OP - Introduction to Aberrations - Week 10

Question 1

What is the effect of aberrations?

Answer 1

Cause Ray’s to depart from ideal paraxial path

Question 2

What are the types of achromatic aberrations: (5)

Answer 2

• spherical aberration
• coma
• astigmatism
• petsval (field) curvature
• distortion

Question 3

What is the difference between positive and negative spherical aberration?

Answer 3

+ve: rays further from the optical axis bend too much

-ve: rays further from the optical axis bend too little

Question 4

Can spherical aberration occur for on or off axis object points?

Answer 4

Yes. Both.

Question 5

Which monochromatic aberrations can only occur with off axis object points?

Answer 5

• field (petzval) curvature

- distortion

Question 6

In regards to the paraxial image plane, what is the difference between field curvature and distortion?

Answer 6

• 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

Question 7

What kind of foci does oblique astigmatism cause?

Answer 7

2 line foci: one in the Sagittal plane and one in the tangential plane

Question 8

Which monochromatic aberrations give point-to-point imagery?

Answer 8

• petzval

- distortion

Question 9

Which monochromatic aberrations do NOT give point-to-point imagery?

Answer 9

• Spherical
• Coma
• Astigmatism

Question 10

What is the interval of sturm? Where can it be found?

Answer 10

• the distance between 2 line foci

- found in astigmatism

Question 11

What does positive distortion look like? What about negative?

Answer 11

\+ve = pin-cushion 
-ve = barrel

Question 12

Explain coma

Answer 12

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

Question 13

Explain Petzval curvature

Answer 13

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’

Question 14

Explain astigmatism

Answer 14

An aberration that causes sagittal and tangential lines to focus at different distances

Question 15

Explain spherical aberration

Answer 15

Ray’s hitting the lens further from the optical axis converge to a point before or after the paraxial image point

Question 16

What is positive spherical aberration? What is negative?

Answer 16

Positive: rays focused too short; Myopia
Negative: rays focused to far; hyperopia

Question 17

What are the 4 common ways of quantifying ray aberration?

Answer 17

• transverse aberration
• longitudinal aberration
• angular aberration
• wave aberration (a.k.a path length aberration)

Question 18

What is the difference between meriodional rays and skew rays?

Answer 18

Meriodional rays intersect the optic axis, skew rays do not

Question 19

How can you quantify the aberration of a meridional ray?

Answer 19

• longitudinal aberration [O’G]

- transverse aberration [O’H]

Question 20

How can you quantify the aberration of a skew ray?

Answer 20

• transverse aberration [Q’Qhat’]

- angular aberration

Question 21

What is wave (path length) aberration of a beam?

Answer 21

It’s the difference in OPL between the central ray of the beam and any other ray of the beam

Question 22

Define OPL.

Answer 22

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.

Question 23

What effect does ray height have on wave aberration?

Answer 23

Increased ray height increases wave aberration

Question 24

What factors can influence the level of wave aberrations? (2) and what kind of aberration do they most affect

Answer 24

1) Aperture stop diameter
- affects spherical ab. the most
2) Distance off-axis
- affects distortion the most

Question 25

What is a measure for the level of aberration in general?

Answer 25

Point spread function (PSF)

= the light distribution in the image of a point source

Question 26

What are some other measures of aberration?

Answer 26

For astigmatism:
1/ interval of sturm
2/ radii of sagittal and tangential image surfaces

For field curvature:
- can quantify with petzval surface

Question 27

What factors affect the levels of monochromatic aberrations in general? (4)

Answer 27

1/ lens shape (shape factor)
2/ position of conjugates
3/ aperture stop position (stop shift factor)
4/ surface shape (asphericity)