church eventually adopted this view of the universe as doctrine as well as Aristotelean ideas (Bennett, 1999, pp. 108,147). Galileo Galilee challenged the church’s earth centered view as well as its notion of perfection. Galileo believed the heliocentric model of the universe since a lot of the apparent problems with Ptolemy’s theory cleared up. The church maintained that anything but a circle would be considered imperfect or flawed. Although Galileo is credited with inventing the telescope, it was more than likely Hans Lippershey who actually invented the telescope a year before (Bennett, 1999, pp. 147-148; Dickinson, 1998, p. 136; Gluck, 1964, p. 102). The telescope has since been one of the ideal methods of viewing the sky. This paper will cover the different types telescopes and their aberrations as well as some key advances in telescope design known as adaptive optics. Basic knowledge of lenses and mirrors as well and the images they form are assumed. Such famous telescopes as the Hubble telescope, have photographed breathtaking views of celestial objects that continue to inspire and awe the public. It is no coincidence that the telescope plays an important role in developing the first scientific discipline: astronomy. The importance of the inner workings of the telescope can not be understated since many amateur astronomers to this day contribute to the discovery of comets. Because of adaptive optics, astronomy is undergoing a new golden age.
Types of Telescopes:
There exists several types of telescopes and each has certain strengths and weaknesses. Generally, the optical elements of a telescope are enclosed in a tube to block out stray light. Telescopes are generally divided into two main divisions: refracting or reflecting (Bennett, 1999, p. 184; Consolmagno, 2000, p. 203; Cutnell, 1998, p. 817). The telescope that Galileo used is known as a refracting telescope. Refracting telescopes are made up of an objective lens and an eye piece. Refracting telescopes come in two varieties depending on the eyepiece used. An astronomical telescope has a plus powered eyepiece and a Galilean telescope would have a minus powered eyepiece (NAVEDTRA, 1997, p. 5-19). The objective lens is the first refractive surface light encounters in a telescope. The focal point will then lie between the objective lens and the eyepiece (Bennett, 1999, p.185; Cutnell, 1998, p. 815; Gluck, 1964, p. 108; Halliday, 1997, p. 889; Ogle, 1979, p. 123; Strong, 2004, p. 338). One of the important factors in determining a good refracting telescope is the size of the objective lens. The larger the lens the more light will reach the eyepiece and the clearer the image will be (Bennett, 1999, p. 186; Consolmagno, 2000, p. 203). It is important to note that the image of a refracting telescope will be inverted with the exception of the Galilean telescope. Sometimes another lens is placed in between the objective lens and eyepiece in order to erect the inverted image (Freeman, 1990, p. 199; NAVEDTRA, 1997, p. 5-19). The approximate length of a refracting telescope would be the sum total of focal lengths (Strong, 2004, p. 338). The largest refracting telescope in the world today has an objective lens that is one meter in diameter and a telescope tube nineteen and a half meters long (Bennett, 1999, p. 185).
Reflecting telescopes are more modern in origin and are more versatile. The reflecting telescope can come in a variety of dimensions and types such as the Dobsonian, Cassegrain and Maksutov. A reflecting telescope is comprised of a primary concave mirror and an eyepiece. The primary mirror is placed at the end of a tube facing the opening and reflects the light onto a secondary mirror, which is considerably smaller, and in turn reflects to an eyepiece. In this case, the primary mirror is the first object that light will hit with the exception of the Maksutov.
The Dobsonian telescope is also known as a Newtonian telescope. In Dobsonian telescopes, the primary mirror focuses the image onto a secondary flat mirror that is angled so that the reflected light ends up at the eyepiece that is on the side of the tube. Dobsonian telescopes are the most popular telescopes on the market. These telescopes provide a good value for the money but are bulkier than other types of reflector telescopes. (Consolmagno, 2000, p. 203; Dickinson, 1998, p. 68; NAVEDTRA, 1997, p. 5-16).
The Cassegrain has a primary mirror with a hole in the center. The primary mirror focuses the image on to a secondary mirror, which is slightly convex. The secondary mirror reflects it back toward a third mirror located behind the primary mirror through the center. The third mirror reflects the oncoming light into the eyepiece. Cassegrains are notably smaller than the Dobsonian and are usually more expensive (Consolmagno, 2000, p. 203; Dickinson, 1998, p. 68).
The Maksutov reflector is also known as a catadioptic reflector. It is similar in design to the Cassegrain and has a corrective plate that does some of the work that the primary mirror would have done. The other difference to the Cassegrain reflector is that the secondary mirror in the Maksutov is flat. This allows the tube to be especially compact. These telescopes are also more expensive than the Dobsonian (Consolmagno, 2000, p. 203; Dickinson, 1998, pp. 69 - 72; NAVEDTRA, 1997, p. 5-16).
Eyepieces:
Eyepieces are responsible for magnifying the image made by the objective mirror or lens with the exception of the Galilean telescope. In the case of the Galilean telescope, the lens attempts to make the converging light parallel by diverging it with a minus lens. In the other types of telescopes the eyepiece is a plus lens that converges divergent light. (Gluck, 1964, p. 109).
Telescope Capabilities:
The telescope has three very important qualities. It’s ability to magnify, the quality or resolution of the image, and the amount of image called the field of view impact the price and quality of the telescope.
The objective lens, mirror and eye piece form a relationship, one of which, is magnification. Angular magnification is equal to the focal length of the objective lens divided by the focal length of the eyepiece. For example, a 100mm focal length lens with an eye piece whose focal length is 25mm, the telescope will provide 4x the amount of magnification (angular magnification = (focal length of primary mirror or objective lens) / (focal length of eyepiece)) (Consolmagno, 2000, p. 203; Freeman, 1990, p. 197; Gluck, 1964, p. 109).
Resolution deals with the quality of the image. There is a theoretical limit to resolution that is dependent upon the primary mirror or objective lens. The larger the primary mirror or objective lens the greater the theoretical limit for resolution. Resolution is measured in arc seconds, which are divisions of a circle. A circle contains 360 degrees. Each degree is then divided into 60 minutes and each minute into 60 seconds. A working approximation of resolution would be to divide 120 arc seconds by the aperture in millimeters. The aperture in the case of a telescope would be the objective lens or primary mirror. So, a 100mm objective lens would have 1.2 arc seconds of resolution. (resolution = (120 arc seconds) / (aperture of telescope)) (Bennett, 1999, p. 188; Consolmagno, 2000, p. 203).
Another quality important to amateur and professional astronomers alike is the field of view. Field of view is the amount of sky one can possibly see given a specific amount of magnification. The field of view is measured by the apparent field, which is the approximate angle the eye piece makes when up to your eye divided by the magnification. The apparent field is usually between 35 - 40 degrees. So given a telescope with a magnification of 4x one can see about 10 arc seconds of view (field of view = (40 degrees or apparent field) / magnification) (Consolmagno, 2000, pp. 203, 204).
Telescope Aberrations:
Telescopes, as with any optical system, can contain aberrations that will hinder the clarity of what is being viewed. The aberrations encountered with telescopes are chromatic aberration, spherical aberration, coma, astigmatism, distortion, and curvature of field.
Chromatic aberrations are primarily the problem of refracting telescopes and eyepieces. White light consists of several wavelengths of light. Each wavelength converges at different focal lengths, if the light is incident to the refractive element, given the same medium. Chromatic aberrations come in two varieties: axial and lateral. Axial chromatic aberration is a series of focal lengths along the axis of the lens. Lateral chromatic aberrations are the differing image sizes that are produced by the wavelengths. So in the case of crown glass, the blue and violet wavelengths will converge before the red wavelength. This is due to the fact that the lens can bend the smallest wavelength for a longer duration than a larger wavelength. Lateral chromatic aberration is the more common complaint. What the individual sees is an unclear image. If the dispersion is significant it is actually possible to see the different colors. This aberration can usually be corrected by choosing a lens that has a lower dispersion value. An achromatic lens may be used as well. An achromatic lens attempts to converge more than one wavelength using lenses of differing compositions into a compound lens. A lens with high dispersion will disperse the colors enough so that the secondary element, usually crown glass, focuses all of them to a point. A good telescope will usually have an achromatic lens and will be considerably more in price (Abramowitz, 2005; Cutnell, 1998, p. 818; Brooks, 1996, pp. 502-503; Lester, 2004).
Spherical aberrations are incident parallel rays in the periphery of the lens or mirror that do not converge where the paraxial rays converge. This aberration becomes apparent in larger objective lenses and primary mirrors. The solution, in the case of the reflector, is to have a mirror that is slightly parabolic. The refracting telescope can use a system similar to the one needed in chromatic aberration or can use lenses that are aspheric. Aspheric lenses taper off in curvature toward the periphery allowing the light to converge at the focal point (Abramowitz, 2005; Brooks, 1996, p. 505; Lester, 2004).
Coma is an aberration in which a point of light is distorted into an almost comet like appearance. This aberration is caused by off axis rays that pass through different zones of a lens or mirror. These different areas will have different magnifications which overlap giving the object a comet like appearance. The solution for this aberration is to have a lens system similar to the correction for spherical aberration in the case of the refractor. A lens system that corrects spherical aberration and coma are called aplanatic For the reflecting telescope, a simple glass plate can be used that provides negligible amounts of chromatic aberration. Another way to address coma in the reflecting telescope is to make the primary mirror less parabolic and the secondary mirror slightly hyperbolic (Abramowitz, 2005; Brooks, 1996, p. 505; Lester, 2004).
Astigmatism is an aberration that contains two different foci between the sagittal and tangential planes; horizontal and vertical respectively. Astigmatism is made apparent when off axis rays strike the surface of the spherical lens or mirror. This is generally corrected using a lens system and in the case of mirrors, a plate (Abramowitz, 2005; Brooks, 1996, p. 505,506; Lester, 2004).
Distortion has the appearance of a barrel or pincushion. This aberration is caused by the uneven magnification across the lens or mirror system. This defect is found in compound systems in which there are many elements that correct for other aberrations. The image is no longer truly represented even though it may be sharp. Distortion may be corrected by making adjustments in the optical system, but may still be present to some degree (Abramowitz, 2005; Brooks, 1996, p. 508; Lester, 2004).
Lastly, curvature of field is an aberration that becomes apparent after other aberrations have been corrected. This aberration makes an image have curvature instead of focusing on a plane. This is corrected using a plate before the primary mirror. The plate will flatten the image back to a plane eliminating the aberration. This aberration is inherent because of the use of lenses which are themselves curved (Abramowitz, 2005; Brooks, 1996, p. 508; Lester, 2004).
Adaptive Optics:
The Hubble telescope was launched into space with the hope of escaping the optical problems inherent in an atmosphere. Although the topic of adaptive optics does not address an optical aberration per se, it has been a significant advance in terrestrial based telescopes. The atmosphere contains many small perturbations caused by temperature differences naturally in the atmosphere. Adaptive optics attempts to circumvent these issues (National Optical Astronomy Observatories, 2000; University of California Regents, 2002).
Adaptive optics systems attempt to cancel out a distorted wavefront. A beacon, which is generally a laser, is sent into the sky in order to detect the incoming wavefronts. After detection, the wavefront is analyzed. In order to cancel out the offending perturbation a deformable mirror is used. The deformable mirror effectively does the opposite of what the distortion is doing. The deformable mirror is made up of several smaller mirrors in a honeycomb arrangement. Each mirror is hooked up to a machine which allows it to move in whatever way the analyzing element determines will cancel out the distortion. There are only a handful of telescopes that have this technology in place such as the WIYN telescope in Kitt Peak, Arizona (Bennett, 1999, p. 194; European Southern Observatory, 2000; National Optical Astronomy Observatories, 2000; University of California Regents, 2002, 2004).
Conclusion:
Hubble Telescope will be retired in late 2007 and will be replaced by the James Webb Space telescope by 2010. The pictures and science provided by the new telescope should be no less amazing than that of Hubble. This new telescope will assist in answering the deeper questions about the origins of our universe. On earth, the birth of adaptive optics hopes to revitalize current telescopes with better image processing that may even surpass what Hubble can do now. Without the telescope, astronomy is simply not possible. The furthest reaches of the universe may come into full view in less than a decade and may usher in another golden age of astronomy and amateur astronomers will continue to scan the skies with telescopes in awe of this glorious universe.
Works Cited
Abromowitz, Mortimer., Spring, Kenneth R., & Davidson, Michael W. (2005). Common
Optical Defects in Lens Systems (Aberrations). Retrieved March 22nd, 2005, from
http://micro.magnet.fsu.edu/primer/lightandcolor/opticalaberrations.html

Bennett, Jeffrey., Donahue, Megan., Schneider, Nicholas., & Voit, Mark. (1999). The
Cosmic Perspective. Menlo Park, CA: An Imprint of Addison-Wesley Longman, Inc.

Brooks, Clifford W., Borish, Irvin M. (1996). System for Opthalmic Dispensing.
Butterworth-Heinemann: Boston.

Cutnell, John D., Johnson, Kenneth W. (1998) Physics (4th ed.). New York: John Wiley
& Sons, Inc.

Consolmagno, Guy., Davis Dan M. (2000). Turn Left At Orion: A Hundred Night Sky
Objects to See in a Small Telescope - and How to Find Them (3 ed.). Cambridge
University Press: Cambridge UK.

Dickinson, Terence. (1998). Night Watch: A Practical Guide to Viewing the Universe (3
ed.). Firefly Books: Willowdal, Ontario.

European Southern Observatory. (2000, September 21st date of last update) An
Introduction to Active & Adaptive Optics. Retrieved February 22nd, 2005, from
http://www.eso.org/projects/aot/introduction.html

Freeman, M. H. (1990). Optics (10 ed.). London: Butterworth.

Gluck, Irvin D. (1964). Optics: The Nature and Applications of Light. New York: Holt
Rinehart and Winston Inc.

Goldman, Noah.(2005) Comets in Ancient Cultures. Retrieved: March 20th, 2005, from
http://deepimpact.jpl.nasa.gov/science/comets-cultures.html

Halliday, David., Resnick, Robert., Walker, Jearl. (1997). Fundamentals of Physics [Part
4] (5th ed.). New York: John Wiley & Sons, Inc.

Lester, J. B., Telescope Aberrations. (2004) Retrieved: February 22nd, 2005, from
http://www.erin.utoronto.ca/~astro/ast110/lectures/aberrations.html

National Optical Astronomy Observatories. (2000, February 16th date of last update)
WIYN Adaptive Optics: Development of a Tip-Tilt System. Retrieved February 22nd,
2005, from http://claret.kpno.noao.edu/wiyn/wttm.html

Naval Education and Training Program Development Center (NAVEDTRA). (1997)
Basic Optics and Optical Instruments. Mineola: Dover Publications.

Ogle, Kenneth N. (1979). Optics: An Introduction For Ophthalmologist. Springfield:
Charles C. Thomas Publisher

Strong, John. (2004). Concepts of Classical Optics. Mineola: Dover Publications.

University of California Regents.(2004, Novemeber 30th date of last update) What is
Adaptive Optics? Retrieved February 27th, 2005, from http://cfao.ucolick.org/ao/

University of California Regents. (2002 date of last update) How Does an Adaptive
Optics System Work? Retrieved February 27th, 2005, from
http://cfao.ucolick.org/ao/how.php

by: Alvaro Cordova

Introduction:
Our ability for stereoscopic vision is essentially the basis for the human species’ ability to adapt and innovate. Evolutionarily speaking, primates first developed a more forward facing eye position in response to their habitat. In order for primate survival in a forest or jungle, it was necessary to estimate distances between trees in order to jump while catching prey or to avoid being eaten. Stereopsis, or depth perception, has allowed for better hand eye coordination in humans. Any disruption to this delicate system would hinder greatly the contribution to the tribe in the case of the caveman, and in more modern times, one’s quality of life. , Strabismus, when left untreated, can lead to amblyopia and is an example of disruption to stereopsis. According to Grosvenor, amblyopia is known to accompany anisometropia … and affects between 2 to 2.5% of the general population. This would translate to about 5.8 to 7.2 million Americans dealing with amblyopia according to the current United States population. Anisometropic amblyopia and strabismic amblyopia, which will be discussed further in this paper, are conditions that for the most part are preventable and very debilitating to the sufferer. The good news about amblyopia is that it is not impossible to treat after the age of six or seven as was previously believed. A growing amount of evidence suggests that with several types of vision therapy, even adults can show some improvement in their visual acuity. This paper will discuss vision therapy, strabismus and amblyopia as well as its effects on the patient.
Vision and the Brain:
Before there is any discussion about the topics mentioned, it is important to know how the brain works in regards to binocular vision. From birth we are able to see, but our vision is not fully developed. The coordination of eye movement develops around the age of six or seven and fully develops at about the age of nine. , During development, the brain is learning how to interpret visual messages. These visual messages are relayed by each eye to the occipital lobe where the visual cortex resides. , , The brain now has to make one coherent image out of the two images that have come from each eye. This process is known as fusion. , , , Diplopia, or double vision, is a symptom of the brain’s inability to process or fuse the two images it is receiving from each eye. The process of fusion, if continuously interrupted, will invariably hinder binocular vision development when left untreated in children ,
What is Vision Therapy:
Vision therapy as defined by Caloroso is somewhat broad in its scope. She describes vision therapy as the total treatment program for a strabismic patient, which may include optical or medical treatment options (passive therapy) and orthoptics (active therapy). This definition involves two distinctions; active and passive therapy. Active therapy is designed to improve visual performance by involving the patient consciously in a sequence of specific controlled visual tasks. Passive therapy involves the use of corrective lenses, prisms or surgical reduction in the strabismic angle. Cassin, defines orthoptics as a treatment that deals with the diagnosis and treatment of defective eye coordination, binocular vision and functional amblyopia by non-medical and non-surgical methods, e.g., glasses, prisms, exercises. These two definitions have a considerable overlap as well as some differences. Surgery, although not considered by Cassin to be a form of vision therapy, will be mentioned as a treatment for strabismus. The terms orthoptics, vision therapy, and vision training are used interchangeably. As mentioned in the definitions, strabismus and binocular vision are treated or enhanced respectively with some form of vision therapy. In other words, binocular vision is the goal of vision therapy when it treats some form of strabismus or visual binocular problem. This paper will use Caloroso’s definition for vision therapy since the active vs. passive distinction is a useful subdivision.
Extraocular Muscles and Nerves Associated with Strabismus:
In regards to strabismus it is important to understand the functions of the extra ocular muscles and the corresponding cranial nerves that innervate them. The eye has a muscular system that allows it to move in various directions in order to line up the object with the visual axes of the eyes.
Those muscles that aid in the movement of the eye include the medial and lateral recti, inferior and superior recti, and the inferior and superior obliques. , , The lateral recti are located temporally, the medial recti are located nasally, the superior and inferior recti are located above and below to the eye respectively. The aforementioned rectus muscles are attached to the sclera and then proceed back into the orbit where they are attached to a bone surrounding the optic nerve called the annulus of Zinn. , , The lateral and medial recti provide temporal and nasal movement respectively which translates to movement on the x-axis. The superior and inferior recti provide up and down motion respectively or movement on the y-axis. The superior oblique is located as its name suggests with the exception that instead of simply attaching straight back it extends over the medial wall and passes through a fibrocartilage ring called the trochlea. The inferior oblique extends from the medial wall of the orbit to the inferolateral aspect of the eye. The oblique muscles are primarily resposible for intorting and extorting the eye. , ,
Cranial nerves are responsible for activating these muscles. Cranial nerve III (CN3), the oculomotor nerve, is perhaps the most important. CN3 innervates the medial, superior and inferior recti, the inferior oblique, the iris sphincter muscle and the ciliary muscle. , , CN4, the trochlear nerve, stimulates the superior oblique. CN6, the abducens, governs the lateral rectus muscle. Other cranial nerves, including the optic nerve or CN2, the trigeminal or CN5, and the facial nerve or CN7, govern vision, sensation, and facial muscles respectively. These muscles and their corresponding nerves are responsible for the ability of the eye to see, feel sensations and move. , ,
In order to understand strabismus it is important to be able to know which muscles and nerves govern convergence. Convergence is the process of turning the eyes inward in order to maintain binocular vision as an object approaches. , , Orthophoria is the state when the eye converges properly. , Strabismus is defined as an eye misalignment or abnormal eye movement that is caused by extraocular muscle imbalance. , , , The eye has trouble converging to some point in space because an extraocular muscle may be abnormally weak or strong. Usually, strabismus shows itself in the first 6 months of life if it is congenital. Strabismus that surfaces after 6 months of life is considered acquired. It is necessary to note that strabismus is a general term. There are two useful divisions of strabismus; tropias and phorias. Tropias are misalignments of the eyes that are caused by muscle imbalances. Phorias are latent tendencies for the eyes to deviate that is prevented by fusion. This paper will primarily focus on some of the causes and treatments of esotropia and exotropia.
Causes and Descriptions of Esotropia and Exotropia:
Five percent of children have some type of strabismus. One of the more common types of strabismus is esotropia. Esotropia is a definite turning of the eye inward or nasally. , , , , This condition is what is socially referred to as crossed-eyes. , , , Several causes for esotropia may exist. Generally, a weak lateral rectus muscle in the strabismic eye is the primary cause. Conversely, an overly active or tense medial rectus muscle may also be involved. , There may also be a problem with the accommodative response. As mentioned before, the oculomotor nerve is responsible for several muscles that provide the following actions: Accommodation, the process of changing the shape of the crystalline lens by the ciliary muscles in order to change its dioptric power and adjust for objects at various distances, convergence, the inward turning of the eyes, and the dilation and contriction of the pupil. Since CN3 has such a strong relationship between convergence and accommodation, there may be an overactive convergence response if there is a significant amount of refractive error. This is most commonly found in hyperopic children. Uncorrected hyperopia will cause the child to accommodate in order to see far away. This then leads to over accommodation and to over convergence. , , , This type of esotropia is categorized as accommodative esotropia. Esotropia can be classified as either intermittent or constant and unilateral or alternating. , Generally, asthenopia, or eye discomfort arising from eye use, is a common complaint among those with intermittent strabismus. , Moreover, genetic influences can’t be ruled out as a possible cause. , , ,
Exotropia, although less common than esotropia, is no less debilitating. Exotropia is a definite turning of the eye temporally. , , , , This is what is socially referred to as wall-eyes. This form of strabismus may be caused by a weakness in the medial rectus muscle. Statistically, myopes are more prone to exotropia. A direct correlation between myopia and exotropia has not been found as of yet. Depending on actions taken to correct these types of exodeviations, it is not uncommon to reclassify the patient as the development of the eye changes. Exotropia may also be alternating or unilateral, intermittent and may have a genetic cause as well.
Vertical Strabismus and Phorias:
Hypertropia and hypotropia are upward and downward deviations of the eye respectively and are categorized as vertical strabismus. , A strong inferior oblique may also be present in vertical strabismus and is generally present in esotropia and exotropia. Phorias are related closely to their tropic cousins. In the case of esophoria and exophoria similar muscle imbalances are present but only partially.
Amblyopia:
Strabismus, when left untreated in children under 7, may cause the sufferer to eliminate one of the two images. This process is known as suppression and is an unconscious response to diplopia. , Once the eye has undergone suppression the eye will effectively become amblyopic. Amblyopia, according to Sardegna, is a condition in which the eye provides poor vision even though there isn’t any pathology or damage to the retina or visual pathways. , , , ,
There are three useful classifications for amblyopia: anisometropic amblyopia, strabismic amblyopia and anisostrabismic amblyopia. , These distinctions are based upon the cause of the amblyopia. In the case of anisometropic amblyopia, one of the eyes contains a significant refractive error, anisometropia, and becomes amblyopic. In many cases anisometropic amblyopia may not have any visible deviation and may go undiagnosed. Strabismic amblyopia is amblyopia that originates because of strabismus. Anisostrabismic amblyopia is a form of amblyopia that is combined because of both anisometropia and strabismus are present. What is interesting about amblyopia is that the type of amblyopia may have different neurophysiological mechanisms. A study of 14 patients, 8 anisometropic and 6 strabismic, were shown checkerboards of various sizes while undergoing an MRI. Choi found that calcarine activity was more suppressed at higher spatial frequencies for anisometropic amblyopia and was more suppressed at lower spatial frequencies in the case of strabismus. The calcarine fissure is the part that separates the upper and lower parts of the brain and are where the optic radiations end. When the anisometropic amblyope viewed the smaller checkerboard there was less activity in the calcarine fissure whereas the strabismic amblyope had less activity viewing larger checker sizes. Although this study had a small sample, it is nonetheless a significant find and further reinforces the classification system for amblyopia.
Amblyopia is better treated before the age of six or seven years of age. , This is commonly called the critical period. Previously, it was commonly believed that after this age it was impossible to regain any vision. It turns out that this critical period can extend beyond what was previously believed. , Begley notes that vision therapy is one of the first to exploit the brain’s rewiring capabilities, known as neuroplasticity and further adds the following.
For decades scientists had thought that the brain undergoes very little change after childhood. They knew that the adult brain could form the new connections that underlie learning and memory, but believed that its basic structure was immutable and fixed, or hard wired.

Also, there seems to be a reasonable explanation for the disparity between Wick, Birnbaum’s finding and Thomas, Chalkley’s findings about the critical period; the use of occlusion therapy, which will be discussed in further detail. As Stevens notes, occlusion therapy is almost always the treatment of choice for amblyopia and treatment is considered complete once visual acuity does not improve. Wick notes that occlusion therapy in older patients is not often recommended because of the difficulty in re-establishing normal visual acuity in the amblyopic eye. and reasons that after 10 years of age occlusion alone is not a very successful treatment for patients with anisometropic amblyopia. Wick performed a study in which 19 patients, all over the age of 6, were studied over 6 years to see how the effects of full refractive correction, using lenses or prisms to align the visual axes, 2 to 5 hours of occlusion therapy and active vision therapy could be used to reverse the affects of amblyopia. Wick found that 8 of the 19 patient’s conditions were able to be reversed completely and the patient with the least amount of improvement improved 75 percent and was 49 years old. Finally, Wick concluded with the following.
Such results might also explain why many clinicians use the terms critical period and plastic period interchangeably because they would have observed limited improvement when (occlusion) therapy was instituted after 10 years of age and then concluded that the critical (development) period and plastic (treatment) period were the same.
Perhaps with further study better methods for treating amblyopia in adults may be found.
Treatments for Strabismus and Amblyopia:
Correcting strabismus or amblyopia may take many forms. The use of surgery, optical devices and active vision therapy are essential and in many cases the optician may play a pivotal role in the success of the treatment. Some of the treatments for esotropia and exotropia include surgery, but it is considered a last resort. A surgeon could either strengthen or weaken muscles by reattaching them to a different area on the sclera in order to correct the imbalance. In some cases it is necessary to perform the procedure more than once or vision therapy may be implemented for any residual imbalance. , , , In the case of accommodative esotropia, full correction for distance and near would be used. Bifocals would short circuit the over convergence response due to excessive accommodation by providing all the plus they need. With bifocals it is important to bisect the pupil to ensure that the segment is used. , The use of prisms may be used initially to encourage fusion.
Active vision therapy may also play a role in correcting the esotrope or exotrope. Hart Chart procedures are examples of accommodative therapy. Two charts with black letters are printed on a white background; one containg large letters and the other small, for far and near respectively. The larger lettered set would be placed at a distance while the smaller lettered could be held. A set of plus and minus lenses assist in the process of changing the amount of accommodation necessary to see the letters clearly. The purpose of these exercises is to either increase the amplitude of accommodation or to improve the efficiency of accommodation.
The Aperture-Rule trainer is an instrument used to train vergence. The instrument can be used with either one or two apertures. An image is placed some distance behind the aperture(s). The single aperture is used to encourage convergence and the double aperture is used to stimulate divergence. This technique has the advantage of dissociating vergence, or eye movements, and accommodation.
Bar readers are tools used as an anti-suppression therapy. The bars are either polarized or composed of anaglyphic, or red and green material. The reader is used with different filters and helps to partially dissociate the image. The patient may or may not see words when the filters are applied, and may be instructed to blink rapidly, or to shake the bars when suppression occurs. This method stimulates the continuous perception of two distinct objects for a short duration of time.
Prism therapy may be used monocularly in order to provide the patient with awareness of his or her eye movements. The patient notices jumps in the images when the eye moves. The technique is used to encourage fixation of the amblyopic eye as well as vergence ranges when used on both eyes.
The Brewster stereoscope is another approach for stimulating binocular vision. There are two distinct images in which the patient attempts to fuse while looking through the scope. The objects can be placed at a variety of distances apart. The distances are based upon the type of vergence that is desired. This instrument can also be used as a form of anti-suppression therapy. The patient points to a control and blinks rapidly when suppression occurs. This is by no means an exhaustive overview of the many active vision therapy techniques available, but is simply given to provide an understanding of the methodology behind some techniques.
The use of occlusion therapy has been successful in the treatment of amblyopia for centuries and is still the primary approach to treating amblyopia. In occlusion therapy, the eye without the amblyopia is covered with an occluder of some type. This forces the amblyopic eye to work and develop. , Several types of occluders exist and will occlude partially or completely depending on the needs of the patient. Generally, full and complete occlusion will be the method of choice since there exists several types of patches and bandage occluders that are more resistant to non-compliance than partial occluders. Full occlusion guarantees that the patient can not use the eye in any way shape or form without removing the occluder. Unfortunately many children will find occluders uncomfortable and may simply avoid using them. In such cases, the use of cycoplegic eye drops may be necessary such as atropine, homatropine and others. These drugs have side-effects that must be weighed carefully and are generally used as a last resort before proceeding to surgery. ,
Conclusion:
Stereopsis is an advantage that mankind has inherited through natural selection. Without this advantage, our ability as humans to use or make tools would be hindered considerably. Such considerations would bear heavily on humanity’s use of technology. Consider the task of driving a vehicle without the ability to determine the proximity of pedestrians. It is undeniable that binocular vision can have a considerable impact on one’s quality of life. Many careers such as that of a pilot, require fully developed depth perception. Sadly, strabismus and amblyopia can shrink the opportunities available to the sufferer. Strabismus can be caught early with a simple eye exam. Waiting after the age of six or seven to treat strabismus or amblyopia may put the patient at risk for permanent vision loss despite tailored vision therapy as in the case of the 49 year old in Wick’s study. Only with further research will the process of suppression be better understood with the hope of yielding techniques for treating or reversing strabismus and amblyopia.
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Thomas, Strabismus and Amblyopia, p. 6.
Caloroso, Clinical Management of Strabismus, p. 6.
Cassin, Dictionary of Eye Terminology, p. 75.
Thomas, Strabismus and Amblyopia, p. 3.
Cassin, Dictionary of Eye Terminology, p. 196.
Thomas, Strabismus and Amblyopia, p. 6.
Cassin, Dictionary of Eye Terminology, p. 250.
Chalkley, Your Eyes, p. 114.
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Thomas, Strabismus and Amblyopia, p. 7.
Cassin, Dictionary of Eye Terminology, p. 102.
Thomas, Strabismus and Amblyopia, p. 7.
Cassin, Dictionary of Eye Terminology, p. 268.
Cassin, Dictionary of Eye Terminology, p. 206.
Chalkley, Your Eyes, p. 76.
Caloroso, Clinical Management of Strabismus, p. 249.
Cassin, Dictionary of Eye Terminology, p. 101.
Grosvenor, Refractive Anomolies: Research and Applications , p.124.
Sardegna, The Encyclopedia of Blindess and Vision Impairment, p. 218.
Thomas, Strabismus and Amblyopia, p. 7.
Cassin, Dictionary of Eye Terminology, p. 101.
Chalkley, Your Eyes, p. 79.
Grosvenor, Refractive Anomolies: Research and Applications , p.125.
Thomas, Strabismus and Amblyopia, p. 7.
Chalkley, Your Eyes, p. 78.
Thomas, Strabismus and Amblyopia, p. 7.
Caloroso, Clinical Management of Strabismus, p. 249.
Cassin, Dictionary of Eye Terminology, p. 102.
Grosvenor, Refractive Anomolies: Research and Applications , p.124.
Thomas, Strabismus and Amblyopia, p. 8.
Caloroso, Clinical Management of Strabismus, p. 250.
Caloroso, Clinical Management of Strabismus, p. 12
Cassin, Dictionary of Eye Terminology, p. 102.
Caloroso, Clinical Management of Strabismus, p. 12
Cassin, Dictionary of Eye Terminology, p. 40.
Linda M. Ross, ed, Crossed Eyes. Ophthalmic Disorders Source Book. Detroit: Omnigraphics Inc, 17, (1997):123-125. p. 123.
Linda M. Ross, ed, Aligning Eyes: Straightening Out Strabismus. Ophthalmic Disorders Source Book. Detroit: Omnigraphics Inc, 17 (1997):127-134 . p.128.
Jill Sardegna, et al, Amblyopia. The Encyclopedia of Blindess and Vision Impairment. 2nd ed. New York: Facts On File Inc, 2002. p. 218.
Thomas, Strabismus and Amblyopia, p. 8.
Caloroso, Clinical Management of Strabismus, p. 223
Cassin, Dictionary of Eye Terminology, p. 104.
Chalkley, Your Eyes, p. 79.
Sardegna, The Encyclopedia of Blindess and Vision Impairment, p. 218.
Thomas, Strabismus and Amblyopia, p. 8.
Cassin, Dictionary of Eye Terminology, p. 104.
Thomas, Strabismus and Amblyopia, p. 8.
Caloroso, Clinical Management of Strabismus, p. 223
Cassin, Dictionary of Eye Terminology, p. 104.
Thomas, Strabismus and Amblyopia, p. 8.
Caloroso, Clinical Management of Strabismus, p. 278.
Cassin, Dictionary of Eye Terminology, p. 135, 136.
Caloroso, Clinical Management of Strabismus, p. 278.
Thomas, Strabismus and Amblyopia, p. 9.
Caloroso, Clinical Management of Strabismus, p. 159.
Thomas, Strabismus and Amblyopia, p. 9.
Caloroso, Clinical Management of Strabismus, p. 175.
Cassin, Dictionary of Eye Terminology, p. 30.
Ross, Ophthalmic Disorders Source Book, p. 117.
Sardegna, The Encyclopedia of Blindess and Vision Impairment, p. 9.
Arthur Stevens, Rod J. Baker, Considerations in the Routine Assessment and Treatment of Anisometropic Amblyopia. Clinical and Experimental Optometry. 82.4, (1999):111 - 118. p. 111
Caloroso, Clinical Management of Strabismus, p. 175
Mi Young Choi, et al., Comparison Between Aanisometropic and Sstrabismic Amblyopia Using Functional Magnetic Resonance Imaging. British Journal of Ophthalmology. 85.9 (2001):1052 - 1056. p1052.
Ibid.
Cassin, Dictionary of Eye Terminology, p. 56.
Choi, British Journal of Ophthalmology, p. 1052
Choi, British Journal of Ophthalmology, p. 1052
Thomas, Strabismus and Amblyopia, p. 9.
Martin H Birnbaum, et al, Success in Amblyopia Therapy as a Function of Age: A Literature Survey. American Journal of Optometry & Physiological Optics. 29.5 (1977):269 - 275. p. 269.
Bruce Wick, et al, Anisometropic Amblyopia: Is The Patient Ever Too Old To Treat? Optometry and Vision Science, 96.11, (1992):886 - 878. p. 866.
Sharon Begley, Reversing Partial Blindness. (Wall Street Journal, 2/1/2005) (pp. 2)
Stevens, Clinical and Experimental Optometry,p. 111.
Wick, Optometry and Vision Science, p. 876.
Ibid, p. 877.
Chalkley, Your Eyes, p. 78.
Ross, Ophthalmic Disorders Source Book, p. 132.
Sardegna, The Encyclopedia of Blindess and Vision Impairment, p. 219.
Thomas, Strabismus and Amblyopia, p. 13.
Caloroso, Clinical Management of Strabismus, p. 257.
Thomas, Strabismus and Amblyopia, p. 13.
Caloroso, Clinical Management of Strabismus, p. 259.
Ibid, p.296
Caloroso, Clinical Management of Strabismus, p. 300.
Ibid, p.302.
Ibid, p. 339.
Ibid p. 296, 309.
Wick, Optometry and Vision Science, p. 870.
Caloroso, Clinical Management of Strabismus, p. 113.
Thomas, Strabismus and Amblyopia, p. 13.
Caloroso, Clinical Management of Strabismus, p. 113, 142.
Thomas, Strabismus and Amblyopia, p. 13.