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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

However, for many industry users, temperature is the parameter of interest. Temperature is calculated by converting the digital signal from each pixel in the camera's sensor into units of radiance (measured in units of watts/cm2-steradian). These radiance values are then converted into apparent temperatures by an iterative process involving numerical integration of the function that relates radiance to temperature (the Planck function) over the waveband of the infrared camera. The emissivity of the target is factored in during this conversion process. Emissivity is a measure of the light emitted by a surface relative to the light from a perfect blackbody emitter at the same temperature. For many common materials, the emissivity is close to unity in the infrared waveband.

AIR-PATH EFFECTS
This method of measuring temperatures is accurate when the target is close to the infrared camera, on distance scales of meters, for example. However, when measuring more distant targets of known temperature, a decrease in the apparent temperature of the target as a function of distance is noticeable.

This effect is caused by absorption of infrared light by the air between the camera and the target, the so-called air path. The effect of the air path is particularly noticeable with infrared cameras operating in the 3 to 5 micron wavelength range, which is the midwave infrared (MWIR) band. This is because carbon dioxide in the atmosphere absorbs infrared light strongly in the 4.2 to 4.4 micron band.

Also, water vapor is absorbed at other wavelengths between 3 and 5 microns. These two molecular species are the prime contributors to air-path effects in the MWIR band. The air path will reduce the apparent radiances (and temperatures) of targets by substantial amounts over kilometer-sized length scales. Fortunately, this effect can be taken into account when measuring temperatures.

Recently, studies were undertaken to measure the magnitude of the air-path effect on MWIR camera temperature measurements over path lengths that are commonly encountered in real-world applications. The measurements were performed in typical winter atmospheric conditions in coastal Southern California (16C, 40 percent relative humidity). The apparent radiance and temperature of a blackbody source were measured at a single setpoint temperature (100C) out to the longest practical distance that the equipment allowed. The relatively high blackbody temperature was chosen so that the camera could be operated at a short exposure time, which makes the camera system insensitive to variations in lens temperature and self-emission from the atmosphere, greatly simplifying the data analysis. The system consisted of a FLIR Systems Merlin Mid camera mated to an Optimum Optical Systems three field-of-view lens set to its narrowest field of view (1.1 degrees horizontal). The Merlin camera was connected to a PC running RTools and the system was radiometrically calibrated with a six-inch square laboratory blackbody, which was placed as close to the lens as possible to minimize the length of the air path during the calibration.



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