Not invasive at all. SRI's "Iris On-the-Move" product is a gateway that people walk through; a long-range infrared camera images their iris as they're passing through. As long as you catch a glimpse of the green light as you're walking, they can match your iris template. See "Iris on the Move: Acquisition of Images for Iris Recognition in Less Constrained Environments", Matey et al 2006.
(I'm trying to vaccinate you from this idea, not convince you. I obviously hate the idea of iris scanners at airports. My point is that unfortunately it can be done, and we should be careful.)
