Imaging Real worldOpicsSensor 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: Real lenses and systems of lenses may be approximated by thin lenses if only paraxial rays (near the optical axis) are considered 1/d o + 1/d i = 1/f

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

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 cos 4  –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 Electron Gun Photoconductive Plate Lens System + +        Scanning Electron Beam

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) SiO 2 (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