An infrared-transmitting polymer based on common low-cost materials may lead to low-cost night-vision lenses that retain focus while imaging at variable distances. The polymer, developed by researchers from the University of Tsukuba, keeps its shape after stretching. Designing conventional infrared night-vision lenses capable of easily changing focus from one position to another is typically difficult and costly. Without fabricating lenses to feature variable-focus capability, details that are important to capturing and understanding an image can be lost. Using a flexible polymer made from common materials is desirable for creating lower-cost, more readily available lenses. The infrared lens in the researchers' system is made from an elastic polymer derived from algae and plant compounds. It is capable of variable focus. Courtesy of University of Tsukuba. The polymer is based on sulfur and compounds derived from algae and plants. The researchers prepared it using a chemical process called inverse vulcanization, in which the constituent compounds are mixed and stirred together as they are heated. They then poured the material into a silicon mold and heated it further. In tests to determine transparency to infrared light, the researchers determined that even a 3.3-mm-thick lens was able to transmit 10% of incoming infrared light. “The lenses have two wavelength ranges that are infrared-transparent,” said senior author Takaki Kanbara. “No lens is completely transparent; 10% transmission is an excellent value for these materials.” To test the lens for variable-focusing, the researchers projected an image through the lens and monitored the image that came through as the lens was elongated. Squalene and other long unsaturated hydrocarbons, which help optimize the cross-linking structure, gave the polymers the necessary elasticity. The lens is able to return to its original shape after being stretched repeatedly by 20%. “The lens retained 54% of the focus variation, which is sufficient for practical uses,” said Takashi Fukuda, senior researcher at the National Institute of Advanced Industrial Science and Technology. “The lens also retained its full initial focus after contracting back to its original shape.” The research was published in ACS Applied Polymer Materials (www.doi.org/10.1021/acsapm.0c00924).