An innovative ultrathin lens could lead to more affordable IR thermal imaging cameras. The 1-mm-thick silicon Fresnel lens was developed by researchers from the French aerospace lab Onera, French thermal imaging sensor company ULIS, the Optical Institute and the French National Center for Scientific Research. A picture taken with the new camera. The system can detect people and some details of a setting, in this case an office. Images courtesy of Optics Letters. Silicon is less expensive than materials traditionally used in IR cameras such as germanium and chalcogenide, and photo printing or molding of the material can be used to create such lenses inexpensively, researchers said. The prototype lens has a 130° field of view, and performs well in low-light conditions, researchers said. It is effective in the long-wavelength IR realm from 7 µm to 14 µm, making it sensitive to differences in temperature. “It could detect people in a room, on a street corner or in a dark alleyway,” said lead researcher Tatiana Grulois, a doctoral candidate at Onera who developed the lens prototype. “In recent years, huge efforts have been made to reduce the price of uncooled long-wavelength IR detectors.” Fresnel lenses typically have not been used for broadband imaging due to “devastating chromatic problems,” researchers said. Grulois said her prototype is a cross between “lighthouse” and “diffractive” Fresnel lenses, taking advantage of relatively deep concentric rings to reduce chromaticity. (a) The new Fresnel lens, 1 mm thick and less than 4 mm in useful diameter, with a euro cent for comparison. (b) The new thermal infrared camera, which integrates the lens. The researchers admit that the new lens does not provide the highest resolution, but can effectively reveal the presence of a person, even in low-light settings. This makes it suited to surveillance, particularly in homes, offices, small businesses and the dark alleyways around them. Additional testing must be done to make the new lens marketable, Grulois said, noting that new thermal IR imagers such as this one will “open completely new and exciting fields of application.” The work was funded by the French Procurement Agency. The research is published in Optics Letters (doi: 10.1364/OL.39.003169). For more information, visit www.onera.fr.
