Don Shift Sends: Primer on Thermal Camouflaging and Evasion Principals

Note: this an adaptation from the non-fiction book Poor Man’s Air Force: A guide to how small drones might be used in domestic unrest or low intensity conflict. Don Shift is a veteran of the Ventura County Sheriff’s Office and an avid fan of post-apocalyptic literature and film. He is a student of disasters, history, current events, and holds several FEMA emergency management certifications. Visit www.donshift.com.

There are two “kinds” of infrared light: near-infrared radiation (NIR) and thermal infrared radiation (TIR) or heat. All drones should be assumed to have low-light observation and navigation capabilities, either thermal or light-amplification “night vision.” Even cheap camera sensors can employ limited digital light amplification. A UAV (drone) flying at night should be assumed to have either or both capabilities.

The most common form of “night vision” takes advantage of near-infrared radiation (NIR). NIR is often used in night vision technology as it has longer wavelengths than visible light and can penetrate through some atmospheric conditions and objects such as clothing, allowing objects to be detected in low-light conditions. Most know this kind of night vision (image intensification) from the green cast footage of videos and movies.

What this article looks at is thermal radiation or what is essentially the difference in temperature of the environment. Thermal infrared (TIR) radiation is emitted by objects as a result of their temperature and is often used to detect and measure heat. TIR sensitive cameras create images based on the temperature differential of objects as they contrast with each other and their surroundings. These cameras may be referred to as FLIR, Forward Looking Infrared, a brand name.

TIR vision operates based on the fact that as an object’s temperature increases, it emits more infrared radiation in a wavelength beyond the visible range of human sight. This additional radiation corresponds to an increase or decrease in the object’s energy state. By detecting and measuring this infrared radiation, the thermal imaging system can differentiate temperature gradients and produce a visual representation. This variation in temperature is commonly referred to as thermal contrast.

How to hide from thermal

Thermal imaging is degraded by precipitation, cloud, and smoke. This can make it difficult for thermal radiation to pass through these conditions and can also cause thermal radiation emitted from objects to scatter and become diffuse. As a result, the contrast and clarity of thermal images may be reduced in rainy, foggy, or misty conditions. The effectiveness of thermal imaging in these conditions depends on the density the fog, precipitation, or smoke, as well as the sensitivity and resolution of the thermal imaging equipment used.

Vegetation offers concealment from near-infrared sensors as it reflects light in that wavelength similarly to visible light. NIR radiation is reflected by the leaves of plants and does not penetrate very far into the vegetation. On the other hand, TIR radiation can penetrate vegetation better than NIR radiation because NIR is absorbed by the water content in plant tissues. The amount of TIR radiation that can penetrate vegetation depends on the density and structure of the vegetation, so a deep forest canopy may provide good concealment.

Thermal crossover is a phenomenon that occurs twice a day, every morning and evening, when environmental background temperatures and an object equalize or blur. During thermal crossover, it can be difficult to distinguish between two closely spaced objects with similar temperatures. This can lead to errors in thermal imaging analysis and interpretation, so activity during periods of low temperature contrast decreases the chance of being seen. Human bodies and hot vehicles will be best camouflaged by the background thermal profile from midday to late afternoon when the ground and air is the hottest.

Thermal hiding is getting behind or under something that blocks the source of the heat. For example, a person crawling into a bunker with a sandbagged roof or a truck parking beneath a bridge is hiding under thermal cover. In order of decreasing effectiveness, thermal cover includes earth overhead cover, earthen embankments (horizontal cover), thick vegetation (a tree canopy), camouflage nets, and smoke.

Getting below ground or underneath natural earthen cover is best as the Earth itself will block the sensor’s view and help dissipate body heat. Horizontal cover blocks the straight line or oblique view of a sensor and helps, except when a UAV is directly overhead. Thick vegetation increases the ceiling of the drone, due to collision danger, and the foliage also helps disperse heat as well as offer concealment.

Thermal blending is attempting to blend the thermal signature in with its background the way camouflage attempts to blend objects or people with the environment. Blending works by masking the heat signature. Cloth-like flexible materials that can be used to expeditiously cover a human or be used to construct a shelter work through blending and masking. Without an insulating air gap, these materials will quickly heat up from the person below them.

A thermal barrier is a material that can be used to block or attenuate the transfer of thermal energy from one side to the other. A thermal barrier can take various forms, such as a layer of insulation, a reflective surface, or a combination of both. A thermal barrier blanket blocks the thermal radiation and a layer of insulation (including an air gap) prevents the heat from warming the thermal barrier. These have to work in concert with each other.

The thermal barrier stops the infrared radiation like a blanket over a flashlight, assuming the material has sufficiently thermally opaque properties. An air gap acts as insulation that also dissipates heat, preventing it from heating the barrier. All insulative materials in contact with a heat source (body) will heat up eventually. Heat bleed is not a phenomenon experienced with NIR; under NIR, for something similar to happen the camouflage would have to reflect IR light or someone would have to emit light from behind concealment, like turning on a flashlight.

In short, unlike “regular” night vision, in which camouflaging principals are very similar to under visible light, thermal camouflaging is more than just breaking up patterns and not being seen. Thermal camouflage is more conscious of larger shapes as silhouettes are more obvious under the limited contrast often seen through thermal sensors. It’s a lot easier to see a bright white spot against a dark background than it is a camouflage pattern against a matching background.

Blending is critical no matter what kind of a barrier is used. Thermal camera systems are not like a pass/fail system that can be defeated simply by hiding the heat signature. Seeing a uniformly black (cold) square in the middle of a forest will be a dead giveaway the same way a drunk driver isn’t hiding from a cop by turning off his headlights.

Exploiting thermal camo is very difficult while moving and most techniques apply to camouflaged static positions. For example, tempered safety glass in car windshields is basically totally thermally opaque, but not everyone can hide inside a car or, as the meme goes, walk around in a suit made of car windshields. Solutions for thermal camouflage while moving aren’t great or for extended use.

To hide from thermal imaging, minimize the difference in temperature between yourself and your surroundings. Totally hiding is impossible without getting out of the line-of-sight of the sensor by going underground, for example, but many techniques can reduce a thermal signature or confuse a sensor operator. To a good thermal sensor, an umbrella may block the body behind it but the camera will still see an umbrella. Outsmart the pilot, not the sensor.

• Camouflage layer>thermal barrier>insulation/air gap>heat source (body)
• Do not expose any shiny Mylar or plastic. Cover these materials with a matte, natural or camouflaged cloth.
• Stay behind or under the thermal barrier without being in physical contact with it.
• Keep an air gap between the heat source (body), the thermal barrier, and the camouflage layer.
• Use tree canopies and vegetation to partially block your heat signature.
• Use camouflage netting to help dissipate heat and break up signatures.
• Do not use thermal barriers while moving; this will create a moving “black hole.” If you must deploy the barrier on the move, stop, deploy it, and remain motionless until the threat passes.

If you must move:

• Increase your distance from the sensor or adversary. Infrared radiation follows the inverse square law. In short, the further a heat emitter is from the source, the smaller its thermal signature is.
• Take advantage of this thermal variation “clutter” presents a sensor operator in distinguishing heat sources. Operate in a high thermal contrast area, such as an urban area among warm cars or around rocks that have been heated by the sun.
• Do not be in an area with a uniform temperature, such as an open field, unbroken ground, a road, or paved area. Although, one exception is in the late afternoon/evening thermal crossover is another time where heat-producing sources may blend with the ground.
• Put objects between you and the sensor.
• If you are being chased, run into deep vegetation. Thick undergrowth or heavy tree canopies will make it very difficult to see through the foliage. Move under tree cover or in shadows whenever possible.