Exploring Fresnel Optics and Lenticular Arrays for Enhanced Visualization

1. Design Realization lecture 27 John Canny 12/2/03

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

3. This time • More Fresnel optics: • Lenticular arrays/diffusers • Prisms • Diffusers • Holograms • Polarization

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

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

6. Lenticular arrays • Simplest version: array of cylindrical lenses.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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

12. Lenticular arrays • Spherical arrays diffuse in both directions: • Large angle: 30-90 • Homogeneous in all directions.

13. 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.

14. 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.

15. 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).

16. 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.

17. An improvisation with Fresnel prisms • Opposing prism arrays create an array of TIR mirrors:

18. 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.

19. An improvisation with Fresnel prisms • Inverted images are formed in front of the array, without the distortion effects of lenses.

20. An improvisation with Fresnel prisms • Two such pairs invert the image twice, producing a right-sided, displaced image.

21. 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…

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

23. 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.

24. 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 (< 1 micron). • e.g. Photograph with SLR camera with Fuji “minicopy” film. The negative is the hologram.

25. 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

26. 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.

27. Polarization by reflection

28. 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.

29. 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.

30. 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.

31. Summary • More Fresnel optics: • Lenticular arrays/diffusers • Prisms • Diffusers • Holograms • Polarization