CCTV, HOW DOES IT WORK?

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1 August 25, 2005 CCTV FLAME DETECTION Video flame detection is an attractive concept. We ve all heard the adage, seeing is believing. In short, we trust our eyes to deliver accurate information. New designs in optical fire detection have brought seeing into flame detection. Several companies, including Det-Tronics, have invested heavily in CCTV technology with the hope that it would deliver an increased level of safety. The hope is still there, however the technology is not. CCTV, HOW DOES IT WORK? CCTVs are digital cameras. Images are captured by the camera on sensing elements called pixels (picture elements). A high-speed microprocessor analyzes the collection of pixel information to determine the presence or absence of a fire. We can get a good idea of how this process works by watching a live TV image. The pixels on the screen create an array of images. Some images never move while others change size, shape, color and intensity. Our brain scans all of the pixel images, looking for telltale signs of an unwanted fire. The CCTV flame detector s microprocessor has the job of interpreting a continuous stream of black and white or color images, looking for a fire. A significant challenge for the microprocessor is the sheer volume of data. There is much more extraneous information in a stream of visible light images than, for example, a filtered Infra Red (IR) image. Compounding the problem is the fact that there is no property of light emitted from fire in the visible spectral range that is fundamentally unique to fire. This means that there is nothing highly specific that the microprocessor can focus its computing power on to determine if there is or is not a fire in the field of view. Every illuminated object in the camera s field of view registers as sensor-input and must be accepted or rejected as a fire image. Objects that are moving, changing shape, color, shade, and size, must all be differentiated from a possible fire. By comparison, the hot CO 2 produced in a hydrocarbon fire creates a distinctive radiometric IR signature in the 4.2 to 4.7micron range. IR filters in an infra-red flame detector screen out most of the radiation clutter generated by other objects in the field of view because they are very dim relative to fire in this spectral region. This very powerful means of differentiation is severely limited in flame detection methods that are restricted to visible light emissions. 1

2 Fig Micron Image Fig. 2 Visible Light Figure 1 is a depiction of the primary property of a hydrocarbon fire that is generating intense IR radiation at 4.4 microns. Other objects in the image are nearly invisible at 4.4 microns. Figure 2 is a visible light CCTV image of the same fire. The microprocessor s challenge is to distinguish the difference between fire images and the numerous non-fire images. FIRE TYPES SUITABLE FOR CCTV: CCTV flame detectors are best suited for detecting bright flames and perform better in low level or no light conditions. Typical sources of bright flames are burning liquid hydrocarbons such as gasoline and n-heptane, which generate bright yellow/orange flames. Methane, propane and many other light-hydrocarbons, however, burn with a translucent blue flame and are difficult to detect by CCTV detectors, because they emit comparatively little light. As ambient light levels increase, the flames can become indistinguishable from their surroundings. A rule of thumb is that a CCTV detector will have difficulty detecting a fire if the flame is difficult to see with the naked eye. A thorough analysis of the hazardous area s fuel sources should be conducted prior to selecting a detection technology. If one of the fuels in the area has blue translucent flame characteristics then CCTV will be severely challenged. Likewise, lighting conditions should also be considered since bright lights or full sunlight conditions may reduce detection range. 2

3 REAL FIRES VS FALSE ALARM SOURCES: An often repeated comment about fire detectors is: it is easy to build a flame detector to see fires, it is difficult to build a detector that does not alarm in the presence of false alarm sources. For example, UV detectors have no trouble detecting most flames, but they also respond to UV rich friendly UV sources such as arcs, sparks, welding, lightning, X-rays. Time delays, detector isolation and orientation have resolved some of these issues. Similarly, low-tech IR detectors often responded to IR rich sources, hot objects such as exhaust vents and heated vessels. Multi Spectrum IR detector technology has virtually eliminated all but the most eccentric attempts to create false alarm scenarios. As described above, one of the challenges associated with CCTV flame detection is the microprocessor s ability to distinguish between real fires and all of the other objects in the field of view. Tests by Det-Tronics have demonstrated numerous CCTV false alarm scenarios, for example the reflection of light off brightly colored clothing. TYPICAL DETECTION RANGE FOR CCTV: Detection ranges are typically verified via independent third party approval tests. The most recognized standards are EN54-10 for Europe and FM3260 for the Americas. Both standards are widely accepted throughout the rest of the world. The tests establish a level of detector performance (speed of response and detection range and cone of vision) to specific fuels. The data is helpful in selecting the correct detector for installation and placement. A CCTV flame detector currently available on the market has a published detection range of 10 meters (33 ft) to a 0.1 m 2 n-heptane pan fire as tested by Factory Mutual (FM). Det-Tronics CCTV detector prototypes have similarly short detection ranges. By comparison, the Det-Tronics X3301 multi-spectrum IR detector s range is 64 meters (210 ft) as verified by FM, or 6 times the range and covers an area 38 times larger (see figures 3 & 4). Forty CCTV flame detectors are required to cover the same area as one X3301! The detection range is even shorter, as described earlier, for less luminous fires produced by fuels such as methane or propane. The reduced detection range and area coverage is so significant that it is often cost prohibitive to use CCTV technology. Finally, even with 50% detector window obscuration, most UV and IR detectors have significantly longer detection ranges than CCTV detectors with clean optics. 3

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5 WHAT IS THE ROLE OF CCTV FLAME DETECTION? The role is to detect unwanted and potentially hazardous fires and provide rapid visual confirmation of a fire hazard to the operator(s), who in turn take appropriate action. Depending on what is seen on the monitor, the operator can decide on what action to take. He might conclude that the alarm is false and override or abort normal fire warning or fire fighting actions. He might conclude that the incident is minor and take action to deal with it, or that it is a major incident and initiate appropriate action and warnings. By today s standards, CCTV flame detector performance is, at best limited. Virtually every industrial IR flame detector available today can outperform current technology CCTV flame detectors in detection range, area of coverage, false alarm rejection, detection of fire types and detection of fires through optical contamination. How do current CCTV flame detectors perform as visual surveillance cameras? Sensing elements used in the most basic point-and-shoot cameras contain 3 to 7 million pixels (megapixels). Today s CCTV flame detector sensing elements contain approximately 1 megapixel sensors. The resulting video images are therefore grainy. Low ambient light and adverse weather conditions can further degrade the image. If both flame detection and surveillance are needed, it is Det-Tronics recommendation to use a high performance flame detector, such as the Det-Tronics X3301 Multi-IR detector and a high performance off the shelf explosion proof surveillance camera. In most cases the results will be a superior system at a lower cost. Furthermore, some conventional flame detectors, such as the X3301, have auxiliary relay outputs that can be used to trigger a surveillance camera to record events. Since these outputs are isolated from alarm relays, their use does not impair or complicate operation of dedicated fire alarm equipment. 5

6 CONCLUSION: There is no perfect fire detector. Accidental fires are by their nature unpredictable. How and where a fire starts, shape, size, rate of growth, materials involved, and the external physical environment are all subject to the vagaries of chance. Therefore a reliable, robust flame detector must operate on principles that permit its operation across the widest possible range of variables. The advantage of combining flame detection and video surveillance in one device is compelling. Unfortunately, current technology does so at extreme expense to both functions. CCTV flame detection capabilities are sub-standard compared to the current conventional detectors, and the utility of the video images is limited by their lowresolution and poor quality. Det-Tronics understands the value of CCTV surveillance in hazard management and recommends the use of high-quality video surveillance cameras where needed. Det-Tronics 10 year CCTV flame detector development project was helpful in uncovering many of the issues with this technology, which, in its view, have yet to be resolved. Det-Tronics will continue to explore all frontiers in flame detection in the hope that it will eventually find a breakthrough that will permit Det-Tronics to put its name on a CCTV flame detector. In the meantime, Det-Tronics will only endorse the use of a dedicated, fit for purpose, flame detector possibly used in combination with a suitable video surveillance camera for operator reassurance. 6