Schott RG9

The 3mm thickness gives it real heft in the filter holder, and it’s cut cleanly to the Ø dimension I ordered. Slotted it into my optical setup and the cutoff is sharp where I needed it. Exactly what I was hoping for.

Used this RG9 as a longpass in my spectroscopy rig, and the 3mm thickness gives it a nice solid feel that doesn't flex under the mount. It cut off exactly at the right wavelength for my setup, no stray light issues.

Snapped this into my filter holder for IR work and the cutoff is exactly where I expected it. Glass arrived clean with no chips or haze, and the polish is good enough that I don't see any scatter in my beam.