Optical system for projector and projector using it

An optical system for a projector, the optical system comprising:

• an array of digital micromirrors arranged in a plane, each micromirror being rotatable between two positions so as to modulate incident light by reflecting it in a first direction or in a second direction depending on a picture signal that switches the positions of the digital micromirrors between the two positions;
• a prism system including, in order from the array of digital micromirrors, a first prism, an air gap layer, a second prism, and an air interface;
• the first prism having a prism surface at said air gap layer that is at a first angle as measured with respect to a normal to the plane of the array of digital micromirrors, and the second prism having a prism surface at said air interface that is at a second angle as measured with respect to said normal, and the second angle is substantially equal in magnitude and opposite in direction to said first angle;
• wherein
• light for illuminating the array of digital micromirrors is deflected at the air gap layer to illuminate the array of digital micromirrors;
• and
• light reflected in the first direction and light reflected in a third direction that lies between the first and second directions is separated at the air interface.