This system is based on submillimeter wave imaging using frequency modulated continuous wave (FMCW) radar technology. We can do imaging at 100 GHz and 300 GHz in both transmission and reflexion.

Description of the FMCW system.

The millimeter wave imaging setup is an all-electronic system able to measure signal in both transmission and reflection. The system has two frequency ranges, from 75 to 110 GHz and from 220 to 330 GHz. Four heads compose the system: two both emitting and receiving and two only receiving signals. The output power is about 2 mWat 100 GHz and 60 μW at 300 GHz. The dynamic range is 70 dB at 100 GHz and 60 dB at 300 GHz. The image is produced by moving the head in a line with a maximum distance of 800 mm. Thus, the sample is raster scanned and the main advantage is the very large scanned area about 600 × 800 mm. The measurement time of one pixel is 240 μs, and a whole scan with best resolution lasts about 25 min. Each head works as a profilometer using frequency-modulated continuous wave to obtain a profile. The spatial resolution is about 5 mm in typical materials at 100 GHz and 2 mm at 300 GHz. 

Photography of the millimeter wave FMCW radar imaging system

Applications of FMCW imaging

FMCW is useful for several applications like non destructive test of composite material, art painting, security, etc.

Non destructive test (NDT)

Composite materials like Glass Fiber Reinforced Polymer (GFRP) can be tested in order to detect and localize cracks, voids and several kind of defects.

Security

This system can be used to see inside parcels or bags.

FMCW image of a bag with two guns.
FMCW image of a bag with two guns.

Art painting

An oil painting was analysed using this system. You can see on figure 4(a) the visible photography of the oil painting. Then, figure 4(b) is the transmission image at 300 GHz. Then, figure 4(c) is the reflection image at 100 GHz and figure 4(d) is the reflection image at 300 GHz. We can detect invisible defects with this technology, like voids between the painting layer and the canvas.

J-P Guillet, M. Roux, K. Wang, X. Ma, F. Fauquet, H. Balacey, B. Recur, F. Darracq and P. Mounaix (2017). Art Painting Diagnostic Before Restoration with Terahertz and Millimeter Waves. Journal of Infrared, Millimeter, and Terahertz Waves, 38(4), 369-379., DOI 10.1007/s10762-017-0358-1 Link to paper

Old oil painting analysed with terahertz waves and FMCW