“Spatial frequency” refers to the number of pairs of bars imaged within a given distance on the retina . One-third of a millimeter is a convenient unit of retinal distance because an image this size is said to subtend one degree of visual angle on the retina.
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Spatial frequency is generally expressed as cycles, or line pairs, per millimeter (lp/mm) in analog environments, but for digital systems, cycles per pixel (c/p) is more appropriate where sensor sizes vary from one detector to another.30 Eyl 2013
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