In
optics
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
, a doublet is a type of
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
made up of two
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.
Simple lenses are prone to aberrations, especially chromatic aberr ...
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 (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...
, optical designers have more latitude to correct more
optical aberrations
In optics, aberration is a property of optical systems, such as lenses, that causes light to be spread out over some region of space rather than focused to a point. Aberrations cause the image formed by a lens to be blurred or distorted, with th ...
more thoroughly.
Types
Doublets can come in many forms, though most commercial doublets are
achromat
An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane.
The most comm ...
s, which are optimized to reduce
chromatic aberration
In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
while also reducing
spherical aberration
In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
and other
optical aberrations
In optics, aberration is a property of optical systems, such as lenses, that causes light to be spread out over some region of space rather than focused to a point. Aberrations cause the image formed by a lens to be blurred or distorted, with th ...
. The lenses are made from glasses with different
refractive indices
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
and different amounts of
dispersion
Dispersion may refer to:
Economics and finance
*Dispersion (finance), a measure for the statistical distribution of portfolio returns
*Price dispersion, a variation in prices across sellers of the same item
*Wage dispersion, the amount of variatio ...
. Often one element is made from
crown glass and the other from
flint glass
Flint glass is optical glass that has relatively high refractive index and low Abbe number (high Dispersion (optics), dispersion). Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint gl ...
. This combination produces a better image than a simple lens. Some
Trilobite
Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...
s, which are now extinct, had natural doublet lenses in their eyes.
Apochromat
An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses.
Explanation
Chromatic aberration is the phenomenon of differen ...
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 is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) to f ...
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.
[Kingslake, R. (2010). ''Lens Design Fundamentals'' (R. B. Johnson, Ed.; 2nd ed.). p. 5 SPIE PRESS. DOI: 10.1016/B978-0-12-374301-5.00005-X] In a hard-cemented doublet, the lenses are held together by an
adhesive
Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.
The use of adhesives offers certain advant ...
with mechanical strength, such as optically transparent
epoxy
Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also coll ...
.
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
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic ...
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.
[Fred A. Carson, Basic optics and optical instruments, page AJ-4]
Properties
Lenses may be cemented together for one or multiple reasons:
# To eliminate the reflection losses of two air-glass surface interfaces.
# To prevent total reflection at the air-film interface due to critical ray angle.
# To effectively combine two stronger lenses into 1 weaker lens to assist in mechanical mounting limitations.
There are several advantages and disadvantages to cementing elements together.
Advantages
# 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.
# Cemented groups may assist to decrease the physical length of the optical system.
# Provides superior control for minimising spherochromatism and other aberrations, allowing the creation of more advanced optical systems using fewer elements.
Disadvantages
# 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.
# Precision cementing is high in manufacturing cost. It is often cheaper to AR-coat two air-spaced elements.
See also
*
Achromatic lens
An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane.
The most comm ...
*
Triplet lens
A triplet lens is a compound lens consisting of three single lenses. The triplet design is the simplest to give the required number of degrees of freedom to allow the lens designer to overcome all Seidel aberrations.
The three lenses may be ce ...
References
Lenses
2 (number)
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