KM lists n = 2.403 at 10.6 µm (2.403427 on the tabulated line). That index is why an uncoated ZnSe window is not “clear.” It is two Fresnel surfaces.
The reflection
For a single air–ZnSe interface, reflectance is ((n−1)/(n+1))². With n = 2.403 that is about 17% per face. Two faces, uncoated, and a large fraction of the beam never enters the crystal. The datasheet already says this. It is not a coating brochure; it is the index.
What to put on the RFQ
Uncoated is correct for some FTIR windows and some process viewports. It is the wrong default for a CO2 resonator or a cutting-head lens. Write AR 10.6 µm both faces, or dual-band 10.6 µm + 633 nm, if a HeNe rides the same path. Do not assume a window “comes coated.”
What we will not invent
Coating curves, residual reflection in ppm, and laser-damage numbers belong on the quotation against a part number. This page is the index and the Fresnel consequence. Grade still matters: laser grade is the absorption number at 10.6 µm; optical grade is the imaging job. Coating does not change the grade.