An observation window for a microwave device exhibiting microwave radiation of a predetermined frequency, the observation window comprising two optically transparent panels to which an optically transparent conductive film has been applied to a single side thereof, each of the transparent conductive films primarily reflecting incident microwave radiation and being substantially parallel and spatially separated from each other by a predetermined distance, the predetermined distance being equal to an odd integer multiple of one quarter of the wavelength of the microwave radiation of the predetermined frequency in the interstice between the transparent films, the predetermined distance having a tolerance of plus or minus 0.15 of the wavelength in the interstice.