GoKawiilHuman eyes don’t register the infrared portion of the light spectrum because infrared photons don’t carry enough energy to trigger the signaling pathway inside our light-sensing cells. But we’ve been able to make devices that give us a visual representation of what’s happening in the infrared.
A team at the Beijing Institute of Technology, led by Xin Tang and Ge Mu, has now built a device that lets people see infrared in a new way. Instead of just translating it to visible shades of green as it’s done in standard night-vision goggles, it translates different infrared wavelengths into distinct parts of the visual spectrum, giving the eye something closer to natural vision.
Researchers achieved that by combining mercury telluride colloidal quantum dots, which absorb infrared light, and a dual-layer OLED, which converts that absorbed energy into visible color. Stacked together with the right internal wiring, they make incoming infrared radiation come out the other side as an ordinary-looking, full-color image.
Invisible light
Vision begins when a photon strikes a light-sensitive pigment in the retina and causes it to change shape. That alters a protein that the pigment is embedded in, starting a nerve impulse that eventually contributes to an image in the brain. The molecular shape change that starts it all needs a minimum amount of energy, roughly 1.6 electron volts; infrared photons with wavelengths longer than 700 nanometers don’t carry enough energy to do it. That leaves over half of the Sun’s radiant energy outside our visual reach, along with anything that emits or reflects heat rather than visible light.
Most existing devices that allow us to see infrared use IR photodetectors wired to visible-light LEDs. This approach makes infrared light visible, but only as brightness: A warmer object glows a bit more, a cooler one glows a bit less, and we represent them all in the same (usually greenish) color. One problem with this is that human eyes are far better at distinguishing subtle differences in hue than they are at picking out brightness differences, so a device that only modulates brightness is leaving most of the eye’s sensitivity unused.