 Low image acquisition speeds may force a tradeoff between time and resolution.Terahertz imaging can be useful for luggage and postal mail screening because it can identify substances on the basis of their characteristic spectra in this frequency band, such as explosives and illicit drugs; for example, several liquid explosives can be distinguished by the change in dielectric response in the terahertz range as a function of alcohol percentage.Although dangerous metal objects, such as knives, can be recognized by their shapes through certain pattern recognition algorithms, it is impossible to see through metallic packages with terahertz waves.