# The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging
## 2 Advantages of the Gyrotrons as Powerful Radiation Sources for Sensing and Imaging
### Active Thermal Imaging Using Gyrotrons  
Another technique that benefits from the high-output power of the sub-THz gyrotrons is the long-range sensing based on active thermal imaging [99; 100]. Its principle is grounded on the fact that when beamed on the target, the millimeter-wave gyrotron radiation generates rapid transient temperature increases in different portions of the irradiated area. The time-dependent thermal field is registered using sensitive infrared (IR) imagers. In principle, this concept can be used in many situations where passive infrared imaging is currently used. The preliminary laboratory proof-of-principle experiments have demonstrated its feasibility [100]. In the measurements, an 83 GHz gyrotron with an output power of 20 kW has been used to rapidly heat various simple and complex targets. Thermal imaging with a sensitive mid-wavelength IR camera reveals clear signatures in a variety of objects illuminated at power levels of 50-200 W over an area of approximately 100 cm\({}^{2}\). In these experiments, the target was located 1.6 m from the source, while the IR camera was located 1.2 m from the target. A variety of objects and configurations, including obscured metallic and dielectric specimens buried in sand, and covered by several layers of cloth have been detected successfully. Since temperature differences of a few hundredths of a degree can be detected, temperature changes are often visible almost immediately after the irradiation [97]. Among the anticipated potential applications is a long-range detection of explosive devices. The comparison of similar systems for active thermal imaging that use a variety of heating sources (e.g., lasers, flashlamps, and longer wavelength microwaves) shows that the millimeter-length waves provided by the sub-THz gyrotrons are particularly well suited for long-range sensing.  
Figure 9: (**a**) Gyrotron-based active real-time imaging system for food inspection; (**b**) shadow images captured from the video taken by the line array detectors placed beneath the conveyor belt. Images courtesy of [PERSON].  
A High-Sensitivity Technique for Time-Resolved Imaging and Measurement of 2D Intensity Profiles of Millimeter-Wave Radiation  
Recently, a novel high-sensitivity time-resolved method for imaging and measuring the spatial distribution of the intensity of millimeter waves by using visible continuum emitted by the positive column of a DC gas discharge in a mixture of cesium vapor with xenon has been proposed and demonstrated experimentally [101; 102]. This imaging technique can be used for measuring the parameters of moderate-power radiation generated by various sources of millimeter waves and has been applied to the identification of the operating mode of a W-band gyrotron with a pulsed magnet as well as to the evaluation of the relative powers of some spurious modes. It has been shown also that this method can be applied to real-time imaging and non-destructive testing with a frame rate higher than 10 fps. In the experiments, two-dimensional shadow projection images of objects opaque and transparent to millimeter waves have been obtained irradiating the studied objects with pulsed watt-level millimeter waves. Moreover, it has been demonstrated that this particular type of shadowgraphy can be used for both single-shot screening (e.g., detection of concealed objects) and time-resolved imaging of time-dependent processes.