Gas is nearly invisible, but it has a temperature, and you can read it from how the gas 'shadows' the thermal background. In the experiment, a camera looks at a warm screen through a jet of carbon dioxide. If the gas is cooler than the background, it appears as a dark spot; if warmer, it appears bright. But there is a special background temperature at which the spot completely vanishes: the gas and the background become indistinguishable. That temperature is the gas temperature.

The device resembles a thermometer that never touches the measured object, only looks at it. The classic 'line reversal' method required complex optics and spectrometers. Here, a narrowband camera sensitive only to the 4.3-micrometer wavelength catches the moment the image disappears. The sharper the camera, the more precise the measurement.

The method doesn't just measure temperature at a single point; it can map the entire heat distribution across a gas flow. The authors tested it on engine exhaust and a human breath — both flows change rapidly, but the camera keeps up. Such imaging could be useful in engineering, medicine, and emissions monitoring.