Lateral Versus Axial Resolution
In terahertz imaging, lateral and axial resolution must be distinguished. Lateral resolution is primarily dictated by the wavelength used. Due to the frequency range used, the wavelength 3 mm (100 GHz) ranges from 100 µm (3 THz). Other factors that determine lateral resolution are the optics used and the imaging concept employed. The axial resolution is determined by the imaging technique used (TDS, FMCW or MIMO). These techniques allow time-of-flight measurement and thus depth-resolved imaging. The axial resolution here ranges from 10 µm to several mm, depending on the imaging technique.
Active terahertz imaging for non-destructive testing can be implemented with different concepts:
- Combination of terahertz source and planar detector
- Array on coherent emitter and receiver
- Mechanically scanning transmitters and receivers
Area detectors in the terahertz range are currently only power detectors, i.e. these detectors only detect the incident power, but no phase of the incoming wave. Thus, no information on propagation time and the like is accessible. This concept is mainly implemented in transmission.
Typically, the arrays operate on coherent transmitters and receivers based on the MIMO (Multiple-Input -Multiple-Output) principle. Usually, the transmitters are switched individually and the detectors are always active, capturing the amplitude and phase of the signals reflected from the target. In this way the images are reconstructed. Here, the measurements are preferably made in reflection.
Most terahertz detectors are point sensors. To generate an image with this type of sensor, an object must be detected in a grid pattern. Here, either the transmitter and receiver are moved relative to the object, or the object is moved if the sensor is stationary. Here, measurements are made in both transmission or reflection.
While the first method only allows a transmitted image, the two methods create a depth-resolved image of the object under investigation. Therefore, this is often referred to as tomography or 3D imaging – which is not quite correct. Since the refractive index in the object is not equal to one (=air), the images are distorted in the beam direction due to the different travel time.