Have you ever seem what happens to a standard green glass lens if you leave the clear plastic lens off? for those that don't know the green lenses are very prone to any spatter that touches them sticking.

It was one of those Apprentice practical jokes back in the day, when doing positional weld training, you'd slip into the workstations at lunch time and remove the plastic lens from someone's shield, of course when welding over head a lot of sparks find their way onto your shield, and without the clear they stick hard to the green glass, within an hour of over heads there is so much spatter stuck on the green lens that it is almost opaque,

It is one of the quirks of the human eye that you don't notice until it's too late, Apparently the brain is programmed to ignore missing pieces of information in your field of vision that arise from the retina having inactive areas where veins, damaged areas and the optic nerve are situated, as a result we all have blind spots that the brain fills in for us, when we look through a welding lens we can only see where the bright light is situated with everything else being black, this makes it really easy for the brain to fill in the spatter areas until you only see a vague bright spot, The best one I ever saw was literally covered in spatter it was hard to believe that any light at all would have passed through it.

It just so happened that the subject of this practical joke was half way through his test sample which was to go out for X Ray and apparently his welding certification was on the line, so there was a bit of a backfire with the "jokee" being told that before doing a critical weld he should have checked his gear properly so whatever the outcome was, he was stuck with it, and the Joker was sent home without pay for the afternoon, They were a bit tough on practical jokes at that place as they should be I suppose, but it all turned out for the best, as the weld came up defect free, or at least acceptable, so no real loss.

Regards
Rick