Low-dispersion glass

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Comparison of visible wavelength dispersion (i.e., the distance between foci for blue and red) of crown and flint glass converging lenses Kron flint.svg
Comparison of visible wavelength dispersion (i.e., the distance between foci for blue and red) of crown and flint glass converging lenses

Low-dispersion glass (LD glass) is a type of glass with reduced chromatic aberration, meaning the refractive index 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 between its constituent colors, resulting in a reduced "rainbow effect" at high-contrast edges. Wavelength dispersion in a certain material is characterized by its Abbe number; LD glass has a higher Abbe number than conventional types. Crown glass is an example of a relatively inexpensive low-dispersion glass.

Contents

Branding

Abbe number versus refractive index for glass lens materials Abbe-diagram 2.svg
Abbe number versus refractive index for glass lens materials

Photographic lenses with LD glass have been branded and marketed with different names to indicate the use of low-dispersion elements in the optical design, including:

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 publishes a diagram of the glasses it manufactures, grouped by glass code, showing refractive index (y-axis) as a function of Abbe number (x-axis). [9] 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

Imaging

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. [10]

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. [11]

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. [12]

Scientific

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. [13]

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. [14]

History

This apochromatic triplet converges three wavelengths of light Apochromat.svg
This apochromatic triplet converges three wavelengths of light

Some glasses have a peculiar property called anomalous partial dispersion. Abnormal dispersion is required to design apochromatic lenses; in contrast to the achromatic doublets, which converge blue and red wavelengths, apochromats converge focus of three or more wavelengths. [15]

Rare earth and radioactive lenses

Thoriated glass is doped with thorium dioxide, 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 is radioactive, optical engineers and designers sought a replacement using different doping elements, and lens designs using thoriated glass been discontinued by the late 1980s. [16]

Kodak Aero-Ektar fitted to K-19B camera for aerial reconnaissance, with puppy An unabashed Korean puppy holds his ground in an "eye to eye" encounter with a huge Fifth Air Force aerial camera... - NARA - 542230.tif
Kodak Aero-Ektar fitted to K-19B camera for aerial reconnaissance, with puppy

As an alternative, after 1930, George W. Morey introduced borate glasses doped with lanthanum oxide and oxides of other rare-earth elements, greatly expanding the available range of high-index low-dispersion glasses; although lanthanum is also radioactive, it has far less activity than thorium. Borate glasses have lower wavelength-refraction dependence in the blue region of spectrum than silicate glasses with the same Abbe number. During WWII, Kodak manufactured high-performance thorium-free optical glass for 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. The use of rare earths allowed development of high-index low-dispersion glasses of both crown and flint types. [17]

The use of low-dispersion glass in long-focal-length lens assemblies was pioneered by Ernst Leitz GmbH (Leitz) after World War II. Leitz laboratories discovered that lanthanum(III) oxide could be used as a suitable replacement for thorium. [18] However, additional elements had to be added to preserve the amorphous structure of the glass and prevent crystallization which would cause striae defects in images captured through those lenses.

These so-called "borate flint" glasses, which Schott classifies as KzF (kurzflint), 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. [19] Another high-performance glass contains a high proportion of zirconium dioxide; however its high melting point requires use of platinum 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 ( CaF
2 ) crystals were used starting in the 1960s for lens elements requiring low dispersion; [20] [21] however, there were significant drawbacks to using fluorite: the low refraction index of fluorite required high curvatures of the lenses, therefore increasing spherical aberration. In addition, fluorite has poor shape retention and is very fragile, requiring special handling to process into lens elements. [22]

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. [23]

See also

Related Research Articles

<span class="mw-page-title-main">Abbe number</span> 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.

<span class="mw-page-title-main">Optical aberration</span> Deviation from perfect paraxial optical behavior

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.

<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.

<span class="mw-page-title-main">Chromatic aberration</span> Failure of a lens to focus all colors on the same point

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 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. Since the focal length of the lens varies with the color of the light different colors of light are brought to focus at different distances from the lens or with different levels of magnification. Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.

<span class="mw-page-title-main">Corrective lens</span> Type of lens to improve visual perception

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.

<span class="mw-page-title-main">Achromatic lens</span> Lens that is designed to limit the effects of chromatic and spherical aberration

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.

<span class="mw-page-title-main">Flint glass</span> Type of optical 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.

<span class="mw-page-title-main">Refracting telescope</span> 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">Prism (optics)</span> Transparent optical element with flat, polished surfaces that refract light

An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides. Not all optical prisms are geometric prisms, and not all geometric prisms would count as an optical prism. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, acrylic and fluorite.

<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">Apochromat</span>

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.

<span class="mw-page-title-main">Eyepiece</span> 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 named because it is usually the lens that is closest to the eye when someone looks through an optical device to observe an object or sample. The objective lens or mirror collects light from an object or sample and brings it to focus creating an image of the object. The eyepiece is placed near the focal point of the objective to magnify this image to the eyes. The amount of magnification depends on the focal length of the eyepiece.

<span class="mw-page-title-main">Doublet (lens)</span>

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.

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.

<span class="mw-page-title-main">Achromatic telescope</span> 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.

Canon FL 300mm lens refers to two telephoto prime lenses made by Canon. The lenses have an FL type mount which fits the Canon FL line of cameras.

<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.

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

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  2. ↑ Sato, Haruo. "NIKKOR - The Thousand and One Nights No.11: NIKKOR-H 300mm F2.8". Nikon Imaging. Retrieved 16 August 2024.
  3. ↑ "Make Your First Interchangeable Lens a Telephoto". VisionAge. No. 3. 1985. pp. 13–16. Retrieved 16 August 2024.
  4. ↑ "Pentax 6×7 [brochure]" (PDF). Pentax Corporation. 1976. Retrieved 16 August 2024– via Pacific Rim Camera, Reference Library.
  5. 1 2 "Groundbreaking 06: Bringing Special Low Dispersion Glass to Life". Sigma Sein. Retrieved 16 August 2024.
  6. ↑ "Tamron Product Brochures". Adaptall-2.com. pp.  1, 2. Archived from the original on November 6, 2007.
  7. ↑ "Tokina Lens Catalog". Tokina Co., Ltd. Retrieved 16 August 2024.
  8. ↑ "FD 500mm f/4.5L". Canon Camera Museum. Retrieved 16 August 2024.
  9. ↑ "Interactive Abbe diagram". Schott AG. Retrieved 17 August 2024.
  10. ↑ Gerald F. Marshall (19 July 1991). Optical Scanning. CRC Press. pp. 65–. ISBN   978-0-8247-8473-7.
  11. ↑ Rob Sheppard (1997). Telephoto Lens Photography. Amherst Media. pp. 19–. ISBN   978-0-936262-53-6.
  12. ↑ "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)
  13. ↑ Horn, Alexander (2009-11-09). Ultra-fast Material Metrology. John Wiley & Sons. ISBN   9783527408870.
  14. ↑ "Binocular Lens and Prism Glass - Helpful Facts for 2022". Birds At First Sight. 2022-05-16. Retrieved 2022-09-28.
  15. ↑ Smith, Gregory Hallock (2006-01-01). Camera Lenses: From box camera to digital. SPIE Press. ISBN   9780819460936 – via Google Books.
  16. ↑ Frame, Paul. "Thoriated Camera Lens (ca. 1970s)". ORAU Museum of Radiation and Radioactivity. Retrieved 17 August 2024.
  17. ↑ Shannon, Robert R. (1997-06-13). The Art and Science of Optical Design. Cambridge University Press. ISBN   9780521588683 – via Google Books.
  18. ↑ Schneider, Jason (September 5, 2018). "Rare Earth Glass Leica Lenses: A Quick and Quirky Overview". Leica Society International. Retrieved 16 August 2024.
  19. ↑ Lankford, John (1997-01-01). History of Astronomy: An encyclopedia. Taylor & Francis. ISBN   9780815303220 – via Google Books.
  20. ↑ Crowther, Jonathan (2017). "Asahi Pentax Ultra Achromatic Takumar 85mm lens - achromatic imaging". JMC Scientific Consulting Ltd. Retrieved 16 August 2024.
  21. ↑ "FL-F 300mm f/5.6". Canon Camera Museum. Retrieved 16 August 2024.
  22. ↑ "Fluorite lenses: Corrective capabilities beyond the limits of ordinary optical glass". Canon Camera Museum. Retrieved 17 August 2024.
  23. ↑ "Optical glasses". GMP Photo. Archived from the original on 2016-11-30.
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