After her arrest at an Occupy Wall Street protest in December, Samantha Wilson expected to be booked, fingerprinted and subjected to a mug shot. But when a police officer raised a small device to her face and began photographing her eyes, she declined.

Ms. Wilson, 32, said her refusal resulted in a threat from the officer.

“He said: ‘It’s not really optional. It’ll take you longer to get out of here if you don’t do it,’ ” she recalled.

The New York Police Department began photographing the irises of people arrested in Manhattan in 2010; officials said then that the images would help prevent suspects from escaping. But the program drew criticism from criminal defense lawyers and civil liberties experts who expressed concern that it could infringe on individuals’ privacy, especially in cases in which the charges were eventually dropped.

More than a year later, as the program has been extended across the city, opponents have renewed their objections and accused officers of sometimes pressuring people to submit to the photographs — which are supposed to be optional — by keeping those who do not comply in custody longer.