Line Pairs per millimeter or LP/mm is a resolution measurement commonly used to evaluate digital radiography sensors. LP/mm is a measurement of spatial resolution, that is how small a space can you perceive. Spatial resolution is important if you want to see a very small item such as the tip of an endo file on a radiograph. It is not so important if you are looking for subtle changes in the density of enamel.
A line pair gauge like this one shows five line pairs (one dark and one light) converging together into a smaller and smaller space eventually reaching a maximum of 20 LP/mm. It is important to note the gauge only goes to 20. If a radiography vendor tells you that his or her sensor has a LP/mm of 23 or 26 one of two things is happening. They are mistaken or they are quoting a theoretical measurement based on pixel size.
Te problem with theoretical is that it is well… theoretical. The actual performance is always less, sometimes a lot less, that the theoretical upper limit. As a result the only reliable measurement of LP/mm is an actual test using the gauge and the highest possible score is > (greater than) 20.
In actual testing sensors range from as low as 8 to >20.
It is also important to note that in order for a human eye to actually see 20 or greater LP/mm the image must be magnified. Good spatial resolution is a result of both a high-quality sensor and well designed software that can display the image in an appropriate way. Some software enhancement tools such as sharpening may actually decrease LP/mm.
Some radiography vendors have made the claim that LP/mm is not an important measurement. In fact it is important for some things and not for others. It is important if you are trying to see the tip of a #8 file or a pattern of bone trabeculation. It is not important if you are trying to see incipient decay.
Thanks to Doug Yoon and Joel Karafin at XDR Radiography