Looks like near-IR of some sort but media calls everything x-ray since it's what people know. X-Rays would go through cards anyway. But you'd get nice pictures of peoples hands though, and cataracts after a night of play.
edit: now I think of it: if the cloth is thin enough you don't even need near-IR. Old fashioned IR camera's (those without any fancy filter) from the '00 showed though some relatively thin opaque synthetic material with a tiny IR source so ...
Yes, my conclusion as well. Especially with a bright enough IR source. The bones reference was as a bit sarcastic reference if they'd used x-rays but since nobody got seriously ill that did not happen..
I'm pretty sure it was just marks on the back of the cards that glasses/contacts then converted into an xray-like view, not any actual technology for seeing through the cards.
edit: now I think of it: if the cloth is thin enough you don't even need near-IR. Old fashioned IR camera's (those without any fancy filter) from the '00 showed though some relatively thin opaque synthetic material with a tiny IR source so ...