Long-wavelength infrared (LWIR)

Infrared cameras are categorized or labeled in many ways, which may be driven by camera design (handheld, fixed install, iPhone), performance attribute (high speed, high sensitivity, high resolution), or application (non-destructive testing, high temperature metal imaging and measurement, gas imaging, microscopic imaging).

Another approach to categorize cameras commonly used for scientific applications is to identify the spectral region from which the camera receives the infrared emissions used to present its infrared image.  LWIR infrared cameras are equipped with infrared detectors responsive to emissions between 8 and 14 µm.

LWIR cameras are by far the most common and are used in a wide variety of applications, including security (night vision), auto & marine navigation, process control, predictive maintenance, building diagnostics, and research. Advances in infrared detector technology, particularly in microbolometer detector design, have made LWIR cameras affordable, which accounts for their broad adoption.

Microbolometer detector design also eliminates the need for cryogenic cooling, which was a requirement for long-wave photovoltaic detectors common before the broad commercial adoption of microbolometer detectors in the late 90’s. Price-sensitive markets such as security, building diagnostics, search and rescue, and firefighting developed as microbolometer detectors replaced more expensive cooled IR detector technology.

The solid-state microbolometer design also eliminated the need for a mechanical detector cooling mechanism, which was not only expensive but impractical for process monitoring applications where temperature data collection was required 24 hours per day, seven days a week.

Plank’s law defines the peak efficiency of radiant energy emitted from a surface as a function of wavelength, and targets in the ambient temperature range emit infrared radiation most efficiently in the long wavelength region between 8 and 14 µm. In addition to affordability, suitability for use in ambient temperature ranges also explains their broad adoption.

Temperature measurement is an important feature for cameras used in process control, predictive maintenance, building diagnostics, and research applications. NIR or Near infrared cameras use detectors in the shorter wavelengths close to (near) the visible light spectrum and can be influenced by light emissions. For this reason, LWIR cameras with detector response most distant from the light spectrum are preferred for temperature measurement applications. Exceptions to this rule include high temperature measurements on lower emissivity surfaces, which are more effectively measured with NIR or SWIR infrared cameras.

Most LWIR cameras using microbolometer detector technology are capable of producing infrared images at frame rates between 30hz and 125 hz (with sub-frame mode), which addresses most applications. However, measurements on high-speed production processes, dynamic research applications such as measurement on explosions or electrical pulses may require faster frame rates, which are possible with photovoltaic or CMOS detectors commonly available only in shorter wavelengths.

Some special high-performance long-wave cameras use cooled Mercury Cadmium Teluride or Quantum well sensors to deliver thermal images with high resolution and very high thermal sensitivity. Both these detector materials have faster response times than the uncooled sensors and will support frame rates up to 1000 hz in full frame mode and up to 40 times faster when a reduced pixel count region of the image is recorded. Although these cameras are expensive and sold in small quantities, they can address certain applications that require fast data capture, unable to be acquired with uncooled long wave detector technology.

Finally, a long-wave cooled Quantum Well detector is available in a design that narrows the spectral response of the camera to a wavelength that is coincident with the absorption band of gases (10.6 µm) such as sulfur hexafluoride or ammonia gas. Because the detector is highly sensitive, it can produce a clearly visible gas cloud from a small leak even when the narrowed spectral range dramatically reduces the infrared energy exposed to the detector.

LWIR cameras will continue to remain the dominant type of infrared camera sold worldwide for the foreseeable future. New variants of LWIR cameras will emerge that address niche applications with special features, new designs, and accessories that make them a perfect fit for the growing list of thermal imaging applications.