Patent ID: 7145145

Claim:
An optical window that can be secured in a wall of a material-processing apparatus to derive information about the stage of processing of the material in the apparatus, wherein the optical window comprises: a radiation-transmitting element that is capable of transmitting radiation along an axis into the apparatus and for transmitting radiation along the axis out of the apparatus, the light emanating from the material being processed, whereby radiation transmitted out of the apparatus can be analysed to measure the progress of the processing of the material within the apparatus, a moveable body carrying the radiation-transmitting element, the body being moveable between a first position in which the element can transmit radiation into and out of the apparatus and a second position in which the element can be cleaned; a cleaning chamber that, when the moveable body is in the second position, lies adjacent to the element whereby the element can be cleaned by contacting with a fluid in the cleaning chamber.