optical element replacement and operational downtime of the lamp due to maintenance fig5 shows the microwave excited hydrogen ultraviolet lamp structure used to explain the fifth example of the first preferred embodiment according to this invention wherein the removal of carbon adhering to an optical element located in vacuum zone 14 by using lamp light emissions will be described further elaboration of structural and operational elements that are similar to those of example 1 will be omitted the specifications of light transmitting window 8 were the same as those explained for example 1 in fig5 optical element 27 is positioned to receive the light emitted by lamp 9 further explanation of the optical element will be omitted since it is similar to that used in example 4 in example 5 to verify the effects the effectiveness of the invention the lamp was operated without a gas supply until the transmission rate of the interference level had declined 50 from an original value of 100 to intentionally degrade the transmission rate by half and then the optical element 27 was positioned within vacuum zone 14 the method of example 2 was used to supply water vapor to vacuum zone 14 under conditions where the partial pressure of the water vapor in vacuum zone 14 was maintained at 1 mtorr experiments were also conducted by adjusting the valve aperture to deliver 5 mtorr 2 mtorr 0 01 mtorr and 0 005 mtorr of water vapor partial pressure but as will be explained.