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Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

IR Gets Lost in the Jet Stream

The air path between the camera and the object to be measured can affect temperature measurements taken with midwave infrared cameras
An MWIR image
Flir Systems
An MWIR image taken at an 860 meter range shows a 100C blackbody source and people.
Figure 2
Flir Systems
FIGURE 2: The apparent temperature versus range for a 100C blackbody target.
Figure 3
Flir Systems
FIGURE 3: Atmospheric transmission versus range fora 3-5 micron MWIR camera system.
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By Austin Richards, Ph.D. and Greg Johnson

Thermal infrared cameras are commonly used for non-contact temperature measurement. This technique produces highly accurate results, which can be even more accurate if the user understands how to compensate for variations caused by the air path between the camera and the target.

Thermal infrared cameras capture pictures using the light that is emitted by all objects. The cameras can be calibrated to measure temperatures of furnaces, motors, fuses, wiring, animals, people and just about anything else.

The temperature calibration process involves pointing the camera at an infrared calibration source of known strength, which is typically measured in units of radiance. The source radiance is then varied to generate a calibration table that relates the camera's signal output to the radiance of the calibration source. These radiance values are converted into temperatures and displayed on the LCD screens of handheld cameras, generally in the form of spot meters. For many handheld camera products, individual images can be stored and analyzed offline.

For high-end infrared cameras that are calibrated to measure temperature, the output is typically a stream of digital images at video frame rates. Every pixel in the image is represented by a digital count value that is linearly proportional to the radiance of the scene. In many applications, it is useful to do the calibration and image processing in software running on a host PC, which is connected to the digital image output of the infrared camera. The user is able to calibrate a camera such as one containing a custom window or filter. This is useful when the camera is connected to an optical train that is specific to the user's system.

The FLIR Systems RTools software suite is designed for this purpose. The infrared camera itself does not require internal calibration; instead, the calibration is done on the system level. After the system is calibrated, it is considered to be radiometric, and can then be pointed at a scene of interest and used to measure the apparent radiance and, by extension, the apparent surface temperature of an object. Radiance measurements are used exclusively in certain specialized applications such as target signature analysis, where the emissivity of the target is not well known and temperature measurement is of secondary importance.

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