at a wavelength of about 450 nm the initial value for curve 52 is significantly below that of the other curves and it is not possible to identify a plateau with constant transmittance this characteristic of the comparative specimen from the prior art curve 52 is also discernible from the yellowish brown colour of this specimen clearly caused by the poorer transmittance specifically in the blue region of the spectrum its r a value is 96 2 whereas the r a values of both of the specimens composed of the purer plastic of thickness 2 mm and 4 mm curves 50 and 51 are 99 6 good r a values begin from 97 particularly preferably from 98 and very particularly preferably from 99 irrespective of the plastics present the yellowness index of the comparative specimen curve 52 is 13 0 indicating marked yellowing and that of the specimens composed of the pure plastic of thickness 2 mm curve 50 is only 0 8 mm and at thickness of 4 mm curve 51 still only 1 1 these low yellowness indexes for the specimens composed of the pure plastic are surprising and serve to emphasize that these materials have good suitability for the production of the optical elements of the invention table 1 lists the transmittance values measured for the abovementioned specimens at a wavelength interval of.