Reduction of glass-to-air surface reflection constituted one of the few fundamental advances made in optics technology this century.
Until a solution to this problem was derived. the optical design of the photographic lenses was restricted invariably to not more than three free-standing components or, in other words, six glass-to-air surfaces. With a larger number of components, multiple reflections at optical surfaces caused so much stray light to reach the image plane that contrast was impaired. Moreover, as a result of double reflection, false-light images were liable to be produced in the vicinity of the film plane: these were then reproduced in the picture as more or less sharply defined light spots usually taking the form of the diaphragm aperture pattern. The number of such double reflections with 6, 8, 10, 12, 14 and more glass-to-air surfaces becomes 15, 28, 45, 66 and 91 times greater. The greater the number of these double reflections, the more likely it is that, apart from the more or less diffused contrast-reducing false light, these discrete ghost image would be superimposed on the image of the subject. In 1817, Fraunhofer notices that reflection at glass surfaces decreases with exposure to atmospheric influences. Similar observations were made later by Lord Rayleigh and Denis H. Taylor.
In 1904, Taylor patented a process in England for anti-reflection finishing by acid treatment. This did not, however, produce constant results and the method gradually faded into obscurity.
The major breakthrough in the field was made at Carl Zeiss in 1935 when Smakula discovered a method suitable for industrial application, wherein thin layers of low-refracting fluorides were vacuum-deposited in specific quantities on to glass surfaces. It was Smakula, too, who was instrumental in fostering the process from laboratory stage to shop floor application and subsequently realizing its widespread application in the optical industry.
Before the end of the Second World War, or just about a decade after the first successful experiments, there were already over a hundred vacuum chambers of Smakula's design in operation. Zeiss was granted a German patent on the process effective November 1, 1935.
The discovery of anti-reflection coating gave rise to intensive research activity in the field of thin optical coatings. The salient aims of this work were to improve the hardness of the coatings so as to make them resistant for outer surface application as well, to find methods of production other than high-vacuum evaporation, and finally to further reduce the residual reflection of the bloomed surfaces.
In Germany, research was conducted partly by Zeiss and Schott of Carl Zeiss Foundation, by the laboratories of those optical manufacturers interested in the application of the invention, and the physics laboratory of Heraeus in Onstmettingen.
In 1939, a significant step was taken toward reduction of residual reflection by Schott who succeeded in producing the first double coating by applying the so-called gas decomposition method. This was followed by the first triple coating in 1943. The theory of triple-layer coatings was thus developed and, after the war was over, reports appeared on the characteristics of multi-layer coatings.
It is today impossible to say whether it was the prospects for industrial production of thin layers that was responsible for the bounding advances made in vacuum technology or inversely whether the advances in vacuum technology promoted the production of thin layers. Nor is it easy to trace the developments, especially of multi-layer coatings, as it always involves long and tedious work from the development of laboratory prototypes until they can be placed on the production line. In short, the art of multi-layer production lies rather in finding the right single layers of correct refractive indexes and fulfilling various other conditions than in the discovery of a specific layer combination.
In the course of the reconstruction of the Zeiss Works in Oberkochen in 1945, the vacuum units were quickly modernized and research was devoted to multi-layer coatings. Initially, double layers went into production. On lenses of low refractive index for narrow spectral range, this was much more effective than a single layer coating. The main field of application in the photographic sector was filters.
Then, about ten years ago, coating with three or more layers was introduced for objectives of microscopes intended for reflected light work. False light interference is much greater in reflected-light microscopy than in photographic applications.
At the beginning of the sixties, Optical Coating Laboratory Inc. of America which had experience in the field of multi-layer coating in association with Balzers AG of Europe, applied these techniques with success in regular coating of glass parts and optics in the aerospace industry.
Carl Zeiss began to attach increasing significance to the switchover from single-layer to multi-layer coating in the photographic sector as the number of glass-to-air surfaces in their lenses grew ever greater. For example, it was not uncommon that ultra-wide angle and zoom lenses had 16 to 18 such surfaces.
The final decision by Carl Zeiss in 1972 to switch over to multi-layer coating had the same significance as the original introduction of the anti-reflection concept had for the optical industry as a whole. It was therefore befitting that Zeiss lenses with the multi-layer coatings, like those predecessor models marked with the red T, should now be marked with the T plus a star (T*).
The great improvement in suppression of reflection brought about by the T* coating has the following advantages for the photographer:
Source: The Finest Optics by ZEISS. Interchangeable Photographic Lenses with Contax/Yashica Mount (1975).
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