A linen tester is a fixed focus magnifying glass that use primary used for inspection of fabrics and printed material. Its a hands free fixed focus magnifier you lay in on the surface and it does not require being held a focal length. The measuring increments allow for thread counting (threads per inch) as well as measuring type and font sizes. They are often used by jewelers as well as fingerprint and document analysis. The most common magnification is 5x-6x, and as magnification goes up the lens size goes down that is due to the laws of physics.