Terahertz NDE for metallic surface roughness evaluation

Metallic surface roughness in a nominally smooth su rface is a potential indication of material degrada tion or damage. When the surface is coated or covered with an opaqu e dielectric material, such as paint or insulation, then inspecting for surface changes becomes almost impossible. Terahertz NDE is a method capable of penetrating the coatin g and inspecting the metallic surface. The terahertz freq uency regime is between 100 GHz and 10 THz and has a free space wavelength of 300 micrometers at 1 THz. Pulsed ter ahertz radiation, can be generated and detected usi ng optical excitation of biased semiconductors with femtosecon d laser pulses. The resulting time domain signal is 320 picoseconds in duration. In this application, samples are inspe cted with a commercial terahertz NDE system that scans the sample and generates a set of time-domain signals that are a function of the signal reflected from the metall ic surface. Post processing is then performed in the time and freque ncy domains to generate C-scan type images that show scattering effects due to surface non-uniformity.