An optical element including a first surface, a second surface, and a third surface, at least one of the first surface, the second surface, and the third surface being a curved surface, and at least one of the first surface, the second surface, and the third surface being a reflecting surface. Also included...http://www.google.de/patents/US6049429?utm_source=gb-gplus-sharePatent US6049429 - Optical element combination optical element and observation system using such optical elements
