Use in optical systems as liquids to transmit light at wave lengths lower than 250 nm, in particular at 157 nm, of perfluoropolyethers having the following formula: X1—O(CF2O)n(CF2CF2O)m(CF2CF2CF2O)p (CF2CF2CF2CF2O)q—X2  (I) wherein: the repeating units —CF2O—, —CF2CF2O—, —CF2CF2CF2O—, —CF2CF2CF2CF2O— are statistically distributed along the chain; —X1 and —X2 are perfluoroalkyl chain end groups equal to or different from each other, having formula —(CF2)zCF3 wherein z is 0 or an integer from 1 to 3; n, m, p, q are 0 or integers comprised between 1 and 100, with the proviso that: at least one among n, m, p and q is different from zero; the ratio between the oxygen atoms and the carbon atoms (O/C) is higher than 0.33; wherein the chlorine atoms, determined by 19F-NMR are substantially absent.