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Module 17 Section 1 |
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Module 17: |
Clinical Optics Part 1 |
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Section 1: |
Lens Aberrations | |||
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The perfect glasses lens would project a
sharp, undistorted image onto the retina. However, the
complexities of the human eye make this very difficult to achieve.
Just think about it! The lens designer (i.e., the refractionist,
the optician, the fabricator, and the manufacturer) has to deal with a
fluctuating tear film, an aspherical cornea, an aging zoom lens, and a
lens correction mounted on the patient's nose. On top of that, the
correcting lens carries some inherent design deficiencies. Pat
yourself on the back when a patient leaves the optical shop satisfied
with his new glasses!
In order to increase our percentage of satisfied glasses wearers, it is helpful to understand what can go wrong with a glasses lens. Lenses are designed in such a way as to refract light to a point focus on the retina. The same characteristics that achieve this (curvature, thickness, diameter, etc.) also have negative effects on the image. These negative effects are called "aberrations". There are seven major lens aberrations that work against obtaining a perfect image on the retina. They are:
The light rays from the peripheral edge of the lens are refracted (bent) to a greater degree than the light rays passing through the center of the lens.
This creates a slight blurring of the image that is minimized by the size of the pupil. The smaller the pupil is, the less blurring there is from spherical aberration (the pinhole effect). This is one reason that the patient with a large pupil, from dilation or otherwise, may see "ghosting" around an otherwise sharp image. The dilated patient with an 8 mm pupil gets spherical aberration from his spectacle lens, the cornea, and his natural lens.
Distortion of the projected image occurs with every lens, but it becomes more and more pronounced as the power (curvature) of the lens increases. The curvature of the lens that causes the beneficial redirection of light rays also has the negative effect of "bending" the image of a straight line.
If there is little or no distortion, the image of a square grid would be projected like this:
If the lens is a minus lens, "barrel" distortion results, with a projected image something like this:
If the lens is a plus lens, "pincushion" distortion results, with a projected image something like this:
How can distortion be minimized?
This aberration is similar to distortion in that straight lines may appear to be curved. The aberration is caused by projecting the image of a flat object onto a curved surface (the retina). It might appear something like this:
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| Marginal
astigmatism is caused when a narrow beam of light enters a lens at an
oblique angle. The rays of light at opposite meridians within the
beam are focused at different points.
Optimizing the front and back curves for a particular prescription can minimize this aberration. What this really means is that the sharpest image is obtained by looking directly through the optical center of the lens, especially with higher powered lenses. |
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Coma is similar to marginal astigmatism in that light entering a lens at an oblique angle is bent to varying degrees depending upon the point of entry. The comparison here is between rays of light in a wide beam instead of a narrow beam.
The effects of coma are largely neutralized by the pupil (pinhole effect) and are not usually a factor in lens design. |
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When white light enters a prism, the light is bent toward the base of the prism. White light is composed of the visible spectrum of wavelengths (colors). The shorter wavelengths (e.g., violet) are bent at a greater angle than are the longer wavelengths (e.g., red).
Since a glasses lens can optically be considered to be two prisms apex to apex (minus lens) or base to base (plus lens), then there is a tendency for a lens to focus the different visible wavelengths at different points, creating a "chromatic aberration" and a somewhat blurry image.
Some lens materials cause more chromatic aberration than others. The "Abbe" value of a lens indicates the ability of the lens to transmit light without chromatic aberration. The Abbe value is between 1 and 100, with high numbers indicating the lens is less likely to cause chromatic aberration. The Abbe value of CR-39 plastic (the most common lens material) is 58. The Abbe value of polycarbonate material is 31. Generally, the higher the index of refraction for a given lens material, the lower the Abbe value is.
Magnification and Minification
As compared to the image created on the retina of an emmetropic eye, plus lenses enlarge (magnify) the image, and minus lenses make the image smaller (minify). The greater the plus power, the greater the magnification. The greater the minus power, the greater the minification is. Although minification is a minor annoyance at best, magnification can be a very useful aberration when image enlargement is desirable (e.g. low vision lenses).
The negative effects are of little consequence with lens powers under 4 diopters. The exception would be the unfortunate patient who has a plus lens correction for one eye and a minus lens correction for the other eye. A significant image size difference makes it difficult to maintain fusion and binocular vision.
Magnification and minification are affected significantly by the vertex distance. The greater the vertex distance is, the greater the degree of magnification and minification. There is a practical application of the vertex effect upon magnification and minification. Image size changes are minimized when the vertex distance is zero. This is the case when the patient wears contact lenses.
When checking the vision of a high myope (above 8 D), keep in mind the minification factor. Some of these patients cannot see the 20/20 letters because they make such a small image on the retina. For a -15.00 D myope, 20/25 vision can be considered to be equivalent to 20/20 vision for an emmetrope.
There you have it, a jumble of aberrations that stand between the patient and a clear, distortion free image. You would think that lens manufacturers would be able to solve some of these problems. Although manufacturers have not totally eliminated the effects of aberrations, they have minimized the effects by developing what is called the "Corrective Curve Theory".
Mathematical formulas are used to calculate what the optimum front and back surface curves would be for a given prescription and lens material. Each manufacturer has their own formulas, so the same prescription may be made differently depending upon which lab is used.
So, what are we as technicians to make of this information? Some useful conclusions can be drawn:
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