Types
Doublets can come in many forms, though most commercial doublets are achromats, which are optimized to reduce chromatic aberration while also reducing spherical aberration and other optical aberrations. The lenses are made from glasses with different refractive indices and different amounts of dispersion. Often one element is made from crown glass and the other from flint glass. This combination produces a better image than a simple lens. Trilobites, which are now extinct, had natural doublet lenses. Apochromats can also be made as doublets.
Doublets can be air-spaced, cemented, or "oiled." Oiled doublets hold the optical fluid in place with surface tension alone. In a cemented doublet, the lenses are held together by an adhesive with mechanical strength, such as optically transparent epoxy. Canada balsam was traditionally used for this purpose. Some doublets use no adhesive between the lenses, relying on external fixturing to hold them together, either because the optical design requires a gap or because thermal expansion differences between the two lenses won't allow cementing. These are called "un-cemented", "air-spaced" or "broken contact" doublets. A sub-type of air-spaced doublet is the dialyte, a design where elements are widely spaced to save on the amount of glass used or where the elements cannot be cemented because they have strongly dissimilar curvatures.
Read more about this topic: Doublet (lens)
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