This is a fused silica substrate cold mirror used to eliminate thermal heat and IR light. When placed on a 45° angle, the thermal heat and IR light goes through the mirror and is dumped onto a heatsink while the visible and UV light is reflected at an angle to continue through the optical system. Since the substrate is fused silica, UV light is not absorbed but continues through the optical system with the visible light.

UV Compatible
Cold Mirrors typically reflect visible light while allowing IR light to transmit through. Because their substrate is typically glass, UV light gets absorbed and does not get transmitted or reflected. Since this cold mirror uses a fused silica substrate, it allows UV light down to 200nm to be reflected as well with the visible light.