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Gaussian profile is a characteristic symmetric "bell curve" shape.

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n0 linear,n2 nonlinear term

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Part 2

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Part 2

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Part 3

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Part 2

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CR-39, or allyl diglycol carbonate (ADC), is a plastic polymer commonly used in the manufacture of eyeglass lenses. The abbreviation stands for "Columbia Resin #39", which was the 39th formula of a thermosetting plastic developed by the Columbia Resins project in 1940.

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Single Layer on Glass,but double layer on plastics Materials.

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Single Layer on Glass,but double layer on plastics Materials.

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Methods of Controlling Surface Reflection

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Methods of Controlling Surface Reflection

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1)For clear ophthalmic crown glass with Refractive index 1.523, the light lost by reflection at the front surface ? (in percentage %).

2)Remaining light when reaches the back surface of the lens? (in Percentage %).

3)The light lost by the back surface of the lens? (%)

4)Light transmitted by the lens? (%)

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Why ARC is needed?

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Try to focus on the image

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Each lenslet is a fraction of a mm. It divides the broad beam of light

Into many sub-beams for measurement. Each lenslet focuses onto the

Video sensor. We analyze the position of each spot.

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The distance between the wavefront surface and a reference plane.

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1)Emmetropia/no aberrations
2) Myopia/Distorted wavefront
Aberrometers measure the shpa of the wavefront

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Hartmann-Shack Aberrommetry:How does it work

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Hartmann-Shack Aberrommetry:How does it work

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-Analyze  light that emerges or is reflected from the retina and passes through the optical system of the eye.

-Produces a “fingerprint” of the aberrations for an individual eye.

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-Custom can correct for
- Myopia or hyperopia

-Astigmatism

-Spherical Aberration

Coma

Custom does not correct for :

-Chromatic aberration

-Diffraction

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- Light is has both particle and wave characteristics

- Wavefront analysis describes the wave behavior of light in the eye

-Actual image displacement of the point source from foveola as it passes through the optical system of the eye compared to the ideal image.

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Consider an object consisting of a perfect point. The image of this object will be at least one point wide. Normally it’s image will consist instead of a spot of several points, brightest in the centre and progressively darker away from the centre. This image function is the point-spread function.

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An Airy disc shows how a point image is degraded by aberration

-When wave encounters and obstruction, the direction of the wave changes.

This is called DIFFRACTION

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-Hartmann first looked at this a century ago

-Shack elaborated on this in the 1980’s working for the air force

-Liang was the first to use the wavefront sensor to measure the human eye in 1994.
-By the late 1990’s commercial development was occuring.

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Wavefront Aberration-Part 2:Beyond sphere and cylinder

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Wavefront Aberration-Part 2:Beyond sphere and cylinder

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-Higher order aberrations have been traditionally ignored clinically.

Now are routinely considered:

- Post lasik increase in higher order aberrations

- Can be easily measured

- Wavefront guided correction available

Patient expectations

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-  Chromatic Aberration

- Spherical Aberration

-  Oblique Astigmatism

-  Coma

-  Curvature of Field

-Distortion

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-Not about sharpness, but faithful reproduction of the shape of the object.

- It occurs when magnification varies with the distance of the object from the optic axis.

-Problem only for high powers

-Tends to falsify the positions of objects and cause vertical lines to wave
- Aphakes.

-Minimized by very steep back base curves.

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-The surface of the image formed by the eye is also curved, fortunately, the retina is also curved.

-For lens systems, using best form lenses with non-spherical shapes can help.

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-A lens aberration that causes a flat object surface to be imaged onto a curved surface rather than a plane.

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-Causes an planar object to project a curved (nonplanar) image. It can be thought of as arising from a "power error" for rays at a large angle. Those rays treat the lens as having an effectively smaller diameter and an effectively higher power, forming the image of the off axis points closer to the lens.

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-For a single lens, coma can be partially corrected by bending the lens. More complete correction can be achieved by using a combination of lenses symmetric about a central stop.

-Coma is not well compensated for in the human eye.

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-The resulting image is called a comatic circle.

-The coma flare, which owes its name to its cometlike tail, is often considered the worst of all aberrations, primarily because of its asymmetric configuration.

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-A lens with considerable coma may produce a sharp image in the center of the field, but become increasingly blurred toward the edges.
Note:Please omit the left side image

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Coma is an aberration which causes rays from an off-axis point of light in the object plane to create a trailing "comet-like" blur directed away from the optic axis.

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- Mitigated by deviating from the spherical shape

-Aspheric Surfaces to the Rescue
- Aspheric lens has a non-spherical lens surface. The main advantage of aspheric lenses is its ability to correct for spherical aberration. Aspheric lenses allow optical designers to correct aberrations using fewer elements than conventional spherical optics because the former gives them more aberration correction than multiple surfaces of the latter. Given that, smaller amount of aspheric lenses can be substituted for many spherical lenses to achieve similar or better optical results, while reducing system size, simplifying the assembly process, and yielding imaging lenses that ultimately cost less and outperform assemblies made of traditional spherical components.

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A dot is no longer imaged as a dot, but as two image lines.

Note:Please omit the right side image.

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This aberration primarily influences the image quality of spherical lenses. When the wearer looks at an angle through the lens, there is a deviation which he perceives as blur. The higher the dioptric power of the lens, the more pronounced this error becomes.

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The amount of spherical aberration in a lens made from spherical surfaces depends upon its shape.  Best form, depends on base curve.

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Spherical aberration in the human eye is reduced  by the aspheric shape of the lens and the cornea.

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-For lenses made with spherical surfaces, rays which are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point.

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Correction of chromatic Aberration (Apochromatic Lens)

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Correction of chromatic Aberration (Apochromatic Lens)

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-The addition of a third lens corrects for three colors (red, blue and green), greatly reducing the fuzziness caused by the colors uncorrected in the achromatic doublet.

- Achromatic triplets, also known as apochromatic lenses, use three lenses to correct aberration even further. Whereas doublets are able to focus two distinct wavelengths, apochromats can focus three wavelengths to achieve greater image clarity. Apochromats are typically more expensive than doublets due to their use of low dispersion lenses constructed of fluorite, flint glass, and optically transparent fluids.

- In the human eye, chromatic aberration is reduced by the lens, which changes index from the nucleus outward.

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-An achromat doublet does not completely eliminate chromatic aberration, but can eliminate it for two colors, say red and blue.

-The idea is to use a lens pair – a strong lens of low dispersion coupled with a weaker one of high dispersion calculated to match the focal lengths for two chosen wavelengths.

-Cemented doublets of this type are a mainstay of lens design
Figure :Achromatic Doublets

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-Dispersive power (abbe value) is based on change in index  for different wavelengths

-If the index  is the same for all wavelengths, there is NO DISPERSION

-The n increases as wavelength decreases

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-A lens will not focus different colors in exactly the same place.

-the focal length depends on refraction and the index of refraction

-Short wavelength has higher n and is refracted more than long wavelength

-The amount of chromatic aberration depends on the dispersion of the glass.

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-There is no such thing as a perfect image
-All light rays passing through optical systems are subject to distortions

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Two- step magnification

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Huygen eyepiece & Ramsden eyepiece

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A simple Maginifier

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Standard Relative Apertures & Irradiance Available on Cameras

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Standard Relative Apertures & Irradiance Available on Cameras