Dear all, thanks for your information. I am actually not interested in optical constants per absorption peak, but in Black-body spectrum averaged values. So averaged over the Planck spectrum (say at 60 degC). I already have the article "Comparative evaluation of the infrared transmission of polymer films" of P.T. Tsilingiris, but his thinnest layer is 0.1 mm and I am interested in layers with lower thicknesses. The work of "SOLAR OPTICAL AND INFRARED RADIATIVE PROPERTIES OF TRANSPARENT POLYMER FILMS" of Gernot Wallner considers layers below 0.1mm and shows that the average optical constant for IR increases with decreasing layer thickness, but this one does not contain polyethylene. So, I am missing the IR averaged optical constant for layers