Advanced Imaging


Advanced Imaging Magazine

Updated: January 12th, 2011 09:49 AM CDT

Lighting the Way

From reading license plates to tracking an oil slick, illumination plays a vital role in all applications
The extensive and heterogeneous population of North American license plate designs is apt to baffle even the most robust video-based recognition engines. Imaged in visible light, background graphics often interfere with the letters and numbers, impeding interpretation of the latter. From top to bottom, here are four examples of license-plate images. Full color, monochrome, infrared and segmented for character identification. (Courtesy Inex/ Zamir (Knoxville, Tenn.)

By Lee J. Nelson
Contributing Editor

Fully retro-reflective and non-retro-reflective plates present their own obstacles as each produces infrared images devoid of useful information. Selective (ranges of) visible wavelengths can help. Again, a single set-up is insufficient for all situations that users may encounter.

In the remote sensing arena, wavelength and illumination are also key. That certainly was true for early satellites, Landsat and SPOT, which were intended to pass the northern hemisphere during late morning hours. Light continues to be the controlling collection management variable for spaceborne platforms plus airborne photography: too little, and the reflecting surface does not return sufficient energy; too much, and the sensor is blinded by glinting, caused by sunlight reflecting directly back towards the observer. System managers often have rather narrow operational windows, and those limits indirectly led to the appeal of active systems (LIDAR and RADAR), which are not influenced by natural light.

Intensity, however, is only one piece of the energy input equation. Directionality is another. Lower incident angles beget increased shadowing and raise the specter of the “unknown”...lurking in the shade. Among the most challenging environments for any passive collection system are those where features never get exposed to daylight (as in sub-terrain resource exploration, mine rescue operations, and underwater investigations). From a military standpoint, illumination and incident angles are vital to successful site/target analyses.

The nagging inability to manage and/or compensate for natural light and incidence helped inaugurate the concept of persistence. Persistent surveillance captures various views of an object and exploits the effects of dynamic illumination. The eastern edge of an area might be imaged during mid-morning; then, the system revisits and captures the same area’s western boundary that afternoon. Using multiple-angle acquisitions, information is accumulated and assembled about the entire region-of-interest. Such an approach highlights the attraction of newer UAVs (unmanned autonomous vehicle systems), often referenced by the media and cited, most recently, in last month’s edition of Advanced Imaging.

Remote sensors are proving to be critical and timely tools for assessing the current Gulf of Mexico marine oil spill. Rapid-response reconnaissance is essential to aide in determining the location and extent of contamination (particularly for the thickest portions of the slick) and to forecast its scale, movement rate, and direction via persistent surveillance and drift prediction modeling.

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