Low-dispersion Glass
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Low-dispersion glass (LD glass) is a type of
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
with reduced
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...
, meaning the
refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
does not change as strongly with different wavelengths of light. In other words, the light passing through the glass has a smaller spread or
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 variat ...
between its constituent colors, resulting in a reduced "
rainbow A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular ...
effect" at high-contrast edges. Wavelength dispersion in a certain material is characterized by its
Abbe number In optics and lens design, the Abbe number, also known as the Vd-number or constringence of a Transparency (optics), transparent material, is an approximate measure of the material's dispersion (optics), dispersion (change of refractive index versu ...
; LD glass has a higher Abbe number than conventional types.
Crown glass Crown glass is either of two kinds of glass: *Crown glass (window) was a type of hand-blown window glass. *Crown glass (optics) is a type of optical glass used in lenses. {{Disambig ...
is an example of a relatively inexpensive low-dispersion glass.


Branding

Photographic lens A camera lens, photographic lens or photographic objective is an optical lens (optics), lens or assembly of lenses (compound lens) used in conjunction with a camera body and mechanism to Imaging, make images of objects either on photographic film ...
es with LD glass have been branded and marketed with different names to indicate the use of low-dispersion elements in the optical design, including: * ''Anomalous Dispersion'' (AD):
Minolta was a Japanese manufacturer of cameras, lenses, camera accessories, photocopiers, fax machines, and laser printers. Minolta Co., Ltd., which is also known simply as Minolta, was founded in Osaka, Japan, in 1928 as . It made the first integrated ...
* ''Extra-low Dispersion'' or ''Extraordinary Dispersion'' (ED):
Nikon (, ; ) is a Japanese optics and photographic equipment manufacturer. Nikon's products include cameras, camera lenses, binoculars, microscopes, ophthalmic lenses, measurement instruments, rifle scopes, spotting scopes, and equipment related to S ...
,
Olympus Olympus or Olympos () may refer to: Mountains In antiquity Greece * Mount Olympus in Thessaly, northern Greece, the home of the twelve gods of Olympus in Greek mythology * Mount Olympus (Lesvos), located in Lesbos * Mount Olympus (Euboea) ...
,
Pentax was a Japanese camera and optical equipment manufacturer. Currently, it exists as the Pentax Life Care Business Division of Hoya's medical endoscope business, as well as the digital camera brand of Ricoh Imaging, a subsidiary of Ricoh. Penta ...
* ''Extraordinary Low Dispersion'' (ELD):
Sigma Sigma ( ; uppercase Σ, lowercase σ, lowercase in word-final position ς; ) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase Σ is used as an operator ...
* ''Low Dispersion'' (LD):
Tamron is a Japanese company manufacturing photographic lenses, optical components and commercial/industrial-use optics. Tamron Headquarters is located in Saitama City in the Saitama Prefecture of Japan. The name of the company came from the surna ...
* ''Super Low Dispersion'' (SD):
Tokina is a Japanese manufacturer of photographic lenses and CCTV security equipment. Lens designations * FX - Full frame * DX - cropped digital * AF - Auto-Focus * AT-X Pro - professional line (constant aperture zooms or primes) * AT-X - consumer l ...
* ''Special Low Dispersion'' or ''Super Low Dispersion'' (SLD):
Sigma Sigma ( ; uppercase Σ, lowercase σ, lowercase in word-final position ς; ) is the eighteenth letter of the Greek alphabet. In the system of Greek numerals, it has a value of 200. In general mathematics, uppercase Σ is used as an operator ...
* ''Ultra-low Dispersion'' (UD):
Canon Canon or Canons may refer to: Arts and entertainment * Canon (fiction), the material accepted as officially written by an author or an ascribed author * Literary canon, an accepted body of works considered as high culture ** Western canon, th ...
Some glasses may include a "Super" modifier (e.g., "Super ED") to designate materials with even lower wavelength dispersion characteristics. There are no industry-wide standards which determine whether a given material may be considered LD glass; these designations should be seen as manufacturer-specific, i.e., special glasses with LD / ED / UD labels have lower dispersion than conventional glasses from the same manufacturer.
Schott AG Schott AG is a German multinational glass company specializing in the manufacture of glass and glass-ceramics. Headquartered in Mainz, Rhineland-Palatinate, Germany, it is owned by the Carl Zeiss foundation, Carl Zeiss Foundation. The company's ...
publishes a diagram of the glasses it manufactures, grouped by
glass code A glass code is a method of classifying glasses for optical use, such as the manufacture of lenses and prisms. There are many different types of glass with different compositions and optical properties, and a glass code is used to distinguish betwe ...
, showing refractive index (y-axis) as a function of Abbe number (x-axis). Most classical crown and flint glasses follow a gentle curve on the right side of the chart, which demonstrates these classical glasses have an inverse dependence between refractive index and Abbe number. Lens materials to the left of those have a higher Abbe number for a given refractive index, and may be considered to be low-dispersion types, including many of the lanthanum-doped glasses.


Applications

File:Prism-rainbow-black-2.svg, White light passing through a prism using conventional glass is dispersed into a
rainbow A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular ...
because the
refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
is a function of wavelength. File:Chromatic aberration (comparison) - enlargement.jpg,
Chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...
from inadequate correction to converge different wavelengths, giving a "rainbow effect" at high-contrast edges. File:Achromat doublet en.svg, Diagram of an
achromatic doublet 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. Wavelengths ...
: crown and flint glasses are used together to converge the red and blue wavelengths at a single focal point. File:US Navy 030122-N-5786V-501 Sabrina Day from Beaufort, S.C., uses a 400mm fixed telephoto lens to photograph daily activity aboard the aircraft carrier.jpg, High-performance
telephoto lens A telephoto lens, also known as telelens, is a specific type of a long-focus lens used in photography and cinematography, in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens ...
es often incorporate LD glass elements.


Imaging

Low-dispersion glasses are particularly used to reduce
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...
, most often used in
achromatic doublet 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. Wavelengths ...
s. The positive element is made of a low-dispersion glass, the negative element from a high-dispersion glass. To counteract the effect of the negative lens, the positive lens has to be thicker. Achromatic doublets therefore have higher thickness and weight than the equivalent non-chromatic-corrected single lenses. In comparison to
telephoto lens A telephoto lens, also known as telelens, is a specific type of a long-focus lens used in photography and cinematography, in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens ...
es, shorter
focal length The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...
objectives benefit less from low-dispersion elements, as their chief problem is
spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. This phenomenon commonly affects lenses and curved mirrors, as these components are often shaped in a spherical ...
rather than
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion, color aberration, color fringing, or purple fringing, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the ...
. The spherical aberration introduced by the LD elements can be corrected with
aspheric lens An aspheric lens or asphere (often labeled ''ASPH'' on eye pieces) 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. ...
elements. The increased sharpness provided by SLD elements allows using lower
f-number An f-number is a measure of the light-gathering ability of an optical system such as a camera lens. It is calculated by dividing the system's focal length by the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical ...
s and therefore faster
shutter speed In photography, shutter speed or exposure time is the length of time that the film or digital sensor inside the camera is exposed to light (that is, when the camera's shutter (photography), shutter is open) when taking a photograph. The am ...
. This is critical, e.g., in
sports photography Sports photography refers to the genre of photography that covers all types of sports. In the majority of cases, professional sports photography is a branch of ''photojournalism,'' while amateur sports photography, such as photos of childre ...
and
wildlife photography Wildlife photography is a genre of photography concerned with documenting various forms of wildlife in their natural habitat. As well as requiring photography skills, wildlife photographers may need field craft skills. For example, some anim ...
. The shallow
depth of field The depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus (optics), focus in an image captured with a camera. See also the closely related depth of focus. Factors affecting depth ...
provided by a telephoto lens also allows the subject of the photography to stand out better against the background. Infrared corrected ''special-low-dispersion glass'' also has benefits to CCTV cameras. The low chromatic aberration of SLD glass allows the lens to always stay in focus, from visible light to infrared.


Scientific

Low-dispersion glasses are also employed in handling
ultrashort pulse In optics, an ultrashort pulse, also known as an ultrafast event, is an electromagnetic pulse whose time duration is of the order of a picosecond (10−12 second) or less. Such pulses have a broadband optical spectrum, and can be created by ...
s of light, in e.g. mode-locked
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
s, to prevent pulse broadening by
group velocity dispersion In optics, group-velocity dispersion (GVD) is a characteristic of a dispersive medium, used most often to determine how the medium affects the duration of an optical pulse traveling through it. Formally, GVD is defined as the derivative of the inv ...
in the optical elements.


Sport optics

In binoculars, ED glass, also sometimes referred to as a high density (HD) glass, is a high quality optical glass that increases light transmission, decreases light dispersion, and so cuts down on chromatic aberration, or "color fringing", which is due to the splitting of the light spectrum. It is used in binocular objective lenses to help focus the light waves of the color spectrum on the human eye, and to deliver bright, sharp images. ED lenses are composed of a specific formulation that contains rare-earth elements. However, there is no ED standard that dictates the materials that must be used in ED lenses. Therefore, the quality of ED glass can vary.


History

Some glasses have a peculiar property called anomalous partial dispersion. Abnormal dispersion is required to design
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. The prefix ''apo-'' comes from the Greek preposition ''ἀ ...
ic lenses; in contrast to the achromatic doublets, which converge blue and red wavelengths, apochromats converge focus of three or more wavelengths.


Rare earth and radioactive lenses

Thoriated glass is doped with
thorium dioxide Thorium dioxide (ThO2), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is mainly a by-product of lanthanide and uranium production. Thorianite is the name of the mineralogical for ...
, resulting in a lens material with high refractive index and low dispersion, suitable for apochromatic designs. Thoriated glass was in use before World War II, but not widely used until the 1950s. Because
thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...
is
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
, optical engineers and designers sought a replacement using different doping elements, and lens designs using thoriated glass been discontinued by the late 1980s. As an alternative, after 1930, George W. Morey introduced borate glasses doped with
lanthanum oxide Lanthanum(III) oxide, also known as lanthana, chemical formula , is an inorganic compound containing the rare earth element lanthanum and oxygen. It is used in some ferroelectric materials, as a component of optical materials, and is a feedstock ...
and oxides of other
rare-earth element The rare-earth elements (REE), also called the rare-earth metals or rare earths, and sometimes the lanthanides or lanthanoids (although scandium and yttrium, which do not belong to this series, are usually included as rare earths), are a set o ...
s, greatly expanding the available range of high-index low-dispersion glasses; although
lanthanum Lanthanum is a chemical element; it has symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements bet ...
is also radioactive, it has far less activity than thorium. Borate glasses have lower wavelength-refraction dependence in the blue region of
spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
than
silicate glass Glass is an amorphous ( non-crystalline) solid. Because it is often transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window panes, tableware, and optics. Some common objects made o ...
es with the same
Abbe number In optics and lens design, the Abbe number, also known as the Vd-number or constringence of a Transparency (optics), transparent material, is an approximate measure of the material's dispersion (optics), dispersion (change of refractive index versu ...
. During WWII,
Kodak The Eastman Kodak Company, referred to simply as Kodak (), is an American public company that produces various products related to its historic basis in film photography. The company is headquartered in Rochester, New York, and is incorporated i ...
manufactured high-performance thorium-free optical glass for aerial photography, but it was yellow-tinted. In combination with
black and white film Black-and-white (B&W or B/W) images combine black and white to produce a range of achromatic brightnesses of grey. It is also known as greyscale in technical settings. Media The history of various visual media began with black and white, a ...
, the tint was actually beneficial, improving
contrast Contrast may refer to: Science * Contrast (vision), the contradiction in form, colour and light between parts of an image * Contrast (statistics), a combination of averages whose coefficients add up to zero, or the difference between two means * ...
by acting as an ultraviolet filter. The use of rare earths allowed development of high-index low-dispersion glasses of both
crown A crown is a traditional form of head adornment, or hat, worn by monarchs as a symbol of their power and dignity. A crown is often, by extension, a symbol of the monarch's government or items endorsed by it. The word itself is used, parti ...
and
flint Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Historically, flint was widely used to make stone tools and start ...
types. The use of low-dispersion glass in long-focal-length lens assemblies was pioneered by
Ernst Leitz GmbH Ernst Leitz GmbH was a German corporation based in Wetzlar, a German centre for optics as well as an important location for the precision engineering industry. History Carl Kellner (optician), Carl Kellner, mechanic and self-taught mathematic ...
(Leitz) after World War II. Leitz laboratories discovered that
lanthanum(III) oxide Lanthanum(III) oxide, also known as lanthana, chemical formula , is an inorganic compound containing the rare earth element lanthanum and oxygen. It is used in some ferroelectric materials, as a component of optical materials, and is a feedstock ...
could be used as a suitable replacement for thorium. However, additional elements had to be added to preserve the
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is a characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymousl ...
structure of the glass and prevent
crystallization Crystallization is a process that leads to solids with highly organized Atom, atoms or Molecule, molecules, i.e. a crystal. The ordered nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regu ...
which would cause striae defects in images captured through those lenses. These so-called " borate flint" glasses, which Schott classifies as KzF (), are however highly susceptible to
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
by
acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
s,
alkali In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The a ...
s, and weather factors. However borate glass with more than 20 mol.% of lanthanum oxide is very durable under ambient conditions. Another high-performance glass contains a high proportion of
zirconium dioxide Zirconium dioxide (), sometimes known as zirconia (not to be confused with zirconium silicate or zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral bad ...
; however its high melting point requires use of
platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
lined crucibles to prevent contamination with crucible material. Lenses which incorporate low-dispersion glass element(s) can be more expensive than equivalent lenses using classical glass elements. This is because several of the mentioned high-performance glasses require the production of high-purity chemicals in substantial quantities.


Calcium fluoride (fluorite) crystals

In parallel to the development of LD glass, artificially-grown
fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs scal ...
()
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s were used starting in the 1960s for lens elements requiring low dispersion; however, there were significant drawbacks to using fluorite: the low
refraction index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light Light, v ...
of fluorite required high curvatures of the lenses, therefore increasing
spherical aberration In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. This phenomenon commonly affects lenses and curved mirrors, as these components are often shaped in a spherical ...
. In addition, fluorite has poor shape retention and is very fragile, requiring special handling to process into lens elements. A good high-refraction replacement for calcium fluoride as a lens material can be a fluorophosphate glass. Here, a proportion of fluorides is stabilized with a
metaphosphate A metaphosphate ion is an oxyanion that has the empirical formula . It was first postulated in 1955 but was not observed until 1979, when it was detected by mass spectrometry. Metaphosphate is an intermediate in the hydrolysis of phosphate esters ...
, with addition of
titanium dioxide Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or Colour Index Internationa ...
.


See also

*
Aspheric lens An aspheric lens or asphere (often labeled ''ASPH'' on eye pieces) 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. ...
*
Achromatic lens An achromatic lens or achromat is a lens (optics), lens that is designed to limit the effects of chromatic aberration, chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into ...
*
Abbe number In optics and lens design, the Abbe number, also known as the Vd-number or constringence of a Transparency (optics), transparent material, is an approximate measure of the material's dispersion (optics), dispersion (change of refractive index versu ...
*
Diffraction-limited system In optics, any optical instrument or systema microscope, telescope, or camerahas a principal limit to its resolution due to the physics of diffraction. An optical instrument is said to be diffraction-limited if it has reached this limit of res ...
* Material dispersion coefficient


References

{{reflist


External links


Lens Specifications
Glass compositions