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====Edge-spread function====
The two-dimensional Fourier transform of an edge is also only non-zero on a single line, orthogonal to the edge. This function is sometimes referred to as the '''edge spread function''' (ESF).<ref>Holst, G.C. (1998). ''Testing and Evaluation of Infrared Imaging Systems'' (2nd ed.). Florida:JCD Publishing, Washington:SPIE.</ref><ref name="ElectroOpticalTestLab">{{cite web|url=http://www.electro-optical.com/html/toplevel/educationref.asp|title=Test and Measurement – Products – EOI|website=www.Electro-Optical.com|access-date=2 January 2018|archive-url=https://web.archive.org/web/20080828124035/http://www.electro-optical.com/html/toplevel/educationref.asp|archive-date=28 August 2008|url-status=dead}}</ref> However, the values on this line are inversely proportional to the distance from the origin. Although the measurement images obtained with this technique illuminate a large area of the camera, this mainly benefits the accuracy at low spatial frequencies. As with the line spread function, each measurement only determines a single axes of the optical transfer function, repeated measurements are thus necessary if the optical system cannot be assumed to be rotational symmetric.
[[File:MTF knife-edge target.jpg|thumb|right|215px|In evaluating the '''ESF''', an operator defines a box area equivalent to 10%{{citation needed|date=August 2013}} of the total frame area of a '''knife-edge test target''' back-illuminated by a '''black body'''. The area is defined to encompass the edge of the target image.]]
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