A Practical Explanation of the Johnson Criteria
The subjective interpretation of ISR imagery can lead to inaccurate methods of determining the accuracy to which differing ISR gathering equipment is capable of producing.
When engaging in Intelligence, Surveillance and Reconnaissance (ISR) activities it is vital to be able to analyse information swiftly and accurately.
However, the accuracy of which an image or images can be analysed can be considered a highly subjective activity.
Influences such as observer experience, image composition Problem even physical ability of the person or persons performing the analysis can all attribute to the accuracy in which an image is analysed. For example, two analysts may interpret one image and propose two distinctly different opinions.
Johnson’s criteria divide the possible interpretation of an image into four categories.
Detection, Orientation, Recognition and Identification.
These categories are described below:
Detection – An object is present
Orientation – The object is symmetrical, asymmetric, horizontal or vertical
Recognition – The type of object can be distinguished, a person versus a car
Identification – A specific object can be distinguished, a woman versus a man, a specific car.
Minimum required image resolution of a target in terms of line pairs or pixels for a 50 percent probability for an observer to differentiate between the categories and is shown below expresses:
Detection – (1 ± 0.25 line pairs OR 2 ± 0.5 pixels)
Orientation – (1.4 ± 0.35 line pairs OR 2.8 ± 0.75 pixels)
Recognition – (4 ± 0.8 line pairs OR 8 ± 1.6 pixels )
Identification – (6.4± 1.5 line pairs OR 13 ± 3 pixels )
The following can be concluded from the image shown to the right:
Detect - a group of objects can be Detected.
Recognise - the objects can be Recognised as vehicles.
Identify - the object can be Identified as a group of white vehicles and a red vehicle.
To use the Johnson Criteria to predict various sensor system performance, some key system performance specifications are required; such as sensor size and Horizontal Field of View. The size of the target is also required. Using the Johnson criteria for a selected category, an indicative distance can be calculated from the previously listed specifications.
Johnson’s Criteria is useful when attempting to objectively categorise the performance of visual ISR systems.
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