Article Archive
Ultra Violet, Infrared & Optical Aids
Detector Technology Evolves To Meet Modern Demands
Vol 20 No 4

Modern "Multi-Spectrum" optical flame detectors evolved from early optical flame detector technologies that used a single ultraviolet (UV) light sensor, said Mark E. Dundas of Houston-based Detection & Measurement Systems, Inc.

"You had an ultra violet light sensor mounted in an explosion-proof enclosure together with its associated electronics," Dundas said. "If the detector saw UV from a fire source it would go to alarm in just a very few seconds."

Detection & Measurement Systems is the Texas representative for Fire Sentry Corporation multi-spectrum optical flame detectors and CCTV based video smoke detectors and for Zellweger Analytics fixed and portable combustible and toxic gas detectors.

UV-based optical flame detectors look at a specific band of ultraviolet light found in fire which is not normally present in an outdoor environment, Dundas said. This wavelength is also found in sunlight, but the atmosphere absorbs it long before it reaches the ground.

The problem for these early detectors was that at least two other sources exist for this UV wavelength in a typical industrial environment -- lightning and arc welding. These commonly produced false alarms from UV-based optical flame detector, Dundas said.

"So manufacturers developed UV/IR flame detectors, adding an infrared sensor that looks at a specific band of infrared light that is typically produced by fire," Dundas said. "In addition, the flame detector looked for flicker in the infrared light sources, simular to the flickering light of a fire. When the UV sensor, the IR sensor and the flicker detection voted positive, the device would go into alarm."

Yet, there were still problems with false alarms. For example, if someone was welding in the distance while, at the same time, sunlight reflected off a nearby body of water , the combination of UV and flickering IR would activate an alarm without a fire. The technology soon got a black eye from these and other false alarm issues, Dundas said.

New technology that came out of the missile defense industry proved to be the salvation for optical flame detectors, Dundas said. From 1970 to 1973 Fire Sentry president and CEO David A. Castleman was an Electro-Optical Electronic Engineer at the U.S. Department of Defense Naval Missile Center's Electro-Optics Department at Point Mugu, California.

"Basically, how missile detection systems work is that arrays of UV, IR and visible light sensors are located in different positions on an aircraft's fuselage," Dundas said. "Now, a missile is a big stick with flame blowing out the rear. That spectral signature travels at the speed of light in all directions."

When a missile approaches an aircraft one or more of the sensor arrays pick up the unique signature from the flame of the missile. That data is conveyed to a microprocessor that does an immediate analysis of that spectral signature, looking through tables of different spectral signatures for different types of missiles.

"Specific missiles will have a specific fuel associated with it," Dundas said. "This serves as a good identifier of the missile itself. The system has to be able to distinguish between a friendly missile fired by a buddy and an unfriendly missile fired from an enemy fighter."

This technology has to work regardless of the spectral chaos that fills the sky in an aerial dogfight, he said.

"You have an environment where the sun is high in the sky emitting tons of spectral noise," Dundas said. "There may be tanks burning on the ground and other jet aircraft flying nearby. Yet, the system has to be able to pick that missile out of all that spectral activity with an exceptionally high degree of false alarm immunity."

Essentially, missile detectors are elaborate fire detectors, Dundas said.

Castleman soon adapted that technology to Fire Sentry optical flame detection products. While most UV/IR flame detectors rely on one sensor for each, Fire Sentry uses two UV sensors, one IR sensor and two visible light sensors. The information collected by the sensor array is fed into a microprocessor for onboard spectral analysis. A fire alarm is declared when streaming spectral data from the environment fits into the generic fire spectral model within the flame detector's microprocessor. This includes hydrocarbon fires, non-hydrocarbon fires or even burning metals.

"Multi-Spectrum technology is highly effective at detecting a fire, while providing exceptional immunity against false alarms," Dundas said. "This is much more sophisticated technology than anything else out on the market."

Fire Sentry is coming out later this year with a new range of optical flame detection products based on IR and visible light spectrum technology.

Meanwhile, Fire Sentry is having great success with its visual smoke detection systems that marry sophisticated motion detection software with closed circuit television cameras.

"The ability for video smoke detection to reach out and detect smoke in a manner similar to optical flame detection is a huge leap in smoke detection technology. It's quite amazing. All other technologies require the smoke to reach the detector in order for detection to occur. o

To reach Mark Dundas, call (713)541-9800 or e-mail him at


P: (979) 690-7559
F: (979) 690-7562

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