Photonics Detects Wildfires
Brent D. Johnson
The fire season of 2002 was the second worst in 50 years, burning more than 7 million acres and engulfing Arizona, Colorado and Oregon. The devastation was a wake-up call to authorities charged with protecting the nation's parks and wilderness areas, and motivated private citizens to take aggressive action to protect their homes and property.
Data collected via satellite can be used for early detection and precise location of fires. This multispectral enhanced image was taken of the Rodeo and Chediski wildfires in June 2002. Courtesy of the National Oceanic and Atmospheric Administration.
After watching three blazes sweep past his Boulder, Colo., home in recent years, R.J. Smith decided to fashion a detection system that would give him advance warning. Smith is a volunteer fireman who worked for a combined 21 years in the fields of laser communications and remote sensing at McDonnell Douglas Corp. and at Ball Aerospace & Technologies Corp.
He looked at infrared sensors as a possible solution, but so many things glow in the IR region that it was a massive processing problem to distinguish a fire signal from other objects, he said. So he turned to the UVC part of the spectrum. UVC has the advantage of being solar-blind, which is a huge advantage for fire detection.
A fire alarm system based on UV emissions detection was designed and commercialized by a homeowner in a fire-prone area of Colorado.
Because the ultraviolet flux from the sun is strong, the device needed to be able to discriminate the radiation with 120-dB rejection of UVB and longer wavelengths. He selected the Flame Sensor UV Tron R2868 from Hamamatsu Corp., which is a nearly solar-blind sensor in the wavelength range of interest: 230 to 250 nm. It uses the photoelectric effect of metal and a gas multiplication effect to detect the weak UV emission from flames between 185 and 260 nm. The detector has low input voltage, does not require optical visible-cut filters and can monitor a nearly hemispherical sunlit field within a range of about one mile without scanning, image processing or expensive optics. In addition, the current amplification within the tube is so high that it doesn't require an amplifier.
Smith combined the sensor and an alarm into a two-part, battery-operated fire detection system called X3. He created the company Fire Scout Inc. to market the devices, and about 100 of the systems already have been deployed on homes in fire-prone areas. They sell for about $200 apiece, compared with $5000 and up for most IR systems. Smith would like to see them used as part of a larger early warning system maintained by the National Interagency Fire Center. In fact, such an integrated system that links ground stations could be a reality by next fire season.
In August, NASA announced that, as part of its mission to protect the home planet, it has developed a sensor web that will combine spectral data from the Moderate Resolution Imaging Spectrometer (Modis) on the Terra and Aqua satellites as well as from the Earth Observing 1 satellite. Real-time transmission of data to a ground-based rapid-response center could detect the precise location of flare-ups within hours.
When wildfire sites are identified, firefighters can move in with portable thermal detection equipment. In wildland firefighting, thermal imagers are good for identifying the fire line point, said Mike Studer, marketing manager for infrared fire and rescue products at Raytheon Commercial Infrared (RCI). This gives the incident commander more information about the position and direction of the fire line without being inhibited by daytime radiation. The cameras are also useful for identifying residual embers that might flare up into a new fire.
RCI makes a series of thermal detectors that are designed to identify the IR signature of fires in the mid-IR range between 3 and 5 µm and the long-wave IR between 7 and 14 µm. They are based on two types of technology. The first, traditional beryllium strontium titanate (BST) detection, has been used in law enforcement and firefighting and is, perhaps, most famous for its application in the Cadillac Night Vision system.
The second is based on amorphous silicon. This enables a smaller form factor, allowing OEMs to reduce camera size and weight. The future benefits of amorphous silicon lie in manufacturability, Studer said. However, it does exhibit slightly different characteristics from BST. It handles a narrower dynamic range and requires additional processing and an electronic iris mode, which produces a stronger image through electronic adjustment of the gain/brightness.
RCI supplies the amorphous silicon detectors to Sage Technologies Ltd. of Willow Grove, Pa., which produces a hands-free unit called the HelmetVue. According to Sage's Wesley Sheridan, to achieve a hands-free device, the designers needed the smallest size and lowest power. He said that they had started with BST, which has better image quality, but that that would have required a thermoelectric cooler, which draws a lot of power.
They finally selected the amorphous silicon-based detector because it could run on two AA batteries. The HelmetVue is being distributed by the Total Fire Group of Dayton, Ohio, as the Fire Warrior.
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