# What is an appropriate set of 4-bar targets for MRTD (Minimum Resolvable Temperature Difference) measurement?

There are two references for selecting a set of 4 bar targets with the spatial frequencies covering a customer s range of interest. The Tri-Services Working Group recommends 4 spatial frequencies in their Guide for Preparing Specifications for Thermal Imaging Sets. These are recommended frequencies, not requirements. They are: 0.2 fo 0.5 fo 1.0 fo 1.2 fo fo is 1/(2 IFOV) where IFOV is the Instantaneous Field of View, (also called the DAS, Detector Angular Subtence). Holst simply refers to measuring MRTD in 3 places. A high frequency measurement (the value is not specified), a mid frequency test (the fo value described above), and a low frequency measurement which he has dependent on viewing distance. Since fo is the theoretical Nyquist frequency limit if a starring array has a 100% fill factor, the 1.0fo and 1.2fo values are adequate for the high frequency limit. The 1.2fo value is somewhat arbitrary, so a range of 1.2fo to 1.5fo should be acceptable (and is actually discussed in the T

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