n ≈ 2.40 at 10.6 µm. Fresnel reflection from one uncoated face is about 17%. Two faces, and you have already given a third of the beam back to the room as ghost and loss. That is before absorption. Coat the optic.
What we will quote
KM grows and cuts the substrate. Coating is quoted to the wavelength set on the RFQ — we do not pretend every window ships with a mystery “BBAR.” Common asks:
- uncoated (spectroscopy, odd bands, you coat)
- AR 10.6 µm, both faces (straight CO2)
- dual-band AR 10.6 µm + 633 nm (HeNe through the same lens)
- broader mid-IR / LWIR stacks for cameras (not a CO2 stack)
A 10.6-only V-coat is usually the lowest loss at the laser line and a poor HeNe window. A dual-band stack is a compromise: you keep alignment, you give back a little at 10.6. Say which failure you cannot live with.
Coatings are not the crystal
A perfect AR on optical-grade material does not turn it into laser grade. The coating sits on the surface. Bulk absorption still cooks the optic from the inside. Specify grade and coating as two lines. If the print only says “AR ZnSe,” we will come back and ask 10.6, LWIR, or dual-band.
Durability: ZnSe is soft. The coating is only as happy as the substrate cleaning and the abrasion the operator inflicts. Gloves, no dry paper, no “just wipe it with a shirt.” We will still take the window back for disposal when the coating is cooked. Do not put it in municipal trash.