Objects in powders detected and imaged with THz radiation

Summary form only given. Detecting objects in powders is an important issue in industrial quality control, especially in food production. The range of the electromagnetic spectrum that can be employed for this purpose is limited, however. Radiation in and around the range of visible light cannot be used because the surfaces of all grains of a powder scatter the beam, so that transmission is very low. X-rays can be used, but it is hard to detect anything but metals. Radio waves are also transmitted, but due to their large wavelength they are not very sensitive to small objects. As a compromise between visible light and radio waves, THz radiation has a typical wavelength of 0.1 to 3 mm, which is larger than the grain size of most powders but smaller than the size of most objects to be detected. This makes THz radiation a very promising candidate for effectively detecting and even imaging small objects embedded in powders.