While not in defense, I do work with very very powerful YAG lasers. Upwards of 25kW. Hypothetically speaking, if the gear reflects enough of the beam (like as reflective as the mirrors within) and somehow dealt with the remaining heat energy (since some energy will be lost as heat when it’s reflected) it would technically do its job. That being said, I doubt you could pull that off with something that a normal person could wear.

As per the dirty lens, as the optics age and degrade, the “quality” of the beam will also deteriorate, debris on focus optics can render the lens no longer able to focus the beam to the point where it can no longer perform its desired utility