Anti-reflection coating

During the twentieth century there has been enormous progress in optical techniques. One of these innovations was particularly significant because it was the basis for everything that was to follow.

by Peter Hennig

Anti-reflection coating During the twentieth century there has been enormous progress in optical techniques. One of these innovations was particularly significant because it was the basis for everything that was to follow.

by Peter Hennig In earlier times there was a general rule for inexperienced photographers: always have your back to the sun. There was a good reason for this as every piece of glass in the lens produced as much flare as normal window glass. Even if there were fewer elements in the lens the total flare and loss of contrast would be considerable. There were not only strong reflections between the different elements, but also between the two surfaces of any single element. For someone who was less experienced, taking photos in direct light was out of the question, and even side lighting was difficult to master.
Since a reflection repeatedly bounces from one lens surface to the next, it is usual to talk of two reflections, the primary reflection and the secondary reflection. As the number of elements increases (for example 1, 2, 3, 4, 5, and 6) you get an increase in element reflection (1, 6, 15, 28, 45, and 66 reflections respectively). It is easy to understand why it was impossible to have more than a few elements in the lenses of the day. A lens with three elements would have fifteen internal reflections, and if you added another independent element the reflections would almost double, to end up with a total of twenty-eight. Three elements were considered the maximum for the reasonable reproduction of contrast. With such a lens you could still risk side-lit photos and in some circumstances even photos in direct light.
The problem was that when it comes to making corrections, three elements did not get you very far - especially not in the 1920s, when the need for faster lenses became acute. The first solution that provided a fairly good result was to cement specific elements together to form a lens group, thereby reducing the number of individual elements. The famous double anastigmats such as the Goerz Dagor or Zeiss Double Protar that gave great improvements in picture quality at the turn of the century, could have up to ten elements but a mere two groups. However, their speed was still only modest. In 1932 a young physicist, Dr. Ludwig Bertele of the Carl Zeiss company, succeeded in making a lens with a maximum aperture of f1.5. The lens was made up of seven elements cemented together in such a way that there were only three groups. The picture quality was amazing, albeit with heavy distortion and field curvature.
This hints at the method's weakness: when two elements are cemented together, both surfaces need to have the same spherical shape, and that means that many opportunities for correction are lost.
Groups can have different shapes and therefore offer totally different ways of correcting optical errors. A telling example of this problem was the Ernst Leitz company's answer to the Zeiss Sonnar 1.5/50. In 1936 they produced what was for the period an extremely well-corrected lens called the Xenon 1.5/50. Of its seven elements, five were independent, giving forty-five internal reflections - three times the number of the Sonnar 1.5/50 - and it was therefore virtually unusable outdoors in sunlight. It was designed purely as a specialised lens for low light, and the film had to be developed in a way that increased the contrast.

Cancelling waves
Another, better way that turned out to be a determining factor in future developments was the attempt to do something about the reflections themselves. One of the characteristics of light is that it moves in waves. In this case it behaves just like other waves. You can strengthen or weaken the waves depending on how they coincide. You can conduct a simple experiment at home: sitting in your bath tub, hold the palms of your hands face down a couple of inches apart, and pat the water surface gently to make waves. The waves will move inwards and meet each other between your hands. If you pat the water with both hands in time with each other (called 'in phase'), there will be a zone where the waves meet each other crest to crest and reinforce each other. If your hands are out of time ('out of phase'), the result will be a zone between your hands with smaller waves; the waves have been diminished. By making the waves reflect off suitable objects, it is possible to angle the returning waves in such a way that they meet the original wave out of time. By doing this you can diminish the size of any reflected wave going in a particular direction. Although waves in water and waves in light are not fully comparable phenomena, this example will still give you an idea of how they work.

The weakening of light through interference
If you want to weaken a flow of light by using interference, your aim will be to make the waves of light meet themselves out of phase. Anti-reflection coating basically works like this: the surface of the lens is covered with a coating of an exact thickness - one quarter of the wavelength that needs to be eliminated - and with a specific index of refraction. Most of the image-creating light passes through the lens, but some is reflected back out, partly off the coating surface and partly off the glass under the coating. In other words, the reflected light consists of two parallel waves with a well-determined phase difference. When these parallel waves are exactly out of phase the light is extinguished as a result of interference and there is no reflection. In practice this will only be true for one wavelength, and since we always work with many wavelengths simultaneously, there is no total extinction of light, only different levels of weakening for all the different wavelengths.

Early experiments
Sometimes we meet with a phenomenon that could be called 'nature's own anti-reflection coating'. Old lenses that have been exposed to damp conditions for a long period sometimes have an oxidised layer on the surface that has a refractive index that is different from the underlying glass. In favourable circumstances this layer will work as an incomplete anti-reflection coating. To the eye the lens looks as if it is covered in spots of varying darkness. The British lens designer and photography encyclopaedist Dennis Taylor (the man responsible for the original triplet lens) observed this phenomenon as early as the late nineteenth century, and tried to create the same oxidation layer under controlled circumstances, but because he did not understand how the phenomenon occurred, he never managed to achieve repeatable results. It is odd that although Taylor could never prove that his method was practical, nor could explain why the phenomenon occurred, he still managed to register a patent. So there is an English patent from the end of the nineteenth century on a treatment to reduce surface reflection from glass.

Science takes over
At the beginning of the twentieth century theories of light extinction by interference in thin layers started to take shape, and in the 1920s the American scientist Dr. Catharine Blodget performed a series of elegant experiments that proved the theories correct. Blodget found a substance with a suitable refractive index that had a curious quality: when it was poured onto water it spread across the surface in such a way as to form a layer exactly one molecule thick. By dipping a plate of glass in and out of the water, Blodget could coat it with successive layers of the substance. Since the thickness of the layer of molecules was known, it was possible to create layers of specific thicknesses. Thus Blodget found a practical way of producing the total extinction of light of any given wavelength by interference. Her elegant and illustrative experiments were highly convincing, and represented a major scientific breakthrough. Unfortunately the layers were not durable, and they were destroyed when they dried. The method therefore had no practical application.

From theory to practice
The interest in producing an industrially applicable method of anti-reflection coating grew rapidly, especially because of the need for fast, well-corrected lenses for the new 35 mm cameras of the 1930s. Two famous German companies, Ernst Leitz in Wetzlar and Carl Zeiss in Jena, were prominent in the development of a method for the industrial production of interference layers. The difficulty was principally to produce layers of the correct thickness when using a durable substance usable under the conditions of mass production.
At Leitz, under the direction of Prof. Max Berek, they experimented with a method where they applied a liquid that would coagulate quickly on the centre of a rotating lens. The centrifugal force made the liquid spread itself evenly over the lens. The idea was that the lens would stop rotating and the liquid would coagulate when the layer had acquired the correct dimensions. The crucial moment would be observed visually in light of a certain wavelength. However, eventually they realised that this method was too labour intensive and unreliable to be used in a mass production. It is not clear if any coated lenses of this type ever left the factory. It is possible that the Hektor 6.3/28 delivered to the German Navy at the end of the 1930s had this coating.
At the beginning of the 1930s, Carl Zeiss developed a process that led to an industrial breakthrough, and is still in use. Under the supervision of Prof. Alexander Smakula they succeeded in applying a very precise layer by evaporating magnesium fluoride in a vacuum chamber. The patent was registered on 5 October 1935 (DRP N°685767). The method was instantly classified as a secret military patent, and the Zeiss T-lens (transparency lens) was only available for military and scientific use within Germany. In 1940 the restrictions were lifted, and in 1941 the first coated Zeiss lens came onto the civilian market. Because of the war, these exclusive items could only be found in Sweden and Switzerland.

The opportune innovation
The anti-reflection treatment became something of an 'Open Sesame!' for lens makers. Suddenly many older lens designs that incorporated too many elements could be given a new lease of life. A good example is Dr. Paul Rudolp's brilliant calculations for the Zeiss Planar dating back to 1896, that were re-calculated to give a faster version at the end of the 1920s by Dr. Willy Merté (the Zeiss Biotar 1.4/50 DRP 1930). This was an idea that would provide the basis for the fast, well-corrected lenses we still use today. The gradual increase in efficiency produced by coating has made it possible to construct a lens with a very large number of elements without difficulty, giving us a lens with a performance that could only be dreamt of before. Anti-reflection coating is a key technique. Without it, modern lenses would not work.

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