I want to reflect on a topic regarding emissivity and the effects that it has when using a thermal imager (TI) outdoors. Emissivity is arguably one of the more challenging factors to overcome when assessing temperature measurements. However, you can compensate for it if you properly understand it and you’re using the right techniques.
Wikipedia’s definition is as follows: “Emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that may include both visible radiation (light) and infrared radiation, which is not visible to human eyes. The thermal radiation from very hot objects is easily visible to the eye. Quantitatively, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature as given by the Stefan–Boltzmann law. The ratio varies from 0 to 1. The surface of a perfect black body (with an emissivity of 1) emits thermal radiation at the rate of approximately 448 watts per square meter at room temperature (77 °C, 25 °F, 298.15 K); all real objects have emissivities less than 1.0 and emit radiation at correspondingly lower rates.”
Did you know the emissivity values for the following materials?
MATERIAL EMISSIVITY VALUE
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
What does this mean to a firefighter using a TI outdoors? During initial size-up while conducting a 360° survey around a structure, we need to remember how a TI interprets images—e.g., white is hot, black is cold, and everything in between is shades of gray and what the TI detects is heat. Consider an example where your TI is showing a white roof on your initial scan. Take note that anything coming out of a mixer such as concrete or asphalt will have a high emissivity value, which includes asphalt roof shingles. Interpreting what your TI is showing you takes some understanding and technique.
Consider some of the following variables:
- What is the time of day?
- What are the angle and the direction of the sun?
- Are there other buildings that are providing shade to the roof?
When you look through the TI, if one side of the roof is cool and dark and the other side is warmer and lighter, could it be the sun that is heating the roof and not a fire in the attic?
Take into consideration all building construction materials such as roofing materials (clay tiles, wooden cedar shake shingles, steel panels, and shingles) besides the commonly used asphalt shingles as well as siding materials (vinyl and aluminum, wood, brick, and stucco). All have a different emissivity and, depending on the variables, all give a different reading on your TI.
Practicing your technique outdoors
To help improve the proficiency in using your TI outdoors, here is a simple in-house training drill you can conduct. Go for a walk with your TI in the morning, mid-afternoon, and late evening on a sunny day looking at the roof area and all the walls (photos 1-2). What you could expect is to get three different readings on your TI. Now, conduct the same experiment on a cloudy day at roughly the same time intervals, and you’ll get three different readings again. What was the construction material of your firehouse?
1 This photo was taken on a cloudy day in mid-morning and 2 this photo was taken on a sunny day in the late afternoon. (Photos courtesy of Bullard.)
Another example of interpreting your TI is using a rock outdoors—same rock, different day and time (photos 3-4).
3 The photo of this rock was taken mid-morning on a cloudy day and 4 this photo was taken in the late afternoon on a sunny day.
An example of how understanding emissivity can increase your situational awareness is when looking for potentially ejected victims at a motor vehicle incident at night. If you notice an anomaly or heat signature in your TI, could it be a heated tree or rock (which can remain warm for extended periods) or possibly a victim’s extremity? Again, the same principle would apply if conducting search and rescue for a missing Alzheimer’s patient or a small child.
The more knowledge and understanding you have regarding emissivity and the effects that it has on your TI operations, the better you will perform at your next response call when working outdoors.
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.
