Actually, most of the 24/7 surveillance cameras that I've seen already include a ring of NIR diodes for night vision and a sufficiently close photo from one would suffice.

Even better: the wavelengths used are in the range of 700-900nm, which, as indicated by my smartphone camera picking up the 850nm diodes in my Leap Motion controller, are within the range of off-the-shelf camera sensors.

For a decent image you need the iris to span 70 pixels. A Full HD camera probably won't cut it, but a 4k one installed at the entrance of a building might well be scanning irises and we wouldn't even know it.