Infrared Thermography Systems
Infrared (IR) thermographic systems, or IR imagers, provide images that represent surface temperatures, or thermograms, by measuring the magnitude of infrared radiation emanating from the surface of an object. Because IR imagers see the radiation naturally emitted by objects, imaging may be performed in the absence of any additional light source. Modern IR imagers resolve surface temperature differences of 0.1°C or less. With this high sensitivity, they can evaluate subtle thermal phenomena, which are only revealed in the form of slight temperature gradients. Some applications that employ IR thermography include: inspections for predictive maintenance, non-destructive evaluation of thermal and mechanical properties, building science, military reconnaissance and weapons guidance, and medical imaging.
Infrared thermography can be used as both a qualitative and a quantitative tool. Some applications do not require obtaining exact surface temperatures. In such cases, it is sufficient to acquire thermal signatures, characteristic patterns of relative temperatures of phenomena or objects. This method of qualitative visual inspection is expedient for collecting a large number of detailed data and conveying them in a fashion that can be easily interpreted. In contrast, accurate quantitative thermography demands a more rigorous procedure to extract valid temperature maps from raw thermal images. However, the extra effort can produce large arrays of high-resolution temperature data, which are unrivaled by contact thermal measurement techniques, such as using thermocouple wires.
A skilled operator of an IR thermographic system, or thermographer, must be conscious of the possibility that reflected or transmitted, rather than emitted, IR radiation may be emanating from an object. These additional sources manifest themselves as signals that appear to be, but are not actually, based exclusively on the temperature of the spot being imaged.
To understand the challenges and possibilities of IR thermography, it is first necessary to review the principles of physics on which it relies.