It is my understanding that retinal scans can be effected by health conditions. Pregnancy, diabetes, glaucoma, retinal degenerative disorder, AIDS, syphilis, malaria, chicken pox, lyme disease, leukemia, lympoma, sickle cell, congestive heart failure, atherosclerosis, and significant cholesterol change can all apparently cause a retinal scan to change. While some employees may find detection of these conditions as a good thing, other employees may find it invasive.
Research seems to indicate that iris scans change over time. Companies that use Iris scanners need to rescan everyone every year or they get false negatives, which may or may not be an issue.
I think using an ID card scan like was mentioned above, makes the most sense.