What is glass?

Glass is an amorphous (non-crystalline) solid material. Glass is commonly used for windows, bottles, modern hard drives or eyewear and examples of glassy materials include soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminum oxynitride. The term glass (late-Latin ‘glaesum’) developed in the late Roman Empire. In Germany that term was used for glossy and transparent amber, which was mainly used - as early glass was too - in jewels.
Strictly speaking, a glass is defined as an inorganic product of fusion which has been cooled through its glass transition to the solid state without crystallizing. Many glasses contain silica (quartz sand) as their main component and glass former.

The optical and physical properties of glass make it suitable for applications such as flat glass, container glass, optoelectronics material, laboratory equipment, thermal insulator (glass wool), reinforcement fiber (glass-reinforced plastic, glass fiber reinforced concrete), art and - off course - optics. But, unfortunately, not all glass can be used for photography. The mixture of various components - often a well-kept secret - is very important. From its start in 1917 Nikon is able to melt its own glass. This ability was even strengthened when Nikon took over Hikari* Glass Co. in 2004. This glass manufacturer, founded in 1962, is specialized in making glass at relatively low temperatures, at which it is possible to mould glass. Nowadays Nikon Corporation can make glass from a wide choice of nearly 300 different components and/or ingredients.

* Hikari = Japanese for 'light'.

Glass ingredients.

Pure silica (SiO2) has a "glass melting point" of over 2300 °C (4200 °F). While pure silica can be made into glass for special applications other substances are added to common glass to simplify processing. One is sodium carbonate (Na2CO3), which lowers the melting point to about 1500 °C (2700 °F) in soda-lime glass; "soda" refers to the original source of sodium carbonate in the soda ash obtained from certain plants. However, the soda makes the glass water soluble, which is usually undesirable, so lime {calcium oxide (CaO), generally obtained from limestone}, some magnesium oxide (MgO) and aluminum oxide (Al2O3) are added to provide for a better chemical durability. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass. It is, however, not suitable for photographic lenses.

As well as soda and lime, most common glass has other ingredients added to change its properties. Lead glass, such as lead crystal or flint glass, is more 'brilliant' because the increased refractive index** causes noticeably more "sparkles", while boron may be added to change the thermal and electrical properties, as in Pyrex. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion, and was formerly used in producing high-quality lenses (e.g. early versions of the Nikkor 1.4/35mm.), but due to its radioactivity it has been replaced by lanthanum oxide in modern glasses. Nikon’s famous ED-glass (ED = extra dispersion) has a high content of lanthanum oxide. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium (IV) oxide can be used for glass that absorbs UV wavelengths (biologically damaging ionizing radiation).

In August 2010 Nikon introduced - with the wide-range Zoom-Nikkor 4.5-5.6/55-300 mm. - a new type of optical element: HRI (High Refractive Index) element which "offers the same benefits as multiple elements made of standard optical glass" as Nikon's press release explains. "It compensates plane curvature and spherical aberration", without mentioning whether this element is made of glass or some other material.

Finally, fining agents such as sodium sulfate, sodium chloride, or antimony oxide are added to reduce the bubble content in the glass.


Flint & Crown

In most older and modern lenses flint glass and crown glass are widely used.

Flint glass is optical glass that has a relatively high refractive index and a 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.
With respect to glass, the term flint derives from the flint nodules found in the chalk deposits of southeast England that were used as a source of high purity silica by George Ravenscroft (1632-1683), circa 1662, to produce a potash lead glass that was the predecessor to English lead crystal aka flint glass.
Traditionally, flint glasses were lead glasses containing around 4–60% lead oxide; however, the manufacture and disposal of these glasses are sources of pollution. In many modern flint glasses, the lead can be replaced with other additives such as titanium dioxide and zirconium dioxide without significantly altering the optical properties of the glass.
Flint glass can be fashioned into rhinestones which are used as diamond simulants.

Crown glass is optical glass and is produced from alkali-lime (RCH) silicates containing approximately 10% potassium oxide. It has low refractive index (1.50 - 1.54) and low dispersion (with Abbe numbers around 60). Crown glass got its name from the crown shape achieved by the blowing process.
As well as the specific material named crown glass, there are other optical glasses with similar properties that are also called crown glasses. Generally, this is any glass with Abbe numbers in the range 50 to 85. For example, the borosilicate glass Schott BK7 is an extremely common crown glass, used in precision lenses. Borosilicate's contain about 10% boric oxide, have good optical and mechanical characteristics, and are resistant to chemical and environmental damage. Other additives used in crown glasses include zinc oxide, phosphorus pentoxide, barium oxide, and fluorite.

A concave lens of flint glass is commonly combined with a convex lens of crown glass to produce an achromatic doublet. The dispersions of the glasses partially compensate for each other, producing reduced chromatic aberration compared to a singlet lens with the same focal length.

Chromatic aberration occurs when different wavelenghts of light (colours) do not focus to the same point. An achromatic and an apochromatic lens are corrected to focus to the same point for at least at two or three colours respectively. There are two types of chromatic aberration: axial (longitudinal), and transverse (lateral). Axial aberration occurs when different wavelengths of light are focused at different distances from the lens (focus shift). Longitudinal aberration is typical at long focal lengths. Transverse aberration occurs when different wavelengths are focused at different positions in the focal plane, because the magnification and/or distortion of the lens also varies with wavelength. Lateral aberration is typical at short focal lengths.

Scientists, like the English theoretical physicist Sir John Brian Pendry (the science of invisibility), are able to create ‘super lenses’ or ‘hyper lenses’ containing ‘metamaterial’.

Nearly all lenses, used for photography, filming and projection are in fact a group of lens elements of which some are glued together. Canada balsam, a turpentine-alike liquid from the Balsam fir (Abies Balsamea) is used as a cold glue for glasses and optical instrument components. It has a breaking index similar to most glasses.

The refractive index of miscellaneous materials:

human eye lens = 1.32 / water = 1.33 / gelatin = 1.54 / fused silica = 1.46 / soft crown glass = 1.52 / barium light flint glass = 1.60 / lanthanum glass = 1.68-1.88 / silver chloride = 2.06 / diamond = 2.42


*In physics and optics, the Abbe number, also known as the V-number or constringence (aka reciprocal dispersion) of a transparent material, is a measure of the material's dispersion (variation of refractive index with wavelength) in relation to the refractive index. It is named for Ernst Abbe (1840–1905), the German physicist who defined it.

**A refractive index (or index of refraction) (first noticed by Willebrord Snell van Royen aka Snellius - Netherlands / 1580-1626) is a measure of how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical soda-lime glass has a refractive index close to 1.5, which means that in glass, light travels at 1/1.5 = 2/3 the speed of light in a vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, an effect that is used in lenses. Second, light reflects partially from surfaces that have a refractive index different from that of their surroundings.