Updated Mary 11,
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New for March 2009! This
page is on polarization is under construction, please help us out
Polarization is a property of
electromagnetic waves as they are radiated in space. EM waves in
space are transverse, such that oscillations in E-fields
and H-fields are perpendicular to the direction the wave travels,
and at 90 degrees to each other. By comparison, sound waves are
longitudinal, they oscillate along the direction that the
sound is traveling.
History of polarizers
Malus discovered polarization in 1808. Malus's law gives the
intensity of light after it has been polarized by an ideal polarizer
For incoherent light, half of
the intensity is lost. It would take calculus to derive that, it
is the average value under the cos^2 curve, integrated over all
possible angles (2 pi radians or 360 degrees).
The origin of polarizers dates
back to the 1850s, and involves passing quinine through a dog and
treating the urine with iodine to create Herapathite crystals. We
did NOT make this up. Aligning the crystals on a film, Edwin
Land, the inventor/genius who started the Polaroid company, was
the first engineer to exploit polarizers, in the 1920s. The crystals
are needle-like, and when they are aligned on a sheet, a polarizer
is the result. Light polarized parallel to the crystals is absorbed,
and light polarized perpendicular to the crystals is transmitted,
which is the opposite of how you might imagine how it works.
Types of polarization
The E-fields all oscillate in
one direction at a fixed place in space over time, or at a fixed
time over distance. Linear polarization can be purely vertical,
horizontal, or any angle in between. Most antennas produce or receive
a single polarization.
The images below are some animated
gifs you can find on the web
site of Hungarian András Szilágyi, who gave us
permission to post his work. Go there and review his tutorial, chances
are you'll learn something!
EM waves at any polarization
angle can be decomposed into two waves at 45 degrees to the original
vector, such that the decomposed waves are 90 degrees apart (orthogonal).
If one of the orthogonal components is phase shifted exactly 90
degrees, the composite vector will appear to rotate at a fixed time
Circular polarization (LCP and RCP)
Circular polarization can be
left handed (LCP) or right handed (RCP), depending on whether the
phase shift between the components is +90 or -90 degrees.
Why would you want CP? In a comm
link then you don't have to worry about your receiver's angle with
respect to the transmitter, the wave will just spin right in!
Elliptical polarization results
when one of the two linear components in CP is greater amplitude
than the other. The ratio of the two voltage amplitudes is called
the axial ratio. When axial ratio is one, you have CP.
One way to look at elliptical
polarization is that the two components of circular polarization
are not exactly 90 degrees apart.
Polarization in radar systems
Most radar systems operate with
a single polarization. This is because the transmitter and receiver
operate out of the same antenna, so whatever polarization goes out
will come back strongly. The
polarization that is identical to that transmitted is called "co-pol"
and the opposite one is called "cross-pol".
Below is a demonstration of a
radar detector interacting with a WalMart door opener. You can see
why you don't want to mount your radar detector vertically! By the
way, the microwave door opening is an example of microwave object
detection, another way microwave engineers improve our lives.
What if you are a policeman and
you are trying to catch more speeders than your buddies? Just turn
your police radar gear 90 degrees and none of the lead-foot scofflaws
with radar detectors will have a chance to jam on their brakes in
time to avoid your trap! Bwa ha ha ha! Another public service announcement
Actually, there is another consideration.
Police radar is vertically polarized, because there is more likely
to be clutter and multipath when horizontal is used, because more
objects happen to be horizontal. So the radar gun might have reduced
range and accuracy when help sideways. Anyone out there have any
experience with this?
Polarization in communication
Polarization requirements depend
on how the system is used. For example, if you wanted to create
a "comm-on-the-move" system that points to a satellite,
the system would likely have to use circular polarization so that
the antenna (and the vehicle it is mounted on) could rotate with
respect to the transmitter. This is exactly what is used in XM radio
and Sirius radio receivers. The penalty is that the satellite must
supply twice the transmit power compared to a single pol system.
That is one reason that XM satellites "Rock" and "Roll"
were the largest commercial satellites ever launched! The other
reason is that the tiny receive antenna has very low gain, it is
Compare satellite radio to satellite
television. Your dish must face the satellite exactly to grab the
signal, and the angle must be near perfect, because the signals
are linearly polarized. We said signals, because there are actually
two, one is vertically polarized and one is horizontally polarized;
this technique allows the network to provide twice as much content
compared to a singular polarization, so you can get even more home
shopping networks, reality shows and other mind numbing garbage.
You'd think with all that bandwidth there might be a "masters
degree channel" where you could learn how a remote control
works instead of getting butter on one while you enjoy a bag of
microwave popcorn. At least the microwave industry is there to help
you out with warm and tasty snacks.
An orthomode transducer
is a often used to separate orthogonally polarized signals in a
communication system. We'll cover that topic as soon as someone
asks about it!
Polarization purity is
the ratio of the desired polarization component to the undesired
component. 40 dB is a good figure of merit.
Polarization of visible light
Light waves are of course electromagnetic
waves, but at much higher frequency than microwaves. The wavelengths
of the visible spectrum span 4000 to 7000 Angstroms (which is 400
to 700 nanometers, thanks for the correction, Morten!) Put in the
same units, X-band microwave radiation has a free-space wavelength
of 300,000,000 Angstroms. Incoherent waves (all visible light with
the exception of laser light is incoherent) contain energy distributed
at all angles, perpendicular (transverse) to the direction of propagation.
Polarized sunglasses have a vertical
polarizer, or polaroid. When light reflects from a flowing
stream, it becomes partially polarized. Most often the polarization
is horizontal, because objects that cause unwanted reflections are
often horizontal, like bodies of water, or roadways. Polarized sunglasses
or (filters on a camera) can remove unwanted reflections (blocking
horizontal polarization) and provide a higher-quality image, but
at reduced intensity. Reduced intensity is not usually a problem,
cameras and eyes adjust automatically.
These pictures were used with
permission from Professor
Ching-Kuang Shene at Michigan Tech. Thank you sir!
niche application of polarizers is 3D glasses for viewing movies.
The original 3D experience crudely separated the two images using
color filters; later polarized glasses with polarizers at +45 and
-45 degrees allowed full color for both eyes. However, if you tilted
your head, you saw double images and the effect was lost and a headache
was often the result. Today, RealD
provides a new 3D experience. Their technology separates the two
pictures into two circular polarizations, left hand and right hand.
The glasses that they distribute filter out the unwanted images
and work regardless of what angle your head happens to be.
In order to change linearly polarized
light to circularly polarized, a quarter-wave retarder is used.
This is a media that presents different indices of refraction to
the light wave, such that one is retarded (phase shifted) by 90
If you want to be a real nerd,
bring home your RealD 3D glasses and play with them in front of
a mirror. Put on the glasses and close one eye - the open eye remains
shaded, but you can see the closed eye clearly! This is because
you're looking OUT through one lens and IN through the other, reversing
the polarization of the reflected wave (as the whole thing is reflected
back at you through the mirror). If you have two mirrors positioned
at a 90 degree angle you can re-reverse the image and you will find
that your OPEN eye is the one that you see clearly in the reflection!