Photographic Image Formation I

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Photographic Image Formation I

Szymon Rusinkiewicz, Tim Weyrich Technology in
Art and Cultural Heritage.Princeton Freshman
Seminar 2006 Acknowledgment some figures by B.
Curless, E. Hecht, W.J. Smith, B.K.P. Horn, and
A. Theuwissen
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Photographic Image Formation
Real world
Optics
Sensor
DarkRoom
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Pinhole Camera

• Acquiring images using a pinhole camera?
• Use light-sensitive film at image plane

Pinhole camera
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Pinhole Camera
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Pinhole Camera
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Pinhole Camera
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Pinhole Camera

• Limitations depending on size of aperture
• Aperture much too small diffraction through
  pinhole ? blurry image

Pinhole camera
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Pinhole Camera Limitations

• Can we have sharp images, no diffraction, and
  enough light at the same time?

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Pinhole Camera Limitations

• Can we have sharp images, no diffraction, and
  enough light at the same time?
• Optical lenses do the trick!

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Lenses

• Focus a bundle of rays from a scene point onto a
  single point on the imager
• Increases aperture without loss of sharpness

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Ideal Thin Lens Law

• Relationship between focal distance andfocal
  length of lens

1/do 1/di 1/f
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Camera Adjustments

• Focus?
• Changes di
• Iris?
• Changes aperture
• Zoom / wide-angle?
• Changes f and sometimes di
• Changes field of view

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Focus and Depth of Field

• For a given di, perfect focus at only one do
• In practice, OK for some range of depths
• Circle of confusion smaller than a pixel

• Better depth of field with smaller apertures
• Better approximation to pinhole camera

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Field of View

• Depends on
• Focal length of lens
• Size of imager
• Object distance?

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Sensors

• Photographic film
• CCD sensors
• CMOS chips

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Photographic Film

• Until recently the most common imager
• Silver salts or dyes darken under light exposure
• After fix step, image prints from negative
• Multiple film layers with filters color images

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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 createselectron-hole
  pair

10V
Photon

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Charge Transfer

• Charge has to be transported off the chip to
  digitizing circuits
• Charge-coupled devices (CCD) build bucket-chains

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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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Where do all the colors come from?

• Electrons dont have a color

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Where do all the colors come from?

• Electrons dont have a color
• We can separate images into red, green, and blue

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3-Chip Cameras

• Prisms separate incoming light into red, green,
  and blue wavelengths
• One detector chip for each color

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Single-Chip Cameras

• A single detector chip
• Small color filters in front of each pixel
• Images have to beprocessed for per-pixel RGB

Bayer mosaic
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Foveon Technology

• Layered sensor
• Similar structure to photographic film

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Development Process

• Classical film requires development in dark room
• What about digital images?

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Development Process

• Classical film requires development in dark room
• Digital images require Digital Darkroom
• Mapping of sensor data to pixel values
• Mapping defined by
• Contrast intensity (dynamic range)
• Gamma
• Simulated film (grain, solarizatoin, )