exercise: for what purposes do we make visual representations?
Post on 20-Dec-2015
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Exercise:
For what purposes do we make visual representations?
Visualisation is as old as humanityPaleolithic cave paintings, Lascaux, France, ca. 30000 years ago.
Image from http://www.culture.gouv.fr/culture/arcnat/lascaux/en/
Paleolithic figurine, “Venus of Willendorf,” ca. 25000 years old.
Image from Wikipedia
These are visual representations of objects that are remembered or imagined – products of abstract visualisation.
Human seeing is built of multiple components
Detection:
Light vs. dark
Motion, direction
Patterns, orientation
Edges
Contrast:
Gradients, shading
Relative distance & size
- parallax, stereopsis
Color discrimination
Object discernmentand
Object recognition, visual computation
Human seeing is built of multiple components: detection, contrast, object discernment
Human seeing is built of multiple components: object discernment
Spring peeper (frog), Pseudacris crucifer
Image from http://www.uri.edu/cels/nrs/paton/photo_sppe.htm
A classic illustration of object-background discrimination
Exercise:
What is an image?
Wavelength is inversely proportional to frequency: λ = c/ν Energy is directly proportional to frequency: E = hν and inversely proportional to wavelength: E = hc/λ
Most imaging depends on manipulating and detecting electromagnetic radiation
E-M radiation is is usually treated as propagating waves.
Refraction changes the direction of propagation at an interface
Light waves travel more slowly in media such as glass or water
Image from http://www.williamson-labs.com/optical-body.htmThis website has some nice introductory optics material.
Index of refraction is a measure of resistance to light propagation
Refractive index = speed of light in vacuum / speed of light in material
⇒Snell's law:
Index of refraction depends on wavelength, and is different for different materials
Wavelength dependence of refractive index for some glasses. This function is also known as a dispersion curve.
visible
Focus is a composite effect of refraction (or of reflection)
http://www.williamson-labs.com
from Hecht and Zajac, Optics
The focusing effect of a lens can reconstruct wavefronts from an object.
This is known as image formation.
1/f = 1/so + 1/s
i (the Gaussian Lens equation)
An image can also be formed by removing all the unfocused rays.
This is the principle of a pinhole camera.
This CCD pinhole camera is only $65 at spygadgets.com
Undesigned lenses can form images
Flower in the background imaged through hanging dew dropsFrom http://www.flickr.com/groups/macroviewers/discuss/72157594313729574/
Thus imperfect eyes can be useful.
Parabolic surfaces are used to focus radiofrequency signals and sunlight.
satellite communication: satellite dish antenna." Online Art. Encyclopædia Britannica Online. <http://www.britannica.com/eb/art-67385>.
Your home satellite dish has the shape of a part of a paraboloid. A radio telescope works the same way, but with a larger dish.
A cylindrical paraboloid reflector makes an effective solar heat collector. The center pipe usually has water or another fluid pumped through it.
Each point on a parabola will reflect normally-incident light to the parabola's focus point.
normal(=90°)
incidence
The surface formed by rotating a parabola about its axis is a
paraboloid.
axis
A parabolic refracting surface (e.g. a glass lens) will also focus parallel rays to a point.
A discontinuous array of reflectors can approximate a larger parabolic dish
The Solar One solar power stationBarstow, California, 1993
PIX Number 00036Sandia National LaboratoriesNational Renewable Energy LaboratoryU.S. Department of Energycopyright, U.S. Department of Energy
The Solar One mirror array as seen on Google Earth.
Likewise, a discontinuous array of refracting surfaces can approximate a lens
A Fresnel lens (right) has most of the refracting surface of a solid spherical lens (left).
A Fresnel lens can give the same focusing power with much less glass, so it can be much thinner and lighter.