Interpreting Thermal Images

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Interpreting Thermal Images
Fire Services Journal, September/October 2004
by Jonathan Bastian

Interpreting Thermal Images

Is it really that easy?

Over the past six years of development, thermal imaging manufacturers have reduced the size of thermal imagers (TIs), while increasing their simplicity by including fewer buttons and making them generally easier to operate. This movement to simplify thermal imagers is advantageous for firefighters as it reduces some of the training associated with placing a TI in service. It is a disadvantage, however, in that it causes a number of firefighters to infer incorrectly that ease of use means easy to use.

Despite the fact that some TIs have just one operational button and form factors that resemble camcorders, effective use of thermal imaging requires more than just charging the battery and “looking at stuff.” While the images a camcorder sees are just like the images your eyes see, a TI sees a very different picture. Since the thermal imager is registering variations in heat, rather than light, understanding and interpreting images requires basic knowledge and extensive practice.

The Basics

When using thermal imagers, firefighters cannot forget that TIs receive heat and make images from the data. A TI is not an x-ray device; it does not send radio waves or signals; it does not see through most materials. It just detects heat from various objects.

After receiving information on how hot or cold all the surfaces are in a given scene, the electronics inside the TI assign colors to the temperatures and then display them on a video monitor, normally an LCD screen. The most common colors are black, white and shades of gray. White is normally hot, black is normally cold, and varying shades of gray complete the picture by displaying the temperatures in between.

Keys to Proper Image Interpretation

There are three key points to interpreting basic thermal images safely and properly. First, the image your TI generates is relative. This means that in one scene, an item might show as “white hot,” yet still be touchable by an ungloved hand. In another scene, an image that appears “white hot” could indicate an object hot enough to cause burns through a firefighting glove. Images 1 and 2 demonstrate how a “white hot” object can be benign, or dangerous, depending on its surroundings. When evaluating an image, always consider an object’s temperature in relation to the scene as a whole.

Image 1: Firefighters prepare to knock down a bedroom fire. “White hot” in this scene includes burning furniture, obviously hot enough to be dangerous or cause injury.

Image 2: A light switch, with electrical current flowing through it, also shows as “white hot.” However, it is barely warm to the touch. In the air-conditioned room, the switch is relatively hot.

Second, remember that a TI does not see through most objects. It sees surface temperatures. This is especially important to remember when evaluating structural features. Certain structural features may “show through,” even though they are hidden from the human eye. Image 3 shows building studs visible through drywall. Note, however, that the TI is not seeing the studs. It is seeing the temperature difference the studs have created on the drywall, compared to the temperature of the void spaces between them.

Image 3: The thermal imager appears to “see” the studs in the wall. It is merely seeing temperature differences on the drywall where the studs make contact. Heat is conducted from the drywall into the studs because the studs are colder. Winter weather outside has cooled the siding, which in turn has cooled the studs.

Last, firefighters cannot forget that TIs read surface temperatures. This has double-meaning for a firefighter. First, if the TI has “crosshairs” or a “target diamond” in the center of the screen, the temperature reading from that spot is an estimated surface temperature. It is not an air temperature. Second, if a firefighter sees a white “cloud” on the display that appears to be smoke or fog, he must assume this indicates very dangerous, superheated gases. Normally a TI will not detect a gas; however, superheated gases can be visible on a TI. The process is similar to the effect of the sun on asphalt, and the heat waves that can be seen coming from the road. Normally, a person cannot see air. Yet when the sun heats asphalt, the asphalt heats the air, making the air visible when viewed from a certain angle. When air is superheated, its properties change and it can appear on thermal imager displays. Firefighters must practice regularly with the imagers assigned to them to become proficient at recognizing superheated gases.

Image 4a, 4b: The thermal layer in 4a (vanadium oxide microbolometer) is slightly more subtle than it is in 4b (ferroelectric BST). Note: your thermal imager, even if it uses the same style detector, may not display superheated gases the same way. Practice with your TI to understand its performance.

Advanced Tips

Having a command of the basic skills addressed so far should enable a firefighter to interpret most thermal images properly. As firefighters become more proficient, they will see images with odd characteristics. Going back to basics, and remembering how the TI functions, will help a firefighter think through a difficult interpretation.

Review Image 5. This image, taken at 21:00 hours on a summer night, is the southwest side of a single-family dwelling. The image shows several unusual characteristics. The lower portion of the building (the concrete foundation) is warmer than the upper portion (the framed living area). Why is the basement portion of the building hotter? Building construction and materials directly affect how a thermal imager views a structure, and concrete absorbs and retains heat longer than siding. Additionally, the right side of the framed structure is slightly cooler than the left. It appears as if there is a shadow on the siding. Why would a thermal imager see a shadow, when shadows are created by light? This shadow is a “heat shadow” created by a tree that has shaded part of the structure from the setting sun. As a result, this portion of the building cooled sooner compared to the rest of the structure.

In short, this image shows that users must consider external factors, including the sun and weather, as well as the effects of nearby objects (other buildings, trees, etc.). Firefighters cannot forget the critical effect of building construction on how heat is conducted and portrayed throughout a structure.

Image 5: The exposed basement leads to exposure of the concrete foundation. Concrete absorbs and releases heat differently from wood framing and siding, which accounts for the temperature differences between the basement and the main floor. The “heat shadow” to the right is from the tree visible on the left of the image.

During exterior operations, a TI can be confused easily by the presence of sky in the image. The sky has no surface and leads to deep space, which has air temperatures near -451°F (-268°C). So, to the TI, a very cold space with no surface is essentially absolute zero (or the coldest temperature theoretically possible). Image 6 shows this effect, with the eaves of the house appearing white hot. Remember that the TI scales temperatures in relationship to other objects in the scene. The presence of the sky next to the roofline makes the TI choose white hot compared to the sky. BST-based thermal imagers tend to make only portions of the object bordering the sky “hot.” Microbolometers tend to make the entire image “whiter” when there is a lot of sky in the image. To reduce the effect, especially with microbolometers, try to crop the scene so that there is almost no sky in the image. This will help generate more familiar and usable thermal images.

Image 6: The eaves and roofline appear white hot on this thermal image. This is not an attic fire, however. Because the roofline is compared to the cold of deep-space, the white hot effect is due to it being “relatively” hot.

Conclusion

Most thermal imagers utilize the white-through-black color scale to help make thermal images easier for a firefighter to interpret; colors are sometimes added to provide emphasis or clarity to dangerous conditions. Easy operations and smaller size of the TI encourage regular use as well. But truly understanding thermal images can be difficult because firefighters are using their eyes to see a representation (black and white thermal image) of something they normally cannot see (infrared energy, or heat). Regular practice and training will help firefighters develop and maintain solid image interpretation skills.

Use your TI often, wisely and safely.

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