Aunik Rahman1, Kenneth Tator2, Anis Rahman1,*
1Applied Research & Photonics, Inc., 470 Friendship Road, Suite 10, Harrisburg, PA 17111
2KTA-Tator, Inc., 115 Technology Drive, Pittsburgh, PA 15275
Non-destructive terahertz reflection interferometry offers many advantages for sub-surface inspection such as interrogation of hidden defects and measurement of layers’ thicknesses. Here, we describe a terahertz reflection interferometry (TRI) technique for non-contact measurement of paint panels where the paint is comprised of different layers of primer, basecoat, topcoat and clearcoat. Terahertz interferograms were generated by reflection from different layers of paints on a metallic substrate. These interferograms’ peak spacing arising from the delay-time response of respective layers, allow one to model the thicknesses of the constituent layers. Interferograms generated at different incident angles show that the interferograms are more pronounced at certain angles than others. This “optimum” angle is also a function of different paint and substrate combinations. An automated angular scanning algorithm helps visualizing the evolution of the interferograms as a function of incident angle and also enables the identification of optimum reflection angle for a given paint-substrate combination. Additionally, scanning at different points on a substrate reveals that there are observable variations from one point to another of the same sample over its entire surface area. This ability may be used as a quality control tool for in-situ inspection in a production line.
Keywords: Terahertz reflective interferometry, Paint and coating layers, Non-destructive thickness determination, Subsurface
layer thickness measurement, Interferometry thickness modeling, Automated scanning and quality control.