The image based factor Opacity refers to the amount of opaque areas in an x-ray image. X-ray systems with higher penetration have the potential to reduce detection problems due to opacity. In addition, it is possible to implement image measurement algorithms in x-ray equipment that warn the human operator (x-ray screener) with a ”dark alarm”, which would be triggered by opaque areas that are deemed too large or dense for unassisted human interpretation. Manual search would follow when a dark alarm was indicated.
http://casra.ch/publications/doc/SchBolHalHelBelHay2008.pdf



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Note added at 16 hrs (2012-02-18 17:16:01 GMT)
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Now, it is possible to resolve the majority of dark alarms (a dark alarm is defined as occurring where insufficient penetration was recorded by the low energy pulse) without providing excessive dose at the external surface of the unit.
http://goo.gl/ahsSx

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Note added at 16 hrs (2012-02-18 17:16:52 GMT)
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"dark alarm" "x-ray"
http://goo.gl/RZ2Gt