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Clearing the air: Dealing with visual atmospheric interference
Visibility-reducing atmospheric interference is one of the leading challenges to effective long-range video surveillance. Applications where this can be apparent include homeland security, border patrol, traffic monitoring, city oversight, coastal waterway control, airfield management… the list goes on.
Whether it’s fog in an ocean port, pollution over a city water reservoir, blinding snow on an airport tarmac or blowing dust in an oil field, if you can’t see a target, you can’t observe it. Even on a “clear” day, at long ranges there can still be sufficient atmospheric effects to reduce visibility and obscure important areas of interest. And the farther you try to see, the worse the situation can become.
Fortunately, there are several methods which can aid in dealing with this. Before a suitable solution may be determined, however, distinction must be made as to the purpose of the video system. For most situations, video surveillance can be broken into these three general functions:
- Detection - There is something approaching the perimeter;
- Recognition - There is a vehicle approaching the perimeter;
- Identification - There is a blue truck with U.N. markings approaching the perimeter.
Atmospheric anomalies can interfere with this entire range of video surveillance applications. Each solution has its own benefits and characteristics, and several are described below.
One of the methods employed in attempting to overcome such issues is the use of special infra-red (IR) or “thermal-imaging” cameras. These devices display images based on a difference in objects’ heat signatures rather than relying on visible light. As such, IR cameras have the benefit of functioning in total darkness. Because they cannot distinguish visible characteristics, they are often best-suited for detection and basic recognition of a threat. For example, a thermal camera can be used to confirm that a tractor-trailer rig is approaching, but generally will not be able to offer further identification, such as color or markings. Thermal imaging equipment is generally more expensive than conventional CCTV solutions.
Another solution commonly employed is in-line video processors and image enhancers to manipulate a standard camera signal. Electronic filters and microprocessor-controlled circuitry are used to help “strip away” the visual interference caused by the fog, smoke or other phenomenon, and then reconstruct the desired underlying image.\
Generally used to process video from the visible light spectrum, these can be very effective in situations requiring identification of specific subjects or threats (although some processors may remove the color component of the video during processing). Usage may be limited in areas of darkness, however, as even long-range IR illuminators may be unable to penetrate the atmospheric aberration the user is attempting to overcome.
A more recent development in this arena is the introduction of “in-the-lens” processing. An example of this newer technology can be found in the Pentax Atmospheric Interference Reduction (PAIR) 55x zoom lens. Introduced in late 2009, PAIR technology is effective against not only fog, smoke and rain, but also solid-particle interference such as blowing sand and snow. Like the in-line processors above, PAIR works in the visible light spectrum, but unlike the stand-alone enhancers, the electronic circuitry and processing power are kept inside the camera lens housing, reducing the need for additional head-end space, and generally reducing the overall cost. Under the right conditions -- and when coupled with the latest EMCCD-equipped video cameras -- PAIR can also be used to provide clearer color images in near darkness.
Turning up the heat
The phenomenon which has proven to be the most difficult to overcome is heat haze, also known as “turbulence” or “scintillation.” How is heat haze generated? Simply put, the refractive index (the amount that light bends as it passes through a substance) of air changes with its temperature. As a surface warms, the air above it is heated unevenly, resulting in visible ripples and waves. Typically, the larger the temperature differential (surface vs. air temperature), the more apparent this becomes. The effect is most noticeable and problematic when attempting to conduct long-range surveillance over stretches of land which have been heated by the sun, such as an airfield tarmac or desert floor. The effects are amplified by distance, as the distortion layer becomes thicker and thicker.
Until now, most solutions to deal with heat haze have been expensive, and many can only be applied to pre-recorded video, as the amount of processing required often results in a delay too significant for live viewing. In the second generation of PAIR, Pentax addresses this concern by maintaining an in-the-lens solution which provides live, color video processing, with effective reduction of heat haze distortion.
Picking the “right” solution
What is the best technology for your application? Since each scenario is different, and needs vary, there is no simple answer. Arrival at the final solution is driven by factors such as the environment and conditions in which the system will be used, the level of detail required for proper information-gathering and the budget constraints for the project. Contacting a security integrator with experience in your particular area of concern is a good first step. Manufacturers will also be eager and willing to assist in making the right choice for your needs.
Andrew Shemo is a technical sales executive with PENTAX Imaging Company. He can be reached at: