2. Atmospheric transmission

The Earth's atmosphere is not transparent at all wavelengths. The transmission of optical radiation through the atmosphere depends on two essential phenomena:

• the absorption of gaseous constituents of the atmosphere, in particular water (H ₂ O) and carbon dioxide (CO ₂ ) ;

• absorption by diffusion due to the presence of the particles it contains, whether molecules or aerosols.

When radiation passes through the atmosphere, absorption bands appear due to the presence of water vapour (figure 2 ). Water vapor is...