As per the Newport website:
The 20B20NC.1 Broadband Beam Sampler is a 2.0 inch (50.8 mm) diameter, 9.40 mm thick, BK7 beam sampler with a λ/10 surface flatness. It is 440-700 nm anti-reflection coated on the back surface. Our broadband beam samplers are designed to split off or sample a beam by Fresnel reflection for beam monitoring applications where minimum disturbance of the transmitted beam is necessary