But even if the target isn't cut, that doesn't mean I won't get eye damage, does it? Or do glasses protect even if they're only a bit opaque to the target wavelength?
That being said, it's all a function of how much light enters your eye. You can look up the exposure limits of the human retina for different frequencies of light to get an idea how little is actually required to damage your eye. The target or sensor should be sensitive enough to react to that much incident light. Just putting something behind the glasses and have it not get cut is definitely not enough to indicate no eye damage.