Thursday, May 31, 2012


SWIR - Short-Wavelength (or Short-Wave) Infrared, wavelengths at or near the short-wave end of the infrared.

You might think the infrared band stretching from the red end of the visible spectrum to three micrometers (µm) is used widely enough that the it would have a well-accepted definition. Dream on. Everybody has their own take, and they can't even agree whether that part of the spectrum should contain one or two bands.

Wikipedia's "Infrared" entry lists several definitions of SWIR, without trying to resolve the conflicts or explain the differences. It first cites a "commonly used subdivision scheme" that defines SWIR as wavelengths from 1.4 to 3 µm, where water absorption is high, with a separate near-infrared (NIR) band at 0.75 to 1.4 µm where water absorption is low. It's not a bad definition, but why is it referenced to a book titled Unexploded Ordnance Detection and Mitigation?  The International Commission on Illumination makes a similar split at 1.4 µm because shorter wavelengths (the IR-A band) can reach the retina, but longer wavelengths (the IR-B band) are absorbed within the eye. Both divisions make sense from the standpoint of illumination.

Other divisions are based on sensor response. Wikipedia cites a split between NIR extending to the long-wave limit of silicon detectors near 1.0 µm, and SWIR from 1.0 to 3.0 µm, given in Miller and Friedman's Photonic Rules of Thumb. A Raytheon wall chart similarly defines NIR as 0.7 to 1 µm, and SWIR as 1 to 2.7 µm. However, makers of other detectors pick other dividing points. An article by Nova Sensors (Solvang, CA) defines SWIR as 0.9 to 1.7 µm, the response range of the InGaAs sensors used in their SWIR cameras. However, sometimes the two ranges overlap. Headwall Photonics (Fitchburg, MA) lists its NIR hyperspectral imaging sensor as responding to 0.9 to 1.7 µm and its SWIR version as responding to 0.95 to 2.5 µm.

Some don't bother splitting the band at all. The International Organization for Standardization (ISO) 20473 standard calls 0.78 to 3 µm NIR, and the McGraw-Hill Dictionary of Scientific and Technical Terms calls NIR 0.75 to 3 µm. Neither lists SWIR.

I could go in agonizingly nit-picking detail, but the lesson is clear -- SWIR (and NIR) can be useful labels for parts of the infrared, but you need to check the numbers to be sure what they mean.

No comments:

Post a Comment