With a good thermal infrared camera, we can explore phenomena that produce only small thermal changes in the environment. One example is the temperature change produce by our footprints when walking. When a shoe or foot contacts the floor, the floor is usually the cooler surface, and so the shoe or foot will warm it up a little. This thermal signature lingers on for a little while, slowly decaying as the footprint temperature returns to the equilibrium temperature of the surroundings.

The measurements shown below were taken with an A6700sc FLIR camera, using an f/2.5 lens. This has a cryo-cooled InSb detector <span>(</span>3-5um spectral range<span>)</span>, running at 100Hz, and using a 1.22ms integration time. The raw data images are 640×512 in size. If I recall correctly, they are encoded as 14-bit values. The stated NETD is 18mK.

The images and videos below show a series of measurements that I took together with David Serrano Garcia, showing him walking down the hallway of our building in both slippers and in socks. This first image gives a view of the scene using a regular visible camera, with the infrared camera shown at the bottom. Note that the videos are large files — this type of data does not compress well, without destroying some of the interesting detail.

Experiment setup for the thermal footprints measurements.

The first video shows a standard thermal view, in which the highest temperature in the scene is scaled to the color white, and the lowest temperature in the scene is scaled to black. Because David’s clothing is so much warmer than the footprints, the footprints are only barely visible. In this video, he is wearing slippers.

Standard thermal view.

Next, we change the display colors in order to enhance any temperature differences from the carpeted floor. Temperatures colder than the carpet will be darker gray, while temperatures warmer than the carpet will be brighter gray. Any temperatures that are more than a few degrees above or below the carpet temperature will be shown as a saturated white or black, respectively. In this enhanced view, we can clearly see the footprints. Although the video here only lasts 26 seconds of real time, the full video shows that the footprints remain just barely visible for up to about one minute of time.

Enhanced thermal view.

Next, David removed his slippers and did the same promenade in his socks. In this enhanced thermal video, we can see that the temperature of the footprints is higher now, so that the footprints remain visible in the enhanced thermal view for about 2 minutes.

Enhanced thermal view.

After these experiments, I unfortunately didn’t have a good chance to try this out on smooth flooring. I expect that the carpeting helps to increase the thermal lifetime of the footprints, and that the thermal signature will fade more quickly on hard flooring, but I haven’t had the chance to give this a try.

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