Advanced Imaging


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.

By Austin Richards, Ph.D. and Greg Johnson

Figure 3 shows the effective atmospheric transmission as a function of range. The effective transmission is the ratio of the apparent radiance and the true radiance weighted by the spectral response of the camera system and the spectral shape of the blackbody source. The radiance data used to make this plot was taken at the same time as the data in Figure 2. The effective atmospheric transmission is 0.55 at a range of 800 meters. The air-path correction factor is therefore the inverse of this, or 1.8. If one measures an apparent radiance of R for a target at that range, then the true radiance is 1.8R. If the emissivity of the target is known, then the true radiance and the emissivity can be used to calculate the surface temperature. The air-path correction factors will depend on the concentration of carbon dioxide in the air and the humidity. For applications requiring high precision, it may be necessary to empirically measure air-path factors at the time that the radiometric data is collected.

Based on the testing, infrared cameras are useful tools for measuring radiance or temperature, even when the target is a significant distance away. Understanding and compensating for atmospheric effects can yield substantial improvements in the accuracy of these measurements.

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