The only principle useable for stand-off detection of CWA is absorption of optical radiation in far-infra-red (FAR IR) part of optical spectra. Absorption is caused by molecules of the CWA.
Reasons:
All CWAs have strong absorptions bands inside FAR IR part of the spectra;
Atmosphere is “clear” inside the FAR IR, thus enabling long range of such sensors.
Consequences:
Detection of CWA is possible only on condition the agent is present in the air in the vapour form (droplets in the air and liquid on the ground do not contribute directly to selective optical absorption);
Physical properties of CWA vapour cloud and terrain will limit the detection capability of any SoD system.