What Are Johnson’s Criteria?
Johnson’s criteria are a set of minimum standards used to evaluate how well a night vision thermal imaging system can detect an object under specific conditions. To assess this, Johnson’s criteria use digital signal processing measurement and analysis to interpret a field test of the night vision equipment.
For the test, a target object, such as a soldier or a tank, is placed at a distance where it is barely detectable by the night vision equipment. To digitally interpret the object, a horizontal or vertical barcode is also placed in the field of view at the same range, and its spatial frequency is increased by adding bars until the total number of bars becomes difficult to distinguish. The spatial frequency of the pattern can then be specified in terms of the number of lines needed for a pattern that covers the object’s minimum dimension, such as the number of horizontal lines needed to cover the height of a tank. This method is used to express resolution in terms of line pairs, where a pair is formed by a bar and the white space between it and the next bar, together equivalent to two pixels on a TV screen.
Johnson’s criteria uses this method measuring resolution to evaluate a night vision system in terms of how many lines it takes to perform four functions:
- Detection: detecting whether an object is present
- Orientation: discerning whether an object is symmetrical or asymmetrical as well as its directional orientation
- Recognition: recognizing which class an object belongs to, such as recognizing a house, truck or man
- Identification: identifying descriptive details of the object, such as whether a vehicle is a pickup or a minivan.
How well a surveillance system performs according to these criteria can affect its quality rating as well as thermal camera cost. These criteria can also be used to help optimize surveillance system set-up by determining where a camera should be placed to capture the desired resolution.