Fraunhofer Institute for Industrial Mathematics ITWMTerahertz waves can penetrate many materials such as plastic, ceramics, textiles and wood and are therefore ideal for non-destructive testing of objects. Terahertz imaging – similar to X-ray and ultrasound testing – is used to generate images of the inside of the object. In this way, manufacturing-related defects or weak points in the object can be detected at an early stage during production. Terahertz technology therefore makes an important contribution to industrial quality control and conserves resources.
Terahertz Imaging
Terahertz radar can be used to test materials for defects and internal structures in a non-contact and non-destructive manner. Pictured: Cylindrical sample being examined for internal defects using terahertz waves. In combination with a rotating stage, tomography data can be generated.
© Fraunhofer ITWM / Thomas Brenner
Keramik-Messung mit Terahertz
Making the Invisible Visible
By scanning the object to be measured, defects in the internal structures can be made visible.
© Fraunhofer ITWM
Terahertz Imaging
Image Generation
Active terahertz imaging is generally used for non-destructive testing, in which the test specimen is actively irradiated and the reflected or transmitted radiation is analyzed. This can be implemented using various concepts:
Mechanically Scanning Transmitters and Receivers
Most coherent terahertz detectors – i.e. radiation receivers that detect both the amplitude and the phase of the measurement signal – are point sensors. To generate an image with this type of sensor, an object must be detected in a grid pattern. Either the transmitter and receiver are moved relative to the object or the object is moved when the sensors are stationary. The measurements can be carried out in both transmission and reflection. In contrast to the transmission image, measurements in reflection provide a depth-resolved image of the object under investigation. It should be noted that the images in the beam direction are distorted due to the different transit time, as the refractive index in the object is not equal to one (=air).
Fast Volume Inspection through the MIMO Terahertz System
Array of Coherent Transmitters and Receivers
Arrays of coherent terahertz transmitters and receivers usually work on the basis of the MIMO principle (multiple-input-multiple-output). Here, the transmitters are usually switched individually while all detectors are always active and record the amplitude and phase of the signals reflected by the target. In this way, complete, three-dimensional terahertz images can be reconstructed by evaluating the signals from all transmitter-receiver combinations.
With this MIMO arrangement of eight transmitters and eight receivers, an approximately 50 cm wide, linear line is realized. If the sample moves relative to this line, the image quality can also be improved by using SAR algorithms for image reconstruction.
Combination of Terahertz Source and Flat-Panel Detector
Flat-panel detectors in the terahertz range currently only exist in isolated cases and as pure power detectors, i.e. these detectors only detect the incident power but not the phase of the incoming wave. This means that no information on the signal propagation time is available. This concept is mainly implemented in simple transmission measurements.
Lateral Versus Depth Resolution
In terahertz imaging, a distinction must be made between lateral resolution and depth resolution.
- The lateral resolution is primarily determined by the wavelength used. In the frequency range commonly used, the wavelength ranges from 3 mm (100 GHz) to 50 µm (6 THz). Other factors that determine the lateral resolution are the optics and imaging concepts used.
- The depth resolution is primarily determined by the technology used (TDS, FMCW or MIMO). These techniques allow a time-of-flight measurement of the terahertz signals and thus a depth-resolved recording. Depending on the recording technique, the depth resolution is between 10 µm and several millimeters.