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Operating Instructions
Appendix A
– Introduction to Thermal Imaging
Appendix A
– Introduction to Thermal Imaging
A thermographic camera or infrared camera is a device that forms an image using infrared radiation, similar to a
common camera that forms an image using visible light. Instead of the 450
–750 nanometer range of the visible
light camera, infrared cameras operate in wavelengths as long as 14,000 nm (14 µm).
A.1 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:
1.
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.
2.
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.
3.
Far-infrared (Far-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. 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 which are then 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 can be shown in a variety of display
modes depending on the intensity of the infrared emission. The combination of all the impulses from all of the
elements creates the image
A.2 Uncooled infrared detectors
Some thermal imaging cameras need cooling systems to reduce the amount of thermally induced noise they
pick up and improve image quality. Uncooled detectors such as the detector in the Thermal Camera do not
require these cooling systems. Uncooled thermal cameras use a sensor operating at ambient temperature, or a
sensor stabilized at a temperature close to ambient using small temperature control elements. Modern uncooled
detectors all use sensors that work by the change of resistance, voltage or current when heated by infrared
radiation. These changes are then measured and compared to the values at the operating temperature of the
sensor.
