Professional Insights into Thermal Imaging with a Drone

Sponsored post by David Lee, Marketing Manager, FLIR Systems —

Thermal imaging is a key technology sUAS operators have wanted to get their hands on for years. Certainly, the uses for thermal are well-known and wide-spread – from search and rescue (SAR) to building inspections, from utility and transportation to precision ag, thermal cameras provide capabilities that no other technology can.

But that can be a double-edged sword. Thermal imaging can help operators see the invisible and do exciting things to save lives and protect resources…but it’s not a magic bullet. Like any other imaging technology, thermal will gives the best results when used in certain conditions, and when the operator knows how to properly interpret the images they’re seeing. In order to do this, operators need to have a basic understanding of how thermal imagers work, and how to set themselves up for success.

A little background…

Put simply, thermal imagers detect and display differences in the thermal energy given off by objects within the field of view. Thermal energy (also called “heat energy”) is around us all the time. In fact, it’s given off by everything about Absolute Zero – the temperature at which all molecular activity stops.

Therefore, any given scene will contain a number of items that are sources of thermal energy. The trick is understanding why an object is giving off that energy and what it signifies.

Into the weeds…

One way people unwittingly cause confusion in the thermal imaging world is by using temperature and thermal or heat energy interchangeably. These are not the same thing. Most people are unaware that “heat energy” and “temperature” are different. Heat is a form of energy. It cannot be measured; only its effects can. Temperature is a measure of the kinetic energy of molecules within an object expressed against a known scale (like Fahrenheit or Celsius). Thermographers use specialized radiometric cameras and software to convert the thermal energy seen with the camera into an accurate temperature measurement, but that’s another topic altogether. Let’s stick with basic thermal imaging for now.

Some objects generate their own heat energy (people and other warm-blooded animals), while others absorb energy from the sun and radiate that back into the atmosphere. Different objects generate or radiate different amounts of energy based on such factors as their composition, their surface textures, and the surrounding conditions. It’s these differences that allow thermal imagers to create the images you see.

If you do any research at all into what makes thermal imaging work you’re bound to run into a very important word: Emissivity. Emissivity can be thought of as the efficiency with which an object gives off heat energy. Warm-blooded animals are typically very efficient radiators, so we say that they’re “highly emissive.” Things get a little more complicated when we start looking at everything else in the world – those things that absorb the sun’s energy and radiate it back into the environment. Some are very efficient and some are not.

With very few exceptions, all of the energy you see in a thermal image has either been emitted (given off) or reflected (bounced back). Usually is an image will contain a combination of the two. Material that is a good emitter (dirt, rocks, concrete) is not a good reflector. Conversely, a highly reflective material (shiny metal, glass) is more likely to show you the radiation it’s reflecting from objects around it rather than its true thermal energy state.

Why do I care?

So, what does all this have to do with using a thermal camera from a drone? Well, let’s take a SAR mission as an example.

Trying to find a person in a wide open area with lots of exposed soil, rocks and other organic material can be extremely difficult if the sun is out and the surface objects have been absorbing thermal energy from the sun for a few hours. Because all of these objects will have high emissivity – meaning that they both absorb and emit energy very efficiently – they can easily overwhelm the radiation given off by the person you’re looking for.

This is where the difference between temperature and thermal energy comes into play. If you’re looking for a person in the desert, with lots of exposed rock and soil, in the early afternoon on a sunny day you could have just as hard a time finding them if it’s 50°F or 90°F because the ground has become saturated with thermal energy (a condition called “solar loading”).

You can set yourself up for success by flying earlier in the day – preferably before your search area is exposed to the sun – or when the sun is below the horizon. Also, if there’s cloud cover the effects of solar loading will be greatly reduced. If you’re not searching an area of bare ground, try positioning the aircraft so you’ll be searching shadowed areas. This will create a natural area of greater energy difference that will make your target easier to spot.

Another important factor is the color palette you select. Many cameras like the Flir Vue Pro and Zenmuse XT include color palettes that are optimized for specific tasks, like industrial inspections or building surveys. While these can be fun to look at and make great images of electrical components with overheating problems, they’re not the best for a search and rescue (SAR) mission. Black Hot or White Hot palette are the better choices for SAR.

The following images illustrate the issue between temperature and thermal energy.

In this image, the person is clearly visible near the front of his truck. Note the areas of open ground that are brighter than the surrounding area. This is where the soil is raised in mounds a few feet high. They have better drainage, are dryer than the lower, darker areas, and are angled to meet the rising sun more directly so they’re absorbing more thermal energy. All of these factors combine to create more solar loading in those areas.

The target has walked away from the truck and is barely visible to the right of the center of the image. He is walking through a slight depression where the soil  is again muddier than the surrounding high spots.

The target has continued walking in the same direction. He is now crossing a raised, rocky area and has effectively disappeared from the view even though he’s standing in the open. As he crossed this area and entered the muddy ground downhill from his current position he became clearly visible again.

Takeaways

So, for everyone excited about using a thermal imager, are we saying not to get one? Absolutely not. However, we want to emphasize that deploying one in a tactical situation or on a roof inspection is something that takes practice and study. Don’t show up at a search location or a job site expecting the person or problem to stick out like a sore thumb unless you use the tools at your disposal correctly. Use online resources or get some training so that you’ll understand what you’re seeing and why it looks that way.

It will be worth it.