[1] Tomography using terahertz radiation can image samples that are opaque in the visible and near-infrared regions of the spectrum.
[citation needed] Terahertz time domain systems (THz-tds) have made significant advances in 2D imaging.
Terahertz imaging can be useful for luggage and postal mail screening because it can identify substances on the basis of their characteristic spectra in this frequency band, such as explosives and illicit drugs;[4][5][6][7][8][9][10][11][12][13][14] for example, several liquid explosives can be distinguished by the change in dielectric response in the terahertz range as a function of alcohol percentage.
It mainly studies the establishment of process models such as refraction, reflection and diffraction when terahertz waves transmit samples, which has certain requirements for reconstruction algorithms.
In diffraction tomography, the detection beam interacts with the target and uses the resulting scattered waves to build a 3D image of the sample.
However, terahertz fault synthetic tomography is not affected by refraction and reflection because of the small incidence angle during projection.
The depth distribution information of the refractive index can be obtained by analyzing the time delay of the peak value of the reflected pulse.
In 2009, J.Takayanagi et al. designed an experimental system that successfully used tomography on a semiconductor sample consisting of three sheets of superimposed paper and a thin two-micron thick layer of GaAs.
[39] However, it remains a challenge to obtain high quality images with this technique due to scattering and diffraction effects required for measurement.
The high order scattering measurement usually results in poor signal to noise ratio (SNR).
Synthetic aperture processing (SA) differs from traditional imaging systems when collecting data.
Terahertz computed tomography records both amplitude and spectral phase information when compared to X-ray imaging.