Inoptics, adoublet is a type oflens made up of twosimple lenses paired together. Such an arrangement allows more optical surfaces, thicknesses, and formulations, especially as the space between lenses may be considered an "element". With additionaldegrees of freedom, optical designers have more latitude to correct moreoptical aberrations more thoroughly.

Doublets can come in many forms, though most commercial doublets areachromats, which are optimized to reducechromatic aberration while also reducingspherical aberration and otheroptical aberrations. The lenses are made from glasses with differentrefractive indices and different amounts ofdispersion. Often one element is a positive lens made ofcrown glass and the other is a negative lens made offlint glass. This combination produces a better image than a simple lens. SomeTrilobites, which are now extinct, had natural doublet lenses in their eyes.^[1]Apochromats can also be made as doublets.

Doublets can be air-spaced, cemented, or "oiled". Oiled doublets hold the optical fluid in place withsurface tension alone. Elements may be cemented by oil or a soft cement in the case when the differential thermal expansion of crown and flint glasses causes hard or cured cements to warp or fracture. With larger elements, it is ideal to separate the lenses using a spacer.^[2] In a hard-cemented doublet, the lenses are held together by anadhesive with mechanical strength, such as optically transparentepoxy.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 becausethermal expansion differences between the two lenses will not allow cementing.^[3] These are called "un-cemented", "air-spaced" or "broken contact" doublets.^[4] A sub-type of air-spaced doublet is thedialyte, 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.^[5]

Lenses may be cemented together for any of several reasons:^[2]

1. To eliminate the reflection losses of two air-glass surface interfaces.
2. To prevent total reflection at the air-film interface due to critical ray angle.
3. To replace a low-power lens that is difficult to mount with an equivalent doublet made from two higher-power lenses.

There are several advantages and disadvantages to cementing elements together:

1. Cementing elements can simplify raytracing operations as the cement may nearly always be ignored, with the tracer treating the ray as if it refracts directly from one element into the next.
2. Cemented groups may assist to decrease the physical length of the optical system.
3. Provides superior control for minimising spherochromatism and other aberrations, allowing the creation of more advanced optical systems using fewer elements.

1. Cementing elements requires a high precision to ensure correct centering of the cemented components. This difficulty greatly increases on groups of more than two elements.
2. Precision cementing is high in manufacturing cost. It is often cheaper to AR-coat two air-spaced elements.

• Achromatic lens
• Triplet lens

• Lenses
• 2 (number)