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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

After the calibration was complete, the camera system was pointed down a long street at the blackbody source, which was powered by a portable generator. The blackbody and generator were rolled down the street on a cart away from the camera system. The measurement distances from the camera to the source were measured by a laser rangefinder.

Radiance and temperature measurements were made at intervals out to a maximum range of 860 meters, at which point the image of the blackbody was so small (3 by 3 pixels) that increasing the range any more would affect the quality of the measurement. An image was captured that shows an infrared image of the blackbody made at this range. The six-foot tall people in the image are included for scale.

The apparent temperature versus range data is shown in Figure 2. The rate of apparent temperature decrease is dramatic within the first 100 meters, and is the result of the presence of carbon dioxide, which absorbs nearly all the infrared signal between 4.2 and 4.4 microns. At longer ranges, water vapor primarily accounts for the reduced apparent temperatures, because it attenuates the infrared signal over the remaining portions of the 3 to 5 micron waveband. The atmospheric conditions are shown in the plot's heading. Aerosols present in the air include droplets of salt water blown by winds onto the shore. The carbon dioxide level is assumed to be around the average level of 360 parts per million by volume. The experiment will be repeated under various conditions to understand how factors like humidity change the results.

ESTIMATING DIFFERENCES
The plot in Figure 2 helps to estimate air-path-induced temperature differences, but only for temperatures right around 100C. Temperature is not a linear function of radiance, and there is no simple way to generalize this plot to work over a range of target temperatures. The correct way to measure surface temperature at different ranges with infrared imaging is to first measure the target's apparent radiance, correct this radiance for air-path effects, and then determine the object's surface temperature based on its emissivity. The RTools software applied air-path corrections to radiometric data.

A more useful parameter to measure for a particular set of atmospheric conditions is the effective atmospheric transmission as a function of range. The transmission is a number that is between zero and unity, where zero corresponds to no transmission whatsoever and unity corresponds to perfect transmission, as is the case in the vacuum of space. The apparent radiance of a target is defined as the atmospheric transmission factor multiplied by the true radiance. The apparent radiance of a target can be measured, however, users generally want to determine the true radiance of a target, and because of this it is useful to calculate the inverse of the transmission factor, which is often called the air-path correction factor. The correction factor is a number that is greater than or equal to unity, with unity indicating perfect transmission. If the air-path correction factor is known for a given distance and a set of atmospheric conditions, then it can be used to determine the true radiance of a target.



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