Due to the volumetric shrinkage of the resin and the induced residual stress during the curing process, the
reflection on the gel-coating layer surface will be imperfect if twists and wrinkles exist on the gel-coating surface. This
phenomenon is denoted as print-through phenomenon (PTP). Currently, the detection of PTP for most of the yacht
industry using the composite materials is performed mainly by visual inspection, and its quality is needed to be
quantified to determine their grades. Therefore, there is a need to develop a lightweight portable optical measurement
system that can be applied quickly to inspect different levels of PTP for the fiber-reinforced plastics (FRP) of the yacht
body. The measurement system was developed based on the scattering principle of a reflected laser fringe projected on
to the workpiece surface. Two indexes, namely the profile peak-valley height and wave-height of the Fast-Fourier
Transform based on the centerline of the extracted image profile, were proposed to quantify the PTP of a test specimen.
The mean line width of the extracted image was applied to evaluate the surface roughness of the test specimen, based on
the scattering theorem. A set of software programmed with Borland C++ Builder language was developed to calculate
the proposed indexes and the mean line width. The developed measurement system has been taken to some yacht
factories to do the on-site measurements. The measurement results were, in general, consistent with the surface
conditions of the polished surfaces.