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ENE 492

ENE 492. Fundamental of Optical Engineering Lecture 3. Lens Aberrations. Aberration happens when the rays do not converge to a point where it should be. We may distinguish the aberrations into 6 categories: Spherical aberration Coma Oblique astigmatism Curvature of field Distortion

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ENE 492

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  1. ENE 492 Fundamental of Optical Engineering Lecture 3

  2. Lens Aberrations • Aberration happens when the rays do not converge to a point where it should be. We may distinguish the aberrations into 6 categories: • Spherical aberration • Coma • Oblique astigmatism • Curvature of field • Distortion • Chromatic aberration

  3. Spherical Aberration • Rays near axis come to focus at a different distance than marginal rays. • In other words, rays that pass through a lens further away from the optical axis come to a focus closer to the lens than paraxial rays.

  4. Spherical Aberration

  5. Spherical Aberration • This spherical aberration can not be completely corrected with a simple lens. • Complete elimination of this aberration can be done by using a compound lens.

  6. Chromatic Aberration • This happens due to various wavelengths in light rays. • The index of refraction varies as a function of wavelength. • A single lens has different focal lengths for different colors.

  7. Chromatic Aberration • Chromatic aberration can be corrected by using two lenses in contact, one made of crown and the other of flint.

  8. Mirrors • A plane mirror reflects the light without focusing it. • The virtual image formed by a plane mirror is located at the same distance behind the mirror as the object is in front of it.

  9. Mirrors

  10. Mirrors • Assume θ << 1

  11. Example • For h = 10 cm., R = 2m. Find f.

  12. Mirror Conventions • Incident beam travels left to right, reflected light travels right to left. • Distance is measured from plane of mirror. • Object is on the left of a mirror, then object distance s < 0. • Image to the left of a mirror, then object distance s’>0 (virtual image). Image to the right of a mirror, then image distance s’< 0 (real image).

  13. Mirror Conventions • For converging mirror, focal length f > 0. • For diverging mirror, focal length f < 0.

  14. Example

  15. Example

  16. Example • Think of the last time you viewed yourself in a mirror. • Assuming that the mirror surface is perfectly flat, what are its radius of curvature and its focal length? • Is the image formed by the mirror real or virtual?

  17. Camera • The simplest type of camera is the pinhole camera. • The sharpness of the image depends on the diameter of the hole. • If it is too large, the image is blurred. • Thus, as the pinhole is small, the image improves in clarity until a certain pinhole size.

  18. Camera • The advantage of a pinhole camera apart from its simplicity is that all objects are in focus on the screen. • The depth of field of the camera is unlimited. • The disadvantage of pinhole camera is that it admits limited light, so that it is not suitable in freezing the action of moving objects. • Some put a converging lens to improve the brightness and sharpness of the image.

  19. Camera • The aperture, which admits light to the film, is variable and coordinated with the exposure time (shutter speed). • The light power incident at the image plane is expressed as

  20. Example • A lens of f = 4 cm is stopped down to an aperture of 0.5 cm (D). What is the relative aperture or f-number?

  21. Camera • From the figure above, the depth of field is (s2 – s1) that may be expressed as

  22. Camera • We clearly see that although the interval is symmetric about s0’ in image space, the depth of field interval is not symmetric about s0.

  23. Human eye Source: Wikipedia

  24. Human eye • There are 3 well-known defects in vision due to errors of refraction of the eyes. • Near-sightedness (Myopia): Longer distance from cornea to retina than usual. • Far-sightedness (Hyperopia): Shorter distance from cornea to retina than normal. • Astigmatism: Unequal (asymmetric curvatures) in the corneal surface.

  25. Human eye Note: The distance for best viewing for adult is 25 cm.

  26. Near-sighted person • GTLE:

  27. Example • A near-sighted person has a far point of 100 cm and a near point of 15 cm. • What f of the lens should be to move the far point to infinity? • With this correction lens, can this person read a book at the normal near point (25 cm from the eye)?

  28. Magnifying glass • The magnifying glass is used for better spatial resolution in image. • On the other words, it helps us to see smaller objects or to be able to distinguish finer features.

  29. Magnifying glass

  30. Example • What does “20x lens” mean?

  31. Microscope • The magnification of small objects is achieved by the use of two postitive lenses: objective lens and an eyepiece.

  32. Microscope • The magnification is approximated by • The standard tube length is 16 cm. • By viewing with the microscope, the image formation may be drawn as

  33. Example

  34. Example

  35. Telescope

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