Design Realization lecture 27

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Design Realization lecture 27

• John Canny
• 12/2/03

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Last time

• Lenses reviewed convex spherical lenses.
• Ray diagrams. Real and virtual images.
• More on lenses. Concave and aspheric lenses.
• Fresnel optics
• Lenses

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This time

• More Fresnel optics
• Lenticular arrays/diffusers
• Prisms
• Diffusers
• Holograms
• Polarization

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Fresnel lenses

• Remove the thick-ness, but preservepower.
• Some artifacts areintroduced, but are invisible
  for large viewing areas(e.g. diplays).

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Lenticular arrays

• Many lenses printed on one sheet.
• Simplest version array of cylindrical lenses.
• Used for budget 3D vision

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Lenticular arrays

• Simplest version array of cylindrical lenses.

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Lenticular arrays

• Lenticular screens are rated in LPI for lines per
  inch. Typical range is 40-60 LPI, at about 10
  per square foot.
• Budget color printers can achieve 4800 dpi.
• At 40 LPI that gives 120 images in approx 60?
  viewing range, or 0.5? per image.

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Lenticular stereograms

• By interleaving images from views of a scene
  spaced by 0.5?, you can achieve a good 3D image.
• At 1m viewing distance, 0.5? translates to 1cm
  spacing between images.
• Eye spacing is about 6 cm.

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Diffusers

• Diffusers spread collimated (parallel) light over
  a specified range of angles.
• Can control viewing angle for a display.
• Gives sense of presence in partitioned spaces.

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Geometric diffusers

• Arrays of tiny lenses (lenticular arrays).
• Can be cylindrical (diffusion in one direction
  only), used in rear-projection screens.
• Surface etching. Using in shower glass,
  anti-glare plastic coatings.
• Holographic surface etching provides
  tightly-controlled diffusion envelope.
• Low-quality surface finish(!) on plastics gives
  diffusion effect.

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Lenticular arrays

• Cylindrical arrays
• Diffusion in one direction only, same as the
  arrays in lenticular stereograms.
• Used in rear-projection screens.
• Large angle 30-90?

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Lenticular arrays

• Spherical arrays diffuse in both directions
• Large angle 30-90?
• Homogeneous in all directions.

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Rough surfaces

• Diffusion depends on the range of angles on the
  surface. Surface should be irregular but not too
  sharp.
• Arbitrary range of diffusion angles. 2-4? typical
  for anti-glare plastic coatings.

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Material diffusers

• Tiny spheres embedded in clear polymer with
  different refractive index.
• Can achieve wide range of diffusion angles.
• Simpler to manufacture than most surface
  diffusers.

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Example Rear projection screens

• Combination of
• Rear fresnel lens - concentrates light toward
  central viewers
• Front lenticular screen spreads light
  horizontally
• Diffusing material spreads light vertically
  (by a smaller angle).

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Fresnel prisms

• Similar idea to lenses. Remove the thickness of
  the prism and stagger the surface facets.
• Useful for bending light over a large area, e.g.
  for deflecting daylight.
• Also used for vision correction.

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An improvisation with Fresnel prisms

• Opposing prism arrays create an array of TIR
  mirrors

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An improvisation with Fresnel prisms

• The array creates images of any point on the
  opposite side but only in cross-section. Two
  crossed arrays create images in 3D.

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An improvisation with Fresnel prisms

• Inverted images are formed in front of the array,
  without the distortion effects of lenses.

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An improvisation with Fresnel prisms

• Two such pairs invert the image twice, producing
  a right-sided, displaced image.

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Holography

• Holograms are based on interference patterns
  caused by the fine structure of the hologram.
• Production methods are generally complicated and
  require
• A coherent laser light source
• Collimating optics
• Careful film processing
• Lots of trial and error

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Holography

• E.g. white-light transmission hologram setup from
  www.3dimagery.com

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Computer-Generated Holography

• Interestingly, there are many software packages
  that can compute CGH holograms (most standard
  optical CAD packages can do this).
• One of the simplest and most robust types of
  hologram is the Fraunhofer hologram. The
  hologram is a kind of Fourier transform of the
  object. It can be accelerated using efficient FFT
  software.

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Computer-Generated Holography

• Current printers are at 4800 dpi, or about 5
  microns, and produce binary images.
• Turning a printed image into a hologram requires
  reduction down to optical wavelengths (lt 1
  micron).
• e.g. Photograph with SLR camera with Fuji
  minicopy film. The negative is the hologram.

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Computer-Generated Holography

• Some commercial vendors will print holograms from
  an image sequence (movie or pan-around a fixed
  object) e.g. www.litiholographics.com

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Polarization

• Remember that light is an electro-magnetic wave
  with both electric and magnetic components normal
  to its motion.
• Normal light has E (electric) components in all
  directions, but it can be polarized under certain
  conditions.

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Polarization by reflection
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Polarization by reflection

• This reflection profile is typical for other
  materials like water or metals.
• Reflected glare is typically mostly
  horizontally polarized.
• Vertical polarized sunglasses eliminate much of
  it.

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Polarization by absorption

• Dichroic materials exhibit different absorption
  for transverse and parallel light polarizations.
  The (artificial) polaroid material typically
  transmits 80 of parallel light, but only 1 of
  transverse light.

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Circular Polarization

• Birefringent materials exhibit different
  refractive indices (hence velocity) for the two
  light polarizations.
• If a birefringent material is the right
  thickness, the slower wave can be delayed exactly
  ¼ wavelength.
• Sending linearly polarized light into this layer
  leads to elliptic polarization.
• If the polarizer axis is at 45 to the
  birefrengent axis, the light will be circularly
  polarized.

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Summary

• More Fresnel optics
• Lenticular arrays/diffusers
• Prisms
• Diffusers
• Holograms
• Polarization