Low dispersion glass
Encyclopedia
Low dispersion glass is a type of glass
with low dispersion
. Crown glass
is an example of a relatively inexpensive low-dispersion glass.
Special low dispersion glass (SLD glass) and extraordinary low dispersion glass (ELD glass) are glasses with yet lower dispersion (and yet higher price). Other glasses in this class are extra-low dispersion glass (ED glass), and ultra-low dispersion glass (UL glass).
, most often used in achromatic doublets. 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
es, shorter focal length
objectives benefit less from low dispersion elements, as their chief problem is spherical aberration
instead of chromatic aberration
. The spherical aberration introduced by the LD elements can be corrected with aspheric lens
elements. The increased sharpness provided by SLD elements allows using lower f-number
s and therefore faster shutter speed
. This is critical, e.g., in sports photography
and wildlife photography
. The shallow depth of field
provided by a telephoto lens also allows the subject of the photography to stand out better against the background.
Low dispersion glasses are also employed in handling ultrashort pulse
s of light, in e.g. mode-locked laser
s, in order to prevent pulse broadening by group velocity dispersion in the optical elements.
. Before their availability, calcium fluoride
in the form of fluorite
crystal
s were used as material for these lenses; however the low refraction index of calcium fluoride required high curvatures of the lenses, therefore increasing spherical aberration
. Fluorite also has poor shape retention and is very fragile. Abnormal dispersion is required for design of apochromat
lenses.
Glass with addition of thorium dioxide
has high refraction and low dispersion and was in use since before World War II
(WW2), but its radioactivity led to its replacement with other compositions. Even during WW2, Kodak managed to make high-performance thorium-free optical glass for use in aerial photography, but it was yellow-tinted. In combination with black and white film
, the tint was actually beneficial acting as a photographic filter
improving contrast
.
Leitz laboratories discovered that lanthanum(III) oxide
can be a suitable thorium dioxide replacement. Other elements however had to be added to preserve the amorphous character of the glass and prevent crystallization
that would cause striae defects.
After 1930, George W. Morey introduced the lanthanum oxide and oxides of other rare earth element
s in borate glasses, greatly expanding the available range of high-index low-dispersion glasses. Borate glasses have lower wavelength-refraction dependence in the blue region of spectrum
than silicate glasses with the same Abbe number
. These so called "borate flint" glasses, or KZFS, are however highly susceptible to corrosion
by acid
s, alkali
s, and weather factors. However borate glass with more than 20 mol.% of lanthanum oxide is very durable under ambient conditions. The use of rare earths allowed development of high-index low-dispersion glasses of both crown
and flint
types.
Another high-performance glass contains high content of zirconium dioxide
; however its high melting point requires use of platinum
lined crucibles to prevent contamination with crucible material.
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
, with addition of titanium dioxide
.
Several of the mentioned high-performance glasses are expensive because highly pure chemicals must be produced in substantial quantities.
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
with low 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...
. 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...
is an example of a relatively inexpensive low-dispersion glass.
Special low dispersion glass (SLD glass) and extraordinary low dispersion glass (ELD glass) are glasses with yet lower dispersion (and yet higher price). Other glasses in this class are extra-low dispersion glass (ED glass), and ultra-low dispersion glass (UL glass).
Application
Low dispersion glasses are particularly used to reduce chromatic aberrationChromatic 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...
, most often used in achromatic doublets. 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
Telephoto lens
In photography and cinematography, a telephoto lens is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long-focus...
es, shorter focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...
objectives benefit less from low dispersion elements, as their chief problem is 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...
instead of 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...
. The spherical aberration introduced by the LD elements can be corrected with aspheric lens
Aspheric 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....
elements. The increased sharpness provided by SLD elements allows using lower f-number
F-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
s and therefore faster shutter speed
Shutter speed
In photography, shutter speed is a common term used to discuss exposure time, the effective length of time a camera's shutter is open....
. This is critical, e.g., in sports photography
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 children playing association football, is a branch of vernacular...
and wildlife photography
Wildlife photography
Wildlife photography is the act of taking photographs of wildlife.Wildlife photography is regarded as one of the more challenging forms of photography. As well as needing sound technical skills, such as being able to expose correctly, wildlife photographers generally need good field craft skills...
. The shallow depth of field
Depth of field
In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...
provided by a telephoto lens also allows the subject of the photography to stand out better against the background.
Low dispersion glasses are also employed in handling ultrashort pulse
Ultrashort pulse
In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...
s of light, in e.g. mode-locked laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s, in order to prevent pulse broadening by group velocity dispersion in the optical elements.
Variants
Some glasses have a peculiar property called anomalous partial dispersion. Their use in long focal length lens assemblies was pioneered by LeitzLeitz
The Esselte Leitz GmbH & Co KG, also Leitz, is a German manufacturer of office products. The company, which is headquartered in Stuttgart, is owned by Esselte since 1998. Today the brand Leitz is used by Esselte mainly for filing and workspace products....
. Before their availability, calcium fluoride
Calcium fluoride
Calcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride...
in the form of fluorite
Fluorite
Fluorite is a halide mineral composed of calcium fluoride, CaF2. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon...
crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s were used as material for these lenses; however the low refraction index of calcium fluoride required high curvatures of the lenses, therefore increasing 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...
. Fluorite also has poor shape retention and is very fragile. Abnormal dispersion is required for design of 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:...
lenses.
Glass with addition of thorium dioxide
Thorium dioxide
Thorium dioxide , also called thorium oxide is a white, crystalline powder. It was formerly known as thoria or thorina. It is produced mainly as a by-product of lanthanide and uranium production.[1]...
has high refraction and low dispersion and was in use since before World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
(WW2), but its radioactivity led to its replacement with other compositions. Even during WW2, Kodak managed to make high-performance thorium-free optical glass for use in aerial photography, but it was yellow-tinted. In combination with black and white film
Monochrome photography
Monochrome photography is photography where the image produced has a single hue, rather than recording the colours of the object that was photographed. It includes all forms of black-and-white photography, which produce images containing tones of grey ranging from black to white. Most modern...
, the tint was actually beneficial acting as a photographic filter
Photographic filter
In photography and videography, a filter is a camera accessory consisting of an optical filter that can be inserted in the optical path. The filter can be a square or oblong shape mounted in a holder accessory, or, more commonly, a glass or plastic disk with a metal or plastic ring frame, which...
improving contrast
Contrast (vision)
Contrast is the difference in visual properties that makes an object distinguishable from other objects and the background. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view...
.
Leitz laboratories discovered that lanthanum(III) oxide
Lanthanum(III) oxide
Lanthanum oxide is La2O3, an inorganic compound containing the rare earth element lanthanum and oxygen. It is used to develop ferroelectric materials, and in optical materials.-Properties:...
can be a suitable thorium dioxide replacement. Other elements however had to be added to preserve the amorphous character of the glass and prevent crystallization
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
that would cause striae defects.
After 1930, George W. Morey introduced the lanthanum oxide and oxides of other rare earth element
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
s in borate glasses, greatly expanding the available range of high-index low-dispersion glasses. Borate glasses have lower wavelength-refraction dependence in the blue region of spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
than silicate glasses with the same Abbe number
Abbe number
In physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...
. These so called "borate flint" glasses, or KZFS, are however highly susceptible to corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
by acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s, alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
s, and weather factors. However borate glass with more than 20 mol.% of lanthanum oxide is very durable under ambient conditions. The use of rare earths allowed development of high-index low-dispersion glasses of both crown
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 flint
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...
types.
Another high-performance glass contains high content of zirconium dioxide
Zirconium dioxide
Zirconium dioxide , sometimes known as zirconia , is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite O2...
; however its high melting point requires use of platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
lined crucibles to prevent contamination with crucible material.
A good high-refraction replacement for calcium fluoride as a lens material can be a fluorophosphate glass
Fluorophosphate glass
Fluorophosphate glass is a class of optical glasses composed of metaphosphates and fluorides of various metals. It is a variant of phosphate glasses.Some fluorophosphate glasses are used as low dispersion glasses. Some show anomalous partial dispersion...
. Here, a proportion of fluorides is stabilized with a metaphosphate
Metaphosphate
A metaphosphate ion is an oxyanion that has the empirical formula PO3−. The structure of a metaphosphate ion can be described as being made up of PO4 structural units in which each unit shares two corners with another unit...
, with addition of titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
.
Several of the mentioned high-performance glasses are expensive because highly pure chemicals must be produced in substantial quantities.
See also
- Aspheric lensAspheric lensAn 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....
- Achromatic lensAchromatic lensAn 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 in the same plane....
- Abbe numberAbbe numberIn physics and optics, the Abbe number, also known as the V-number or constringence of a transparent material, is a measure of the material's dispersion in relation to the refractive index...
- Diffraction-limited system
- Leica Camera
- Material dispersion coefficientMaterial dispersion coefficientIn an optical fiber, the material dispersion coefficient, M, characterizes the amount of pulse broadening by material dispersion per unit length of fiber and per unit of spectral width. It is usually expressed in picoseconds per ....