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.
Various elements can be used to form glass, including silicon, boron, phosphorus, germanium and arsenic, mostly in oxide form, but also in the form of selenides, sulfides, fluorides and more. These materials give glass its characteristic non-crystalline structure. The addition of materials such as alkali metals, alkaline-earth metals or rare earths can change the physico-chemical properties of the whole to give the glass the qualities suited to its function. Some optical glasses use up to twenty different chemical components to obtain the desired optical properties.
In addition to optical and mechanical parameters, optical glasses are characterized by their purity and quality, which are essential for their use in precision instruments. Defects are quantified and classified according to international standards: bubbles, inclusions, scratches, index defects, coloring, etc.
History
The earliest known optical lenses, dating from before 700 BC, were produced under the Assyrian Empire: they were made of polished crystals, usually quartz, rather than glass.
It wasn't until the rise of the Greeks and Romans that glass was used as an optical material. They used it in the form of spheres filled with water to make lenses for lighting fires (burning glass), as described by Aristophanes and Pliny, or to make very small, indistinct characters larger and sharper (magnifying glass), according to Seneca.
Although the exact date of their invention is not known, glasses are said to have been described in 1299 by Sandro di Popozo in his Treatise on Family Conduct: "I am so altered by age, that without these lenses called spectacles, I would no longer be able to read or write. They have recently been invented for the benefit of poor old people whose eyesight has become bad". At the time, however, "glasses" were actually made from beryl or quartz.
The only lens available at the time, ordinary soda-lime glass, was unable to compensate for optical aberrations. However, it evolved slowly over the centuries. It was first lightened by the use of ashes, which contain manganese dioxide that transforms ferrous oxide (FeO) into ferric oxide (Fe2O3), which is much less colorful. Then, around 1450, Angelo Barovier invented "crystalline glass" (vetro cristallino) or "Venetian glass" (cristallo di Venezia), improving on the previous process by purifying the ashes by leaching to obtain a purer potash. Lime was introduced, first for economic reasons in the 14th century, then as a technical improvement in Bohemia in the 17th century (Bohemian glass), eliminating a very large proportion of impurities. This practice did not arrive in France until the middle of the eighteenth century. It was at this time that the Manufacture Royale de Glaces de Miroirs (Compagnie de Saint-Gobain S.A.) began to produce glass composed of 74% silica, 17.5% soda and potash, and 8.5% lime.
Thus, the first complex optical instruments, such as Galileo's telescope (1609), used ordinary soda-lime glass (the first crown glass), composed of sand, soda, potash and sometimes lime, which, although suitable for glazing or bottles, was hardly suitable for optical applications (distortion, blurred effect, irregularities, etc.). In 1674, the British inventor George Ravenscroft, wishing to rival Venetian and Bohemian crystal while being less dependent on imported raw materials, replaced lime with lead(II) oxide to compensate for glass's lack of resistance to humidity, thus inventing lead crystal (the first flint glass, named after the high-purity English siliceous stone used), brighter than ordinary glass, composed of silica, lead oxide and potash.
Chester Moore Hall (1703-1771), using the two types of glass available (soda-lime crown and lead flint), invented the first achromatic doublet. His work was taken up by John Dollond in his Account of some experiments concerning the different refrangibility of light, published in 1758.
The real revolution in optical glass came with the development of industrial chemistry, which facilitated the composition of glass, allowing properties such as refractive index and dispersion coefficient to be varied. Between 1880 and 1886, the German chemist Otto Schott, in collaboration with Ernst Abbe, invented new glasses containing oxides such as "anhydrous baryte" (barium oxide BaO) and anhydrous boric acid (B2O3), with which he developed barium crowns, barium flints and borosilicate crowns.
Between 1934 and 1956, other oxides were used. Then, by adding phosphates and fluorides, phosphate crowns and fluorine crowns were obtained. As optics became increasingly complex and diverse, manufacturers' catalogs expanded to include 100 to 200 different lenses; glass melts increasingly included special components such as oxides of heavy elements (high refractive index and low dispersion), chalcogenides (sulfide, selenide, telluride), halides such as fluorides (low refractive index and high dispersion) or phosphides, cerium-doped glasses to obtain radiation-resistant lenses, and so on.
Since the 1980s, however, glass catalogs have tended to become increasingly limited.
Properties The most important physical properties of glass for optical applications are refractive index and constringency, which are decisive in the design of optical systems, and transmission, glass strength and non-linear effects.
Index and constringency
The refractive index indicates the refractive power of a glass, i.e. its ability to deflect light rays to a greater or lesser extent. This deflection can be deduced from Descartes' law. The refractive index is a wavelength-dependent quantity, creating chromatic aberrations in a system by refracting rays more or less according to their wavelength: This is the phenomenon observed when light is decomposed by a prism. Several laws have approximated this relationship to wavelength, notably Cauchy's law and Sellmeier equation.
The refractive index of a glass is given for the yellow line known as the d line of helium (then noted nd) or for the green e line of mercury (then noted ne), depending on usage and the two main standards used.
The dependence of refractive index on wavelength requires a measure of the glass's dispersion, i.e. the difference in deviation between two wavelengths. A highly dispersive glass will deflect short wavelengths to a great extent, but long wavelengths to a lesser extent. The measure of dispersion is the Abbe number, or constringence. The main dispersion is the difference nF-nC (helium lines) or nF'-nC' (cadmium lines), and constringencies for the same lines as the refractive index are deduced by and . A high Abbe number means that the glass is not very dispersive, and vice versa. Table of spectral lines used to measure refractive indicesWavelength (nm)2325,41970,11529,61013,98852,1101706,5188656,2725643,8469632,8589,2938587,5618546,074486,1327479,9914435,8343404,6561365,0146DesignationtsrCC'DdeFF'ghiSourceHg IR LineH IR LineCs IR LineRed He LineRed H LineRed Cd LineLaser He-Ne LineYellow Na LineYellow He LineGreen Hg LineBlue H LineBlue Cd LineBlue Hg LineViolet Hg LineHg UV Line
Lenses are usually divided into two groups with the generic names of crown and flint, referring respectively to low-dispersion, low-index lenses and high-dispersion, high-index lenses. Typically, the distinction is made at around νd=50: lenses below this value are flints, the others are crowns.
These two parameters alone are needed to differentiate between lenses: two lenses with equal nd and νd are identical. Glasses are represented on the so-called Abbe diagram, a graph with abscissa nd and ordinate νd, where each glass is denoted by a point on the graph.
Oxide glasses fall into a range of nd from 1.4 to 2.0 and νd from 20 to 90, with SiO2 being the oxide glass with the highest contringency and lowest index. Fluoride glasses can go up to νd>100 and nd<1.4, BeF2 being the fluoride glass at the highest constringency and lowest index. Chalcogenide glasses have indexes exceeding 2, a large proportion of which cannot be shown on an Abbe diagram due to their absorption in visible wavelengths preventing a relevant νd measurement. For optical materials that are opaque in the visible range, constringence is measured at longer wavelengths.
This classification also has its limitations when it comes to active optical glasses (birefringence, acousto-optic effect, and non-linear effects), optical filters or graded-index lenses, so we restrict the term "classical optical glass" to the aforementioned glasses, i.e. those with limited index and dispersion, which can be described essentially by their dispersive behavior and refractive index.
Transmission and absorption
Another very important characteristic of optical glass is its absorption and transmission behaviour. The use to which the future lens will be put determines its behavior: filters that absorb in certain spectral bands, lenses that are highly transparent in the visible, ultraviolet or infrared, resistance to radiation.
As a general rule, the transmittance of the glass is given by the manufacturer, noted τi or Ti, a value that depends on the thickness of the material and whose measurement makes it possible to take into account the loss of transmission due to absorption and diffusion by internal defects in the glass. Since the transmittance term takes into account the refractive index via the Fresnel coefficient, it is also dependent on the wavelength and thickness of the sample via the formula where is the transmittance and thickness.
Transmittance windows are of particular interest when it comes to choosing the right glass for applications such as far-infrared or far-ultraviolet. These windows are the result of the absorption of the materials making up the glass, which increases in the infrared and ultraviolet. Absorption in these two wavelength ranges is due to distinct phenomena, and can evolve differently depending on environmental conditions.
Ultraviolet absorption
In the ultraviolet, or UV, the drop in transmission is due to the electronic transitions of the elements making up the glass: valence electrons absorb wavelengths whose energy corresponds to their band gap. According to solid-state band theory, electrons can only take on certain specific energy values in particular energy levels, but with sufficient energy, an electron can move from one of these levels to another. Light waves are charged with an energy hν, inversely proportional to wavelength (ν=c/λ), which can enable an electron to pass from one level to another if this energy is sufficient and therefore if the wavelength is short enough. A silica glass absorbs wavelengths below 160 nm, a glass based on boron trioxide (B2O3) absorbs below 172 nm, a phosphorus pentoxide glass (P2O5) absorbs below 145 nm.