Short Wave Infrared (SWIR)

The Short-Wave Infrared (SWIR) imaging band includes wavelengths spanning from approximately 1.0µm to 2.5µm. There’s no clear line between the SWIR and the NIR (Near Infrared) bands, like there is between the MWIR and LWIR bands; in fact, there is overlap, with NIR being considered to cover from about 0.75µm to 1.5µm. In fact, some sources will use either terminology (NIR or SWIR) to cover the entire spectral band between 0.75µm to 2.5µm. While this can be confusing, there is no strict convention that distinguishes them. 

Despite that confusion, there are differences in the applications of the two wavelength bands. Though both NIR and SWIR are considered part of the infrared portion of the spectrum, neither of them are generally effective as thermal imagers. This is due to the temperature ranges of most surveillance targets (background environment, people, animals, and vehicles). Targets at these temperatures emit relatively low amounts of thermal energy, which are more detectable at longer wavelengths (and with specialized detectors), where ‘thermal’ imagers operate.

From a surveillance perspective, the NIR band is often used in ‘night vision’ (image intensified) devices. Such NIR detectors often use photomultiplier tubes, which amplify existing light, and convert the signal to visible wavelengths, to make them useful as a surveillance tool. In some cases, even with this amplification, they still require the use of NIR illuminators to reflect off the targets, to give enough NIR light to detect.  CCD and CMOS cameras have some sensitivity in the NIR band as well, though they certainly require NIR illumination to operate.

In contrast to this, SWIR imagers have several different surveillance applications. The slightly longer wavelengths of SWIR imagers allow them to see through fog, to a degree much better than either visible or thermal imagers. This makes SWIR highly useful in coastal and maritime surveillance applications, where fog banks are common. For long range, nighttime surveillance, SWIR detectors are sometimes paired with laser illuminators (in the SWIR waveband) and gated; this means that the detector’s response is timed carefully with the laser to only detect pulses of light that have traveled a certain distance. By doing this, the detector can reject reflections that are from shorter sources, such as reflections through a thick fog bank, and only detect illumination that has traveled to the range of the expected target.

From a military surveillance perspective, SWIR can detect lasers from sources such as range finders, pointers, and designators which operate at wavelengths beyond the range of NIR (night vision) devices. This can prove useful in detecting an opponent’s targeting lasers and is often used by manufacturers of those lasers to test them and to align them with other sensors.

Additionally, SWIR imaging finds utility in industrial applications, including quality control, remote sensing, and machine vision. The unique capabilities of SWIR imagers can assist in inspecting products, assessing material properties, monitoring production processes, and identifying anomalies that may not be apparent to the human eye or other imaging technologies.