Measuring the thickness of clear freshwater ice using geometric optics and a spectrometer

An instrument1 which exploits geometric optics was devised to measure the thickness of slabs of optically transparent material such as Plexiglas or freshwater ice using a high-intensity light source. The apparatus consisting of a prism, a micrometer single-axis translation stage, and an optical spectrometer measures the spatial span between two chosen Fraunhofer line wavelengths in the prism-refracted visible spectrum reflected through the test slab. This span was found to be proportional to slab thickness. Measurements were made on various thicknesses of Plexiglas sheets ranging from 8 to 49 mm thick as well as nearly-bubble-free macrocrystalline freshwater ice in the thicknesses ranging from 28 to 53 mm thick. Results indicated a linear relationship between spectral span and slab thickness. This device may be suitable for detecting and measuring point-location ice thickness on aircraft wings, as well as for other industrial and research purposes.