Abstract: The invention pertains to a process for determining the dye uptake of polyethylene terephthalate fibers in which the fibers are irradiated with high-intensity monochromatic light. A light-sensitive sensor is used to measure the Raman scattering and compare it with that of an earlier established model on the basis of Raman spectra of polyethylene terephthalate fibers of known composition and structure. A model is used to calculate the density, which constitutes a quantitative measure of the dye uptake. In the calculation use is made of measurement points from a number of regions in the spectral measuring region of 598 to 1900 cm−1.

Abstract: In a Fabry-Perot etalon or interferometer the metal mirrors are each provided with a dielectric coating on the side facing away from the enclosed gap. The dielectric coating leads to the advantage of peak transmittance being improved without the finesse being adversely affected. The dielectric coating is transparent for the wavelength (.lambda.) used and, preferably, has a thickness of about 0.25.lambda./n, n being the refractive index of the dielectric coating material. The coating, preferably, is of a material having a high index of refraction, examples thereof including zirconium dioxide, titanium dioxide, zinc sulfide, or zinc selenide.