Low-dispersion glass

Last updated October 21, 2023

Low-dispersion glass (LD glass) is a type of glass with a reduction in chromatic aberration (less rainbow effect). Crown glass is an example of a relatively inexpensive low-dispersion glass.

Application

Low-dispersion glasses are particularly used to reduce chromatic aberration, 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.^[1]

In comparison to telephoto lenses, shorter focal length objectives benefit less from low-dispersion elements, as their chief problem is spherical aberration rather than 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-numbers 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.^[2]

Low-dispersion glasses are also employed in handling ultrashort pulses of light, in e.g. mode-locked lasers, to prevent pulse broadening by group velocity dispersion in the optical elements.^[3]

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.^[4]

In binoculars, ED (extra-low dispersion) 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.^[5]

Variants

Some glasses have a peculiar property called anomalous partial dispersion. Their use in long-focal-length lens assemblies was pioneered by Leitz. Before their availability, calcium fluoride in the form of fluorite crystals 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 has poor shape retention and is very fragile. Abnormal dispersion is required for design of apochromat lenses.^[6]

Glass with addition of thorium dioxide has high refraction and low dispersion and was in use since before World War II, but its radioactivity led to its replacement with other compositions. Even during WWII, 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, improving contrast by acting as an ultraviolet filter.

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 elements 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 acids, alkalis, and weather factors. However borate glass with more than 20 mol.% of lanthanum oxide is very durable under ambient conditions.^[7] The use of rare earths allowed development of high-index low-dispersion glasses of both crown and flint types.^[8]

Another high-performance glass contains high proportion 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.^[9]

Several of the mentioned high-performance glasses are expensive because highly pure chemicals must be produced in substantial quantities.

Related Research Articles

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, polished, or molded to the required 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.

<span class="mw-page-title-main">Fluorite</span> Mineral form of calcium fluoride

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.

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.

A corrective lens is a transmissive optical device that is worn on the eye to improve visual perception. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye. Intraocular lenses are surgically implanted most commonly after cataract removal but can be used for purely refractive purposes.

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. Wavelengths in between these two then have better focus error than could be obtained with a simple lens.

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.

$Refracting telescope Type of optical telescope$

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 spyglasses 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.

<span class="mw-page-title-main">Objective (optics)</span> Lens or mirror in optical instruments

In optical engineering, an objective is an optical element that gathers light from an object being observed and focuses the light rays from it to produce a real image of the object. Objectives can be a single lens or mirror, or combinations of several optical elements. They are used in microscopes, binoculars, telescopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.

<span class="mw-page-title-main">Thorium dioxide</span> Chemical compound

Thorium dioxide (ThO₂), also called thorium(IV) oxide, is a crystalline solid, often white or yellow in colour. Also known as thoria, it is produced mainly as a by-product of lanthanide and uranium production. Thorianite is the name of the mineralogical form of thorium dioxide. It is moderately rare and crystallizes in an isometric system. The melting point of thorium oxide is 3300 °C – the highest of all known oxides. Only a few elements (including tungsten and carbon) and a few compounds (including tantalum carbide) have higher melting points. All thorium compounds, including the dioxide, are radioactive because there are no stable isotopes of thorium.

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.

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.

Lanthanum(III) oxide, also known as lanthana, chemical formula La₂O₃, 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 for certain catalysts, among other uses.

<span class="mw-page-title-main">Crown glass (optics)</span> 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.

$Achromatic telescope A refracting telescope design that reduces cromatic aberration$

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

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.

<span class="mw-page-title-main">History of photographic lens design</span>

The invention of the camera in the early 19th century led to an array of lens designs intended for photography. The problems of photographic lens design, creating a lens for a task that would cover a large, flat image plane, were well known even before the invention of photography due to the development of lenses to work with the focal plane of the camera obscura.

Thoriated glass is a glass material used in the manufacture of optical systems, specifically photographic lenses. It is useful to this process due to its high refractive index. Thoriated glass is radioactive due to the inclusion of thorium dioxide, oxide of radioactive element thorium. It has therefore been succeeded as a material of choice by glass including lanthanum oxide. Thoriated glass can contain up to 30% by weight of thorium. The thoriated glass elements in lenses over time develop a brown tint reducing transmission and interfering with neutral color reproduction.

Optical glass refers to a quality of glass suitable for the manufacture of optical systems such as optical lenses, prisms or mirrors. Unlike window glass or crystal, whose formula is adapted to the desired aesthetic effect, optical glass contains additives designed to modify certain optical or mechanical properties of the glass: refractive index, dispersion, transmittance, thermal expansion and other parameters. Lenses produced for optical applications use a wide variety of materials, from silica and conventional borosilicates to elements such as germanium and fluorite, some of which are essential for glass transparency in areas other than the visible spectrum.

References

↑ Gerald F. Marshall (19 July 1991). Optical Scanning. CRC Press. pp. 65–. ISBN 978-0-8247-8473-7.
↑ Rob Sheppard (1997). Telephoto Lens Photography. Amherst Media. pp. 19–. ISBN 978-0-936262-53-6.
↑ Horn, Alexander (2009-11-09). Ultra-fast Material Metrology. John Wiley & Sons. ISBN 9783527408870.
↑ "Archived copy". www.oemcameras.com. Archived from the original on 3 March 2016. Retrieved 17 January 2022.{{cite web}}: CS1 maint: archived copy as title (link)
↑ "Binocular Lens and Prism Glass - Helpful Facts for 2022". Birds At First Sight. 2022-05-16. Retrieved 2022-09-28.
↑ Smith, Gregory Hallock (2006-01-01). Camera Lenses: From box camera to digital. SPIE Press. ISBN 9780819460936 – via Google Books.
↑ Lankford, John (1997-01-01). History of Astronomy: An encyclopedia. Taylor & Francis. ISBN 9780815303220 – via Google Books.
↑ Shannon, Robert R. (1997-06-13). The Art and Science of Optical Design. Cambridge University Press. ISBN 9780521588683 – via Google Books.
↑ "Optical glasses". GMP Photo. Archived from the original on 2016-11-30.

External links

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[Marshall1991-1] Gerald F. Marshall (19 July 1991). Optical Scanning. CRC Press. pp. 65–. ISBN 978-0-8247-8473-7.

[Sheppard1997-2] Rob Sheppard (1997). Telephoto Lens Photography. Amherst Media. pp. 19–. ISBN 978-0-936262-53-6.

[3] Horn, Alexander (2009-11-09). Ultra-fast Material Metrology. John Wiley & Sons. ISBN 9783527408870.

[4] "Archived copy". www.oemcameras.com. Archived from the original on 3 March 2016. Retrieved 17 January 2022.{{cite web}}: CS1 maint: archived copy as title (link)

[5] "Binocular Lens and Prism Glass - Helpful Facts for 2022". Birds At First Sight. 2022-05-16. Retrieved 2022-09-28.

[6] Smith, Gregory Hallock (2006-01-01). Camera Lenses: From box camera to digital. SPIE Press. ISBN 9780819460936 – via Google Books.

[7] Lankford, John (1997-01-01). History of Astronomy: An encyclopedia. Taylor & Francis. ISBN 9780815303220 – via Google Books.

[8] Shannon, Robert R. (1997-06-13). The Art and Science of Optical Design. Cambridge University Press. ISBN 9780521588683 – via Google Books.

[9] "Optical glasses". GMP Photo. Archived from the original on 2016-11-30.

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