Can IR See Through Fog?
The short answer is “yes.”
But a more accurate answer is “sometimes.”
The IR camera on the Massachusetts State Police helicopter couldn’t really see through the boat cover that concealed the suspected Boston Marathon bomber: What it really saw was the tiny increase in the surface temperature of the shrink wrap where it had been warmed by the heat from his body. But in some conditions, some thermal imagers can see through some mediums, like fog for example. It all depends on the device, the size and temperature of the target, and the type of fog—whether it is warm or cool, whether it is made up of large droplets or small ones, and its overall density—because these factors govern the extent to which the infrared energy is bent and scattered on its way from the target to the camera. In dense fog, where naked-eye visibility is reduced to a minimum, thermal imagers aren’t likely to see very much more. But in thinner fog, the thermal image improves dramatically: When the naked eye can see 300 yards, a thermal imager may well see more than a mile. And when the fog lifts to give naked-eye visibility of half a mile or so, the thermal imager picks up targets at anything up to 5 miles.
This article originally appeared in the August 2013 issue of Power & Motoryacht magazine.