1. Infrared: from source to detector

First of all, let's clarify what we mean by "infrared". The definition of this spectral range can vary widely from one scientific community to another. In this article, we focus on wavelengths greater than 1 μm, i.e. beyond the sensitivity range of silicon, and limit ourselves to 14 μm as an upper limit. This spectral range is subdivided into several atmospheric transmission windows:

• SWIR (Short-Wave InfraRed) between 1 and 2.5 μm ;

• MWIR (Mid-Wave InfraRed) between 3 and 5 μm ;

• LWIR (Long-Wave InfraRed) between 8 and 14 μm (figure 1 ).