8. Aberrations of Optical Systems Flashcards
Chromatic aberration
when white light is refractive at an iptical interface it is dispersed into its component wavelengths or colours
in chromatic aberration, the shorter the wavelenth
the more it is deivated
chromaitc aberration is not desierable - T/F
True
lenses used in instruments are designed to eleminate it
Dispersive power of a material is not linked to its refractive index - T/F
True
materials can have a high refractive index but low dispersive power
achromatic lens systemts
composed of elements of varying material combine so that the dispersion is neutralised while the overall refractive power is preserved
what were the earliest achromatic lenses made fromt
flint and crown glass
refraction by the eye is subject to chromatic aberration - T/F
True
what is the total dispersion from red to the blue image
approx 2.00D
where does the emmetropic eye focus
on yellow green 555nm
why does the emmetropic eye focus of yellow green
as this is the peak wavelenght of the phtopic relative luminosity curve
where does the focus lie between
between the blue and red foci, slight nearer to the red end
what is the duochrome test and how is it perfromed
assess for chromatic aberration to check refraction in myopic patients
Upper rank mounted on red and lower on green
Patients views the letweer by red and green light respectivtly and are asked to say which they see clearer
why is red and green used in the duochrome test
as their wavelength foci straddle the yellow greed by equal amoiunts - 0.4D either side
how sensitive is the duochrome test
can pick up refraction of 0.25D of less
a myopic eye sees which more clearly
green
a hypermetropic eye sees which more clearly
red
at the end of myopic refraction, which should they see more clearly
RED > green
If overcorrected, made hypermetropic and using accommodation for distance
Spherical aberration
Prismatic effect – rays passing at periphery are deviated more
ways to reduce spherical aberration
o Use of stops
o Adjusting the lens
o Doublet lens
how do stops help to reduce spherical aberration
only the paraxial zone is used
how does changing the lens help to reduce spherical aberration
plano-convex > biconvex
how do doublet lenses help to reduce spherical aberration
principal lens and a weaker lens of opposite power and different refractive index
ways ocular spherical aberration is reduced
o Anterior corneal surface is aspheric (aplanatic)
o Axial sone of the lens (nucleus) greater refractive power than periphery (cortex)
o Iris acts as a stop – optimum size is 2-2.5mm
o Stile-crawford effect – retinal cones more sensitive to light entering the eye paraxially
oblique astigmatisim
o Occurs when light enters the eye obliquely i.e. no parallel to the principal axis toric effect sturm’s conoid
higher glasses prescriptions produce
higher amounts of oblique astigmatism
Worse for biconvex and biconcave lenes
pantoscoptic tilt
Glasses are made with the lower borders of the lenses tilted towards the cheek to reduce obliquity of the reading potion
best form lenses
reducing both spherical and oblique aberrations, usually meniscus form
ocular oblique astigmatism is reduced by
o Aplanatic curvature of the cornea
o Retinal is a spherical surface - circle of least confusion of the strum’s conoid falls on the retina
o Astigmatic image falls on the peripheral retina – visual appreciation of astigmatic image is limited
coma aberation
spherical aberration for light coming from points not lying on the principal axis
o Rays passing through the periphery of the lens are deviated more
o This produces unequal magnification
o Not optically significant due to reasons for ocular oblique astigmatisim
low order aberrations
myopia, hyperopia, astigmatism
image distortion caused by a convex lens
pin-cushion
image distortion caused by a concave lens
barrel distortion
curvature of field
o Plane objects give rise to a curved image
o Related to – refractive index of lens and cruvate of the lens surface
o Curvature of the retina compensates for curvature of field
