Transcript 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 • First recording in 1826 onto a pewter plate (by Joseph Nicephore Niepce) 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 Lenses • Focus a bundle of rays from a scene point onto a single point on the imager • Result: can make aperture bigger Ideal Lenses • Thin-lens approximation • Gaussian lens law: 1/do + 1/di = 1/f • Real lenses and systems of lenses may be approximated by thin lenses if only paraxial rays (near the optical axis) are considered Monochromatic Aberrations • Real lenses do not follow thin lens approximation because surfaces are spherical (manufacturing constraints) • Result: thin-lens approximation only valid iff sin 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 Spherical Aberration • Results in blurring of image, focus shifts when aperture is stopped down • Can vary with the way lenses are oriented Coma • Results in changes in magnification with aperture Coma Petzval Field Curvature • Focal “plane” is a curved surface, not a plane Distortion • Pincushion or barrel radial distortion • Varies with placement of aperture Distortion • Varies with placement of aperture Correcting for Seidel Aberrations • High-quality compound lenses use multiple lens elements to “cancel out” these effects • Often 5-10 elements, more for extreme wide angle Catadioptrics • Catadioptric systems use both lenses and mirrors • Motivations: – Systems using parabolic mirrors can be designed to not introduce these aberrations – Easier to make very wide-angle systems with mirrors Wide-Angle Catadioptric System 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) 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 Sensors • Vidicon • CCD • CMOS Vidicon • Best-known in family of “photoconductive video cameras” • Basically television in reverse ++++ Scanning Electron Beam Electron Gun Lens System Photoconductive Plate 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 MOS Capacitors • MOS = Metal Oxide Semiconductor Gate (wire) SiO2 (insulator) p-type silicon MOS Capacitors • Voltage applied to gate repels positive “holes” in the semiconductor +10V ++++++ Depletion region (electron “bucket”) MOS Capacitors • Photon striking the material creates electron-hole pair +10V Photon ++++++ + Charge Transfer • Can move charge from one bucket to another by manipulating voltages Charge Transfer • Various schemes (e.g. three-phase-clocking) for transferring a series of charges along a row of buckets CCD Architectures • Linear arrays • 2D arrays – – – – Full frame Frame transfer (FT) Interline transfer (IT) Frame interline transfer (FIT) Linear CCD • Accumulate photons, then clock them out • To prevent smear: first move charge to opaque region, then clock it out Full-Frame CCD • Problem: smear Frame Transfer CCD Interline Transfer CCD Frame Interline Transfer CCD CMOS Imagers • Recently, can manufacture chips that combine photosensitive elements and processing elements • Benefits: – Partial readout – Signal processing – Eliminate some supporting chips low cost Color • 3-chip vs. 1-chip: quality vs. cost 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 Correcting Chromatic Aberration • Simple way of partially correcting for residual chromatic aberration after the fact: scale R,G,B channels independently