Tuesday, April 24, 2012


MWIR or MIR - Mid-Wave-InfraRed or Mid-InfraRed, a range of wavelengths nominally in the middle part of the infrared spectrum, defined in a variety ways.

The terms hiding behind acronyms sometimes can be more confusing than the acronyms themselves. The diverse definitions for "mid-infrared" illustrate the problem. The name clearly implies that MWIR/MIR should be  the middle of the infrared spectrum, but the infrared is a vast region, sprawling from 700 nm at the long end of the visible range to around 1 mm at the upper end of the radio/microwave band. What is the middle?

On a logarithmic scale, it seems simple enough--10 to 100 µm fits right in the middle. But nobody uses that definition. Wikipedia, today's great arbiter of popular culture, defines the MWIR as 3 to 8 µm, but that section of the "infrared" article bears a warning dating from July 2006 that it needs to be cleaned up! The 3–8 µm definition follows the recommendations of the International Commission on Illumination, which considers the MWIR as the short end of the IR-C band from 3 µm to 1 mm.

However, other definitions abound. In its tabulation of optical spectrum bands, ISO, the International Standardization Organization, defines MWIR as 3-50 µm. NASA's Infrared Processing and Analysis Center at Caltech defines the astronomical mid-IR as stretching from 5 µm to 25 or 40 µm. Military system designers traditionally define 35 µm as MIR, the window used by heat sensors on missile guidance systems. The McGraw-Hill Dictionary of Scientific and Engineering Terms does not list mid-infrared, but defines "intermediate infrared radiation" as from 2.5 to 50 µm.

Who's right? It depends on your viewpoint. In my recent mid-infrared lasers and applications webcast, I spoke from the laser industry viewpoint set the boundaries as 2 to 12 µm--starting beyond the telecommunications band and extending to include the carbon-dioxide laser band, as shown in the image. But when I wrote about uncooled infrared cameras in the April issue, I wrote from the detector viewpoint, and called the thermal imaging band from 7.5 to 14 µm "long-wave infrared."  In truth, detectors used in the 35 µm and 7.5- to 14-µm bands differ much more than the laser used. But on reflection I have to wonder why the spectral bands should differ between the light source and the detector.  

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