Line pairs in digital sensors: what are they, what do they mean, and what do I need to know about them?

By James Ramey, Director of Digital Imaging, Dentimax LLCLine pair is a term that dental sales representatives throw around when they are trying to sell digital sensors. The sales rep will also usually wait to hear what other brands of sensors interest you before throwing this word out. The reason they do this is that the competition they are going up against may have a better number in this statistic than their sensor does! That’s all it really is — another statistic such as resolution, pixels, active area, and so on. But we are here to discuss the statistic of line pairs. This brings me to the initial question: What is a line pair?A line pair is, simply, the result of a test that manufacturers perform to determine a couple of different things: 1) Whether the specific sensor they are looking at meets or exceeds the standard they have set for their product, and 2) what is the number of line pairs they can publish as their statistic. To do this they use what is called a line pair tester.A line pair tester is a piece of film (also referred to as a target) that has markings on it (made out of lead) for every millimeter. In the first millimeter, there is one lead line present. If you take an X-ray of the line pair film and you can clearly see the line, then the sensor successfully has one line pair per millimeter. More lines are added with every millimeter, so the next section will have two lines within the millimeter, then three, four, five, and so on. Most of the line pair testers stop at 20 line pairs per millimeter; but with newer and better sensors, the manufacturers of these test devices have increased the number of line pairs to 25 or even 30.When the test is performed, the tester takes an X-ray of the line pair film and then looks at it. Viewing the X-ray of the test device, the tester looks up the scale from one millimeter to five to 10 and so on, and then tries to see how many individual lines can be clearly seen before they start to blend together and look like a solid block or a bunch of lines that are touching each other (the lines should not touch each other in a true line pair test). This fact alone makes the test somewhat subjective.