Polarizing microscope, maker: Voigt & Hochgesang/Göttingen, and Seibert/Wetzlar, approx. 1890, 30.0 × 10.0 × 16.0 cm, brass, etc. KIT Archives 27059/160.
The polarizing microscope from the physicist Otto Lehmann’s (1855–1922) estate was once the centerpiece of an extensive research environment at Karlsruhe Polytechnic. It played a key role in fundamental research on liquid crystals. In 1888, a letter from his colleague Friedrich Reinitzer informed Lehmann about the first observation of the liquid crystals phenomenon. This fascinated him so much that he devoted the rest of his scientific career to pioneering researches in the field. In Lehmann’s time, the term “liquid crystal” was almost provocative, as it juxtaposed the contradictory concepts of a crystal as a solid and a liquid as a flowing substance. Lehmann risked his scientific reputation in attempting to convince initially skeptical experts that certain substances have a liquid-crystalline phase between the solid and liquid aggregate states, in which they flow while also exhibiting regular, crystalline structures. But the future potential of controlling the optical properties of liquid crystals with electrical voltage and taking this as the basis for creating viewing screens was not yet foreseeable. Lehmann’s primary interest lay in identifying substances with liquid crystal properties, observing their behavior, and presenting his findings to both the scientific community and the public at large. He used microscopes with rotatable specimen plates to examine the optical responses of preparations in polarized light. He also used refined heating devices to heat up and cool down specimens under controlled conditions. Lehmann even built some of this specialized equipment required for his microscopic work himself and provided for photographic documentation of his preparations. In this way, Lehmann combined the now often disjunct roles of scientific theorist, laboratory technician, and scientific communicator. kn
This phrase succinctly captures the complex and challenging journey of liquid crystals, from the early careful observations by the botanist Friedrich Reinitzer, to current advancements in screen technology. In the midst of investigating the two melting points of cholesteryl benzoate — a cholesterol derivative he discovered in carrots — and its temperature-dependent coloration in polarized light, Reinitzer reached out to Otto Lehmann, at that time still a private lecturer (Privatdozent) in Aachen, in March 1888. He requested Lehmann to “kindly examine” more closely the enclosed samples using his crystallization microscopes. Lehmann had been acquainted with microscopy since his school days through his father and had received advanced training in crystallography at Strasbourg University under the mineralogist Paul Heinrich von Groth. In his dissertation of 1872 he had already described how to adapt the microscope to observe crystallization phenomena by adding a gas burner for heating and an airflow system for cooling the specimens. By 1888, Lehmann’s numerous publications proved him to be an established expert in microscopic crystallography, with extensive experience and the necessary instruments. After moving from Aachen to Dresden and then, upon his appointment to Karlsruhe as Heinrich Hertz’s successor, Lehmann published the results of his investigation of Reinitzer’s samples in August 1889. He confirmed Reinitzer’s observations and was convinced that cholesteryl benzoate exhibited both liquid and crystalline properties. Although he could not fully explain this state, he described it as “flowing crystals.” Lehmann demonstrated that the observed phenomena — multiple melting points and optical anisotropies in the liquid state — were not due to impurities but represented special states of matter (referred to as mesophases by Georges Friedel). He defended this finding vigorously and eloquently for many years against skeptics and mockers. Starting in 1908, Lehmann sought to disseminate his research results on liquid crystals outside Germany. In 1909, he accepted an invitation to the Sorbonne in Paris to offer an extensive seminar including demonstration experiments. This visit was a great success, laying the foundation for the French school of liquid crystal science, with pioneers such as Charles Mauguin, Georges Friedel, and François Grandjean. In 1904, the Merck company started offering substances with liquid crystal phases in its product line to support scientific research. Interest in liquid crystals waned in the 1930s due to a lack of practical applications, until in the 1960s, Richard Williams and George Heilmeier in the United States developed the first electrically controlled displays. In 1962, Williams discovered an electro-optical scattering effect in liquid crystals at the Radio Corporation of America’s Princeton laboratories, raising hopes by his colleague Heilmeier and, from 1967 on in the firm, for commercial use as large-scale display elements and even as a potential replacement for cathode ray tubes in televisions. With similar enthusiasm, a German chemical manufacturer announced in 1969 that research on “flat television screens would soon be hanging ready on a nail,” following the development of a liquid crystal material operable at room temperature. As we now know, these predictions were overly optimistic. The laws of nature presented engineers with numerous challenges that would take at least another thirty years to overcome. Many ideas were turned into prototypes, patented, and tested, but only a few led to practical solutions. The next significant step toward the low-power, high-contrast liquid crystal display (LCD) came from the research department of Hoffmann-La Roche in Basel, Switzerland. Inspired by a publication by Charles Mauguin in 1911, Wolfgang Helfrich developed and patented together with Martin Schadt the twisted nematic (TN) cell on December 4, 1970. Schadt then systematically developed new liquid crystalline substances with the required properties and purity, improving the performance of the TN cell. The low power consumption and good contrast of TN cells quickly attracted interest from Japanese manufacturers of pocket calculators and wrist watches. Hoffmann-La Roche granted the first licenses for the TN cell as early as 1974. In 1985, Terry Scheffer and Jürgen Nehring at Brown-Boveri in Switzerland introduced the supertwisted birefringence effect, enabling higher multiplex rates and screens with a 640 × 480-pixel matrix. This development paved the way for portable computers. The evolving state of digital electronics at the time was finally matched by appropriate visual display units. In the early 1990s, the first liquid crystal displays (LCDs) with color display and control by thin-film transistors made of amorphous silicon became available. By the late 1990s, LCDs had begun to be used in offices and other workplaces. In 2008, global sales of LCD televisions equalled those of cathode ray tube televisions for the first time. Since then, large-format flat TV screens have found their place on walls worldwide. By 2022, global sales of all electro-optical displays totaled around 160 billion US dollars, with liquid crystal displays accounting for more than 90 %. The use of liquid crystals in windows with controllable light transmission, in electrically adjustable optical lenses, in optical data processing elements, and in controllable components for the microwave range, suggests that further advancements in this field can be expected in future. Michael E. Becker, Henning Wöhler
The crystallization microscope with temperature equalization and polarization apparatus, which Otto Lehmann spent years consistently improving, was the only efficient instrument available that was ideally suited to examine the phenomena that Reinitzer had observed in cholesteryl benzoate. Michael E. Becker