5. Measuring principle

This gas detection system uses the scene background as an infrared source, and an image processing algorithm highlights the presence of any gas clouds on the line of sight. The quantity of gas present is assessed by "double differential" processing of the infrared images, differential in the spatial and spectral domains.

• Spatial differentiation

  A simple radiometric measurement cannot directly "see" a cloud superimposed on a scene if both are at the same temperature. In fact, let's pose the simplified equation for the luminance observed when looking at the scene background through the cloud: $\begin{array}{l}L ={\displaystyle \int \left\{L_{\text{CN}}\left(T_{\text{F}}\right)\cdot {\tau }_{\text{N}}+L_{\text{CN}}\left(T_{\text{N}}\right)\cdot {\epsilon }_{\text{N}}\right\}\text{d}\lambda } \\ =\end{array}$