THERMAL IMAGING FOR THE FIRE SERVICE, PART 3: THERMAL CHARACTERISTICS
BY STEVEN P. WOODWORTH
Part 1 appeared in July 1996; Part 2 appeared in August 1996.
Thus far, we have discussed the basics of thermal imaging (why vision is impaired on the fireground) and the electromagnetic spectrum (what we can see on the fireground). In our discussion of the electromagnetic spectrum, we stated that thermal imaging was an interpretation of infrared energy. We also defined what infrared energy is and where it falls within the electromagnetic spectrum. This month, we will discuss the sources that emit the infrared energy and how they are important to the firefighter. The image the firefighter using a thermal imaging device sees obviously will be a representation of the infrared sources. Understanding these sources will provide the firefighter with a distinct advantage in interpreting the image.
The three basic kinds of infrared emitters of interest to the fire service are active emitters, passive emitters, and direct-source emitters. It is important to be able to distinguish among the types of emitters and how each will affect the operation.
Active emitters always emit infrared energy with little variation. The human body is an example of an active emitter. The human body will always emit infrared energy (assuming the person is alive) without great variation. Generally, the human body does not vary widely in temperature. Certain factors can affect the infrared energy emitted from a person. Heavy clothing such as turnout gear can cause the person to build up heat and will tend to insulate the body. The turnout gear of a firefighter and heavy clothing on a civilian will mask the image. If the turnout gear is wet, the water will tend to mask the image. The turnout coat and pants on a firefighter might appear as a gray image with dark spots where water is present. The individual`s face might appear white, indicating a higher temperature. The body temperature is the same throughout; however, water and heavy clothing are masking portions of the image.
Passive emitters are objects that have infrared emissions that can vary greatly–for example, a piece of steel. At room temperature, approximately 70°F, the steel would appear cold or as a darker image than a person in the same room. If we apply heat to the steel with a torch, we will greatly change the infrared emissions from it. Soon, the steel will appear white in our thermal image, indicating increased temperature in comparison with the surroundings.
Thermal imaging devices can be used to detect fires in void spaces. The ability of the device to detect fire in a void space depends on several factors. The first is the minimum resolvable temperature difference (MRTD) of the device, as discussed in Part 2. The molecular density and the mass of the building components (passive emitters) also will affect the ability to detect fires in void spaces. Materials that have closely arranged molecules are considered dense and usually are well-suited for shielding, such as lead. Lead is more dense than wood, whereas wood is more dense than paper.
This same principle applies to building materials. The type of construction generally gives some clue as to the density of the building materials. Thermal imaging can be performed through some denser materials when the objective is to locate a direct source. A human being will not provide enough infrared energy to provide an image through most construction materials. The exception might be in a collapse situation where the person is against light material such as gypsum board for an extended period of time. Depending on the thermal imaging device used, the operator may be able to pick up residual images of victims. This cannot be done through materials. However, it can be useful in identifying victims` locations or returning to find additional victims.
A material may have greater shielding capabilities as mass increases. Wood may need to be one foot thick to have the same shielding capabilities as one inch of lead. This principle is important for firefighters to understand for thermal imaging. Buildings often are remodeled by simply applying new materials over old ones. In the city of Atlanta, it is not uncommon to find one or two layers of gypsum board over plaster and wood lath or one or two layers of gypsum board over tongue and groove wood boards, an example of increasing the mass of building materials. Another example of increasing mass during remodeling or renovation is the constant layering of roofing materials or partition walls in office buildings.
A fire in the attic of a typical residential structure could easily be detected through gypsum board or plaster and lath using a quality thermal imaging device operated by a trained operator. The fire will heat the ceiling material, which in turn will emit infrared energy. Gypsum board and plaster and lath are not very dense materials and generally have little mass compared with other building materials. Therefore, they will absorb heat energy rather quickly, in turn emitting infrared energy and providing a good thermal image. This enables firefighters entering a structure to use thermal imaging devices to evaluate structural conditions and identify types of construction such as trusses. Firefighters will also be able to locate fires in structures quicker.
A fire behind a concrete wall probably will not be visible to the detector. The density and mass of a concrete wall typically are far greater than for gypsum board or plaster and lath. Hence, far greater amounts of energy would be required to raise the temperature of the concrete to the point where a thermal image could be of value. Lightweight concrete can be thermally imaged; however, the operator must realize tremendous amounts of energy are required to present the image.
I once used a thermal imaging device in Atlanta to determine where to place a ventilation hole on a commercial structure with a lightweight concrete roof. Shortly after locating the fire and starting a hole, all personnel had to evacuate the roof. The fire below was providing a good thermal image through the lightweight concrete; however, we must keep in mind it is also bringing the building down. When dealing with that amount of fire in lightweight construction, the safety of personnel can be in jeopardy. Understanding how the density and the mass affect thermal imaging can be of great value to the operator on the fireground.
Direct sources produce great amounts of energy. Examples of such sources are fire, chemical reactions (that do not have open flame, such as the neutralization of a corrosive material), and the sun. How a thermal imaging device is affected by a direct source is a function of the electronics within the device, primarily the gain. Most thermal imaging devices can handle the focused energy when aimed at a direct source; however, the image presented will depend on the operating principle of the unit.
TYPES OF DEVICES
One type of device, the Pyro Electric Vidicon (PEV), is known for a “whiteout” when pointed at a direct source. The unit must then be aimed away from the source to clear out. This happens when the energy being received by the unit is shown across the entire viewing area. The unit is showing exactly what it is reading–i.e., great amounts of energy.
The other type of device presently finding widespread use in the fire service is the focal plan array chip (FPAC) technology. This unit provides a virtual reality image instead of a pictorial representation. This operating principle offers the distinct advantage of being able to view a direct source without the image “whiting out.” If a firefighter were using a device with FPAC to search for victims or a downed firefighter, it would not matter if the victim were between the rescuer and the fire. The rescuer would still have an image of the victim with the fire in the background. The PEV-type device could “white out,” and the rescuer would lose the image of the victim. The FPAC costs slightly more than the PEV; however, it costs less to maintain. The vacuum tube in PEV units must be replaced at regular intervals to ensure accuracy.
No thermal imaging device should be pointed directly at the sun. This can permanently damage the sensor, which then may have to be repaired by the manufacturer. Next time, “Interpreting the Image.” n
STEVEN P. WOODWORTH is a firefighter with the City of Atlanta, Georgia, assigned to Squad 4. He is an adjunct instructor at the Georgia Fire Academy and a volunteer firefighter for Fayette County. He is co-author of Fighting Fires with Foam (Van Nostrand Reinhold: New York City, 1992).
Two layers of gypsum board over plaster and lath. (Photo by author.)
Gypsum board has been placed over tongue and groove and lath, making access extremely difficult. Thermal imaging is by far the quickest and easiest method for checking for fire in voids and fire extension. (Photo by author.)
An example of a firefighter using a PEV-type thermal imaging device. PEV devices are handheld units. (Photo by Bob Athanas.)
A firefighter using a FPAC-type thermal imaging device. This device is helmet-mounted to keep the firefighter`s hands free for other tasks. (Photo courtesy of CairnsIRISTM.)