An optical head comprises a fixed optical casing and a movable optical casing accommodating a deflecting mirror and an objective lens which moves separately from the deflecting mirror. Windows are inserted in the boundary between the interior and exterior of an optical path to cover a light-exposed area...http://www.google.de/patents/US5175717?utm_source=gb-gplus-sharePatent US5175717 - Optical head having a fixed optical casing and a movable optical casing
