154
A. Saha
Similar to the THz-TDS imaging system, challenges are faced for real-time THz
spectroscopy and conductivity imaging. To acquire images faster, a 2D array detector
is required, and also by using a fast optical delay line, it is required to obtain the
spectrum rapidly. Again, for operation in real-time problems may occur as to find
out the complex refractive index, iterative algorithms are needed. But sometimes
solutions can be obtained analytically.
3.3 THz Pulsed Imaging
In this technique, the times required by the THz pulses to arrive at the detector are
measured. This is also named as time-of-flight imaging (TOF). In fact, the time delay
among the reflected pulses helps to understand the sample internal structure. Here,
the measurement is performed using reflection geometry.
Initially, THz pulse imaging was demonstrated by capturing image of the internal
structure of a 3.5 in. floppy disk [51]. In this technique, the time delay of the pulse
t is directly related to the optical path travelled by the pulse. It is considered that
the refractive index n does not change over the entire THz frequency range under
consideration. In such a case, the distance d between the sample surface and the
sample internal surface can be calculated by measuring the time delay between the
pulses reflected from the sample surface and the sample internal surface, respectively.
The distance can be calculated from the following expression:
d =
ct
2n
(17)
THz pulse imaging technique has practical applications in various fields like in
art conservation, pharmaceutical industry, etc. to name a few. As a non-destructive
method in the field of art conservation, THz pulsed imaging has been used to study
the layers of paint in ancient masterpieces [52–56]. By using THz imaging technique, the hidden information from different paintings can be investigated. By X-ray
imaging technique also the same investigation is possible, but as X-ray has ionizing
property, it may destroy layers of paint and also radiometric dating may be compromised by this. Additionally, it is to be noted that THz pulsed imaging technique
provides supplementary information like the paint thickness, whereas X-ray basically investigates the paint losses. Another very important application of TPI has
been demonstrated in the pharmaceutical industry [57–60]. Since most of the dry
tablets are semi-transparent to THz radiation, it is possible to take images of the
tablets up to high penetration depths. This proves to be very beneficial in the quality
control of the tablets.
However, for real-time imaging, several things are to be taken care of. In this technique, optical delay line is a very important component as the TPI method detects the
reflected echoes generated in the temporal data of the THz-TDS system. Mechanical
A. Saha
Similar to the THz-TDS imaging system, challenges are faced for real-time THz
spectroscopy and conductivity imaging. To acquire images faster, a 2D array detector
is required, and also by using a fast optical delay line, it is required to obtain the
spectrum rapidly. Again, for operation in real-time problems may occur as to find
out the complex refractive index, iterative algorithms are needed. But sometimes
solutions can be obtained analytically.
3.3 THz Pulsed Imaging
In this technique, the times required by the THz pulses to arrive at the detector are
measured. This is also named as time-of-flight imaging (TOF). In fact, the time delay
among the reflected pulses helps to understand the sample internal structure. Here,
the measurement is performed using reflection geometry.
Initially, THz pulse imaging was demonstrated by capturing image of the internal
structure of a 3.5 in. floppy disk [51]. In this technique, the time delay of the pulse
t is directly related to the optical path travelled by the pulse. It is considered that
the refractive index n does not change over the entire THz frequency range under
consideration. In such a case, the distance d between the sample surface and the
sample internal surface can be calculated by measuring the time delay between the
pulses reflected from the sample surface and the sample internal surface, respectively.
The distance can be calculated from the following expression:
d =
ct
2n
(17)
THz pulse imaging technique has practical applications in various fields like in
art conservation, pharmaceutical industry, etc. to name a few. As a non-destructive
method in the field of art conservation, THz pulsed imaging has been used to study
the layers of paint in ancient masterpieces [52–56]. By using THz imaging technique, the hidden information from different paintings can be investigated. By X-ray
imaging technique also the same investigation is possible, but as X-ray has ionizing
property, it may destroy layers of paint and also radiometric dating may be compromised by this. Additionally, it is to be noted that THz pulsed imaging technique
provides supplementary information like the paint thickness, whereas X-ray basically investigates the paint losses. Another very important application of TPI has
been demonstrated in the pharmaceutical industry [57–60]. Since most of the dry
tablets are semi-transparent to THz radiation, it is possible to take images of the
tablets up to high penetration depths. This proves to be very beneficial in the quality
control of the tablets.
However, for real-time imaging, several things are to be taken care of. In this technique, optical delay line is a very important component as the TPI method detects the
reflected echoes generated in the temporal data of the THz-TDS system. Mechanical