The technological development utilizing the terahertz wave has been vigorously in progress in recent years. Technologies that utilize the terahertz wave and exploit the physical property thereof that the terahertz wave can be transmitted through various substances are expected to find promising applications particularly in the field of imaging.
Japanese Patent Registration No. 03387721 discloses a technique relating to a method of visualizing the inside of an object such as hand-carry baggage or some other personal belonging, using the terahertz wave. According to the above-cited patent document, a transparent image of an object can be obtained by irradiating a pulse-shaped terahertz wave onto a point of the object, detecting the terahertz wave transmitted through the object and shifting the object.
Methods of examining a sealed letter have also been proposed besides the techniques of examining hand-carry baggage. Japanese Patent Application Laid-Open No. 2004-286716 discloses a method of detecting a prohibited drug such as a narcotic hidden in a sealed letter. This method utilizes the absorption spectrum that the prohibited drug such as a narcotic characteristically shows relative to the terahertz wave to identify the prohibited drug in the inside without opening the sealed letter.