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A jeweler's loupe is a type of magnifying glass which is used by jewelers to assess the quality of jewelry and loose stones. There are plenty of other uses for a loupe, ranging from closely examining print quality on proofs to looking at botanical specimens in the field. Many shops stock jeweler's loupes in a variety of magnifying powers, along with instructions on how to use them. Learning to utilize a jeweler's loupe does take some time, but it allows users to explore all sorts of interesting objects on a magnified scale.
There are several variants on the basic design of a jeweler's loupe. Some are designed to fit into eyepieces or visors, for example, for jewelers who want to have their loupes readily accessible. Others are designed as units on stands which can be positioned over the object of interest, forcing the user to bend over to look into the eyepiece.
Lenses for loupes can be ground from glass or plastic; high quality loupes tend to have glass lenses. A good jeweler's loupe will also have multiple lenses, as a single magnifying lens can cause distortions of colors and shape. Triplet loupes have three lenses, and they are quite common among professionals; it is also possible to find loupes with even more lenses. The frame of the lens is typically black, reducing refractions which could distort the image.
A jeweler's loupe will usually have markings on it which are designed to indicate the power of magnification it offers. Some are capable of switching between powers, but most simply offer 10 times magnification, making the object of interest 10 times larger under the lens than it is in real life. The higher the power, the lower the depth of field and focal length, meaning that the lens must be placed closer to the object to get a crisp image.
Under a loupe, a jeweler can see things like impurities, blemishes, chips, and inclusions, and the quality of workmanship on a finished piece can also be assessed. With the assistance of a loupe, jewelers can decide how much a piece of jewelry is worth, and they can determine whether or not it is fake, when it was made, and glean other useful information.
”Loupe” is pronounced “loop,” incidentally. The word is believed to come from an Old French word meaning “flawed stone,” although it may also be related to the Old Dutch lupen, “to peer.”