“It’s not the heat; it’s the humidity” has become a popular response to the sweltering heat of summertime, but determining the air’s moisture content quickly and accurately is not necessarily an easy task.Researchers in Ireland may have a solution to gauging this index. They have fabricated a hologram that rapidly changes color in response to slight fluctuations in humidity and that returns to its original state once the moisture has dissipated.The color of a hologram changes in response to the humidity level. RH = relative humidity. Reprinted with permission of Applied Physics Letters.The material could have a range of applications; for example, it could be fabricated as a disposable sensor for use in shipping to safeguard packages against damage from moisture. Researcher Izabela Naydenova and her colleagues at the Dublin Institute of Technology created the hologram with a self-processing acrylamide-based photopolymer on a glass substrate. The device’s ability to change color is the result of the swelling or shrinking of the photopolymer medium when it is exposed to various humidities. As reported in the Jan. 21 issue of Applied Physics Letters, the scientists placed the photopolymer in an environmental chamber where the relative humidity could be adjusted from 5 to 100 percent.They found that, when the humidity was increased from 5 to 80 percent, the hologram’s spectral response shifted by 130 nm. They also found that temperature had practically no effect on the humidity response in the temperature range studied: The spectral peak shifted by only 1.6 nm when the humidity was kept constant at 30 percent while the temperature was increased from 23 to 50 °C. The investigators prepared photopolymers ranging in thickness from 30 to 180 μm, and they found that the thickness directly influenced the response time of the material. After optimizing the thickness, they reported that responses were achieved in a few seconds.