Chapter 8: Aberrations of Optical Systems Flashcards
What are 6 types of optical aberrations in visual systems?
- chromatic aberration
- spherical aberration
- oblique astigmtism
- coma aberration
- image distortion
- curvature of field
What is chromatic aberration?
when white light is refracted at an optical surface it is dispersed into its component wavelengths or colours - shorter wavelengths are deviated more on refraction (i.e. blue > red)
therefore a series of coloured images are formed when white light is incident on a spherical lens
What is the relationship between the dispersive power of a material and its refractive index?
dispersive power is independent of its refractive index
What is used to correct chromatic aberration in lens systems?
achromtic lens systems
What are achromatic lens systems?
lenses of varying material combiend so dispersion is neutralised, while overall refractive power is preserved
e.g. combining convex lens (high refractive power, low dispersive power) with concave lens (low refractive power but higher dispersive power), can enutralise aberration
What is the total dispersion from the red to the blue image in the human eye due to chromatic aberration?
approx 2.00 D
How does the human eye overcome ocular chromatic aberration?
emmetropic eye focuses for the yellow-green (555nm) as this is the peak wavelength of the photopic relative luminosity curve (=between blue and red foci, slightly nearer red)
What is a clinical test that makes use of the chromatic aberration of the eye?
Duochrome test
How does the duochrome test work to test visual acuity?
patient reports which is clearer; myopic eye will see red more clearly, hypermetropic eye will see green letters more clearly
if green is clearer, patient needs +0.25D until they’re the same
if red is clearer, patient needs -0.25D until the same
Why is the duochrome test particularly useful in myopic patients?
experience eye strain if overcorrected + forced to use accommodation for distance vision
should be able to see red letters ore clearly than the green at the end
Why doesn’t colour blindness invalidate the duochrome test?
it depends on the position of the iamge with respect to the retina - should ask whether upper or lower rank of letters appears clearer
What is spherical aberration?
= prsimatic effect of a spherical lens; rays passing through the periphery of a spherical lens are deviated more than those passing through the paraxial zone
What are 4 ways of reducing spherical aberration of a lens?
- ‘stops’: occluding the periphery of the lens
- lens form: plano-convex > biconvex
- aplanatic lens surfaces
- doublet: 2 lenses cemented
How can stops be used to reduce spherical aberration?
they occlude the periphery of the lens so only the paraxial zone is used
How can lens form be adjusted to reduce spherical aberration?
plano-convex lenses preferable to biconvex
What is an aplanatic lens surface to reduce spherical aberration?
spherical surfaces are abandoned and the lenses are grount with aplanatic surfaces - peripheral curvature less than central curvature
What is a doulet when reducing spherical aberration?
principal lens and a weaker lens of different refractive index cemented together
weaker lens should be of opposite power; it also has spherical aberration so will reduce power of the periphery of the principal lens more than the central zone
(usually achromatic as well as spheric)
What are 4 factors which reduce spherical aberration in the human eye?
- Anterior cornea flatter peripherally (aplanatic)
- Nucleus of lens has higher refractive index than lens cortex - axial zone has greater refractive power
- Iris acts as a stop
- Stiles-Crawford effect (type 1): retinal cones more sensitive to light entering the eye paraxially than obliquely
What is the impairment of visual acuity when the pupil is dilated due to?
almost entirely due to spherical aberration
What is optimum pupil size?
2 - 2.5mm
What is the Stiles-Crawford effect?
retinal cones are more sensitive to light which enters the eye paraxilaly than to light which enters obliquely through the peripheral cornea
directional sensitivity limits the visual effects of residual spherical aberration in the eye
What is oblique astigmatism?
aberration occuring when rays of light traverse a spherical lens obliquely; when a pencil of light strikes the lens surface obliquely, a toric effect is introduced. Emerging rays form a Sturm’s conoid with 2 line foci
What causes oblique astigmatism to occur with spectacle lenses?
when the line of sight is not parallel with the principal axis of the lens; also in patients with restricted eye movement who adopt a compensatory head posture and look obliquely through peripheral portions of spectacle lenses
What is the relationship between spectacle lens power and oblique astigmatism?
the higher the spectacle lens power, the greater the unwanted cylindrical power induced by the aberration
What is pantoscopic tilt?
in daily life most adults spend most time looking slightly downward from the primary position so loewr borders of the lenses are tilted towards the cheek
Which types of spectacle lenses cause worse oblique astigmatism than others?
biconvex and biconcave = much worse than meniscus lenses
What are 3 factors making oblique astigmatism minimal in the human eye?
- Aplanatic curvature of the cornea
- Spherical surface of the retina means the circle of least confusion of Sturm’s conoid falls on the retina
- Astigmatic image falls on the peripheral retina which has poor resolving power compared with the retina at the macula so visual appreciation of the astigmatic image is limited
What is the definition of 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 the central rays and come ot a focus nearer the principal axis, causing unequal magnification of the image formed by different zones of the lens
causing elongated composite image rather than circula (like a coma or comet)
What are 2 key ways that coma aberration can be corrected in lenses?
- Limiting rays to the axial area of the lens
- Using the principal axis of the lens rather than a subsidiary axis
What is the effect of ocular coma aberration and why?
not of practical importance - same reasons as oblique astigmatism (due to aplanatic corneal curvature, spherical retinal surface and Stiles-Crawford)
What is image distortion?
when an extended object is viewed through a spherical lens, the edges which are viewed through the peripheral portion are distorted
this is due to increased prismatic effect of the periphery of the lens producing uneven magnification of the object
What type of image disortion is produced by a concave lens?
barrel disortion
What type of image distortion is produced by a convex lens?
pin cushion distortion
What is meant by ‘curvature of field’?
plane object gives rise to a curved image, occurs even when spherical aberration, oblique astigmatism and coma aberrated
Why does cuvature of field occur even when spherical aberration, oblique astigmatism and coma have been eliminated?
it is dependent upon the refractive index of the lens material and the curvature of the lens surfaces
What compensates for curvature of field in the eye?
curvature of the retina
What is Stiles Crawford effect number 2?
there is a shift in the hue and saturation of monochromatic lights depending on the angle of incidence
What determines the position of the circle of least confusion from an astigmatic lens?
focal distance of the spherical equivalent of the lens (e.g. +3.00/-2.00x90 would be ½ = 50cm)
