While the Europe-wide use of full body scanners at airports is still being very controversially discussed because of gross infringements into the privacy sphere, the Institute for Photonic Technologies (IPHT) in Jena is currently developing a terahertz security camera that can detect potentially dangerous objects, without recording anatomical details. 

Full body image, produced with the prototype developed at the Institute for Photonic Technologies (IPHT). Copyright: IPHT

They are already in use at some European airports. These, "nude scanners", are appearing in many places as the method of choice for uncovering terrorists. They are scanning devices that can literally look under peoples' clothes, for the most part with the use of low intensity narrowband millimeter wave radiation. This makes it possible, of course, to discover metallic objects, such as pistols or other weapons, but detailed naked pictures of the person can also be produced - in photo quality.

The prototype developed at the Institute for Photonic Technologies (IPHT) in Jena differs from these devices in two ways: Firstly it is not based on microwave technology, which for the most part is still in the gigahertz range - the researchers work in the infrared range with frequencies in the terahertz range instead. In this respect the Jena researchers' detector does not actively throw beams at a person, but rather passively records what the person radiates. The benefit: Unlike illuminated images, passively created images are free of all shadows, and therefore "blind" to anatomical details. Furthermore there is also no danger to health, because of the passive imaging mode.

The Institute's researchers have succeeded with their method in resolving a problem that emerged previously with the development of terahertz cameras. Even in the infrared range a body's own emissions are relatively weak. With terahertz frequencies these are one dimension weaker again, and can no longer be detected by conventional detectors, as they are lost in the background noise.

The researchers found a way out of this dilemma by developing ultra-sensitive, cooled detectors on a bolometric basis. Bolometers are among the most sensitive radiation detectors, and measure the temperature increase generated by an incident radiation in an absorber volume. Because the bolometer developed at the IPHT works at very low temperatures close to absolute zero, the thermal noise can be limited. The researchers applied the most modern superconducting electronics for temperature measurement. The absorber for the terahertz (THz) waves consists of small, lithographically produced dipole antennas. The scientists use a molybdenum superconductor for measuring temperatures.

The detector is read with an extremely low-noise amplifier on superconductor basis. In combination with a computer-controlled data recording, the detector produced at the IPHT enables the detection of a very small radiant power of approx. 10-16 watts. As the human body's own emissions in the THz range are around 10-14 watts, this detector can generate a purely passive THz image of the human body.

As a demonstrator for the primary functionality of a THz camera, the Jena researchers have since constructed a prototype with a 7-pixel receiver array and a mirror system for a passive image of objects from a distance of five meters. The mirror system is oriented here on telescopic forerunners used in radio astronomy.

The scientists still have a whole series of tasks ahead of them before a THz camera is ready. For one thing the recorded image section must be enlarged and the number of pixels must be increased. And the image recording speed must also be higher.  They plan to reach 25 shots a second by 2010.