Optical parameters of the DLCH films were obtained in the wide spectral range. Dielectric response in UV–VIS range is mainly related to electronic transitions and, therefore, provides information about σ and π electrons. Although the fitting with the PJDOS model gives parameters directly related to the band structure of DLCH their values are not quite reliable if the optical data have not been measured up to VUV and XUV energies . The band gap related to π to π* transitions is one of the parameters which value can be worth noting. The CH83 and CH87 films exhibited almost identical values, 1.14 eV and 1.15 eV, respectively. Since optical data in VUV/XUV range were not available other fitting parameters are not reported. The optical parameters of the films, refractive index and 17 alpha-propionate coefficient, were correctly determined in the spectral range covered by available experimental data and they are compared in Fig. 5a. The extinction coefficient of both the DLCH films was very similar whereas higher refractive index, n = 1.88 at 615 nm, was obtained for the film CH87 as compared to n = 1.81 for CH83. It was already reported that higher refractive index of a DLCH film is related to a higher hardness . This relation is also supported by our results because the hardness of CH87 was 17.8 GPa whereas only 15.7 GPa was determined for CH83 (Table 3). Another correlation between optical and mechanical properties was made by Schneider et al.  linking the extinction coefficient with the Young's modulus. The prepared DLCH films presented very similar extinction coefficient which was compared with the measured Young's modulus (91 GPa for CH83 and 105 GPa for CH87). This small difference of modulus was observed as a slight increase of the CH83 extinction coefficient between 250 and 600 nm.