A quality linen tester with a biconvex white-glass lens and a black aluminum frame (anodized). In the foot (31x31 mm window) is a glass plate with a mm/inch scale with a length/dimension of 20 mm (1 inch). It is foldable and comes with a protective pouch for safe storage or transport.
Linen Testers are strong magnifiers with a measuring scale. Historically they
were (and still are) used to check the thread size and the number of woven threads per square inch. They are also used extensive in the graphic industry, desktop publishing, film and plate, photo negatives, agriculture professions etc.