Imaging

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Imaging
Real world
Opics
Sensor
Acknowledgment some figures by B. Curless, E.
Hecht, W.J. Smith, B.K.P. Horn,
and A. Theuwissen
2
Optics

• Pinhole camera
• Lenses
• Focus, aperture, distortion
• Vignetting
• Flare

3
Pinhole Camera

• Camera obscura known since antiquity
• First recording in 1826 onto a pewter plate (by
  Joseph Nicephore Niepce)

4
Pinhole Camera Limitations

• Aperture too big blurry image
• Aperture too small requires long exposure or
  high intensity
• Aperture much too small diffraction through
  pinhole ? blurry image

5
Lenses

• Focus a bundle of rays from a scene point onto a
  single point on the imager
• Result can make aperture bigger

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Ideal Lenses

• Thin-lens approximation
• Gaussian lens law
• Real lenses and systems of lenses may be
  approximated by thin lenses if only paraxial rays
  (near the optical axis) are considered

1/do 1/di 1/f
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Monochromatic Aberrations

• Real lenses do not follow thin lens approximation
  because surfaces are spherical (manufacturing
  constraints)
• Result thin-lens approximation only valid iff
  sin ? ? ?

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Monochromatic Aberrations

• Consider the next term in the Taylor series, i.e.
  sin ? ? ? - ?3/3!
• Third-order theory deviations from the ideal
  thin-lens approximations
• Called primary or Seidel aberrations

9
Spherical Aberration

• Results in blurring of image, focus shifts when
  aperture is stopped down
• Can vary with the way lenses are oriented

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Coma

• Results in changes in magnification with aperture

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Coma
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Petzval Field Curvature

• Focal plane is a curved surface, not a plane

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Distortion

• Pincushion or barrel radial distortion
• Varies with placement of aperture

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Distortion

• Varies with placement of aperture

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Correcting for Seidel Aberrations

• High-qualitycompound lensesuse multiplelens
  elements tocancel outthese effects
• Often 5-10 elements,more for extreme wide angle

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Catadioptrics

• Catadioptric systems use bothlenses and mirrors
• Motivations
• Systems using parabolic mirrors can be designed
  to not introduce these aberrations
• Easier to make very wide-angle systems with
  mirrors

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Wide-Angle Catadioptric System
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Other Limitations of Lenses

• Flare light reflecting(often multiple
  times)from glass-air interface
• Results in ghost images or haziness
• Worse in multi-lens systems
• Ameliorated by optical coatings (thin-film
  interference)

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Other Limitations of Lenses

• Optical vignetting less power per unit area
  transferred for light at an oblique angle
• Transferred power falls of as cos4 ?
• Result darkening of edges of image
• Mechanical vignetting due to apertures

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Sensors

• Vidicon
• CCD
• CMOS

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Vidicon

• Best-known in family of photoconductive video
  cameras
• Basically television in reverse

? ? ? ?
Scanning Electron Beam
Electron Gun
Lens System
Photoconductive Plate
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Digression Gamma

• Vidicon tube naturally has signal that varies
  with light intensity according to a power law
  with gamma ? 1/2.5
• CRT (televisions) naturally obey a power law with
  gamma ? 2.5
• Result standard for video signals has a gamma of
  1/2.5

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MOS Capacitors

• MOS Metal Oxide Semiconductor

Gate (wire)
SiO2 (insulator)
p-type silicon
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MOS Capacitors

• Voltage applied to gate repels positive holes
  in the semiconductor

10V

Depletion region (electron bucket)
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MOS Capacitors

• Photon striking the material creates
  electron-hole pair

10V
Photon

?
?
?
?
?
?
?

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Charge Transfer

• Can move charge from one bucket to another by
  manipulating voltages

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Charge Transfer

• Various schemes (e.g. three-phase-clocking) for
  transferring a series of charges along a row of
  buckets

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CCD Architectures

• Linear arrays
• 2D arrays
• Full frame
• Frame transfer (FT)
• Interline transfer (IT)
• Frame interline transfer (FIT)

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Linear CCD

• Accumulate photons, then clock them out
• To prevent smear first move charge to opaque
  region, then clock it out

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Full-Frame CCD

• Problem smear

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Frame Transfer CCD
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Interline Transfer CCD
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Frame Interline Transfer CCD
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CMOS Imagers

• Recently, can manufacture chips that combine
  photosensitive elements and processing elements
• Benefits
• Partial readout
• Signal processing
• Eliminate some supporting chips ? low cost

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Color

• 3-chip vs. 1-chip quality vs. cost

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Chromatic Aberration

• Due to dispersion in glass (focal length varies
  with the wavelength of light)
• Result color fringes near edges of image
• Correct by building lens systems with multiple
  kinds of glass

37
Correcting Chromatic Aberration

• Simple way of partially correcting for residual
  chromatic aberration after the fact scale R,G,B
  channels independently