What are the applications for BK7 (K9) windows with steps?
Laser systems: for beam control and resonant cavity design.
Optical sensing: in sensors that need to be precisely mounted or sealed.
Vacuum systems: step design helps to achieve better sealing.
Aerospace: to meet mounting and performance requirements in special environments.
Microscopy: customised windows for special sample viewing.

Price comparison between BK7 and K9
BK7: International common material, higher price (especially imported brands such as SCHOTT), supply chain globalisation.
K9: Chinese local materials, lower cost, suitable for domestic procurement, but less international recognition.