1 / 37

Aberrations

Aberrations. Q1. In terms of Abbe Number, the best combination of positive and negative elements to make an effective achromatic lens doublet would be:. Positive element 57.55, negative element 25.76 Positive element 25.76, negative element 57.55

hall
Download Presentation

Aberrations

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Aberrations

  2. Q1. In terms of Abbe Number, the best combination of positive and negative elements to make an effective achromatic lens doublet would be: • Positive element 57.55, negative element 25.76 • Positive element 25.76, negative element 57.55 • Positive element 57.55, negative element 57.55 • Positive element 25.76, negative element 25.76

  3. Q2. Transverse chromatic aberration (TCA) can be used to quantify image degradation in an image plane, for example the retina. In terms of aperture (y) dependence: • TCA is independent of aperture diameter • TCA  y • TCA  y2 • TCA  y3

  4. Transverse CA Page 3.91 • Different ways to quantify TCA: • Chromatic difference of magnification = difference in image height for 486.1 nm vs. 656.3 nm (for a given object) • To evaluate the effect of pupil diameter on TCA of the eye, look at the size of the red blur patch at the blue focus and vice versa

  5. Page 3.93 TCA ↑ with pupil diameter Figure 3.84 - Transverse chromatic aberration (TCA) for the reduced eye for (a) moderate pupil and (b) large pupil diameter. For a point source, TCA can be measured in terms of the blue “patch” at the red chromatic focus, or red patch at the blue chromatic focus.

  6. Aperture dependence of CA Page 3.89 LCA is a property of light  unaffected by aperture diameter TCA bears the same relationship to LCA as TSA to LSA: LSA  y2 LSA  y3 (differ by factor of y) LCA independent of y; TCA  y (differ by factor of y)

  7. 2 1 Prism Achromatic Doublet Page 3.95 • The key to an achromatic doublet is to combine a low dispersion prism (crown) with a high dispersion prism (flint) • A larger angle crown prism (1) provides excess deviation with lowdispersion (CA) • Meantime, a smaller angle flint prism (2) can provide equal and opposite dispersion with low (opposing) deviation

  8. Flint n2  1  2 n1 Crown Prism Achromatic Doublet - Principle • If the CA induced by the crown prism is equal to that produced by the flint prism  net CA = 0 (base-to-apex) • If the mean deviation (d) of the crown prism > dflint we can produce deviation without net dispersion • This defines a prism achromatic doublet

  9. seen as white d red blue white Prism Achromatic Doublet Flint n2  1  2 n1 Crown

  10. Achromatic Doublet Lens Fig 3.86 Page 3.96 Same principle as achromatic doublet prism F2 () Flint F1 (+) Crown

  11. Cemented Achromatic Doublet Lens Fig 3.86 Page 3.96 Red and blue focus at the same location. Other spectral colors focus at slightly different locations F2 () Flint F1 (+) Crown

  12. Cemented Achromatic Doublet Lens As for the prism doublet, we want: LCA1 =  LCA2

  13. Q3. A patient views a distant polychromatic target and green light focuses on the retina. A low power lens is now placed in front of the eye and red light focuses on the retina. What is the lens power? • Cannot be determined from the information provided • Positive • Negative

  14. Chromatic + Spherical Aberration Effects of SA and CA on Ocular Image Formation

  15. Page 3.94 Figure 3.85

  16. Paraxial Focus Spherical Refracting Surface Aperture n = 1 n' > 1 400 nm 700 nm

  17. Chromatic aberration causes a“spread of paraxial foci” along the axis (blue to left; red to right) n = 1 n' > 1 Paraxial Focus 400 nm 700 nm

  18. Marginal Paraxial Marginal Paraxial Spherical aberration spreads foci according to“incident height” (lateral distance from axis) Effect of SA on green light Paraxial Focus 400 nm 700 nm

  19. Summary: Chromatic Aberration • Longitudinal origin: property of light • Quantifying: • Longitudinal CA (LCA): independent of aperture: • Transverse CA: TCA  y • Quantify as: chromatic difference in magnification; size of blue patch at red focus, etc.

  20. Summary: Chromatic Aberration • Correcting CA: Achromatic doublet: • Prism: combine more powerful, lower dispersion prism (for deviation) with higher dispersion prism (to neutralize dispersion) • (+) Lens: combine more powerful, lower dispersion plus lens (for convergence) with higher dispersion, weaker negative lens (to neutralize dispersion) Both cases: LCA1 = LCA2

  21. Scattering of Light Page 3.97

  22. Scattering of Light Molecular (Rayleigh) light scatter through the air is strongly wavelength-dependent. We see it’s effect every day: the sky is blue distant mountains appear blue the sun appears yellow the colors of sunset are yellow, orange and red Using these observations, describe the wavelength-dependence of Rayleigh scatter

  23. Objectives Explain the wavelength-dependence of Rayleigh (molecular) scatter based on everyday observations Use this information to predict the behavior of short wavelength (blue) and long wavelength (red) ophthalmic surgical lasers Describe the Tyndall Effect (large particle Mie scatter) and its use in detecting anterior chamber inflammation Compare the wavelength-dependence and directionality of Rayleigh and Mie scatter

  24. Scattering Due to inhomogeneity of a medium Two types of scatter: Rayleigh scatter: molecular level (< 100 nm) - weak and non-directional (random direction) Mie scatter: larger particles – strongly directional

  25. Rayleigh Scattering: Blue Sky Light shone through a dark, dust-free room takes on a bluish appearance If the room is entirely evacuated of air, the bluish appearance vanishes Light scatter by molecules in the air produces the same effect → makes the sky appear blue Because the scattered light is blue, the effect must be strongly wavelength dependent

  26. Rayleigh Scattering Blue sky  Rayleigh scatter

  27. Rayleigh Scatter: -dependence Also called dipole scattering

  28. Rayleigh Scatter: -dependence The intensity of light scattered by molecular particles in the atmosphere varies inversely with the fourth power of wavelength:

  29. Rayleigh Scatter: -dependence

  30. Rayleigh Scattering: Yellow Sun Blue is scattered ~ 9 times more than red The farther light travels through the air, the more blue is lost by scatter This is why the sun and distant white lights appear yellow

  31. The sun from 25,000 feet

  32. Rayleigh Scattering: Colors of Sunset • The colors of a sunset are due to the long path ofblue-depletionthrough the atmosphere of sunlight • In a heavily polluted or dust-laden atmosphere, the sunset may be an intense red (dust particles reflect blue-depleted sunlight)

  33. Long path through atmosphere

  34. Rayleigh Scattering -Sunset

  35. Rayleigh Scattering: Distant Mountains Distant mountains appear bluish because they are being viewed through a “filter” of scattered blue light This is best seen in a clean (unpolluted) atmosphere, where Rayleigh scatter is most prevalent

  36. Distant mountains seen through blue “filter” Rayleigh Scattering: Distant Mountains

  37. Rayleigh Scatter and the Eye Shorter wavelength ophthalmic lasers (blue range) scatter more strongly, reducing penetration depth Longer wavelength lasers (red range) are more useful to treat sub-retinal lesions because they elicit less Rayleigh scatter

More Related