The NYPD says the images will be used to help avoid cases of mistaken identity. The process takes about five seconds. Suspects will be scanned using handheld devices when they're arrested and again shortly before they're arraigned to make sure the irises match.
The iris is a colored membrane that regulates how much light enters the eye.
Police say the packages of software, handheld devices and cameras cost $24,000 each and 21 systems will be used around the city.
Central booking in Manhattan started taking photos Monday.
Brooklyn, the Bronx, Staten Island and Queens will get the devices later.
Police say the photos cause no damage to the eye.