Optical instrument comprises a first tube (12) containing a biosensor (8), a second tube (13) and a third tube (14) holding a lens (7). The first tube has a first open end, a first hermetically sealed end and an inner peripheral surface covered by a metal. The second tube has a second open end, a second end hermetically sealed with a covering lens (6) and an outer peripheral surface covered with metal. The third tube has an inner peripheral surface and is inserted into the second tube. Preferred Features: The inner peripheral surface of the first tube has a gold surface layer. The first tube is covered with metal over its whole surface.
