Doublet (lens)

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An achromatic doublet Achromat doublet en.svg
An achromatic doublet
An old Carl Zeiss Tessar camera lens with four elements, comprising two doublets. The front doublet is air-gapped and divergent; the rear doublet is glued and convergent. This arrangement was better at correcting spherical and chromatic aberrations and astigmatism than previous lens designs. Obiettivo fotografico doppio, anastigmatico, asimmetrico, a quattro lenti - Museo scienza tecnologia Milano 05924.jpg
An old Carl Zeiss Tessar camera lens with four elements, comprising two doublets. The front doublet is air-gapped and divergent; the rear doublet is glued and convergent. This arrangement was better at correcting spherical and chromatic aberrations and astigmatism than previous lens designs.

In optics, a doublet is a type of lens made up of two simple 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 additional degrees of freedom, optical designers have more latitude to correct more optical aberrations more thoroughly.

Contents

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. Some Trilobites, 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 with surface 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 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. [3] These are called "un-cemented", "air-spaced" or "broken contact" doublets. [4] 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. [5]

Properties

Lenses may be cemented together for one or multiple 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 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

  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.

Disadvantages

  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.

See also

Related Research Articles

Abbe number material dispersion property

In optics and lens design, the Abbe number, also known as the V-number or constringence of a transparent material, is an approximate measure of the material's dispersion, with high values of V indicating low dispersion. It is named after Ernst Abbe (1840–1905), the German physicist who defined it. The term V-number should not be confused with the normalized frequency in fibers.

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 the nature of the distortion depending on the type of aberration. Aberration can be defined as a departure of the performance of an optical system from the predictions of paraxial optics. In an imaging system, it occurs when light from one point of an object does not converge into a single point after transmission through the system. Aberrations occur because the simple paraxial theory is not a completely accurate model of the effect of an optical system on light, rather than due to flaws in the optical elements.

Lens Optical device which transmits and refracts light

A lens is a transmissive optical device that 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), usually arranged along a common axis. Lenses are made from materials such as glass or plastic, and are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism, which refracts light without focusing. Devices that similarly focus or disperse waves and radiation other than visible light are also called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses.

Chromatic aberration Failure of a lens to focus all colors on the same point

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 wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength. Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing. Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.

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 into focus on the same plane.

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. A concave lens of flint glass is commonly combined with a convex lens of crown glass to produce an achromatic doublet lens because of their compensating optical properties, which reduces chromatic aberration.

Tessar

The Tessar is a photographic lens design conceived by the German physicist Paul Rudolph in 1902 while he worked at the Zeiss optical company and patented by Zeiss in Germany; the lens type is usually known as the Zeiss Tessar.

Refracting telescope

A refracting telescope is a type of optical telescope that uses a lens as its objective to form an image. The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes, the refracting telescope has been superseded by the reflecting telescope, which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.

Gradient-index optics

Gradient-index (GRIN) optics is the branch of optics covering optical effects produced by a gradient of the refractive index of a material. Such gradual variation can be used to produce lenses with flat surfaces, or lenses that do not have the aberrations typical of traditional spherical lenses. Gradient-index lenses may have a refraction gradient that is spherical, axial, or radial.

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.

Eyepiece Type of lens attached to a variety of optical devices such as telescopes and microscopes

An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.

Catadioptric system Optical system where refraction and reflection are combined

A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as searchlights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as "catadioptric", such as surveillance catadioptric sensors.

Large format lens

Large format lenses are photographic optics that provide an image circle large enough to cover large format film or plates. Large format lenses are typically used in large format cameras and view cameras.

Aspheric lens Type of lens

An aspheric lens or asphere is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens.

Crown glass (optics) Type of glass

Crown glass is a type of optical glass used in lenses and other optical components. It has relatively low refractive index (≈1.52) and low dispersion. Crown glass is produced from alkali-lime silicates containing approximately 10% potassium oxide and is one of the earliest low dispersion glasses.

Achromatic telescope

The achromatic telescope is a refracting telescope that uses an achromatic lens to correct for chromatic aberration.

Triplet lens Compound lens consisting of three single lenses

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.

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. The word dialyte means "parted", "loose" or "separated".

The design of photographic lenses for use in still or cine cameras is intended to produce a lens that yields the most acceptable rendition of the subject being photographed within a range of constraints that include cost, weight and materials. For many other optical devices such as telescopes, microscopes and theodolites where the visual image is observed but often not recorded the design can often be significantly simpler than is the case in a camera where every image is captured on film or image sensor and can be subject to detailed scrutiny at a later stage. Photographic lenses also include those used in enlargers and projectors.

Low-dispersion glass is a type of glass with low dispersion. Crown glass is an example of a relatively inexpensive low-dispersion glass.

References

  1. Clarkson, E. N. K.; Levi-Setti, R. L. (1975), "Trilobite eyes and the optics of Descartes and Huygens", Nature , 254 (5502): 663–7, Bibcode:1975Natur.254..663C, doi:10.1038/254663a0, PMID   1091864
  2. 1 2 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
  3. Fred A. Carson, Basic optics and optical instruments, page 4-32
  4. A guide to instrument design – Scientific Instrument Manufacturers' Association of Great Britain, British Scientific Instrument Research Association, page 184
  5. Fred A. Carson, Basic optics and optical instruments, page AJ-4