Aberrations of Optical Systems

Othman Al-Abbadi, M.D

• 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.

Index

• 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

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 earliest achromatic lenses were made by combining elements of flint and crown 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 2-D.• 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

photopic luminosity curve

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 (and thus rendered hypermetropic), forcing them to use their accommodation for near vision.

• Red & green are used because their wavelength foci straddle the yellow-green by equal amounts ~0.4D

• Myopics see red letters more clearly, & vice versa.

• The test is sensitive to an alteration in refraction of 0.25 D or less.

Directions for use :• If the near vision correction is too strong, the subject will

spontaneously see the letters with greater contrast and blacker on the red background.

If the letters are seen clearer on the green background, it means that the near correction is too weak.

• Color blindness doesn’t invalidates the test due to its dependence on the position of the image with respect to the retina NOT on color discrimination.

• A color-blind should be asked which side’s letters appears clearer.

• The eye with overactive accommodation may still require too much minus sphere in order to balance the red and green. Cycloplegia may be necessary.

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 lens:Peripheral rays have shorterfocal length than paraxial rays.

Correction of Spherical Aberration• Spherical aberration may be reduced by occluding the

periphery of the lens by the use of 'stops' so that only the paraxial zone is used.

• 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.

aspheric doublet lens.

Ocular application• The effect of spherical aberration in the human eye is

reduced by several factors:

• (1) The anterior corneal surface is flatter peripherally than at its centre, and therefore acts as an aplanatic surface. • (2) The nucleus of the lens of the eye has a higher

refractive index than the lens cortex… Thus the axial zone of the lens has greater refractive power than the periphery.

• (3) The iris acts as a stop to reduce spherical aberration. The impairment of visual acuity that occurs when the pupil is dilated is almost entirely due to spherical aberration (Optimum pupil size is 2–2.5 mm.)• (4) 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). • This directional sensitivity of the cone photoreceptors

limits the visual effects of the residual spherical aberration in the eye.

Oblique astigmatism• Occurs when rays of light traverse a spherical lens obliquely…

a toric effect is introduced forming a Sturm’s conoid

• Occurs with spectacle lenses when the light of sight is NOT parallel with the principal axis of the lens.

• Worse with higher power lenses. • Less with meniscus (convex-concave) lenses. • NB 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:

1.Aplanatic surface of the cornea reduces oblique astigmatism as well as spherical aberration

2.Retina is a spherical surface ; the circle of least confusion of the Sturm’s conoid formed by oblique astigmatism falls on the retina.

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

Coma • Spherical aberration applied to light coming from points NOT

lying 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.

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:

1.Aplanatic surface of the cornea

2.Retina is a spherical surface

3.Coma image falls on peripheral retina which has poor resolving power compared to the macula; visual appreciation of astigmatic image is limited

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