A team at the Beijing Institute of Technology, led by Xin Tang and Ge Mu, has built a device that lets people see the infrared in a brand-new way. Instead of translating that band of light into the traditional greenish shades of conventional night vision, the new device converts different infrared wavelengths into distinct parts of the visible spectrum, delivering to the eye a color image much closer to natural vision.
The secret lies in combining two technologies. On one side, mercury telluride colloidal quantum dots, which absorb infrared light. On the other, a dual-layer OLED, which converts that absorbed energy into visible color. Stacked together with the right internal wiring, they make the incoming infrared radiation come out the other side as an ordinary, full-color image.
To understand why this matters, it helps to recall the basics of vision. Vision begins when a photon strikes a light-sensitive pigment in the retina and causes it to change shape. That change requires a minimum amount of energy, roughly 1.6 electron volts. Infrared photons with wavelengths above 700 nanometers do not carry enough energy to do it. That leaves more than half of the Sun's radiant energy outside our visual reach, along with anything that emits or reflects heat rather than visible light.
Current devices that allow us to see infrared use IR photodetectors wired to visible-light LEDs. That approach makes infrared light visible, but only as brightness: a warmer object glows a bit more, a cooler one glows a bit less, and everything is represented in the same color, usually greenish. The problem is that human eyes are far better at distinguishing subtle differences in hue than at picking out differences in brightness. A device that only modulates brightness leaves most of the eye's sensitivity unused.
Tang's team's solution starts with the way infrared light is registered in the first place. Conventional semiconductors used in IR detectors have continuous energy bands, absorbing a broad, undifferentiated swath of the spectrum. The mercury telluride quantum dots, being tiny — roughly 4 nanometers — break those energy levels into discrete steps. Photons of different wavelengths excite different electronic transitions, preserving information about which specific wavelength arrived.
The other half of the challenge was translating that signal into a color image. The team built the OLED with two stacked emissive layers, one doped with a red-emitting phosphor and the other with a cyan-emitting one, separated by an energy barrier of about 0.82 electron volts. When only a few charge holes arrive, they are all captured by the red layer, and the device emits a low-brightness red glow. As the number of holes grows — whether because the infrared light is brighter or contains shorter wavelengths — the barrier saturates and the holes begin crossing into the cyan layer. The result is a mixture of red and cyan that shifts the color and increases luminance.
The applications go far beyond the military goggles that come to mind. Medicine, surveillance, night security and astronomy could all benefit from an infrared view that preserves more visual information. The question that remains is how long it will take for this technology to leave the labs and reach affordable commercial devices — and whether, one day, seeing heat in color will be as common as the green night vision we know today.
Sources: Ars Technica, Archyde, Briefly
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