OP - Introduction to Aberrations - Week 10 Flashcards
What is the effect of aberrations?
Cause Ray’s to depart from ideal paraxial path
What are the types of achromatic aberrations: (5)
- spherical aberration
- coma
- astigmatism
- petsval (field) curvature
- distortion
What is the difference between positive and negative spherical aberration?
+ve: rays further from the optical axis bend too much
-ve: rays further from the optical axis bend too little
Can spherical aberration occur for on or off axis object points?
Yes. Both.
Which monochromatic aberrations can only occur with off axis object points?
- field (petzval) curvature
- distortion
In regards to the paraxial image plane, what is the difference between field curvature and distortion?
- field curvature gives point to point imagery but not in the paraxial image plane
- distortion gives point to point imagery in the paraxial plane, but not the paraxial position
What kind of foci does oblique astigmatism cause?
2 line foci: one in the Sagittal plane and one in the tangential plane
Which monochromatic aberrations give point-to-point imagery?
- petzval
- distortion
Which monochromatic aberrations do NOT give point-to-point imagery?
- Spherical
- Coma
- Astigmatism
What is the interval of sturm? Where can it be found?
- the distance between 2 line foci
- found in astigmatism
What does positive distortion look like? What about negative?
\+ve = pin-cushion -ve = barrel
Explain coma
Coma occurs when you have an off axis object point. Ray’s going through the middle of the lens will hit the image point Q’
But as the Rays hit the more outer parts of the lens, they form an image point either above or below the point Q’ (depending on whether its positive or negative coma).
The resulting shape looks like a comet
Explain Petzval curvature
The focal point is treated as the radius of a circle. Instead of hitting the paraxial image plane, rays will converge to various points of a curved surface (part of the circle)
This curved surface passes through the Gaussian image point O’
Explain astigmatism
An aberration that causes sagittal and tangential lines to focus at different distances
Explain spherical aberration
Ray’s hitting the lens further from the optical axis converge to a point before or after the paraxial image point
What is positive spherical aberration? What is negative?
Positive: rays focused too short; Myopia
Negative: rays focused to far; hyperopia
What are the 4 common ways of quantifying ray aberration?
- transverse aberration
- longitudinal aberration
- angular aberration
- wave aberration (a.k.a path length aberration)
What is the difference between meriodional rays and skew rays?
Meriodional rays intersect the optic axis, skew rays do not
How can you quantify the aberration of a meridional ray?
- longitudinal aberration [O’G]
- transverse aberration [O’H]
How can you quantify the aberration of a skew ray?
- transverse aberration [Q’Qhat’]
- angular aberration
What is wave (path length) aberration of a beam?
It’s the difference in OPL between the central ray of the beam and any other ray of the beam
Define OPL.
Optical Path Length
- is the product of physical path length and refractive index.
- it relates to the number of wavelengths travelled by a Ray
OPL = n X dist.
What effect does ray height have on wave aberration?
Increased ray height increases wave aberration
What factors can influence the level of wave aberrations? (2) and what kind of aberration do they most affect
1) Aperture stop diameter
- affects spherical ab. the most
2) Distance off-axis
- affects distortion the most
What is a measure for the level of aberration in general?
Point spread function (PSF)
= the light distribution in the image of a point source
What are some other measures of aberration?
For astigmatism:
1/ interval of sturm
2/ radii of sagittal and tangential image surfaces
For field curvature:
- can quantify with petzval surface
What factors affect the levels of monochromatic aberrations in general? (4)
1/ lens shape (shape factor)
2/ position of conjugates
3/ aperture stop position (stop shift factor)
4/ surface shape (asphericity)
