Office Hours
• Office hours are posted on the website.
– Molly: Tuesdays 2-4pm
– Dr. Keister: Wednesdays 10am-12
– Prof. Goldman is out of town this week, so there are
no office hours tomorrow at 2pm
• All office hours are in the help room downstairs.
Image Filters: Sharpening
• Sharpening filters in Photoshop work just like lateral
inhibition works in your retina.
• Edges between dark and light areas are enhanced by
making the light areas lighter and the dark areas
darker
Sharpen Filter
• Image files are basically arrays (matrices) of numbers,
so image processing is just number processing
• Let’s consider the following mathematical “filter”:
-1
2
-1
+1
-1
Sharpening: Original
2
2
2
2
2
4
6
6
6
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
11
6
6
Sharpening: Results
2
2
2
2
0
4
8
6
6
9
8
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
11
6
6
Chapter 13:
Scattering and Polarization
• Light Scattering
– Blue skies
– Red sunsets
• Polarization
– Review EM waves
– Polarizing filters
Light Scattering
• We have been treating light as rays: straight lines
• If you zoom way in, recall that light is really a
WAVE
• Ray optics don’t work anymore, and we consider
something called scattering
– Scattering is the reason the sky is blue, sunsets are red,
why you can see a laser beam in the air
Ray Optics vs. Scattering
Ray optics
Scattering
Rayleigh Scattering
• The simplest type of scattering is called “Rayleigh
scattering”
• The rule you need to remember about Rayleigh
scattering is that:
– The shorter the wavelength of the incident
light, the more light is scattered
In other words,
– Blue light scatters more than red light
Rayleigh Scattering: Blue Sky
• Blue light scatters more than red light
• This is the reason that the sky is blue
• The light from the sun contains all visible
wavelengths
• It scatters from particles in the atmosphere
• Blue light scatters more than red, so we see
predominantly blue light when we look at the sky
Blue Sky
Concept Question
We see the sky as blue because of sunlight scattering off
of particles in the atmosphere. The moon has no
atmosphere. Standing on the moon, what color does the
sky look when the sun is shining? (during the moon’s
“daytime”)
A. Blue
B. Red
C. Black
D. Some other color
Rayleigh Scattering: Sunsets
• Blue light scatters more than red light
• This is also the reason that sunsets are red
• At sunset, light from the sun has to travel through
more of the atmosphere, and the blue light
scatters away before the light reaches your eyes
• One way to think about it is that your sunset is
blue sky somewhere else on the earth, so they
are getting the blue light, and only the red and
orange get to you
Sunset
Polarization
• To understand polarization, we need to review
something we did back in lecture 3!
• Light as an electromagnetic wave
Electromagnetic Waves
• The polarization is defined as the direction of
oscillation of the electric field
Polarized “along the z-axis”
Along which axis is this light wave polarized?
x
A. X
B. Y
C. Z
Electric field: pink
Magnetic field: blue
y
z
Polarized Light Waves
• Light waves are considered polarized if their electric
fields are all oriented in the same direction
y
y
x
Linearly polarized in
the y-direction
x
Linearly polarized in
the x-direction
Polarized Light Waves
• The electric field can be aligned in any direction in the
xy-plane
• We can describe this polarization as being composed
of an x-component and a y-component
y
y
x
x
Un-polarized light
• Light waves are un-polarized if their electric fields
are rapidly and randomly varying
Polarizing Filters
• A polarizing filter (used in sunglasses, camera
filters, 3D movies, etc) is a device that transmits
light with one polarization only, and rejects all
other polarizations
• There are two main ways to do this:
– Absorb the incorrectly polarized light
– Reflect the incorrectly polarized light
Polarizing Filters
Absorptive Polarizing Filters
• Also know as “Polaroid” film, after the company
that manufactures it.
• Made by processing plastic films containing
special crystals
• Inexpensive, used in sunglasses and camera filters
• Because they absorb the “rejected” light, they are
not suitable for high power applications, as they
will absorb the light energy and melt or deform
Reflective Polarizing Filters
• Also called “wire-grid” polarizers
• More expensive to manufacture
• Can be used with high incident power
• Used primarily for laser applications
Wire-Grid Polarizers