Infrared Spectrum

Infrared is a term derived from the Latin word “infra” (meaning “below”); it refers to the portion of the electromagnetic spectrum with longer wavelengths than red light. Spanning a broad range of wavelengths, the infrared spectrum allows a wide range of phenomena to be measured or observed, with applications ranging from night vision, remote temperature measurement, and even determination of what materials an object contains, as well as many others.

Generally, the infrared spectrum consists of four defined imaging bands, each with its own unique characteristics and applications. The first band, known as near infrared (NIR), ranges from approximately 0.75µm to 1.5µm. NIR imaging finds widespread use in a variety of fields, including remote sensing, surveillance, and medical imaging.

The short-wave infrared (SWIR) band spans from about 1.0µm to 2.5µm; though there is some overlap between NIR and SWIR, distinction is made between them due to differences in applications, and sometimes in detector materials.  SWIR imaging offers advantages in areas such as industrial inspection, agriculture, and mineralogy. It’s ability to see reflected light through fog also provides application in maritime surveillance applications.

The spectral band from approximately 3.0µm to 5.0µm is called the mid-wave infrared (MWIR) band. MWIR imaging excels in applications such as thermal imaging, gas detection, and defense systems. Virtually all materials that detect in this spectral region require cooling to very low temperatures to operate but can provide incredible sensitivity.

Finally, the long-wave infrared (LWIR) band spans from around 7.5µm to 14µm. LWIR imaging holds tremendous significance in fields such as building diagnostics, electrical inspections, and surveillance. Its ability to capture the thermal signatures emitted by objects and living beings, even in complete darkness, enables the identification of heat leaks, moisture intrusion, and concealed targets. Some materials which can detect in this spectral range do not require cooling, making these types of imagers more affordable (and therefore much more common), though they are typically not as sensitive as MWIR systems and have limitations in humid environments.

Infrared and thermal imaging are not necessarily the same thing. While any imager that operates on wavelengths longer than about 750nm can be considered an infrared imager, thermal imagers only operate in the MWIR or LWIR bands.  While all objects emit some amount of thermal energy, most common objects such as people, vehicles, and background objects emit detectable energy only in the MWIR or LWIR bands; therefore those bands are used for thermal imagers. Objects that are extremely hot will begin to emit detectable amounts of energy at shorter wavelengths; the hotter the object, the shorter the wavelengths that it will emit at. This is illustrated by the common example of a piece of iron or steel that is left in a fire for an extended period; it will get hot enough to glow (emit light) even in the relatively short visible wavelengths.

The diverse range of applications and capabilities offered by the infrared spectrum highlights its profound impact on various industries and scientific endeavors. From the near-infrared’s ability to differentiate chemical compounds to the long-wave infrared’s capability to detect hidden thermal signatures, each imaging band within the infrared spectrum unlocks a unique perspective on the world around us.