A linen tester is a strong magnifier

Magnifier

A magnifier is a device used for magnification.Magnifier can also refer to:*Magnifying glass, an optical device for magnification*Screen magnifier, software that magnifies part of a computer screen...

 with a measuring scale.

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

Weft

In weaving, weft or woof is the yarn which is drawn through the warp yarns to create cloth. In North America, it is sometimes referred to as the "fill" or the "filling yarn"....

 and warp

Warp (weaving)

In weaving cloth, the warp is the set of lengthwise yarns that are held in tension on a frame or loom. The yarn that is inserted over-and-under the warp threads is called the weft, woof, or filler. Each individual warp thread in a fabric is called a warp end or end. Warp means "that which is thrown...

 threads within a certain area of fabric. Today, it is more commonly used to measure the line width and check the registration

Printing registration

In color printing, registration is the method of correlating overlapping colors on one single image. There are many different styles and types of registration, many of which employ the alignment of specific marks.-Purpose:...

 of color separations in the field of printing and publishing.

It consists of a strong magnifier and a glass disc which has a measuring scale engraved in it. Nowadays, the scale is usually divided into millimetre

Millimetre

The millimetre is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length....

s, older linen testers had an inch

Inch

An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

 scale. Simpler models have a square opening in the foot, with a scale applied to one or more edges of the opening.

Linen testers are also used in other branches of industry. For example, they can be used to measure the line weight of barcode

Barcode

A barcode is an optical machine-readable representation of data, which shows data about the object to which it attaches. Originally barcodes represented data by varying the widths and spacings of parallel lines, and may be referred to as linear or 1 dimensional . Later they evolved into rectangles,...

s.