ダブプリズム(Daheng)

This type of prism is a half of a common right-angle prism with the right angle corner truncated parallel to the hypotenuse face, enters one of slope faces, has total internal reflection(TIR) on that face, and emerges from the other slope face parallel to its incident direction. Because of one reflection on the hypotenuse face, the image is flipped and inverted, but not laterally transposed. The emerging light is continued in the incident direction.

The most interested property of the Dove prism is that, if the prism is rotated about the incident axis through an angle, the image rotates twice of that angle. This makes it as a beam rotator which is widely used in optical instruments. A Dove prism must be used in parallel light or a collimated beam.