August Köhler, a German scientist and expert microscopist born in 1866, is best known for his development of the superior microscope illumination technique bearing his name (Köhler illumination) and which is still in general use today. In 1893, when he developed the technique, Köhler was working as an assistant to Professor J. W. Spengel at the Institute of Zoology at the University of Geissen, where he was also pursuing advanced studies. The success of Köhler's academic and occupational pursuits was heavily dependent upon his ability to produce high quality photographs in the microscope. The task, however, was not easy due to the relatively primitive methods of illumination and limited light response of photographic emulsions available at the time. A high degree of evenly-distributed brightness is necessary to produce good photomicrographs, but the non-uniform illumination field exhibited by gas-lamps and other primitive light sources of the period tended to induce flaws and uneven backgrounds in photomicrographs.

In order to overcome the problems he was experiencing with photomicrography, Köhler developed a unique microscope configuration that employs both a field and an aperture iris diaphragm (also known as double diaphragm illumination) to produce an evenly illuminated field of view and a brighter image without obscuring glare. In the Köhler illumination technique, a collector lens focuses an image of the lamp filament in the condenser aperture (front focal plane), which in turn projects an image of the field diaphragm into the specimen plane. The scheme relies on two independent sets of conjugate planes, one for the apertures and another for the images, which traverse the microscope optical train from the light source to the observer.

Köhler published his work in a German microscopical journal at the end of 1893 and a short English summary of his work appeared in the Journal of the Royal Microscopical Society the following year. Yet, other researchers in the field did not immediately realize the significance and importance of the Köhler's illumination methodology. In fact, after receiving his doctorate, Köhler's name seemed as if it might vanish into obscurity as he left the University of Geissen to work as a grammar school teacher in Bingen, Germany.

Several years later, however, Köhler came to the attention of Zeiss Optical Works and his life soon changed. The company, through the combined efforts of its co-founder, Ernst Abbe, and glass-specialist Otto Schott, had already vastly improved microscopes of the period through a solid foundation built on precise optical theory and the utilization of appropriate glass formulations. Nevertheless, the company needed to improve illumination techniques before optimum resolution at high magnifications could be realized. Thus, Köhler was invited to join Zeiss, where he began working in 1900. He initially contributed his illumination technique, but later led the company's efforts in other areas of microscope development. Köhler remained with Zeiss for the next 45 years producing several additional innovations during his career.

Although Köhler is most famous for his illumination method, he made several other significant contributions to the world of science. In 1904, Köhler and a Zeiss colleague, Moritz von Rohr, worked together to design a microscope that operated in the ultraviolet region of the electromagnetic spectrum. The microscope was illuminated by ultraviolet radiation generated through a cadmium arc and lenses fabricated with fused quartz. Although this first ultraviolet microscope had significant limitations, it opened up an entire new realm of possibilities for ultraviolet and fluorescence microscopy. Five years later, Köhler was the first to discover grid radiation, which has proved extremely valuable in the medical field, especially in the treatment of tumors. Then, in 1911, Köhler suggested that Zeiss should make all of the matched objectives on a microscope nosepiece parfocal (with matching focal planes), enabling an image to remain in focus when an observer changed magnification. Zeiss trusted the opinion of their microscopy leader and soon implemented the idea with great success.

Feature Articles on Köhler Illumination

Köhler Microscope Illumination - Illumination of the specimen is the most important variable in achieving high-quality images in microscopy and critical photomicrography. The Köhler illumination technique was first introduced in 1893 by August Köhler of the Carl Zeiss corporation as a method of providing the optimum specimen illumination. This technique is recommended by all manufacturers of modern laboratory microscopes because it can produce specimen illumination that is uniformly bright and free from glare, thus allowing the user to realize the microscope's full potential.

Illumination with Transmitted Light - Adjustment of a transmitted light microscope for Köhler illumination is a logical and relatively easy process that should be practiced until mastered by all serious students of microscopy. Each time a microscope is turned on, it should be carefully inspected to ensure proper alignment of all optical components, and to guarantee that the lamp is centered and the condenser and field diaphragm are properly adjusted for Köhler illumination.

Illumination with Reflected Light - In brightfield reflected light microscopy (including contrast enhancing techniques such as differential interference contrast and darkfield), proper use of the two variable diaphragms enable the use of the highly desirable Köhler illumination. It is important to note, that in reflected light systems, the objective serves a dual function: on the way to the specimen as a matching well-corrected condenser properly aligned; on the way back to the microscope as an image-forming objective in the customary role of an objective projecting the image-carrying rays toward the eyepiece.

August Köhler's Original Manuscript - Translated from German, the original article appearing in Zeitschrift für wissenschaftl. Mikroskopie, was reprinted in the Köhler Illumination Centenary commemorative issue by the Royal Microscopical Society in 1994. Included in the article is the original woodcut drawing that depicts the microscope optical train, as well as a re-labeled color version for added clarity. Dr. Köhler entitled the article "A New System of Illumination for Photomicrographic Purposes" because of the importance that his unique illumination technique afforded to recording images on film emulsions in the early twentieth century. Köhler illumination is still the method of choice for microscope illumination in traditional film photomicrography and newer digital imaging techniques.

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