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 f₀ 0.5 f₀ 1.0 f₀ 1.2 f₀
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 f₀ is the theoretical Nyquist frequency limit if a starring array has a 100% fill factor, the 1.0f₀ and 1.2f₀ values are adequate for the high frequency limit. The 1.2f₀ value is somewhat arbitrary, so a range of 1.2f₀ to 1.5f₀ should be acceptable (and is actually discussed in the Tri Services Guide).
The low frequency measurements establish a limiting asymptote, so having both frequencies is useful. The values don’t have to be exactly 0.2f₀ and 0.5f₀. If the number of targets available for testing is limited, then a single value somewhere between 0.2f₀ and 0.3f₀ should be used.
Summary:
| Preferred 4 Target Set | Minimal 3 Target Set | ||
| 1 | nominal 0.2 f₀ (range 0.18f₀ to 0.22f₀) | 1 | nominal 0.3 f₀ (range 0.25f₀ to 0.4f₀) |
| 2 | nominal 0.5 f₀ (range 0.45f₀ to 0.55f₀) | 2 | nominal 1.0 f₀ (range 0.9fo to 1.1f₀) |
| 3 | nominal 1.0 f₀ (range 0.9f₀ to 1.1f₀) | 3 | nominal 1.2 f₀ (range 1.2f₀ to 1.5f₀) |
| 4 | nominal 1.2 f₀ (range 1.2f₀ to 1.5f₀) | ||
Where:
f₀ = 1/(2*IFOV)
IFOV = (Detector Element Size)/(Focal Length)
