You can't check a mirror against another mirror. The solution is a window that transmits light with a known, flat reference face.
The 4 in TX Flat (100 mm clear aperture) covers most bench work at $3,350. The 6 in version (153 mm) extends coverage for larger optics at $5,650. And if you want to turn a 4 in Fizeau into a 6 in instrument outright, the BeamEx 1000 beam expander does exactly that.
4 in or 6 in
Fused silica window, front surface λ/20
Both change the polarization of light. Only one stays accurate when your lab warms up. Here is the difference in plain terms.
Cemented plates can delaminate. Contacted plates drift. Air spacing removes both failure modes — and improves alignment accuracy.
Tunable laser or wide spectrum? A single material can't hold retardation across hundreds of nanometers. Two materials can.
