Beamsplitters are part-mirrors that reflect part of incident energy and transmit the rest.  They are designed for 45 angle of incidence.  Beamsplitters Plate are highly sensitive to polarization.

Reflectance: (Rs+Rp)/2

Transmittance: (Ts+Tp)/2
Typically R/T=30/70%; 50/50%;  70/30%.

Tolerance: ±5% @45°