The Outdoor Effects Emissivity Has on Thermal Imaging
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 the 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
Paint, Aluminum 0.27-0.67
Paint, Oil 0.92-0.96
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?
This article by Manfred Kihn was published in Fire Apparatus Magazine in November issue 2021.