I photographed New York City with a thermal-imaging camera to see how a technology designed for surveillance renders the human form.

The New York City Police Department has a vast monitoring network that links 8,300 cameras and 500 license-plate readers around the city with software that can track movement. It's called the Domain Awareness System. Under a separate program, the department operates a  fleet of surveillance helicopters equipped with forward-looking infrared (FLIR), a type of thermal-imaging technology.

Thermal-imaging cameras were originally developed for the battlefield. Aerial-drone operators and snipers track targets using infrared radiation, which can distinguish between ambient temperatures and heat generated by the human body. The Department of Commerce classifies high-resolution FLIR cameras as “dual-use,” meaning they have both civilian and military applications, and therefore restricts their export. In 2014, however, a low-resolution version was introduced for sale as a clip-on accessory for mobile phones, which I used for this project.

These images first appeared in the April issue of Harper's magazine.