aberrations Flashcards
what is an abberration?
the deviation of an image from perfection
what is needed for a lens to produce a clear image?
for each individual point in the object plane to be imaged at a point on the image plane
name the two types of aberrations
monochromatic and chromatic
what are monochromatic aberrations caused by? when do they occur?
due to geometry of the lens so occur when light is both reflected and refracted and also occurs with monochromatic light
what are chromatic aberrations caused by and when do they occur?
caused by dispersion and occur when the lens refractive index varies with it’s wavelength
what are the two chromatic aberrations?
transverse and longitudinal aberrations
what are the types of non chromatic aberrations
-spherical aberrations
-coma
-oblique astigmatism
-distortions
How does the focal length of a lens change with wavelength?
in red light it has a longer wavelngth so optical materials show a smaller refractive index than for the shorter wavelnegth of blue light
why does polychromatic light cause chromatic aberrations?
as there is more than one wavelength of light passing through the lens so a number of different coloured images at varying positions and sizes are formed
whats the equation for longitudinal chromatic abberations of a lens? (LCA)
LCA = F/V where V is the constringence
what two ways is TCA (transverse chromatic aberration) expressed as?
-linear TCA which is the difference in image heights of red and blue colours of light
-the angle (alpha) between the emergent red and blue rays
How can you minimise the dispersive effects of a lens?
by using a cemented achromatic doublet by putting a plus and minus lens that have a common curvature together
what is required for an achromatic doublet to work?
the chromatic aberrations have to be equal and opposite so
LCA lens 1 = -LCA lens 2w
what’s the equation for the power of an achromatic doublet lens?
F1= (FE x V1)/(V1-V2)
when is the longitudinal spherical aberration positive? is the lens spherically over or under corrected?
when the focal margin lies to the left of the focal point
the lens is spherically under corrected
what lenses will always have SA
lenses with two spherical surfaces
how do you best reduce SA in lenses with two spherical surfaces? What problem does this cause?
-restrict the rays to the paraxial region
-problem is this reduces the amount of light passing through the lens and can introduce other problems such as distortion
what is coma? when does it occur?
an aberration where the object is not imaged as a circle of list confusion but instead as a comet shaped patch made up of a series of comatic circles where each circle arises from refration of off axis point at a particular zone of the lens
-occurs when objects are off-axis and distant
understand the concept of skewed rays
give a formula with tangential coma (CT) linking lens aperture and height of paraxial image
CT is directly proportionate to y^2 x h’
where y is the lens aperture and h’ is the heught of the paraxial image
give a formula linking tangential coma with saggital coma
CT = 3Cs where Cs is the saggital coma
what is sagittal coma?
the displacement from Tp’ of the saggital image point for refraction by the marginal zone
What is oblique astigmatism (OA)? what causes OA?
an aberration and visual defect caused by lack of curvature of the cornea
when objects point way off the lens axis, so the image spreads out in a direction along the lens axis
understand the concept of oblique astigmatism
