34
of blocked pixels, and two lenses. The THz beam, after passing through the object
mask and the random pattern, is collected at the receiver. The positions of the screen,
the focusing lens and the receiver along the beam path are calculated according to
the lens law, in order to efficiently focus the THz beam onto the receiver antenna.
The object mask is made of (opaque) copper tape on a transparent plastic plate. In
this paper, our object mask has a hole shaped as a Chinese character which means
“light”, 1.5 cm both by height and width, as shown in Figure 4.2(a). The screens used
for this experiment are a set of four hundred random patterns printed in copper on
standard printed-circuit boards (PCBs). Printed on a uniform grid on the PCBs, each
pattern contains 32 × 32 pixels. The size of each pixel is Imm × lmm.' A “copper”
pixel corresponds to pixel value zero on the random pattern, while a pixel without
copper corresponds to the value one, since the PCB material is fairly transparent
to the THz beam. Each PCB moves on an automatic tranlation stage to change
from one random pattern to another to ensure accurate alignment. For each random
pattern, the system acquires one THz waveform, consisting of the superposition of
the radiation transmitted through all of the un-metallized pixels.
In CS, every row of the measurement matrix Φ (that is, every random pattern)
is used to form only one measurement, consisting of a complete time-domain wave-
form. In amplitude-only imaging, for each random pattern, the magnitude of the
detected THz waveform at a particular frequency is extracted to obtain one CS mea-
surement. For complex or phase imaging, the real and imaginary components of the
THz waveform at the selected frequency form one CS measurement.