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NOTE: CLICK ON THE IMAGES FOR A LARGER VIEW
IMAGE FORMATION BY THE EYE
IMAGE FORMATION BY THE EYE
The eyes are your portal to this world. They allow you to experience this universe like no other organ can. Be it a cascading waterfall or a starry night, your eyes aid you to visualize your experiences.
The following Video will helps explain how the image is formed by the eye. It also shows how this process can be affected and result in refractive errors. It also demonstrates how to correct these errors
OPTICS 101
OPTICS 101
REFRACTION:
The bending of light, as it passes from one medium to another. If the medium the light is passing into is more dense (like from air to glass), the light will "bend" into the medium or vice versa.
LENS:
A refractive surface that is designed to either converge or diverge light. A convex lens converges light and is shaped like a rugby ball. A concave lens diverges light and is shaped like two prisms sitting on each other's apex.
All lenses have a focal point. This is the point at which parallel rays of light are focused by the lens. The location of this point is dependent on many factor including the type of lens and the medium in which the lens is placed (air, water etc)
RADIUS OF CURVATURE:
Radius of curvature defines how "curved" the surface is. the less the radius of curvature, the more the curve the surface. The more curved the surface, the more it will refract light (i.e. greater its power)
THE OPTICS OF IMAGE FORMATION
THE OPTICS OF IMAGE FORMATION
The transparent & refractive media of the eye (cornea and the lens) are ultimately responsible for focusing light on the retina. An eye that has no refractive error (i.e. one in which light is brought into focus on the retina) is called an emmetropic eye (em= equal metropia = measure). All other states of the eye, in which the image is not formed on the retina are called ametropias (a= not metropia = measure).
The transparent & refractive media of the eye (cornea and the lens) are ultimately responsible for focusing light on the retina. An eye that has no refractive error (i.e. one in which light is brought into focus on the retina) is called an emmetropic eye (em= equal metropia = measure). All other states of the eye, in which the image is not formed on the retina are called ametropias (a= not metropia = measure).
GENERAL CONSIDERATIONS:
When we talk about image formation by the eye, certain parameters are to be kept in mind:
1. The light rays must be parallel (coming from infinity)
2. The eye must be at rest (not accommodating, i.e. not focusing for near objects, look below for more info about accommodation)
When non-parallel rays of light (ones not coming from infinity) hit the lens, the eye begins to accommodate (i.e. focus for near). Thus for all refractive errors, the light rays are considered to be coming from infinity.
The second parameter ensures that eye is NOT accommodating. As you will see later this is one cause of serious confusion when learning about refractive errors
1. THE EMMETROPIC EYE:
This is the "ideal" eye. Parallel light rays focus on the retina, with the eye at rest. No visual aid is required to focus the light
2. THE MYOPIC EYE:
In this case parallel light rays, (with the eye at rest) are brought into focus in front of the retina
3. THE HYPEROPIC EYE (HYPERMETROPIC EYE):
In this case parallel light rays, (with the eye at rest) are brought into focus behind the retina
The figure above shows these three refractive states. Figure "A" shows an emmetropic eye, Figure "B" shows a hyperopic eye, while figure "C" shows a myopic eye.
HYPEROPIA & ACCOMMODATION -THE SOURCE OF ALL EVIL:
Remember how I said that all these states consider the eye to be at rest (i.e. not accommodating)? Consider figure "B". Can you think of a way the eye can naturally force the light rays to become focused on the retina? If you thought accommodation you are right!
Accommodation is the process by which the eye focuses for near object like when you are reading a book or looking at the screen of your monitor. The eye does not (or should not) use accommodation for focusing light coming from distant objects. Accommodation increases the total power of the eye and helps to focus the light rays onto the cornea.
Hyperopes are also called far-sighted. Technically this term is incorrect. Why? If you paid attention to the definition above, hyperopia is defined as a condition in which parallel light rays are brought into focus behind the retina, with the eye at rest (not accommodating). Thus hyperopes DO NOT see clearly for far until they start to accommodate which sort of is against the definition. The term might be useful to describe refractive errors to "normal" people, but for a student of medicine (or optics) proper terminology must be understood and used.
4. ASTIGMATISM:
The cornea is shaped like a sphere (more or less). But what if its shape is more like that of a rugby ball? A new type of error called "Astigmatism" is born. (A= Not, Stigma = spot). Here light rays are refracted differently in the two meridians of the cornea. Confusing? Look at the figure below.
There are two set of light rays shown by the pink and green colors. Remember how this cornea is shaped like a rugby ball. If it were a sphere the two set of light rays would refract equally at the corneal surface, as the radius of curvature in all dimensions on a sphere are the same. The corneal meridian (horizontal or vertical axis) which is more curved bends light more. The less curved meridian bends light less. Thus two sets of focii are produced. The location of these focii determine the type of astigmatism.
In the diagram above the vertical meridian is more curved and thus refracts light more, bringing it in focus in front of the retina (Green). The horizontal meridian is comparatively flatter and thus brings light into focus behind the retina (Pink). The rest of the possibilities are described below:
I. MIXED:
If one corneal meridian focuses light in front of the retina and one behind it the type of astigmatism is mixed
II. SIMPLE MYOPIC:
If one corneal meridian focuses light on the retina and the other in front, the type of astigmatism is simple myopic
III. COMPOUND MYOPIC:
If both corneal meridians focuses light in front of the retina, the type of astigmatism is compound myopic
IV. SIMPLE HYPEROPIC:
If one corneal meridian focuses light on the retina and the other behind it, the type of astigmatism is simple hyperopic
V. COMPOUND HYPEROPIC:
If both corneal meridians focus light behind the retina the type of astigmatism is compound hyperopic.
Astigmatism requires a special type of a lens, called sphere-cylindrical lens to fix.
More information regarding astigmatism & refractive errors is available here
LOOKING BEYOND GLASSES -LASER CORRECTION OF VISION
LOOKING BEYOND GLASSES -LASER CORRECTION OF VISION
Glasses and contact lenses are not the only way to correct refractive errors. Lasers can be used to reshape the surface of the cornea to achieve the same effect as those produced by lenses. Within a given range of refractive errors, Lasers are becoming a standard for vision correction. With advancement in technology it is possible to correct a wide variety of refractive errors and achieve a high level of patient satisfaction.
Glasses and contact lenses are not the only way to correct refractive errors. Lasers can be used to reshape the surface of the cornea to achieve the same effect as those produced by lenses. Within a given range of refractive errors, Lasers are becoming a standard for vision correction. With advancement in technology it is possible to correct a wide variety of refractive errors and achieve a high level of patient satisfaction.
This video shows how a laser can be used to reshape the corneal surface in order to negate the effect of refractive errors.
The task at hand is to find evidence regarding how satisfied patients are with their laser correction of vision.
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