Military Applications of Thermal Imaging: No Place to Hide

The ability to see in the dark gives combatants a massive advantage over adversaries who can’t. The United States military is unsurpassed in the night vision technology available to its forces.

Our last article provided a grounding in the basics of night vision. To summarize, there are three night vision technologies: 1) direct infrared illumination, 2) ambient light intensification, and 3) thermal imaging.  Of these, the second, ambient intensification, is the most widely used. When you see operators using binocular NODs clipped to their helmets, those devices are amplifying ambient light. The key word is “ambient.” For those NODs to work, there must be some light for the device to amplify.

While this article is written to be read separately, it would be useful for the reader to review our other article. It can be found here.

Will night vision goggles work in a completely dark room?

In that article, we asked a question: “Will night vision goggles allow you to see in a windowless room in the complete absence of light?”

The short answer is “no.” Night vision goggles use ambient intensification. If the windows are boarded up, and the lights are turned off, there is no light in the room to amplify.

Let’s review what we mean by “light.” Electromagnetic radiation comes in a range of wavelengths called a “spectrum.” Figure 1 reproduces the spectrum we talked about in our first article.

When we see a red dress, the material of that dress absorbs all the wavelengths from 0.4 to 1.0 micrometers (microns) and reflects only those of, say, 0.65 microns. Our eyes pick up that reflected light, and our brain interprets it as “red.”

Now this is important. If the dress absorbed everything and reflected only the 0.8 wavelength, the dress would look black. Why? The dress absorbed all the wavelengths we can see. Look at Figure 1. We can’t see 0.8 microns.

The light in the 0.8 micron wavelength is what we call “Near-Infrared” or “Near-IR”. If we shine a 0.8 micron light into the dark room, that 0.8 micron light will reflect from the material. Our naked eyes can’t see it, but we can see it with appropriate equipment. That’s how NODs like the AN/PVS-31 (Figure 2) work. They have a sensitivity range that extends from 0.4 to about 1.0 micron. They amplify ambient light across the whole visible and Near-IR spectrum. They will pick up the 0.8 micron light from the dress and display its image to our brain.

That’s how aiming lasers work. We mount a device on our weapon system that generates a laser of 0.8 microns and zero it so our bullet hits the target where that laser falls. The laser is in the Near-IR, so it is invisible to the naked eye. Our NODs pick it up in the dark.

Notice the NODs amplify ambient light that is reflected from objects. The visible and Near-IR spectra are reflective. They require existing light to reflect from an object so our NODs can see it.

If there is no ambient light in that windowless room, there is nothing to reflect, so our NODs are useless.

Reflective versus emissive

Electromagnetic radiation is energy. Light is energy. Some light wavelengths are absorbed and others are reflected. Our eyes register the visible reflected wavelengths. NODs like the AN/PVS-31 register reflected light from the visible and Near-IR spectra.

It should not surprise us that the spectrum extends beyond the Near-IR region. Figure 3 shows the visible spectrum, the Near-IR spectrum, and the rest of the IR spectrum.

A word about units. Figure 3 is labeled in nanometers. 1,000 nanometers (nm) equal 1 micrometer (micron). So Near-IR extends from 0.76-1.5 microns, Mid-IR from 1.5 to 3.0 microns, and Far IR from 3 microns to 1,000 microns.

Radiation from the visible to Mid-IR spectra is reflective. Far IR radiation is emissive.

Everything emits in the far IR range. Far IR radiation is emitted as heat. It’s thermal energy. If we have devices that can “see” emissive IR, we can see in the pitch dark with no light source whatsoever. We’ll see the heat.

That’s where thermal night vision comes in.

Thermal night vision and digital photography

Infrared digital photography is fun and easy. Some digital cameras have an Anti-IR filter over their sensor, others don’t. The Anti-IR filter screens out IR light so only visible wavelengths hit the sensor. You can’t use that camera for infrared photography without removing the filter. Other cameras have no Anti-IR filter and you can use them directly. You place an IR filter (not an Anti-IR filter) in front of your lens. The IR filter looks black because it screens out all visible light and lets IR through. It’s exactly the same as the filter on those World War II sniper scope illuminators. You take a picture, and you get an infrared image.

Now, shine IR light across a dark room and ask a friend to walk across. Take a series of IR photographs in rapid succession. If you flip through them in sequence, you will see a crude movie of your friend walking in the dark. That’s how direct IR illumination night vision works. Almost all night security cameras work that way. They have an IR illuminator and an IR camera.

You’ll notice a problem. Your friend’s walk will look jerky because the number of images you took depended on how quickly you could cycle your camera’s shutter. Security cameras have the same problem. How quickly you cycle the shutter is called the frame rate. If your friend runs across the room, your movie will be awful. The faster he moves, and the slower your frame rate, the jerkier your movie.

NODs like the AN/PVS-31 are analog, not digital. That means they amplify ambient light in real-time, and there are no interruptions in capturing the images your brain sees. That’s a key advantage of ambient intensification over direct illumination. It’s a key advantage of analog over digital.

Now, let’s say we have a thermal IR camera that is sensitive to far IR wavelengths. Turn out the lights. Take an image of the room that displays the radiation emitted by the objects in the room. It will be an image of their temperatures. Cold objects will look black, hot objects will look white. The image will be as detailed, sharp, and contrasty as the temperature gradients in the scene. The camera projects the image on a screen that you can look at.

Figure 4 shows a thermal image shot outdoors at night. The warmer subjects show up as white against the cooler and darker foliage.

Now, ask your friend to walk across the room. Take a series of thermal photos with no IR illumination and project them on your eyepiece. Just like the IR digital camera, you will see a jerky movie of him walking. Just like the digital movie, the faster you cycle the shutter, and the slower he walks, the better the quality. The difference is, you have done away with the IR illuminator.

That is a thermal night vision device. In practice, current technology limits its frame rate to about 30 frames per second. If you have done manual photography, you know that’s pretty slow. 30 frames per second translates to a shutter operating at a speed of 1/30^th of a second. That’s not fast enough to stop fast movement. For example, it’s useless for sports photography. The thermal night vision device has the same problem as a digital direct illumination night vision device.

Thermal sights come in different configurations. Figure 5 shows an Eotech ClipIR thermal scope. It can be attached to the front of a low-power daylight scope to provide thermal imaging without disturbing the weapon’s zero.

Ambient Light Intensification (NODs) versus Thermal Night Vision

So when are ambient light NODs the best choice, and when is thermal best? There’s a lot being written about this topic. There’s hair-splitting and aggressive sales. From a practical perspective, the choice isn’t that hard.

Patrol and CQB

Intuitively, when on patrol or in fast-breaking, close-quarters situations, NODs seem to have the edge. There is no latency, and movement is detected in real time. Problems of reduced peripheral vision can be partially overcome with quad tubes. There are problems of lower resolution and shorter effective range. Again, in CQB, these would not appear to be deal-breakers.

Figure 6 shows the use of an aiming laser. Note that the laser is invisible to the naked eye. It is only visible here because it is being photographed through night vision.

Extremely low-light situations are deal-breakers for ambient intensification. A deep cave or long tunnel with no lights is an example. Going into the basement of a house where the bad guy has broken all the light bulbs is another. In such cases, you have to go to direct IR illumination. That risks giving your position away.

Overwatch, sniping and hunting

A thermal night vision device would work best in cases where the operator is motionless and viewing a relatively static scene. Let’s say he’s on the top floor of a building, set back from an open window, looking through a thermal optic. He’ll get a sharp, contrasty image of whatever’s in his field of view.

Thermal optics work at night and in daylight. If an animal is hidden behind foliage, its body heat will show up on a thermal scope. Any time there is a temperature differential in the scene, the thermal optic will pick it up. This video shows how thermal imaging detects targets despite cover foliage and camouflage that defeats ambient intensification night vision.

It is worth noting that the subject in the video is not moving quickly. He is walking away from or toward the device. Subjects moving toward an observer are effectively slower than subjects moving across his field of view.

Any time the subject and surroundings are the same temperature, the optic will have a hard time telling them apart unless the subject moves. It is the temperature differentials that create contrast.

In this video, Special Forces soldier Tim Kennedy gives a fun introduction to the pros and cons of night vision and thermal. It’s a short clip, but if you followed our earlier discussion of thermal frame rates, it will make perfect sense!

People split hairs a lot, but those kids figured things out quickly.

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About the Author

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Cameron Curtis has spent thirty years on trade floors as a trader and risk manager. He was on the trade floor when Saddam’s tanks rolled into Kuwait, when the air wars opened over Baghdad and Belgrade, and when the financial crisis swallowed the world. Having written fiction as a child, he is the author of the Breed action thriller series, available on Amazon.

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Disclaimer: SOFREP utilizes AI for image generation and article research. Occasionally, it’s like handing a chimpanzee the keys to your liquor cabinet. It’s not always perfect and if a mistake is made, we own up to it full stop. In a world where information comes at us in tidal waves, it is an important tool that helps us sift through the brass for live rounds.