Optical transmission of amorphous polyimidazopyrrolone foils (pyrrons) were investigated in the spectral range 200-3300nm. The investigated materials exhibit a high transmission level (up to 85%) in a wide range from about 500 to 2750nm. The short wavelength edge of transmission depends on the pyrron chemical structure being different for ether-, amide- and ester bridges. When analysing the experimental data, we applied the approach used for amorphous materials in order to determine the electronic structure parameters which may be related to the influence of polyimidazopyrrolone bridge structure and structural disorder on the optical properties and probable electronic transitions.