Line Pairs per Millimeter (lp/mm). This is a measure of resolution, which refers to pairs of lines pressed tighter and tighter together until they eventually merge into a single line. The more lines that can be pressed into a millimeter and still be seen as individual lines, the higher the resolution. Digital radiography sensors can acquire images ranging from 8 to <20 lp/mm. Line pair gauges like the one shown here have a maximum setting of 20. Any vendor who tells you his sensor has a resolution of 22 or 25 etc is either wrong or basing the claim on theoretical limits not actual performance. The unaided human eye can distinguish about 10 to14 lp/mm. Is that extra resolution lost? Not really that’s where the software takes over.
For more information look here.