Terahertz technology improves 3-D tomography

MARCH 24--Terahertz imaging is finding applications in security scanners.
March 24, 2003
2 min read

MARCH 24--Terahertz imaging is finding applications in security scanners. It can detect metallic objects beneath clothing and potentially detect explosives or biological weapons by deriving high-resolution 3-D tomographic imaging with terahertz waves without the need to rotate the sample or surround it with a detector array. A single-projection imaging technique, outlined by S. Wang and X. C. Zhang from Rensselaer Polytechnic Institute (www.rpi.edu) in Applied Physics Letters 8, p. 1821, uses broadband terahertz pulses and a binary lens. It could prove useful for imaging objects that are large or located in a difficult location. Experiments carried out in the 0.5- to 2-THz frequency range suggest that the technique can capture image slices with a depth resolution and image-plane resolution of a few millimeters.

The method relies on the focusing properties of a binary lens, which is a Fresnel zone plate that uses a series of concentric ring structures to focus light. Unlike an ordinary refractive lens, the focal length of a binary lens exhibits a linear wavelength dependence. Using the lens with a multiple-frequency lighting source and a CCD camera can produce tomographic image slices of a 3-D object.

In experiments, the Rensselaer team used a 30-mm-diameter silicon binary lens with a focal length of 2.5 cm at 1 THz. Wang and Zhang generated images of a stack of three acrylic plastic rectangles (60 x 45 x 2 mm), each containing a different shaped aperture. The masks were placed at 3, 7, and 14 cm from the lens, and images were captured on a CCD camera at frequencies of 0.75, 1.24, and 1.57 THz.

At each frequency, the binary lens imaged a pattern corresponding to a certain depth, while images from other patterns remained blurred. The resolution in the image plane is about 1 mm, which is limited by diffraction.

Although the binary method is easy to implement and requires less computer processing power than other tomographic imaging techniques, it does have a shortcoming. Unlike other terahertz-imaging methods it does yield spectroscopic information about the sample.

Sign up for Vision Systems Design Newsletters
Get the latest news and updates.

Voice Your Opinion!

To join the conversation, and become an exclusive member of Vision Systems Design, create an account today!