Exercise: For what purposes do we make visual representations?

For what purposes do we make
visual representations?
Visualisation is as old as humanity
Paleolithic cave
paintings, Lascaux,
France, ca. 30000
years ago.
Image from
These are visual
representations of objects
that are remembered or
imagined – products of
abstract visualisation.
Paleolithic figurine,
“Venus of Willendorf,”
ca. 25000 years old.
Image from Wikipedia
Human seeing is built of multiple components
Light vs. dark
Gradients, shading
Motion, direction
Relative distance & size
Patterns, orientation
- parallax, stereopsis
Color discrimination
Object discernment
Object recognition,
visual computation
Human seeing is built of multiple components:
detection, contrast, object discernment
Human seeing is built of multiple
components: object discernment
A classic illustration of
object-background discrimination
Spring peeper (frog), Pseudacris crucifer
Image from
What is an image?
Most imaging depends on manipulating and
detecting electromagnetic radiation
E-M radiation is is usually treated as propagating waves.
Wavelength is inversely proportional to frequency: λ = c/ν
Energy is directly proportional to frequency:
and inversely proportional to wavelength:
E = hν
E = hc/λ
Refraction changes the direction of
propagation at an interface
Light waves travel more slowly in media such as glass or water
Image from
This 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
dependence of
refractive index for
some glasses. This
function is also
known as a
dispersion curve.
Focus is a composite effect of
refraction (or of reflection)
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/si (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
Undesigned lenses can form images
Flower in the background imaged through hanging dew drops
From http://www.flickr.com/groups/macroviewers/discuss/72157594313729574/
Thus imperfect eyes can be useful.
Parabolic surfaces are used to focus
radiofrequency signals and sunlight.
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.
satellite communication: satellite dish antenna."
Online Art. Encyclopædia Britannica Online.
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 normallyincident light to the parabola's focus point.
The surface formed by rotating
a parabola about its axis is a
A parabolic
refracting surface
(e.g. a glass lens)
will also focus
parallel rays to a
A discontinuous array of reflectors can
approximate a larger parabolic dish
The Solar One solar power station
Barstow, California, 1993
PIX Number 00036
Sandia National Laboratories
National Renewable Energy Laboratory
U.S. Department of Energy
copyright, 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