SBIR has chosen the edge target for use in its IRWindows standard target set. The signal from scanning across the edge target is differentiated to get the line spread function which would have been provided by a slit target. A Fourier Transform is then performed on this line spread function to generate the MTF.
There are several advantages to the edge target over the slit target. The first is its general applicability. A slit target must be matched to the element size of the IR sensor to provide an accurate spread function. The slit width should be no more that 0.1 IFOV, with a narrower width even better. The target must be carefully manufactured to insure constant width over the height of the slit. A slit this small severely limits the flux passing through it, decreasing the signal-to-noise ratio. A separate target must be manufactured for each element size of interest.
The edge target avoids all of these problems. A single edge can be used for any size element, and the manufacturing constraints are not so severe. The signal can be increased independent of the target size, insuring the maximum signal noise ratio. The only drawback of the edge target is that the differentiation step increases the noise sensitivity of the MTF measurement, which is dealt with independently using other methods.
For the IRWindows system, the advantages of the edge target over the slit target far outweigh the disadvantages.