OPTICAL GLASSES

By Heinz Richter

One of the most important substances needed to create photographic images is glass.  It is used in the manufacture of photographic lenses, camera viewfinders, SLR mirrors, filters, negative carriers in enlargers, condensers in enlargers and projectors, slide mounts, and more.  As a matter of fact, photography, as we know it today, would not be possible without glass.

The manufacture of glass was already known to the Egyptians about 3000 BC and was further perfected by the Phoenicians who are credited with the invention of the glass blowpipe.  During Roman times, glass making was elevated to an art form.  Especially the manufacture of highly decorated small glass vessels (fondi d’oro) became very popular.  This culminated in the 13^th century in Venice with the manufacture of highly artistic glass items.  Yet even in the 16^th century, glass, even window glass, was considered to be a luxury item.  It was also in the 16^th century that the first glass lenses appeared in the camera obscura, which was a major step in the development of photography.

According to the dictionary, “glass is a hard, brittle, transparent or translucent, greenish solid solution made by melting a mixture of silica (sand) and various silicates” like soda, borates, phosphates and potash, with metallic oxides added to give the product special properties.  Colorless glass is obtained by adding manganese, which neutralizes traces of iron in the sand.  Different tints are obtained by adding various metallic oxides.  For instance, gold and copper will result in a red color, manganese violet, cobalt blue, iron and chromium green.

Depending on the composition, some glass will melt at temperatures as low as 500° C (900° F), others melt only at 1650° C (3180° F). After careful preparation and measurement, the raw materials are mixed and undergo initial fusion before being subjected to the full heat needed to turn the matter into glass, called vitrification.  In the past, melting was done in clay pots heated in wood or coal-burning furnaces. Pots of fireclay, holding from 0.5 to 1.5 metric tons of glass, are still used when relatively small amounts of glass are needed for handworking. In modern glass plants, most glass is melted in large tank furnaces, first introduced in 1872.  They can hold more than 1000 metric tons of glass and are heated by gas, oil, or electricity. The glass batch is fed continuously into an opening (doghouse) at one end of the tank, and the melted, refined, and conditioned glass is drawn out the other end. In long forehearths, or holding chambers, the molten glass is brought to the correct working temperature, and the mass is then delivered to the forming machines.

The above described methods are useful for the manufacture of all but optical glass. The manufacture of optical glass is a delicate and exacting operation. The raw materials must be of the highest purity, and it takes great care to prevent imperfections to be introduced.  Small air bubbles and residual unvitrified matter will cause distortions in a lens. Striae, the streaks caused by incomplete chemical homogeneity in the glass, will also cause serious distortion, and strains in glass caused by improper annealing will further impair optical qualities.

Annealing is a process of heat treatment by which glass is rendered less brittle and more resistant to fracture. Annealing minimizes internal defects in the atomic structure of the glass and leaves it free from internal stresses that might otherwise be present because of prior processing steps.

Glass is annealed by heating it to high temperatures and then cooling it slowly.  Large masses of glass are cooled within the heating furnace.  Sheets are usually annealed in a continuous-process furnace. They are carried on a moving table through a long chamber in which the temperature is carefully graded from initial heat just below the softening point to that of room temperature at the end. The annealing time varies widely according to the thickness of the individual piece.  Window glass, for example, requires several hours, plate glass, several days, and glass mirrors for reflecting telescopes as well as certain glasses for photographic lenses several months.

Optical glass was originally melted in pots for prolonged periods, during which it was constantly stirred by a refractory rod. After lengthy annealing, the glass was broken into pieces. The best fragments were further reduced, reheated, and pressed into the desired forms. In recent years a method has been adopted for the continuous manufacture of glass in platinum-lined tanks, using platinum-lined stirrers in the cylindrical end chambers (or homogenizers). This process produces greater quantities of optical glass that are cheaper and superior to glass produced by the earlier method.

The principal specifications for optical glasses are its light bending power, or refractive index, for different wavelengths or colors of light and its dispersion, the difference in refractive index for certain wavelengths.  All of us have seen dispersion of light in form of a rainbow from light shining through a prism and being separated in the spectral colors.  Dispersion is one of the most difficult problems to deal with in designing photographic lenses.  Dispersion will introduce chromatic aberration which, when not controlled, will lead to poor sharpness and loss of detail.  The best optical glasses available today display very high refractive indices with relatively low dispersion.  Within the last twenty years some glass manufacturers have succeeded in producing glasses with a peculiar property called anomalous partial dispersion, first pioneered by Leitz thirty years ago.  Such glasses provide the lens designer with new and powerful means to improve the color correction of chromatic aberrations of camera lenses.  Prior to the availability of glasses with anomalous partial dispersion some manufacturers used calcium fluorite crystals for the manufacture of lens elements in telephoto lenses.  Especially Canon was making wide use of this.  Other manufacturers, however, do not regard them a suitable for photographic lenses because they have very poor shape retention and because their refractive index is only ne1.43.  Because of this relatively low refractive index, lens elements made of calcium fluorite require very strong curvatures.  This has the disadvantage of increasing spherical aberration and causing substantially more surface reflections.

The atlas of optical glasses shows the refractive index and dispersion of many of today’s glasses.  The vertical axis shows the refractive index for a wavelength of 546 nm while the horizontal axis shows the Abbe number (for Professor E. Abbe of Zeiss), or the relative reciprocal dispersion of the glass.  This is a comparison between the refraction of the glass for yellow/green light in the center of the visible spectrum and the difference between its refraction for blue/violet and deep red light.  Please note that these dispersion values are reciprocals:  The higher the Abbe number, the less dispersion.  Sometimes optical engineers need high dispersion numbers, sometimes low ones, to balance the characteristics of different optical glasses in order to produce a high degree of image correction.

The large, circled dots indicate glasses used in Leitz lenses today, the small circled dots indicate Leitz glasses not currently in use.  The plain dots indicate glasses from other manufacturers.

The advances in glass technology can easily be seen when comparing the refractive indices of glasses over the years.  The Leitz Elmar 50mm f/3.5 from 1926 used SK15 glass with a refractive index of ne1.625, this increased to ne1.694 with LaK9 glass in 1953 with the Summicron 50mm f/2.  In the 1969 Summicron the refractive index had grown to ne1.7479 with LaFN2 glass and the special 900403 glass developed for the 1976 Noctilux 50mm f/1 increased the refractive index to ne1.9005 which, to my knowledge, is the highest refractive index for any glass used for photographic production lenses.  Leitz uses a proprietary number to avoid divulging the make-up of this glass.

It was a long, arduous journey to arrive at the modern glasses at the disposal of today’s lens designers.  Glasses with a relatively high refractive index were available even prior to W.W.II.  However, this could only be achieved by the addition of thorium oxide to the glass.  Thorium oxide unfortunately is highly radioactive, and lens designers began to search for a suitable replacement.  One of the problems encountered was that by replacing the thorium oxide with a variety of other elements, the glass turned opaque.  Kodak had some limited success during W.W.II.  They were able to make a thorium oxide free glass with similar properties, but it was yellow in color.  This glass, however, was used to make lenses for some aerial cameras.  Since these were used exclusively with b&w films, the “built-in” yellow filtering actually was found to be beneficial for this specific use.

At the Leitz glass research laboratory similar experiments started in 1949.  After lengthy experimentation they found that lanthanum oxide offered the best chance for success as a replacement for thorium oxide. They found that glasses could be made with a refractive index of ne1.7.  However, these glasses could not be made in large quantities because they tended to crystallize.  This makes it difficult to prevent crystal grooves, or striae.  Other substances like zirconium oxide, yttrium oxide and tantalum oxide had to be added to produce more stable high refractive glasses in large numbers.  The resulting LaK9 glass resulted in the 50mm f/2 Summicron in 1953, the first production lens free of thorium oxide.  Leitz granted the German Schott company a license to produce the LaK9 glass.

For lenses with a maximum aperture of f/1.4 glasses with a refractive index close to ne1.8 are necessary.  However, these are difficult to produce.  Evaporation at the surface of the molten glass, while the temperature is slowly lowered, can easily cause striae.  These can only be prevented by constant stirring to produce consistent blending of the molten glass.  This requires additional steps, so that one oxide will prevent another from crystallizing.  These methods, for instance, have enabled Leica to use glasses with refractive indices of ne1.80 and ne1.82 and a dispersion value of ny45.  For glasses with refractive indices higher than ne1.8 it is difficult to obtain enough chemicals with sufficient purity at a reasonable cost.  Subsequently such glasses are very expensive.

The glass developed by Leitz for the 50mm f/1 Noctilux is such a glass.  It uses a much higher zirconium oxide content, which allows for a refractive index of ne1.9005 and a dispersion value of ny40.  These glasses have a very high melting point of 1600° C (2912° F).  At such high temperatures molecular migration of the material used for the crucibles can contaminate the glass mixture.  To prevent this, crucibles of platinum or at least platinum lined crucibles are used.  The stirring tools also have to be made either of platinum or must be platinum coated.  In addition, the shape of the stirring tools in relation to the viscosity of the molten glass is very important for the homogenization of the glass.  The temperature of the melt in the crucible has to be lowered considerably before it can be poured out.  Then the poured slabs have to be cooled at a carefully controlled rate, which may last days or even months in order to prevent molecular tension.

As mentioned before, most lens manufacturers don’t consider calcium fluorite suitable for lens production.  Instead they prefer a true glass with an amorphous structure.  It was discovered that fluorites remained stable in metaphosphate suspensions.  By incorporating several fluorites, it is possible to optimize the proportions to avoid the creation of striae during the cooling process.  This eventually led to the development of a true glass with anomalous partial dispersion and a refractive index of greater than ne1.544.  With the addition of titanium oxide Leitz was able to produce a glass with an anomalous partial dispersion value of ny66.6 and a refractive index of ne1.544.  This glass was used in the 180mm f/3.4 Apo Telyt R, originally developed for the U.S. Navy, which to this day ranks as one of the best lenses ever produced for a 35mm camera. Many of the large glass manufacturers have little interest in the manufacture of such exotic glasses because optical glass is only of minor importance to them.  Construction glasses, television tubes, bottles etc. are the main income generators.  When these companies develop optical glasses, they will do it only for those which will be in high demand.  It is the exotic wishes of companies like Leica, Zeiss, Schneider, Rodenstock, Nikon, Canon etc. that these glasses were developed in recent years.

The next time we complain about the high prices of some of the top lenses on the market, we should remember the difficulties encountered in just the manufacture of the glass.  Add to that the great care which has to go into the production of the individual glass elements and the mechanical components of the lens, and the prices somehow don’t seem to be all that high after all.