Partial polarization beamsplitter for an optical system

A method for directing light comprising:

• receiving, by a beamsplitter, light from a sample and an overlay projector, the beamsplitter having a substrate comprising transparent material, the substrate having an axis and an interior plane with a central partially reflective portion and an outer transmissive portion, the substrate comprising a first surface configured to receive light from the sample, a second surface configured to transmit light towards oculars, a third surface configured to receive light from the overlay projector, and a fourth surface configured to transmit light towards a camera, the light from the sample having a first polarization and a second polarization, the light from the overlay projector having the second polarization;
• transmitting, by the outer transmissive portion, light from the first surface to the second surface to direct light from the sample towards the oculars;
• transmitting, by the central partially reflective portion, a portion of light from the first surface to the second surface to direct light from the sample towards the oculars;
• transmitting, by a polarization filter of the central partially reflective portion, light having the first polarization;
• reflecting, by the central partially reflective portion, a remaining portion of light from the first surface to the fourth surface to direct light from the sample towards the camera;
• reflecting, by the polarization filter, light having the second polarization;
• transmitting, by the outer transmissive portion, light from the third surface to the fourth surface to direct light from the overlay projector towards an aperture;
• blocking, by the aperture, light received from the fourth surface;
• and
• reflecting, by the central partially reflective portion, light from the third surface to the second surface to direct light from the overlay projector towards the oculars.

A beamsplitter comprising:

• a substrate comprising transparent material, the substrate having an axis and a plurality of surfaces comprising:
• a first surface configured to receive light from a sample;
• a second surface configured to transmit light towards oculars;
• a third surface configured to receive light from an overlay projector;
• and
• a fourth surface configured to transmit light towards a camera;
• an interior plane within the substrate, the interior plane positioned 40 to 50 degrees relative to the substrate axis, the interior plane comprising:
• a central partially reflective portion configured to:
• transmit a portion of light from the first surface to the second surface to direct light from the sample towards the oculars;
• transmit light having a first polarization and reflect light having a second polarization, the light from the sample having the first polarization and the second polarization, the light from the overlay projector having the second polarization;
• reflect a remaining portion of light from the first surface to the fourth surface to direct light from the sample towards the camera;
• and
• reflect light from the third surface to the second surface to direct light from the overlay projector towards the oculars;
• and
• an outer transmissive portion configured to:
• transmit light from the first surface to the second surface to direct light from the sample towards the oculars;
• and
• transmit light from the third surface to the fourth surface to direct light from the overlay projector towards an aperture configured to block the light;
• and
• the aperture, the aperture forming an opening having substantially the same shape as the central partially reflective portion, the opening up to 20 percent smaller than the central partially reflective portion.