Optical system element, for receiving a pressurised functional fluid

An element of an optical system including:

• a first enclosure for receiving in circulation a functional fluid and
• at least one inlet and/or outlet window located on the first enclosure, and configured to allow a light beam to pass into and/or out of the first enclosure;
• wherein:
• the inlet and/or outlet window includes two viewports which laterally delimit a spacer cavity adjacent to the first enclosure,
• said spacer cavity is configured for receiving a second fluid, having a predetermined optical index, and is provided with a pressure adjusting device therewithin,
• the second fluid has an optical index substantially equal to that of one of the functional fluid and a ambient medium surrounding said element, within plus or minus 10%, and
• the pressure adjusting device is adapted for the pressure within the spacer cavity to be substantially equal to a pressure of the other of the functional fluid in the first enclosure and the ambient medium, within plus or minus 10%.