1.1 The Infrared Spectral Region

The infrared (IR) spectral region roughly spans the wavelength range between 1 $\mu$m and 300 $\mu$m. It can be roughly subdivided into the near-IR (NIR, 1-5 $\mu$m), the mid-IR (MIR, 5-30 $\mu$m) and the far-IR (FIR, 30-300 $\mu$m) regions, with longer wavelengths referred to as the sub-millimetre (sub-mm, 300 $\mu$m-1 mm) and millimetre (mm) regions. Figure 1.1 shows the transmission of Earth's atmosphere at near- and mid-IR wavelengths. Water vapour and carbon dioxide are especially important absorbers throughout the IR, with water vapour making the atmosphere completely opaque over large wavelength ranges from almost all ground-based observing sites. Other absorbers, such as ozone, cause more limited (in wavelength) trouble, but can be important for specific scientific programs. In addition, the atmosphere emits molecular lines and a very broad thermal continuum. The thermal emission from the observer's astronomical telescope also contributes significantly to the thermal continuum emission at wavelengths longer than a few $\mu$m.