Lecture 5- Aberrations Flashcards
What is aberrations?
are deviation from perfection in an optical system
when light focuses at different points forming a blurred image
What is an ideal image formation?
when all rays of light meet at one point producing a sharp image
What is a non-ideal image formation?
when rays hit different parts of lens, they refract at different amounts, forming a blurred image point
How is a non-ideal image formed?
So lenses tend to be thinner at edges so therefore they will travel less distance then to the thicker part of lens which is in the middle
What is the wavefront representation of an ideal lens?
unaberrated
ideal optical system
wavefront is spherical
What is the wavefront representation of a real lens?
aberrated
non-spherical wavefront
a slight deviation of the spherical shape of the wavefront
Why is the wavefront of a real lens useful?
it allows measuring the aberrations of the optical system
What is a spherical wavefront?
parallel rays of light focusing at one point
What is a monochromatic aberration?
aberrations that arise due to geometry (shape of lens o
they occur for a single wavelength
What do monochromatic aberrations not depend on?
the wavelength of light
What are the 5 types of monochromatic aberrations?
Spherical aberration Coma Astigmatism Field curvature Distortion
What are the monochromatic aberrations knows as?
seidel aberrations
What are the effects of seidel aberrations?
expanded size for a point image- becomes blurred
curved image plane
extended images are no longer geometrically similar to object
What is the spherical aberration?
rays on optical axis where image rays converge causing blurred image point
What happens to the 2 extreme rays in the non-ideal image formation?
they converge infront of the second focal point
How do we limit spherical aberration?
limit the rays via paraxial region
within the reigion, imaging systems are ideal
What can human eye do to limit the extreme rays from coming in?
placing an aperture
What does the human eye have that is similar to the aperture?
a pupil as it limits the amount of light getting into the eye
What is coma?
when rays come into optical axis at an angle
the bigger the angle the more coma
What does coma do?
increases for off-axis rays and has a comet like appearance
Why does it appear comet like?
there is a distortion on image plane which looks like a comet
-the off-axis rays are close together in the image plane, then closer to the optical axis, then become further away creating this comet-like shape
What is astigmatism?
when 2 planes of best focus, one in horizontal a one in verticle
What is field curvature?
it increases for off-axis rays
the rays which are off-axis, will come at an angle and will be slightly focused in front of image plane
What structure is the field curvature useful for?
retina (curved at back)
as it minimises field curvature
therefore minimises aberration
as the retina is curved, the curved image plane can get everything in focus.
What is distortion?
happens when there is a different magnification with rays coming off-axis
What is pincushion distortion?
when there is magnification oustide
What is barrel distortion?
when there is magnification in the centre
greater magnification in centre
how do we limit chormatic aberration?
by using achromatic doublet lens
What happens when you shine white light through a lens made of crown glass?
so depending on refractive index of different wavelenths, you have different focal points for different wavelengths
What is chromatic aberration dan effect of?
resulting from dispersion as lenses have different refractive indicies for the different wavelengths of light
What does it mean if there is a higher refractive index?
the denser the material, the slower light travels through it (more steep e.g blue light)
What can a prism do?
disperse light into different wavelengths
What is achromatic doublet lens?
counter-balances the differences in speed of colours/different wavelengths
What happens when light goes through an aperture?
it diffracts on the edges
What happens if the smaller the aperture?
diffraction increases
What is the effect of diffraction?
perfect object point= airy disk has the effect of diffraction
What does the aperture do?
minimises aberrations
What happens when you have a large aperture?
you have more aberrations
So how do you minimise the aberrations?
by making the aperture smaller
What happens when you make the aperture smaller?
diffraction increases which means you get an Airy Disk
What is an airy disk?
is the image of a point object due to the effect of diffraction
What is the airy disk if its a symmetrical image?
bright centre dot
What happens if it appears asymmetric?
then you know it has aberrations
