Aberrations of Optical Systems Including the Eye Flashcards
What is chromatic aberration
When white light is refracted at an optical interface, it is dispersed into its component wavelengths or colours The shorter the wavelength of the light, the more it is deviated on refraction. Thus a series of coloured images are formed when white light is incident upon a spherical lens
How to correct chromatic aberration
Achromatic lens system
They are composed of elements (lenses) of varying material combined so that the dispersion is neutralised while the overall refractive power is preserved
For example, by combining a convex lens of high refractive power and low dispersive power with a concave lens of low refractive power but higher dispersive power
How were the earliest achromatic lenses made
combining elements of flint and crown glass
What is the approximate dispersion of light between the red and blue image in the eye
2.00 Dioptres
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
In the duochrome test which colours do the myopic/hypermetropic eye see better
A myopic eye sees the red letters more clearly than the green while a hypermetropic eye sees the green letters more distinctly
What is spherical aberration
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
How to correct spherical aberration
1) by occluding the periphery of the lens by the use of ‘stops’ so that only the paraxial zone is used.
2)Lens form may also be adjusted to reduce spherical aberration, e.g. plano-convex is better than biconvex
3) 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
How does the eye physiologically reduce spherical aberration
(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) the 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)
What is oblique astigatism
an aberration which occurs when rays of light traverse a spherical lens obliquely. When a pencil of light strikes the lens surfaces obliquely a toric effect is introduced. The emerging rays form a Sturm’s conoid with two line foci
What is pantoscopic tilt
In daily life adults spend most time looking slightly downward from the primary position, and spectacles are therefore made with the lower borders of the lenses tilted towards the cheek
How is oblique astigmatism reduced by the eye
1) The aplanatic curvature of the cornea reduces oblique astigmatism as well as spherical aberration
2) The retina is not a plane surface, but a spherical surface. In practice the radius of curvature of the retina in the emmetropic eye means that the circle of least confusion of the Sturm’s conoid formed by oblique astigmatism falls on the retina
3) the astigmatic image falls on peripheral retina which has relatively poor resolving power compared with the retina at the macula. Visual appreciation of the astigmatic image is therefore limited
What is coma
Coma is really 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 the central rays and come to a focus nearer the principal axis.
This results in unequal magnification of the image formed by different zones of the lens. The composite image is not circular but elongated like a coma or comet
What kind of image distortion is produced by concave lenses
barrel distortion
What kind of image distortion is produced by convex lenses
pincushion distortion
