The Basics of Infrared Physics and Technology
Overview
Discussions about Infrared are often overly broad and tainted by Hollywood fantasy. There is specific physics behind the study of infrared and behind the technology used to display the images. It is important to understand the basics of the field in order to when choosing the best technology for your particular needs. This paper will help provide that understanding.
Terminology
• Black body – A perfect emitter of infrared energy. A mirror is a perfect reflector. A Grey body is anything between a perfect emitter and a perfect reflector.
• Emissivity – The percentage of energy emitted by a body. A black body has an emissivity of 1; a mirror has an emissivity of 0. Most objects have an emissivity between 0.7 and 0.9.
• Planck’s Law – A body that emits energy will emit more energy in all electromagnetic bands the higher the temperature of the body. For example, the sun emits all electromagnetic radiation bands from ultraviolet (short wavelength) through the visible to all infrared spectra (very long wavelengths) and has a surface temperature of ~6,000°K.
• Energy bands (for the purposes of this discussion)
o Visible – Energy that the human eye can perceive (0.4μm – 0.6μm wavelength). Sometimes broken into three bands; red (long), green (medium), and blue (short).
o Near Infrared (NIR) – 0.7μm – 1.0μm wavelength
o Short-wave Infrared (SWIR) – 1.0μm – 1.7μm wavelength
o Extended Short-wave Infrared (eSWIR) – 1.0μm – 3μm wavelength
o Mid-wave Infrared (MWIR) – 3μm – 5μm wavelength
o Long-wave Infrared (LWIR) – 8μm – 14μm wavelength
o Very Long-wave Infrared (VLWIR) – 14μm – 20μm wavelength
o Terahertz – 20μm – 100μm wavelength
• Multispectral – imaging in 2 – 4 energy bands
• Hyperspectral – imaging in more than 10 energy bands or across the entire energy spectrum
• FPA (Focal Plane Array) – a sensing device made up of discrete pixels to image one or more energy bands.
• Pixel – an individual sensing element that is part of an array. Pixel size varies by technology and manufacturer.
• FPA resolution – generally defined by number of pixels in an array. i.e., 320x240, 640x512, 1024x768, etc.
• Cooled vs. Uncooled sensor – Cooled sensors are maintained at cryogenic temperatures (4°K – 100°K) while Uncooled sensors nominally operate at ambient temperatures.
• HOT (High Operating Temperature) – cryogenically cooled systems that operate at temperatures 120°K – 150°K.
