In Memory of the LEITZ GLASS LABORATORY

A SOURCE FOR OPTICAL EXCELLENCE

The Leitz Glass Laboratory, which operated until 1989, was the driving force of Dr. Gustav Weissenberg, of the Mineralogical Institute of Marburg, Germany.  The Institute grew crystals, and Weissenberg’s idea was for it to create optical glasses as well as to meet the demand by optical designers for high refraction glasses to correct aberrations.  He wanted to replace the highly radioactive thorium oxide in existing glasses with an oxide free of thorium.  So, in 1949, the Leitz brothers established a glass laboratory within their company to pursue Weissenberg’s research. 

After lengthy experimentation the glass laboratory discovered that lanthanum oxide offered the best chances for success.  They found that glasses could be created with a refractive index of n 1.7.  However, such glasses could not be made in large quantities because their strong propensity for crystallization made it difficult to prevent crystal grooves, or striae.  Other substances, such as zirconium oxide yttrium oxide, and tantalum oxide, had to be incorporated to produce more stable high-refraction glasses in large numbers.  Leitz then granted the German company Schott a license to produce the new LaK 9 glass exclusively for Leitz.

Other Leitz successes

Meanwhile, the Leitz optical designers wanted to design lenses that were even faster than f/2, and the dreamed of doubling the speed to f/1.4.  This required glasses with refractive indices close to n 1.8, and for this they had to find the right technology.  Evaporation at the surface of the molten glass while the high temperature in the crucible is being lowered can easily cause striae, which can only be prevented by stirring to produce constant blending of the molten mass.  In order to accomplish this, technicians had to incorporate interference factors, so that one oxide would prevent another from crystallizing.  Laborious experiments led to three new types of Leitz optical glass, with refractive indices of ne 1.80 and ne 1.82 and a dispersion value of ny 45.  These new types of glass made the design of the 50mm f/1.4 Summilux lens possible

Platinum crucible

Stirring rod

Pouring of molten glass

But for glass types with indices of refraction higher than ne 1.8, it was difficult to obtain enough chemicals with sufficient purity at a reasonable cost.  At the time, one kilogram (2.2 lbs.) of 99.99 percent tantalum oxide cost the equivalent of nearly $200.00.

Still a new type of optical glass

In 1966, Leitz introduced the 50mm f/1.2 Noctilux, which incorporated aspherical surfaces.  Then years later, it was producing the 50mm f/1.0 Noctilux , which did not involve aspherical surfaces.  This was made possible by still another new type of optical glass: one with a higher zirconium oxide content.

The company was seeking a lens with a maximum aperture of f/1.2, for which optical glass with a particularly high index of refraction was necessary.  Ultimately, it wanted to double the maximum aperture of the 50mm f/1.4 Summilux lens to f/1.0.  The higher zirconium oxide content permitted production of Noctilux 900403 glass with a refractive index of ne 1.9005 and a dispersion value of ny 40.

These developments were not without difficulties, however.  The new lenses utilized glass with a very high melting point of nearly 1600 C (2912 F).  That came close to the melting point of the platinum crucibles.  Also, the shape of the stirring tools in relation to the viscosity of the molten glass was important for the homogenization of the glass.  So, the temperature of the melt in the crucible had to be lowered considerably before it was poured out.  Workers then had to cool the slabs at a carefully controlled rate over 10 to 12 days and nights in order to prevent molecular tension.  Furthermore, the glass was still quite susceptible to crystallization.

Creating lenses with apochromatic correction

Another area of investigation at Leitz was correction of chromatic aberrations in apochromatic lenses with long focal lengths.  Fluorite, grown in single crystals, possesses such anomalous partial dispersion for correction of the fourth color in optical glass.  Leitz competitors liked to use these calcium fluorite crystals for lens elements in telephoto lenses, but Leitz did not regard them as suitable for photographic lenses because they have very poor shape retention.  Also, because of their rigid index of refraction of only ne 1.43, the fluorite crystals require strong curvatures, which are disadvantageous.  Leitz preferred to search for a true glass with an amorphous structure.

The company discovered that the fluorites remained stable in metaphosphate suspensions.  By incorporating several fluorites, it was able to optimize the proportions so as to avoid creation of striae during the cooling process.  Eventually it succeeded in developing a true glass with anomalous partial dispersion and a refraction index greater than ne 1.544.  With that, Leitz became the first lens maker to manufacture such lenses.  The 800mm f/6.3 Telyt-S was such a lens [see blog article “Dancing Bear and his Magic Lens”].  It attracted a great deal of attention at the 1972 Olympic Games in Munich.

A few years later, with the addition of titanium oxide, Leitz was able to produce glass with an anomalous partial dispersion value of ny 66.6 and a refraction index of 1.544.  This glass was used in the 180mm f/3.4 APO-Telyt, which ranks as the top lens to this day.  Since then, the company has made a number of lenses with apochromatic correction, and it is creating new ones regularly.

Overall performance of the glass laboratory

Overall, the Leitz Glass Laboratory developed 35 new glasses from 50,000 experimental melts.  For a time, these glasses were used exclusively in Leica lenses.  Without the laboratory, in fact, most of the modern lenses of the period 1949 to 1989 would not have been possible.  Because of its work, the company produced up to 10 metric tons of glass a year, partially because of close communication between glass researchers and lens designers.  Other large glass manufacturers were often behind Leitz/Leica because optical glass is of only minor importance to them.  Phototropic glasses, construction glasses, television tubes, baby bottles and the like are their real moneymakers.  When these companies do undertake to develop optical glasses, it is only for those for which they anticipate a large demand.  It was the exotic wishes of the Leitz Company that permitted it to develop the glasses it did during the 40 years it held sway in this area.

Once it was possible with new technology to derive maximum performance from optical lenses, however, Leica closed the Leitz Glass Laboratory.  The growing number of glass manufacturers and new methods of processing glass an manufacturing lens elements also made this desirable from a financial standpoint.  Still, the company looks back with pride to the breakthroughs of the researchers and designers at the laboratory.

From LEICAVIEW Fall 2001

For more information, see Leica Barnack Berek Blog articles

OPTICAL GLASSES  2-27-2012

MANUFACTURE AND PERFORMANCE OF PHOTOGRAPHIC LENSES  2-29-2012   

DANCING BEAR AND HIS MAGIC LENS  3-19-2012