From: wjc@ll.mit.edu (Bill Chiarchiaro)
Subject: Re: mm-wave imaging
Date: Fri, 29 Jul 94 14:00:45 -0400

rats@ihlpm.att.com wrote:

# Well, since Neal Knox confirms my original suspicion that the gun detector
# is an imaging millimeter wave radiometer, I think we can begin designing
# countermeasures... pieces of styrofoam cut in the shape of handguns with
# metal foil glued to it... aluminized plastic cut in the shape of guns(?)
# ... would RFI shielding foam such as that sold by Emerson & Cummings in
# low-thickness sheets attenuate the signal from such passive detectors?...


clochmul@chem.duke.edu (Charles Lochmuller) wrote:

# Is the idea of shielding foam to prevent detection or to protect the
# wearer? If the former, why wouldn't a holster that absorbs radiation
# be just as obvious as a firearm?  and why not susopect anyone that has
# absorbing clothing?


  My MIT graduate thesis was on the topic of millimeter-wave
radiometry, so I can't resist...

  First, as rats stated, a radiometer is a passive sensor.  Therefore,
there is no signal _from_ the sensor for you to attentuate.  Also,
there is nothing against which you can protect the wearer (see
clochmul's questions).  Such concepts might apply to an _active_
sensor, such as a radar.

  A radiometer is, in effect, a fancy thermometer.  It reports the
radiometric temperature of the object or scene its field of view.
This information can be presented in the form of an image.  An
object's radiometric temperature is the product of its physical
temperature and its emissivity at the frequency of observation.
Emissivity is a comparison of how good a thermal radiator a given
object or medium is as compared to a perfect blackbody, and it
ranges in value from 0 to 1.  Emissivity is related to the loss
(attenuation), reflectivity, and transmissivity of the object or
medium.

  If you would like to know the full details behind what I'm about to
relate, go to a library and research the topic of radiative transfer.
In a nutshell:

  A material such as carbon-impregnated foam is going to be lossy and
have a high emissivity (near 1).  Thus, a radiometer looking at a
piece of that material would report a temperature very close to the
material's physical temperature.  Also, whatever happens to be behind
the material will be unseen by the radiometer.

  On the other hand, a low-loss, reflective material (perhaps a metal)
is going to have a very low emissivity (near 0).  Hence, the
material's own physical temperature is going to be unimportant.  The
radiometer will see the radiometric temperature of whatever object or
scene happens to be reflected by the material.  Again, anything behind
this material will be unseen by the radiometer.

  By arrangement of absorptive (lossy) and reflective materials, it is
possible to modify the radiometric appearance of an object or scene.
For instance, it is possible to make an object blend in with its
background.  A good techincal library can provide a number of books
which describe the millimeter-wave radiometric characteristics of
various materials and terrain.  Also, there are bodies of data about
the millimeter-wave radiometric appearance of people.  For instance,
during the 1970s, Professor Barrett at MIT experimented with
millimeter-wave radiometers for the early detection of breast cancer.
I have seen papers about similar projects in the various IEEE journals
within the last few years.


Bill Chiarchiaro
wjc@ll.mit.edu