It is composed by
two 30° dispersion prisms and a right angle prism.
When a beam incident in one of the two surfaces
(S1 & S2) at Brewster angle, then the beam
will output at Brewster angle too from another
surface, and beam is deviated 90 degree. It is
often used as a dispersion prism.
Polarizer 
30° dispersion prisms and a right angle prism.
When a beam incident in one of the two surfaces
(S1 & S2) at Brewster angle, then the beam
will output at Brewster angle too from another
surface, and beam is deviated 90 degree. It is
often used as a dispersion prism.