Choose the ND65 broadband attenuator when you need a stronger fixed reduction. OPTOStokes specifies ND65 at 350–1100 nm, T = 65%, OD ≈ 0.18, 1.1 mm thickness, and 60/40 S/D, with use cases in sensor protection, laser power tuning, optical calibration, and machine vision. Choose ND90 when you only want mild attenuation: it holds 90% ± 3% transmission with OD 0.046 over the same 350–1100 nm range and works well for light balancing, ghosting reduction, and protective windows. In plain terms, ND65 solves overexposure more aggressively; ND90 preserves more signal. OPTOStokes lists stock support for ND65 and large-inventory OEM support for ND90, so you can start with standard parts and move to custom sizing if your mechanics demand it.