Infrared Energy and Solar Loading

 
Manfred Kihn
 

For those of you who have been in the fire service for more than 15 years, like I have, you most likely remember a time when a thermal imaging camera (TIC) was considered a luxury tool.

Carl Nix

Many fire departments couldn’t afford the technology, and those that could purchase a TIC didn’t fully understand how to effectively use the tool. Today, the use of thermal imaging technology is readily accepted in the fire service as a lifesaving tool that protects firefighters from unseen dangers. I’m grateful for this opportunity to share my TIC knowledge with you each month by exploring a variety of TIC topics from firefighter disorientation to search and rescue to overhaul and much more. Together, we will explore the benefits a TIC can bring to a firefighting crew at the fire scene.

My responsibility as a trainer is to arm firefighters with an in-depth understanding of thermal imaging technology and its benefits to the fire service. My training is designed to teach firefighters how to interpret what this life-saving tool is telling them so they stay safe at a fire scene. I encourage you to share with me your experiences using a TIC and any questions you have about this technology. The more we share with each other our experiences using this technology, the safer we will be when responding to a call.

Railroad tracks, for example (photo 1), that are active will have a low emissivity value as they would be shiny and therefore not retain much heat from the sun.

1 Photos courtesy of Bullard.

BASIC TERMS

Let’s go back to the basics of thermal imaging and examine infrared (IR) energy and solar loading. Emissivity (E) is a measure of a material’s ability to absorb and emit IR energy. Conversely, reflectance (R) is a measure of a material’s ability to reflect IR energy. Good reflectors are not good emitters and vice versa. Good reflectors appear to be the same temperature as their surroundings.

Emissivity and reflectance values range from zero to one, but in general, we only refer to a material’s emissivity value. High emissivity values (i.e., close to 1) indicate that a material emits IR much more than it reflects it. (See box for some examples.) Railroad tracks, for example (photo 1), that are active will have a low emissivity value as they would be shiny and therefore not retain much heat from the sun.

MATERIAL

EMISSIVITY

Brick, red rough

.093

Brick, fire

0.75-0.80

Clay tiles

0.33

Concrete

0.94

Paint, aluminum

0.27-0.67

Paint, oil

0.92-0.96

Sandstone

0.67

Wood

0.80-0.90

Aluminum alloy, oxidized

0.40

Copper, oxidized

0.87

Copper, polished

0.07

Iron, oxidized

0.74

Iron, not oxidized

0.05


The next term is solar loading. As the sun shines during the daytime, objects absorb and reradiate infrared energy, usually resulting in a rise in temperature in the object. After the sun sets, this temperature rise reverses as the absorbed energy reradiates and the object cools.

The amount of energy absorbed during the day is proportional to the emissivity of the object. Remember, good emitters are also good absorbers and vice versa. As a result, better thermal contrasts usually occur several hours after sunset. For example, at midday (photo 2), a firefighter may blend in with the background because his apparent body temperature is nearly the same as the brick wall behind him.

Now, let’s look at what happens to the image of the firefighter at sunset (photo 3). He appears much darker than the brick wall behind him.

Four hours after nightfall, this same brick wall will have radiated much of its loading and will then appear cooler than the person standing in front of the wall. You can see how the firefighter (photo 4) clearly stands out from the wall, which is now appearing very dark.

At midday, a firefighter may blend in with the background because his apparent body temperature is nearly the same as the brick wall behind him.
At sunset the firefighter appears much darker than the brick wall behind him.
You can see how the firefighter clearly stands out from the wall, which is now appearing very dark.

APPLICATION

So, what does this all mean to a firefighter using a TIC during initial size-up doing a 360° search? We need to remember the definitions of emissivity and solar loading to understand how the TIC interprets images. For instance, your TIC is showing you a white roof on your initial scan. Remember, anything coming out of a mixer such as concrete or asphalt will have a high emissivity value, including roof shingles that are asphalt. Before you determine what your TIC is showing you, be sure to consider the following variables: the time of day, the angle of the direction of the sun, and the other buildings that are providing shade to the roof.

Other applications where emissivity and solar loading apply include search and rescue for a missing person and motor vehicle accidents where the driver or passengers may have been ejected from the vehicle. Always remember that TICs generate video images based on differences in temperature of the viewed scene.


MANFRED KIHN is a 19-year veteran of the fire service, having served as an ambulance officer, emergency services specialist, firefighter, captain, and fire chief. He has been a member of Bullard’s Emergency Responder team since 2005 and is the company’s fire training specialist for thermal imaging technology. He is certified through the Law Enforcement Thermographers’ Association (LETA) as a thermal imaging instructor and is a recipient of the Ontario Medal for Firefighters Bravery. If you have questions about thermal imaging, you can e-mail him at manfred_kihn@bullard.com.

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