Lecture 5 - Aberrations Flashcards
What is aberrations
Deviation from perfection - no optical system is perfect
What is aberration part of
Optical imaging system e.g. lens, eyes
What happens in ideal image formation
All rays meet at 1 point = SHARP + CLEAR IMAGE
Rays converge to 2nd focal point
Parallel rays hitting different part of lens - refracts by different amounts
What happens in non - ideal image formation
BLURRED IMAGE
Rays converge to several points on optical axis - not all at one point - only 2 most extreme rays ( top and bottom ) converge at 2nd focal point, instead of all rays - REALITY - NOT PERFECT
What does the level of blur in an image depend on
How many aberrations in optical system
Is ideal lens aberrated or non - aberrated
Unaberrated
What happens in ideal lens
ALL parallel rays of light to point image - perfect
Wavefront 90 degrees to each other
Spherical wavefront
What are monochromatic aberrations
Aberrations that arise due to geometry ( shape of lens/mirror )
Do monochromatic aberrations depend on wavelength of light
No
What are the 5 types of monochromatic aberrations and what are they called
Spherical aberration Coma Astigmatism Field Curvature Distortion These are called Seidal aberration
What are the effects of seidal aberrations
- Expanded size for a point image - blurred
- Curved image plane
- Extended images no longer geometrically similar to object
Spherical aberration
- Extreme rays - misbehaving - blurred image point ( all image rays dont converge at one point )
- Real/ non - ideal
- Includes aperture stop - gets rid of extreme rays by adding aperture ( hole ) = clear image
- Paraxial region - area of extreme rays - outside rays misbehaving
- Want to limit outside rays outside of dotted line to limit spherical aberrations
Coma
- Comet like appearance
- Greater angle - more coma distortion on image plane
- Coma increases for OFF AXIS RAYS - rays coming into optical axis at an angle = closer together in image plane and closer to optical axis
Astigmatism
- Cylindrical lens - different powers according to different meridians
- 2 planes - horizontal and vertical - choose point in between these 2 planes
Field curvature
- Curved field of focus
- Distortion looking curved
- Field curvature increases for OFF AXIS RAYS
- Advantage: Retina - curved at back - helps off axis rays focus in front of retina = clear image - minimise this type of aberration
- CURVED IMAGE PLANE = FOCUS, CLEAR IMAGE
Distortion
- Occurs when linear magnification is a function of the OFF AXIS distance
- Pin - cushion distortion - image magnified OUTSIDE of image, distant parts of image displayed the most
- Barrel distortion - image magnification decreases = image shrinks = greater magnification in CENTRE
Chromatic aberration
effect resulting from dispersion. Lens have different refractive indices for different wavelengths of light
Does light travel through blue glass slower or quicker
Blue glass - more refraction - slower
Does light travel through red glass slower or quicker
Red glass - less refraction - faster
How does refractive index affect speed of light
Higher refractive index = higher density = lower speed light travel
What does white light cause
Dispersion due to many wavelengths
What is used to minimise chromatic aberration
Achromatic doublet lens
What is achromatic doublet lens
2 different types of glasses stuck together
How does achromatic doublet lens work
Counterbalance different colours and speed = decrease wavelength - eliminate chromatic aberration
What is an airy disk
The image of a point object due to effect of diffraction
Never perfect - always effects of aberrations - best optical system
Link between diffraction, airy disk diameter and aperture size
The larger the size of aperture, the smaller the diameter of the airy disk so the smaller the diffraction
