A printer’s loupe & a linen tester at first glance look to be the same. When used as a linen tester threads in cloth are counted. When used as a printers loupe the magnifier reveals the quality of printed photos. A closer look reveals the difference. Read on to see pictures & more & what makes them different.

Note: some of the images have our original domain “indigo.com”. We are now using indigoinstruments.com.

Can You Spot the Printer’s Loupe?

Linen tester-printer's loupe

Linen tester/printer’s loupe

It’s What You Look At That Counts!

A linen tester has a single lens, usually 25mm (1 inch) in diameter with 6 times power & ruler markings along its bottom inside edge. You count the number of threads (per inch or mm) to rate the quality of the cloth.

Checking the quality of a color image is done almost the same way but with a twist.

Fibres & Inks Magnified

The top half shows a wool sweater magnified so you can count the threads. The bottom half shows the red, blue & yellow colored dots that make up a printed image.

What Has Isaac Newton Got To Do With It?

Among his many achievements, Sir Isaac Newton is known for discovering that white light consists of many colors as shown using a prism. A magnifier lens, to a lesser degree, also behaves a little bit like a glass prism.

Glass Prism Light Refraction

In this picture, you can see the typical effect of a glass equilateral prism on sunlight. It refracts white light into the colors of the rainbow. The same thing happens with magnifying lenses but it isn’t as obvious.

An equilateral prism disperses the different colors of the spectrum at slightly different angles. The red, blue & yellow dots that are used to create color images have to be printed precisely to achieve a clear image. When an image is fuzzy, a print graphics expert will look at the separation of the dots to identify the problem. This can only be done reliably if the dots are actually where they appear.

A second lens is combined with the first to correct this effect. In the top picture, you can see 3 magnifiers with double lenses which is why they are thicker. An experienced printer can measure the space between the dots & the necessary adjustments in the print process.

A printer’s loupe can count threads but why pay more if a linen tester can do the job?

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