In this blogpost, we explain basic principles of stand-off detection. We also compare active and passive stand-off detection principles and discuss differences of the two approaches for stand-off detection of gases from distances without any physical contact with the detected gas (e.g. chemical warfare agent).
Principles of operation of stand-off detectors
Stand-off detection relies on absorption of optical radiation by molecules of a gas. The key difference between active and passive stand-off detection systems is utilization of own source of optical radiation in case of active stand-off detection system.
Figure 1. shows an illustrative examples of a passive stand-off detection system. Environment emits optical radiation. The emitted radiation passes via a gas, which causes absorption of a part of the optical radiation. Optical radiation that passes through the cloud is received by a passive stand-off detector. Finally, the detector evaluates the received optical radiation.
Figure 2. shows a diagram of operation of an active stand-off detector. The key differences is usage of a dedicated and controlled light source (e.g. CO2 laser). The active stand-off detector emits short pulses of light that passes thought a gases (e.g. chemical warfare agent). The transmitted optical pulse is reflected from the environment, passes through the cloud second time and finally is received by the active stand-off detector.
The key differences of active stand-off detectors compared to passive detectors are:
Dedicated and controlled source of optical radiation (e.g. CO2 laser)
Optical radiation passes via the cloud 2-times (stronger signature of the cloud on the received optical radiation)
The above mentioned differences results in better sensitivity and extended detection range of active stand-off detectors compared to passive stand-off detectors.