Seeing active fires through the smoke

28 October 2015 by Bill

There is a new way to look at the world. Seeing beyond the pixels, it shows us things that we cannot observe with our eyes. Collecting satellite images using short-wave infrared (SWIR) wavelength bands offers unique remote sensing capabilities such as material detection, smoke penetration, and thermal sensitivity. SWIR satellite images at high resolution are a new capability, and at first may seem unusual or difficult to interpret. One must think, instead of “how would I traditionally interpret this,” “what is the image telling me.”

SWIR is a type of light that we cannot see with our eyes. The visible light spectrum that we can normally see ranges from violet to red. However, there is light called near infrared (NIR) that is beyond red. We cannot see NIR, but we can feel it as heat coming off an incandescent light bulb. SWIR is light that lies beyond NIR, which we also cannot see. Many satellites collect images using visible and NIR light, but SWIR much rarer.



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SWIR light covers a range where many materials have naturally occurring signatures. These signatures, in some cases, let us identify the type of material. Examples include steel, plastic, fiberglass, paint, clay, and certain types of rock. These signatures are not present in the visible and NIR range of light. In addition, the SWIR sensors respond to thermal emission from extremely hot features. Examples include wildfires, industrial flares, and volcanic lava. While we can only feel the heat, SWIR can see the heat.

Figure 1: Regular RGB imagery with smoke on Tomnod
Figure 1: Regular RGB imagery with smoke on Tomnod

Finally, there is the smoke penetration capability. Visible light does something called scattering when it encounters smoke or water vapor clouds. Scattering is when the light bounces off in all directions. That is why clouds and smoke are white and opaque. But, because of the unique SWIR wavelength compared to the size of smoke particles, SWIR is not scattered by smoke. It passes right through the smoke, virtually unchanged. In other words, the smoke is transparent to SWIR. Unfortunately, due to their differing composition water vapor clouds remain opague. This smoke transparency is a huge benefit because it lets us see through a smoke plume and down to the ground, where emergency response crews need our help. Combine the transparency with thermal response, and we can observe hot spots beneath a smoke plume. This capability enables us to map illicit fires, detect unexpected fires, or direct response crews more efficiently in extinguishing fires.

Figure 2: SWIR imagery on Tomnod renders smoke transparent and lets us see the active fires underneath
Figure 2: SWIR imagery on Tomnod renders smoke transparent and lets us see the active fires underneath

High resolution SWIR is a new remote sensing capability, showing us things we cannot see with our eyes. It tells us about materials, moisture, heat, and renders smoke transparent. Most importantly, it gives us another tool to enable our vision of seeing a better world.

Here’s an example of what can be seen using natural color and SWIR:

Bill is a Senior Staff Scientist for the Research and Development group at DigitalGlobe.