Light Basics
In order to understand thermal imaging, it
is important to understand something about
light. The amount of energy in a light wave
is related to its wavelength: Shorter
wavelengths have higher energy. Of visible
light, violet has the most energy, and red
has the least. Just next to the visible
light spectrum is the infrared spectrum.
Infrared light can be
split into three categories:
Near-infrared (near-IR)
- Closest to visible light, near-IR has
wavelengths that range from 0.7 to 1.3
microns, or 700 billionths to 1,300
billionths of a meter.
Mid-infrared (mid-IR) -
Mid-IR has wavelengths ranging from 1.3
to 3 microns. Both near-IR and mid-IR
are used by a variety of electronic
devices, including remote controls.
Thermal-infrared (thermal-IR)
- Occupying the largest part of the
infrared spectrum, thermal-IR has
wavelengths ranging from 3 microns to
over 30 microns.
The key difference between thermal-IR and
the other two is that thermal-IR is emitted
by an object instead of reflected off it.
Infrared light is emitted by an object
because of what is happening at the atomic
level.
Thermal Imaging - Here's how it
works:
A special lens focuses the infrared light
emitted by all of the objects in view.
The focused light is scanned by a phased
array of infrared-detector elements. The
detector elements create a very detailed
temperature pattern called a thermogram. It
only takes about one-thirtieth of a second
for the detector array to obtain the
temperature information to make the
thermogram. This information is obtained
from several thousand points in the field of
view of the detector array.
The thermogram created by the detector
elements is translated into electric
impulses.
The impulses are sent to a
signal-processing unit, a circuit board with
a dedicated chip that translates the
information from the elements into data for
the display.
The signal-processing unit sends the
information to the display, where it appears
as various colors depending on the intensity
of the infrared emission. The combination of
all the impulses from all of the elements
creates the image.
Types of Thermal Imaging Devices
Most thermal-imaging devices scan at a
rate of 30 times per second. They can sense
temperatures ranging from -4 degrees
Fahrenheit (-20 degrees Celsius) to 3,600 F
(2,000 C), and can normally detect changes
in temperature of about 0.4 F (0.2 C).
There are two common types of
thermal-imaging devices:
Un-cooled - This is the
most common type of thermal-imaging
device. The infrared-detector elements
are contained in a unit that operates at
room temperature. This type of system is
completely quiet, activates immediately
and has the battery built right in.
Cryogenically cooled -
More expensive and more susceptible to
damage from rugged use, these systems
have the elements sealed inside a
container that cools them to below 32 F
(zero C). The advantage of such a system
is the incredible resolution and
sensitivity that result from cooling the
elements. Cryogenically-cooled systems
can "see" a difference as small as 0.2 F
(0.1 C) from more than 1,000 ft (300 m)
away, which is enough to tell if a
person is holding a gun at that
distance!
Unlike traditional most night-vision
equipment which uses image-enhancement
technology, thermal imaging is great for
detecting people or working in near-absolute
darkness with little or no ambient lighting
(i.e. stars, moonlight, etc, )
Below is a history of infrared technology
and developments: Please use your cursor to
view the developments for a particular year.
