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Understanding IR camera calibration and corrections help ensure accurate temperature measurements and thermographic mapping.
Quantitative Measurements with IR Cameras
For best results, IR camera users need to think carefully about the type of measurements they need to make, and then be proactive in the camera’s calibration process. Of course, the first step is selecting a camera with the appropriate features and software for the application. An understanding of the differences between thermographic and radiometric measurements is very helpful in this regard.
Thermography is a type of infrared imaging in which IR cameras detect radiation in the electromagnetic spectrum with wavelengths from roughly 900 to 14,000 nanometers (0.9–14 μm) and produce images of that radiation. Typically, this imaging is used to measure temperature variations across an object or scene, which can be expressed in degrees Celsius, Fahrenheit, or Kelvin.
Radiometry is the measurement of radiant electromagnetic energy, especially that associated with the IR spectrum. It can be more simply defined as an absolute measurement of radiant flux. The typical unit of measure for imaging radiometry is radiance, which is expressed in units of Watts/(sr-cm2). (The abbreviation “sr” stands for steradian; a non-dimensional geometric ratio expressing the solid (conical) angle that encloses a portion of the surface of a sphere equivalent to the square of the radius.)
In simple terms, one can think of thermography as “how hot” an object is, whereas radiometry is “how much energy” the object is giving off. Although these two concepts are related, they are not the same thing. IR cameras inherently measure irradiance not temperature, but thermography does stem from radiance. When you thermographically calibrate an IR system you are calibrating / measuring based on effective blackbody radiance and temperature. Therefore, the emissivity of the target object you are measuring is vital to achieving accurate temperatures. (Emissivity or emittance is the radiative property of an object relative to a perfect blackbody.)
Entry level IR cameras with microbolometer detectors operate according to non-quantum principles. The detectors respond to radiant energy in a way that causes a change of state in the bulk material (e.g., resistance or capacitance). Calibration software in these cameras is oriented toward thermographic imaging and temperature measurements. High-end IR cameras with photon detectors operate according to quantum physics principles. Although they also provide high quality images, their software is typically more sophisticated, allowing accurate measurements of both radiance and temperature.
Some reasons why radiance measurements are important include:
• Given a linear sensor, measured radiance is linear with incident energy. Temperature is non-linear with raw digital image counts, even with a linear sensor...
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