waves pass through clothing just like tv, radio and mobile phone
signals. Metal objects reflect strongly showing up in contrast to
the body. these two properties make millimetre wave imaging ideal
for security scanning.
When we take a normal photograph we usually see a contrast between
objects due to the difference in the amount of light they reflect.
That light typically comes from either the sun or sky (outdoors)
or lightbulbs (indoors).
Millimetre wave cameras see the heat in a scene and objects appear
at different temperatures depending on how much heat they emit or
how much they reflect the heat from the surroundings. Outdoors the
contrast is provided by the difference between the heat of warm
objects and reflections of the sky above which is very cold - 200°C
below freezing. indoors the contrast is much less since the temperatures
of objects and the room are similar, so an artificial mm-wave light
source is sometimes used to increase contrast.
These millimetre wave images show cold as white and hot as black.
Outdoors the sky acts as a very cold floodlight which reflects off
your head giving a strong contrast with our surroundings and a clear
image. Indoors the head reflects the temperature of the room which
is closer to our own temperature so there is little contrast and
an indistinct image.
X-ray machines are conventionally used to search for dangerous
items and have been very successful in identifying knives, guns
and other metal items in hand luggage. Metal detectors have been
used to identify objects about a person and rely on detection of
a mass of metal however their effectiveness depends on size of the
object, the type of metal and its orientation. Also metal detectors,
which detect magnetic field variation, cannot distinguish between
metal objects such as belts or keys and threatening objects. As
most of us have experienced, this can slow down the flow of people
With the increased threat of terrorism there is a need for more
effective techniques to detect hostile objects. Also modern weapons
include plastic or ceramic guns and plastic and liquid explosives,
all of which cannot be detected with conventional metal detectors.
Millimetre-waves are non-ionising, unlike x-rays, and should pose
no health dangers.
Millimetre-wave security systems have been tested at many UK airports.
As the waves are effectively seeing through clothes this has posed
privacy issues. Would you mind someone looking through your clothes
for the sake of security? Improvements to the system mean that a
computer will be used to identify the objects rather than a human
behind a screen. Also it is important to remember that mm-waves
are intrinsically pixelated and so do not reveal the naked body
Images on this page courtesy of Qinetiq