A loupe is a small, simple magnifier for looking closely at fine detail. It usually magnifies more than an ordinary magnifying glass. It is made to be held or worn right up against the eye. Watchmakers and jewellers are among its typical users.
How strong is a watchmaker’s loupe?
The loupes used by watchmakers and jewellers magnify between 2 and 20 times. A loupe is usually set in an eyepiece. It can be lodged in the eye socket, clipped to a pair of spectacles or simply held in the hand. Some watchmaker’s loupes carry an extra lens. It can be swung into line with the main lens to raise the power. For example, an 8X lens added to a 4X lens gives 12X.
Why do stronger loupes use several lenses?
A single lens produces a lot of optical aberration, which spoils the image. For this reason, loupes with one lens are mostly limited to low power. Higher-power loupes combine several lenses to keep aberration under control. In 1812 William Wollaston suggested putting two plano-convex lenses together, with their flat faces towards each other. In 1829 Coddington came up with a cheap lens with a V-shaped groove ground around its middle. It is still in use. In 1864 the German physicist Steinheil produced an achromatic, aplanatic triplet lens for loupes. That design is also still used.
Where it comes from
Small, low-power magnifiers of this kind have been in use for centuries. The word loupe is much newer. It has only been used in English since the early twentieth century, from about 1905–10. It comes from French, where it first meant a flawed gem or a lump of hot metal. Its roots go further back, to Germanic. Today the name simply refers to the magnifier itself.
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