Basic of stand-off detection

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.

Fig. 1. Passive Stand-off Detector is using background emitted infra-red radiation (like thermal camera). The larger is detector’s instantaneous field of view, the higher optical power is collected and system’s operation is less affected by noise.

Fig. 1. Passive Stand-off Detector is using background emitted infra-red radiation (like thermal camera). The larger is detector’s instantaneous field of view, the higher optical power is collected and system’s operation is less affected by noise.

 

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.  

Fig. 2. Active Stand-off Detector is using infra-red radiation generated by built-in lasers; therefore, the detector’s instantaneous field of view can be very small. Lasers generate enough of power so that range of the detector can be very large and operation is not affected by noise.

Fig. 2. Active Stand-off Detector is using infra-red radiation generated by built-in lasers; therefore, the detector’s instantaneous field of view can be very small. Lasers generate enough of power so that range of the detector can be very large and operation is not affected by noise.

 

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.