On paintings, black light exposes retouched areas. However, be wary. Masking varnishes can hide a well-executed repainting job even from a black light, which will only reveal these repairs if the varnish is removed. Black lights reveal marks invisible to the naked eye. Loupes, often used in jewelry, for example, are another common kind.
Loupes come in several powers, or strengths.
Farmer finds the 10-power ideal for most purposes. It can magnify the dots in an image and thus differentiate between fine prints and watercolors. A magnifier may also show the fine carbon flecks in a diamond that detract from its value. A slightly weaker eight-power lens allows you to see grosser markings such as patent dates on furniture hinges or hallmarks on silver platters. Folding loupes are available, too, as are convenient styles worn around the neck.