In optics

Optics

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

, a doublet is a type of lens

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 made up of two simple lens

Simple lens

In optics, a simple lens or singlet lens is a lens consisting of a single simple element. Typical examples include a magnifying glass or a lens in a pair of simple reading glasses....

es paired together. Such an arrangement allows more optical surfaces, thicknesses, and formulations, especially as the space between lenses may be considered an "element." With additional degrees of freedom

Degrees of freedom

Degrees of freedom can mean:* Degrees of freedom , independent displacements and/or rotations that specify the orientation of the body or system...

, optical designers have more latitude to correct more optical aberrations

Aberration in optical systems

Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...

, more thoroughly.

Doublets can come in many forms, though most commercial doublets are achromats, which are optimized to reduce chromatic aberration

Chromatic aberration

In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

 while also reducing spherical aberration

Spherical aberration

thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...

 and other optical aberrations

Aberration in optical systems

Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...

. The lenses are made from glasses with different refractive indices

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 and different amounts of dispersion

Dispersion (optics)

In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...

. Often one element is made from crown glass

Crown glass (optics)

Crown glass is type of optical glass used in lenses and other optical components. It has relatively low refractive index and low dispersion...

 and the other from flint glass

Flint glass

Flint glass is optical glass that has relatively high refractive index and low Abbe number. Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00...

. This combination produces a better image than a simple lens. Trilobite

Trilobite

Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

s, which are now extinct, had natural doublet lenses. Apochromat

Apochromat

An apochromat, or apochromatic lens , is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses.-Explanation:...

s 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

Surface tension

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

 alone. In a cemented doublet, the lenses are held together by an adhesive

Adhesive

An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

 with mechanical strength, such as optically transparent epoxy

Epoxy

Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....

. Canada balsam

Canada balsam

Canada balsam, also called Canada turpentine or balsam of fir, is a turpentine which is made from the resin of the balsam fir tree of boreal North America...

 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

Thermal expansion

Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

 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

Dialyte lens

A dialyte lens is a compound lens design that corrects optical aberrations where the lens elements are widely air-spaced. The design is used to save on the amount of glass used for specific elements or where elements can not be cemented because they have dissimilar curvatures...

, 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.