How much protection do I need?

Huge question. The answer is NOT “I need X OD at Y wavelength because this is what it says on my glasses.”

Every facility/company should have a designated Laser Safety Officer (LSO) who ideally calculates the nominal hazard zone (NHZ) for their Class 3b and 4 lasers based on the industry user standards. ANSI Z136.1 “For the Safe Use of Lasers” is the best standard for one to become familiar with initially. If you define your NHZ as your machine, then any optical density requirements of windows/view ports are based generally, on the type of laser, laser hazard wavelength(s) and beam divergence to that window. If one defines a room as the NHZ, then the optical densities required are based on the distance to the closest room window.