A linen tester is a strong magnifier with a measuring scale and a built-in stand. The linen tester was invented to check the quality of woven fabrics. It is used in the textile industry to measure the number of weft and warp threads within a certain area of fabric. Today, it is more commonly used to measure the line width and check the registration of color separations in the field of printing and publishing. Linen testers are also used in other branches of industry. For example, they can be used to measure the line weight of barcodes.