ABERRATIONS AND OPHTHALMIC LENS DESIGN

PresenterJayendra JhaOptometrist

C L Gupta Eye InstituteMoradabad (U.P)

THE PERFECT IMAGE

• There is no such thing as a perfect image

                           

                  

All light rays passing through optical systems are subject to distortions

ABERRATION

• Imperfections of image formation are due to several mechanisms

• The refracting system of the eye is also subject to aberrations, but there are correcting mechanisms built into the eye itself.

TYPES OF ABERRATION

• Chromatic Aberration• Spherical Aberration• Oblique Astigmatism• Coma• Image Distortion• Curvature of Field

CHROMATIC ABERRATION

• When white light is refracted at an optical interface, it is dispersed into its component wavelengths or colors .

• The shorter the wavelength of the light, the more it is deviated on refraction.

• Thus a series of colored images are formed when white light is incident upon a spherical lens

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

CHROMATIC ABERRATION

• Some patients can detect this • Dispersion usually increases in

high index • May be noticeable with IOL’s

CORRECTION OF CHROMATIC ABERRATION

• The dispersive power of a material is independent of its refractive index.

Thus, there are materials of high dispersive power but low refractive index, and vice versa.

ACHROMATIC LENS

• Special optics design of two mated lens concave and convex, which more precisely focus the wavelengths of light onto the same plane.

• Achromatic lens systems are composed of elements (lenses) of varying material combined so that the dispersion is neutralized while the overall refractive power is preserved

• The first achromatic lens was formed by combining crown and flint glass.

OCULAR APPLICATION

• Refraction by the human eye is also subject to chromatic aberration, the total dispersion from the red to the blue image being approximately 2D.

• The emmetropic eye focuses for the yellow–green (555 nm) as this is the peak wavelength of the photopic relative luminosity curve.

• This wavelength focus lies between the blue and red foci, being slightly nearer to the red

• Examined by duochrome test

DUOCHROME TEST

• The Duochrome test can be used to verify the near addition.

• It is based on the chromatic aberration of the eye.

• The test is of particular use in the refraction of myopic patients, who experience eye strain if they are overcorrected, forcing them to use their accommodation for near vision.

DUOCHROME TEST

Too much minus green is clearer

Too much plus red is clearer

CHROMATIC ABERRATIONDUOCHROME TEST

Red clarity= green claritythen image is positionedcorrectly.

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SPHERICAL ABERRATION

• 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.

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

                                                                   

                                                                 

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SPHERICAL ABERRATION

• It was seen that the prismatic effect of a spherical lens is least in the paraxial zone and increases towards the periphery of the lens.

• Thus, rays passing through the periphery of the lens are deviated more than those passing through the paraxial zone of the lens

• In other words, the parallel light rays of incoming light do not converge at the same point after passing through the lens. Because of this, Spherical Aberration can affect resolution and clarity, making it hard to obtain sharp images.

• Results in out-of-focus image.

SPHERICAL ABERRATION- CORRECTION

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

CONTD…

• To achieve the best results, spherical surfaces must be abandoned and the lenses ground with aplanatic surfaces; that is, the peripheral curvature is less than the central curvature .

• Aspherical lenses are lenses with complex curved surfaces, such as where the radius of curvature changes according to distance from the optical axis.

ASPHERIC DOUBLET LENS

• Another technique of reducing spherical aberration is to employ a doublet. This consists of a principal lens and a somewhat weaker lens of different refractive index cemented together .

• The weaker lens must be of opposite power, and because it too has spherical aberration, it will reduce the power of the periphery of the principal lens more than the central zone.

• Usually, such doublets are designed to be both aspheric and achromatic.

OCULAR APPLICATION• Spherical aberration in the human eye is reduced by several

factors:• The anterior corneal surface is flatter peripherally than at its

centre, and therefore acts as an aplanatic surface. • The nucleus of the lens of the eye has a higher refractive

index than the lens cortex.• The iris cuts the peripheral to reduce spherical aberration.

The impairment of visual acuity occurs when the pupil is dilated is due to spherical aberration

• Retinal cones are much more sensitive to light which enters the eye paraxially than to light which enters obliquely through the peripheral cornea (Stiles–Crawford effect).

OBLIQUE ASTIGMATISM

• Occurs when rays of light traverse a spherical lens obliquely, a toric effect is introduced forming a Sturm’s conoid

• 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 perceives as blur. The higher the dioptric power of the lens, the more pronounced this error becomes.

CONTD…

• Occurs with spectacle lenses when the line of sight is not parallel with the principal axis of the lens.

• Worse with higher power lenses. • Less with meniscus (convex-concave) lenses. • Size of pupil makes no difference• Can be corrected by Pantoscopic tilt of the

glasses due to the fact that adults spend most of their time looking slightly downward from the primary position.

OCULAR APPLICATION

Occurs in the eye but its visual effect is minimal Due to:• Aplanatic surface of the cornea reduces oblique

astigmatism as well as spherical aberration• Retina is a spherical surface, the circle of least

confusion of the Sturm’s conoid formed by oblique astigmatism falls on the retina.

• Astigmatic image falls on peripheral retina which has poor resolving power compared to the macula, visual appreciation of astigmatic image is limited.

COMA

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.

COMA • Spherical aberration applied to light coming

from points not laying on the principal axis.• Rays passing through the periphery of the lens

are deviated more than central rays & come to a focus nearer the principal axis.

• Results in unequal magnification of the image formed by different zones of the lens.

• Differs from spherical aberration in that the image formed is laterally displaced.

• A lens with considerable coma may produce a sharp image in the center of the field, but become increasingly blurred toward the edges.

COMA- CORRECTION

• 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.

OCULAR APPLICATION

May be avoided by limiting to the axial area of the lens.Not of clinical significance due to the same reasons for oblique astigmatism… which are:• Aplanatic surface of the cornea• Retina is a spherical surface• Coma image falls on peripheral retina which has poor

resolving power compared to the macula, visual appreciation of astigmatic image is limited

IMAGE DISTORTION

• 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• Minimized by very steep back base curves

IMAGE DISTORTION

• Prismatic effects of lens periphery causes uneven magnification

• Concave Lens: Barrel Distortion• Convex Lens: Pin cushion

Distortion• Clinically its is problematic for

high refractive errors i.e. Aphakics

• Patient can adapt to a small amount of Distortion.

IMAGE DISTORTION

Plus Lens Minus Lens

REFRENCES• Optics and Refraction by A.K Khurana

• Mechanical Optics by Borish

• Borish Clinical Refraction

• Article of Institute of Applied Physics

• Internet

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