A possibility would be that you might want to make multiple changes in light level at random intervals to make it harder for e.g, a video being presented to the camera that matches the input it would expect.
Coz they could add a brainwave scanner that measures the reaction to the input of the eye, or have a higher resolution camera that measures blood flow being active in the eye. Many avenues of addressing this. But as a previous comment mentioned - generally easier to have a physical person next to the iris. After all, not like there is a shortage of humans to do jobs in the World today.