BACK to the HOMEPAGE

OPTICAL GLASS

 Optical glass is an inorganic product (usually more or less completely transparent), which is produced by melting together selected inorganic material and cooling the molten product without allowing its crystallization.

Often the question is raised whether glass is a super-cooled liquid or not. The answer seems to be more complex than most people anticipate. Some reports, authors and teachers claim that one can find old church windows which reveal a difference in thickness top-bottom is proof for the claim that glass is a very viscous liquid. These windows can be found and yes, they are thinner on their top than on the bottom but there is no evidence that this does not come from imperfect manufacturing processes at that time. Another argument is more important and cannot be neglected: In general, there is a very clearly defined transition between a liquid and a solid state - the molecules arrange themselves in a certain lattice and instead of "moving around", vibrate about their position - the solid state has been reached (first order phase transition). On the other end, material, which is called liquid, can be characterized by a measure, called viscosity, which measures the resistance to flow (see below about that unit). Usually a liquid, when cooled reaches a point when crystallization starts but it can also become "super-cooled" and no crystallization starts as no nucleation sites exist. At "room temperature" this material becomes an amorphous solid which maybe called "glass". There is no first order phase transition and therefore a lot of references, books, teachers other material call glass a super-cooled liquid. But glass molecules are rigidly enough bound to enter a quasi-stable status which can exist for a very long time - the molecules are fixed but in a disordered arrangement. That it is quasi-stable can be seen from the fact that it can undergo a spontaneous de-vitrification into a crystalline solid. Another argument for this statement is the fact that the manufacturing of glass requires a certain cooling gradient to get to the "glasseous" state - if cooled too slowly, it will crystallize as well. A third argument - in this case for the supporters of "liquid" is that the borderline viscosity for calling matter liquid was set more or less arbitrarily at 10¹³ Poise.

So, what is glass now - there are several denominations possible - in view of thermodynamics, one is most likely allowed to call glass a ultrahighly viscous liquid or an amorphous solid or much easier, glass - a state which is neither liquid nor solid - plenty of room for discussions and agreements and disagreements, right?

Now back to optical glass - These before mentioned inorganic components are primarily glassifiers like silica, boric or phosphoric oxides mixed with various metal oxides. In the following, only optical glass is discussed, not ordinary window or bottle glass, laser glass or other variations like opal glass or filter glass.

Compared to “normal” glass, optical glass is extremely expensive - hundreds to thousands of Dollars per pound - and sophisticated to manufacture. For special glass types often 10kg Platinum pots have to be used but in general, the use of ceramic containers, which are Pt lined, is more common. An alternative are containers or pots, which can be used only once and then need to be discarded

As mentioned before already, not only is the melting process quite sophisticated, the cooling process is an art in itself. This cooling, often calling “fine-cooling” has to observe strict temperature gradients and timing – not too slow to avoid crystallization of the product, but also not too fast to keep the glass homogeneous enough for optical usage. This temperature range, which is covering most of the viscosity range of the glass, is positioned between two temperatures – the transformation temperature and the aggregation temperature. The viscosity between these two points is covering a range from 10 Poise to about 10¹³ Poise (1 Poise = 1cm־¹gsec ־¹) and a temperature range from over 1400°C to about 350-600°C, depending on the kind of optical glass produced.

This process needs to be extremely accurate and reproducible to ensure that the optical constants of the glass are as identical as feasible - lot after lot, to the 5^th digit behind the comma – not an easy task.

But in reality it is a fact, that despite all accurate controls and monitoring of many parameters, the melting process is very slightly different for each block of glass - so each block gets a production number for identification, which is linked to a full set of optical constants, which are measured and documented. This procedure allows the optics designer by fine-tuning the tolerances to maintain the optical performance of the design according to the available glass material. This fine-tuning is mandatory and essential for all high-performance lenses and optical systems – the better the optical correction of a system, the more sensitive the influence of all parameters – first and foremost the optical ones.

 For the following discussion I shall use the data and denomination of the probably most important and longest known manufacturer of optical glass – Schott in Germany. Other major manufacturers of optical glass are Corning USA, Pilkington and the two Japanese companies, Hoya and Ohara.

There is a long list of parameters, which are characterizing a block of optical glass – the more important ones are the refractive index, the dispersion coefficient, the acid resistance factor, the hardness, the optical transmission coefficient, the degree of purity (or impurity due to inclusions and tiny air bubbles).

Glass, which is characterized by a certain set of parameters within a certain range, is called a “type of glass” – like the well know “flint glass” and “crown glass”.

In the following there is a list of well-known glass types, made, as already mentioned before, by one of the most important manufacturers of optical glass – Schott in Germany - and their respective abbreviations according to Schott.

The denomination of the types of glass by those four other companies are similar to the one from Schott – as example, BK-7 (or better N-BK7, which indicates that this type is free of arsenic and lead) has the general denomination “glass 517642”.

When I use certain well-known indices like the refractive index, I presume that the reader remembers what they mean from the respective lectures in Physics.

These six digits indicate that the refractive index n, at a certain, defined wavelength λ_(d)=587,6nm, that n_(d) is 1,517… (The first three digits of the designation) and that the dispersion ν_(d) at the same, defined wavelength is 64,17…, which rounded up is 64,2. The exact rules are described in the MIL standard MIL-G-174.

One can sometimes find instead of the 6 digits 9 digits as designation of the glass – the before explained 6 plus after a period sign 3 additional ones after the initial 6. These 3 digits are indicating the first digit before the comma and two digits after the comma of the density – like 318 means density of 3,18.

Here an example: N-BK7 (Schott) is (with 9 digits) listed in general as glass 517642.251, which means a refractive index of 1,517 plus dispersion 64,2 plus density 2,51.

Below is a comparison of the designations for the “standard” optical glass, according to Schott, BK7, and its designations according to the other four major manufacturers: 

 Types of Glass

 As you can see from the names of the types of glass listed in the following table, these names are more of historic nature than purely derived from their chemical composition – with good reasons – most of the well-known glass types have already been developed in the 19^th and early 20^th century, or even earlier in time.

This was also the time period, where Schott in Germany (now part of the Zeiss Group of Companies) was more or less the leading power in creating new glass types and therefore, these glass types had originally German names – added are the English translations.

 All these “older” types and all newer glass types are listed in sc. “Glass-catalogues” in x-y diagrams. The x-axis is indicating the dispersion ν and the y-axis the refractive index n_(d). These glass-catalogues contain besides the already mentioned x-y diagrams tables with the mean refractive index, Abbe’s number and chemical and physical properties of the various glass types. As dispersion ν and refractive index n is dependent on the wavelength of light, the catalogues contain sets of them – for at least the major wavelengths in the blue-green-red spectrum area, but usually many more, up to 20 wavelengths from UV to IR.

The following diagrams are taken out of the Schott glass catalogue – shown here are three very different types of glass – a “standard” optical glass, BK7, and then two extremes – FK51 at the very low end of the refractive index and the other one at the opposite end.

 BK7 can be regarded as “standard” optical glass:

 As one can see here, the refractive indices for this type of glass are listed from UV at 248,3nm to IR at 2,3254μ.

The internal transmittance is listed for two values of thickness, 10mm and 25mm for the same range UV to IR.

Important are also the values of the relative partial dispersion and the constants of the dispersion formula – the meaning of these values will be explained a bit later.

FK56 is a special optical glass with a lower refractive index but significantly higher Abbe number ν, 94.95 compared to 64.17 for BK7

N-FK56(Schott)

Comparing the values of BK7 with FK56, you can see the differences in all major constants and values – not only the refractive indices.

 And on the other end of the x-y diagram, where one can find the glass types with very high refractive index is SF66 positioned – with one of the highest refractive index available n=1,9228, but with a very low Abbe number ν=20,88

 SF66 (Schott)

Now to the explanation of the other constants, data, values and abbreviations on these glass characteristics spread sheets:

The dispersion could be explained as the ability of glass (and of other material as well) to alter the direction of a ray of light depending on its wavelength. Another definition would explain it as a consequence of a wavelength-depending refractive index.

The sc. Abbe-number, German "Abbezahl", written as a Greek “n”, ν, which is used in all those lists to quantify the dispersion, is defined as ν_(d)=(n_(d)-1)/(n_(F)-n_(C)). Here d, F and C are standing for certain wavelengths – they are listed in each of the spreadsheets above.

This difference, n_(F)-n_(C) is also called principal dispersion or in German "Hauptdispersion".

In our days, usually not ν_(d), which is based on the d-line of the spectrum, is used for optics calculations, but ν(e) which is based on the e-line and thus consequently the value for ν(e)=(n(e)-1)/(n(F’)-n(C’)).  Not a huge difference, but enough to be of importance for optical calculations – in case of N-BK7, ν(e)=63,96 and ν(d)=64,17.

Optical systems need to be corrected not only for one or two pairs of wavelengths but in most cases for the entire visible spectrum – which means that more pairs of wavelengths need to be taken to calculate the dispersion coefficients resulting from these pairs – called relative partial dispersion P_x,y  with x and y being a pair of wavelengths – as can be seen for N-BK7, there are 10 P values – relative partial dispersion for various wavelengths pairs – listed. For better corrected optical systems, systems for which the secondary spectrum has been eliminated as well, glass with anormal partial dispersion has to be used – these ΔP values (deviation of relative partial dispersion from the “normal” line) are listed as well.

For those who like mathematical formulas, a common way to describe the curve of refractive index versus wavelengths is the
s.c. Sellmeier Dispersion Formula, which is pretty accurate for wavelengths from about 360nm to 1500nm if three terms are
used as written here

n(λ)² -1 = B₍₁₎λ²/( λ²-C₍₁₎) + B₍₂₎λ²/( λ²-C₍₂₎) + B₍₃₎λ²/( λ²-C₍₃₎)

Or the s.c. Schott formula, which is more or less the standard for wavelengths from about 380nm to about 750nm, it is

n_(λ)² -1 = A+A₍₁₎λ² + A₍₂₎λ^-2 + A₍₃₎λ^-4+ A₍₄₎λ^-6+ A₍₅₎λ^-8.

B and C are coefficients, which one can find in the listings of the glass data and A are the constants of the empirical Schott dispersion formula.

Another important value is the internal transmittance – this value reflects in a quantitative manner the ability of the glass to transmit light of different wavelengths. The glass data contain this internal transmittance τ(i) for wavelengths from 2,5μ (IR) to the UV cut-off, depending on the glass, somewhere below 400nm (0,4μ). In case of N-BK7, this cut-off is around 280nm. To get an impression how the thickness of glass influences this transmittance, the data are listed for two different thicknesses – 10mm and 25mm.

It is well known, that the refractive index is changing with temperature – therefore the temperature coefficients of the refractive index are listed as well. Glass does not only change its physical dimensions with temperature, it also changes its optical characteristics significantly enough to know about and take precautions.

If an optical system needs to be corrected as close to perfection as possible, optics designers like to add elements, which are made not out of glass but out of an inorganic crystal – usually Calcium Fluorite. CaF2 has very attractive optical properties – high transmittance from deep UV far into the IR (135nm to 9400nm) and is providing a very low dispersion – this means that an optical system can be brought to “absolute” perfection regarding color correction by using one or more elements of this crystal. In our days, all those crystal elements are artificially grown, not anymore of natural origin. CaF2 is soft and expensive – which makes it necessary to implement such a lens element well protected inside an optical system. That was a very short detour into one of the most important non-glass materials for optical systems. Other ones of importance for certain optical specialty systems are quartz, sapphire and diamond – but discussing them here would lead too far. Back to optical glass!

Other values in the glass data list are coefficients and indicators which allow conclusions regarding acid=, alkali=, stain=, climatic and phosphate resistance – important to know about when one needs to decide on a glass type for use as outermost glass surface in an optical system – as example, it would be very unwise to use as front surface of a camera lens a glass type which is very soft, not enough acid and alkali resistant and which is not resistant to climatic influences – such a front lens would be ruined within very short time. But inside of a cemented multi-lens system, such a glass type could be used if necessary.

There are many more data and values, which are used and necessary to fully characterize optical glass – important ones like the bubble class, the local deviations of the refractive index, called striae, then the amount of inclusions and last but not least stress birefringence. It is evident, that the less microbubbles can be found in a block of glass, the better, the more homogeneous the refractive index is within a block of glass, the better etc. As it is not possible to produce glass which is 100,00000% free of all these deficiencies, their amount, size and density is defining the quality of optical glass as well. For photographic purposes, tiny bubbles or inclusions are usually of no importance for the photographic quality of the optics whereas striae is of importance – just to give an example. It also depends on the location within the optical path which deficiency is more important to consider and which not – striae close the field plane is not as bad as it is in a lens close to the aperture plane whereas bubbles and inclusions in glass which is close to the field plane is much worse than the same amount in a lens close to the aperture plane.

It would exceed the purpose of this short introduction to go into the depth of optics production – cutting, grinding and polishing out of glass blocks, plates and barrels. What was attempted is a short introduction into some of the important data and characteristics of optical glass.

 BACK to the HOMEPAGE