1 / 37

Imaging

Imaging. Real world. Opics. Sensor. Acknowledgment: some figures by B. Curless, E. Hecht, W.J. Smith, B.K.P. Horn, and A. Theuwissen. Optics. Pinhole camera Lenses Focus, aperture, distortion Vignetting Flare. Pinhole Camera. “Camera obscura” – known since antiquity

lot
Download Presentation

Imaging

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. 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

  6. 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

  7. Monochromatic Aberrations • Real lenses do not follow thin lens approximation because surfaces are spherical (manufacturing constraints) • Result: thin-lens approximation only valid iff sin   

  8. 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

  10. Coma • Results in changes in magnification with aperture

  11. Coma

  12. Petzval Field Curvature • Focal “plane” is a curved surface, not a plane

  13. Distortion • Pincushion or barrel radial distortion • Varies with placement of aperture

  14. Distortion • Varies with placement of aperture

  15. Correcting for Seidel Aberrations • High-qualitycompound lensesuse multiplelens elements to“cancel out”these effects • Often 5-10 elements,more for extreme wide angle

  16. 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

  17. Wide-Angle Catadioptric System

  18. 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)

  19. 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

  20. Sensors • Vidicon • CCD • CMOS

  21. Vidicon • Best-known in family of “photoconductive video cameras” • Basically television in reverse  + + + + Scanning Electron Beam Electron Gun Lens System Photoconductive Plate

  22. 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

  23. MOS Capacitors • MOS = Metal Oxide Semiconductor Gate (wire) SiO2 (insulator) p-type silicon

  24. MOS Capacitors • Voltage applied to gate repels positive “holes” in the semiconductor +10V + + + + + + Depletion region (electron “bucket”)

  25. MOS Capacitors • Photon striking the material creates electron-hole pair +10V Photon + + + + + +        +

  26. Charge Transfer • Can move charge from one bucket to another by manipulating voltages

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

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

  29. Linear CCD • Accumulate photons, then clock them out • To prevent smear: first move charge to opaque region, then clock it out

  30. Full-Frame CCD • Problem: smear

  31. Frame Transfer CCD

  32. Interline Transfer CCD

  33. Frame Interline Transfer CCD

  34. CMOS Imagers • Recently, can manufacture chips that combine photosensitive elements and processing elements • Benefits: • Partial readout • Signal processing • Eliminate some supporting chips  low cost

  35. Color • 3-chip vs. 1-chip: quality vs. cost

  36. 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

More Related