The aim of this European SMT (Standards, Measurements and Testing) project was the development of testing methods for measuring the ageing and the condensation behaviour of plastic films. For the ageing task, the degradation rates of the most important physical, optical and mechanical properties of the film were measured. Amongst others, harmonisation had to be achieved on measuring the optical properties of plastic films. For this purpose identical samples of four different co-polymer films and also special antifog films were measured by the partners. Measured were the direct and diffusive light transmittance, haze, the UV- transmittance and the IR-transmittance (longwave). Harmonisation has been achieved on the terminology, methods, set-up, repetitions, wavelength bands, etc. of the optical properties of greenhouse plastic films.