# The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging
## 2 Advantages of the Gyrotrons as Powerful Radiation Sources for Sensing and Imaging
### Imaging, Food Inspection, and Quality Control Using Gyrotron Radiation  
Many industrial processes, most notably food production and processing due to their importance, the large volume of products, numerous serious safety, and quality control concerns could benefit enormously from non-destructive screening and inspection. Such a perspective has stimulated the development of various novel THz technologies [89; 90; 91] that avoid the usage of ionizing radiation (X-rays), which has a detrimental effect on the living matter. However, most of the techniques utilize broadband THz radiation (e.g., TDS (time-domain spectroscopy) systems) or laser light of low power  
Figure 8: Schematics of the remote detection of radioactive material hidden in a container. and higher frequencies that, respectively, have small penetration depth. In this respect, the gyrotron radiation (with its sub-THz and THz frequencies and several orders of magnitude higher output powers) offers complementary/alternative solutions. Although these advantages have not been fully realized so far, a series of recent investigations have demonstrated the potential of the systems for food inspection and control based on imaging with gyrotrons [12; 92; 93; 94; 95; 96]. Generally, there are two possible arrangements of THz imaging optics, namely mirror-based and lens-based [97]. An example of a system of the first kind has been proposed in [12]. It consists of a gyrotron (with an output power of about 100 W at 200 GHz and 1 W at 400 GHz, respectively), an off-axis parabolic mirror (OAP), a cylindrical parabolic mirror (CPM), a flat reflector, a normal parabolic mirror, and a pyro detector used as a THz camera. The wave beam of the gyrotron is converted into a parallel one by the OAP and then by the CPM into a line beam, which is suitable for illuminating the objects placed on a conveyor belt. The reflective imaging system comprises a flat reflector and a parabolic mirror. The experiments carried out with this system have demonstrated the feasibility of fast THz imaging for the inspection of foreign substances in food. In particular, metal and rubber have been detected at a resolution of 0.8 and 1 mm, respectively for pieces with sizes ranging from 2 to 10 mm. In a series of works by [PERSON] et al. (see, e.g., [92; 93; 94; 95; 96]) the capabilities of a non-invasive real-time imaging system that uses a gyrotron as a radiation source have been investigated in detail and demonstrated successfully. For instance, in [92], an active real-time imaging system employing a CW 460-GHz gyrotron and a pyroelectric array camera with \(124\times 124\) pixels has been presented. It can be operated in both transmission and reflection setups. The high output power of the gyrotron (16 Watts in CW operation with a 13-kV 100-mA electron beam) allows overcoming effectively the limit of the sensitivity of the array detectors at room temperature. The capabilities of this system for real-time security screening (e.g., standoff detection of concealed weapons) and identification of a foreign substance in visually opaque dry or frozen food coming out of a production line are demonstrated convincingly by images and real-time videos captured at a rate of 48 Hz (i.e., 48 frames per second) by using a built-in chopper of the camera [95]. It has been noted that by taking the videos at a lower rate and averaging over several frames, even clearer images can be obtained. This allows identifying properly the presence of objects placed inside mail envelopes using real-time videos with a running time of only several seconds. Another problem that stems from the low sensitivity of the detectors is the low signal-to-noise ratio, which leads to a poor contrast along the edges of the images. However, it has been shown that a workaround for enhancement of the contrast is to round the intensity fluctuations in several shots.