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.
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