Long-wave infrared (LWIR)

Long-wave infrared, LWIR, is the band, roughly 8 to 14 micrometres, that industrial thermal cameras default to for a practical reason rather than a peak-emission one: it is an atmospheric transmission window that stays usable from near-ambient temperature up to several hundred degrees Celsius — the range a cooling weld, a heat-affected zone or an interpass surface actually sits in — on detectors that need no cryocooler and lose the least contrast to welding fume. It is not the band where those surfaces radiate most strongly: by Wien’s displacement law, a surface above roughly 90 °C already peaks below 8 micrometres, so a hot weld pool or the arc itself peaks well inside the short- or mid-wave band instead, not LWIR. LWIR wins the ambient-to-moderate range on cost, detector simplicity and fume tolerance, which is why it, not another band, is the default choice for weld and process thermography.

Detector technology reinforces the choice. Most LWIR cameras used in industrial and welding settings are uncooled microbolometer arrays: no cryocooler, and lower cost and size than the cooled detectors older thermal-imaging systems needed, at the price of somewhat lower sensitivity and frame rate than a cooled sensor delivers. For monitoring a cooling bead or an interpass surface, that trade is favourable; for imaging an open arc directly, it is not, which is where short-wave infrared takes over instead.

LWIR is the reason a “thermal camera” and a “welding camera” are frequently two different instruments on the same cell, a distinction covered fully under infrared camera: a visible or SWIR camera resolves the bright arc and pool, while a LWIR camera resolves the comparatively dim heat-affected zone around it, and neither band substitutes for the other. In wire arc additive manufacturing, the same LWIR imaging that monitors a single weld extends naturally to layer-to-layer thermal accumulation, because the temperature range across a build stays inside the band LWIR is built for.

As with any thermal measurement, an absolute-temperature reading from a LWIR camera still requires a known emissivity for the surface in view — the wavelength band changes what can be seen, not the need to account for how the surface radiates.