August Karl Johann Valentin Köhler (4 March 1866 – 12 March 1948) was a German professor and early staff member of
Carl Zeiss AG
Carl Zeiss AG (), branded as ZEISS, is a German manufacturer of optical systems and optoelectronics, founded in Jena, Germany in 1846 by optician Carl Zeiss. Together with Ernst Abbe (joined 1866) and Otto Schott (joined 1884) he laid the f ...
in
Jena
Jena () is a German city and the second largest city in Thuringia. Together with the nearby cities of Erfurt and Weimar, it forms the central metropolitan area of Thuringia with approximately 500,000 inhabitants, while the city itself has a popu ...
, Germany. He is best known for his development of the
microscopy
Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...
technique of
Köhler illumination Köhler illumination is a method of specimen illumination used for transmitted and reflected light (trans- and epi-illuminated) optical microscopy. Köhler illumination acts to generate an even illumination of the sample and ensures that an image o ...
, an important principle in optimizing microscopic resolution power by evenly illuminating the field of view. This invention revolutionized light microscope design and is widely used in traditional as well as modern digital imaging techniques today.
Early life and education
Köhler was born in 1866 in
Darmstadt
Darmstadt () is a city in the States of Germany, state of Hesse in Germany, located in the southern part of the Frankfurt Rhine Main Area, Rhine-Main-Area (Frankfurt Metropolitan Region). Darmstadt has around 160,000 inhabitants, making it th ...
, Germany, where he attended the Ludwig-Georgs-Gymnasium until 1884. He studied at the Technical University in Darmstadt and at the universities of
Heidelberg
Heidelberg (; Palatine German language, Palatine German: ''Heidlberg'') is a city in the States of Germany, German state of Baden-Württemberg, situated on the river Neckar in south-west Germany. As of the 2016 census, its population was 159,914 ...
and
Giessen
Giessen, spelled Gießen in German (), is a town in the German state (''Bundesland'') of Hesse, capital of both the district of Giessen and the administrative region of Giessen. The population is approximately 90,000, with roughly 37,000 univers ...
covering a wide range of fields from
zoology
Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anatomy, structure, embryology, evolution, Biological clas ...
and
botany
Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek w ...
to
mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the proces ...
,
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
, and
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
.
[Zeiss Archives](_blank)
/ref>
Teaching and academic career
In 1888, August Köhler graduated with a teaching degree and subsequently taught at gymnasiums in Darmstadt and Bingen before going back to university. He started his academic career as a student, instructor and assistant to professor J.W. Spengel at the Zoological Institute at the university of Giessen
Giessen, spelled Gießen in German (), is a town in the German state (''Bundesland'') of Hesse, capital of both the district of Giessen and the administrative region of Giessen. The population is approximately 90,000, with roughly 37,000 univers ...
, Germany. The object of his doctorate thesis was the taxonomy of limpet
Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended indep ...
s, a project that depended heavily on microscopic imaging and prompted Köhler into trying to improve the quality of images taken via photomicrography
A micrograph or photomicrograph is a photograph or digital image taken through a microscope or similar device to show a magnified image of an object. This is opposed to a macrograph or photomacrograph, an image which is also taken on a mi ...
. The result of this work was published in 1893.
After receiving his doctorate degree
A doctorate (from Latin ''docere'', "to teach"), doctor's degree (from Latin ''doctor'', "teacher"), or doctoral degree is an academic degree awarded by universities and some other educational institutions, derived from the ancient formalism ''l ...
from the University of Giessen
University of Giessen, official name Justus Liebig University Giessen (german: Justus-Liebig-Universität Gießen), is a large public research university in Giessen, Hesse, Germany. It is named after its most famous faculty member, Justus von L ...
in 1893, Köhler worked a number of years as a grammar school teacher in Bingen. In 1900, he was invited to join the Zeiss Optical Works company in Jena
Jena () is a German city and the second largest city in Thuringia. Together with the nearby cities of Erfurt and Weimar, it forms the central metropolitan area of Thuringia with approximately 500,000 inhabitants, while the city itself has a popu ...
, Germany, by Siegfried Czapski
Siegfried Czapski (28 May 1861 – 29 June 1907) was a German physicist and optician.
Childhood, school and university in Breslau (1870–1881)
Czapski was the son of Simon Czapski (1826–1908) and his wife Rosalie Goldenring (1830-1916) on th ...
based on his earlier work on improving microscope illumination. He stayed with Zeiss as a physicist
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.
Physicists generally are interested in the root or ultimate caus ...
for 45 years and became instrumental to the development of modern light microscope design. From 1922 until his retirement in June 1945, he was also professor
Professor (commonly abbreviated as Prof.) is an Academy, academic rank at university, universities and other post-secondary education and research institutions in most countries. Literally, ''professor'' derives from Latin as a "person who pr ...
for microphotometry at the University of Jena
The University of Jena, officially the Friedrich Schiller University Jena (german: Friedrich-Schiller-Universität Jena, abbreviated FSU, shortened form ''Uni Jena''), is a public research university located in Jena, Thuringia, Germany.
The un ...
. He became honorary professor of the Medical Faculty at the University of Jena in 1922 and received an honorary medical doctor degree in 1934. In 1938, he assumed the head position for the Department of Microscopy, Microphotography and Projection.
Köhler illumination
At the time of the invention of his revolutionary illumination scheme as a graduate student at the University of Giessen, Köhler was working on overcoming problems with microphotography. Microscopes were illuminated by gas lamp
Gas lighting is the production of artificial light from combustion of a gaseous fuel, such as hydrogen, methane, carbon monoxide, propane, butane, acetylene, ethylene, coal gas (town gas) or natural gas. The light is produced either directly ...
s, mirrors or other primitive light sources, resulting in an uneven specimen illumination unsuited for producing good quality photomicrograph
A micrograph or photomicrograph is a photograph or digital image taken through a microscope or similar device to show a magnified image of an object. This is opposed to a macrograph or photomacrograph, an image which is also taken on a mic ...
s using the slow-speed emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althoug ...
s available at the time. Over the course of his work for his doctorate degree, Köhler developed a microscope configuration that allowed for an evenly illuminated field of view and reduced optical glare from the light source. It involved a collector lens for the lamp that allowed the light source to be focused on the front aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane.
An opt ...
of the condenser. This in turn allowed the condenser to be focused on the specimen using a field diaphragm and condenser focus control. This superior illumination scheme is still widely used in modern microscopes and forms the basis for phase contrast Phase-contrast imaging is a method of imaging that has a range of different applications. It exploits differences in the refractive index of different materials to differentiate between structures under analysis. In conventional light microscopy, ph ...
, differential interference contrast
Differential interference contrast (DIC) microscopy, also known as Nomarski interference contrast (NIC) or Nomarski microscopy, is an optical microscopy technique used to enhance the contrast in unstained, transparent samples. DIC works on the p ...
, epifluorescence
A fluorescence microscope is an optical microscope that uses fluorescence instead of, or in addition to, scattering, reflection, and attenuation or absorption, to study the properties of organic or inorganic substances. "Fluorescence microsc ...
, and confocal microscopy
Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM) or laser confocal scanning microscopy (LCSM), is an optical imaging technique for increasing optical resolution and contrast of a micrograph by means of using a sp ...
.[Douglas B. Murphy (2001): ''Fundamentals of light microscopy and electronic imaging'', Wiley-Liss, Inc., New York, ]
Köhler's groundbreaking work on microscope illumination was published in the '' Zeitschrift für wissenschaftliche Mikroskopie'' in 1893 in Germany, followed by an English summary of his work in the '' Journal of the Royal Microscopical Society'' one year later. Its significance was not noted until several years later when Köhler was invited to join the Carl Zeiss AG
Carl Zeiss AG (), branded as ZEISS, is a German manufacturer of optical systems and optoelectronics, founded in Jena, Germany in 1846 by optician Carl Zeiss. Together with Ernst Abbe (joined 1866) and Otto Schott (joined 1884) he laid the f ...
company based on his invention. A century after its first publication, a translation of Köhler's original article, ''A New System of Illumination for Photomicrographic Purposes'', was reprinted in the ''Köhler Illumination Centenary'' commemorative issue by the Royal Microscopical Society
The Royal Microscopical Society (RMS) is a learned society for the promotion of microscopy. It was founded in 1839 as the Microscopical Society of London making it the oldest organisation of its kind in the world. In 1866, the society gained it ...
in 1994.[Pioneers in optics: August Köhler](_blank)
/ref> Today, the Köhler illumination is considered one of the most important principles in achieving the best optical resolution on a light microscope.
Other contributions to microscope development
When Köhler joined Zeiss in 1900, Ernst Abbe
Ernst Karl Abbe HonFRMS (23 January 1840 – 14 January 1905) was a German physicist, optical scientist, entrepreneur, and social reformer. Together with Otto Schott and Carl Zeiss, he developed numerous optical instruments. He was also a co-ow ...
and glass specialist Otto Schott
Friedrich Otto Schott (1851–1935) was a German chemist, glass technologist, and the inventor of borosilicate glass. Schott systematically investigated the relationship between the chemical composition of the glass and its properties. In this wa ...
had already paved the way for microscope improvements through their contributions to precise optical theory and the development of appropriate glass formulas. Köhler's expertise and his illumination technique helped to improve the microscope optics to achieve optimum resolution, using the entire resolving power of Abbe's objectives.
Köhler remained an active staff member of Zeiss for 45 years, contributing numerous innovations during this time. These include the development of a microscope operating with ultraviolet light
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
(together with his colleague Moritz von Rohr
Moritz von Rohr (4 April 1868 – 20 June 1940) was an optical scientist at Carl Zeiss in Jena, Germany.
A street in Jena is named after him: Moritz-von-Rohr-Straße, near Carl-Zeiss-Promenade and Otto-Schott-Straße.
Life
Moritz von Ro ...
), pioneering what would become the starting point for fluorescence microscopy
A fluorescence microscope is an optical microscope that uses fluorescence instead of, or in addition to, scattering, reflection, and attenuation or absorption, to study the properties of organic or inorganic substances. "Fluorescence microscop ...
, and the discovery of grid illumination, a method that would later be used in the treatment of tumor
A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...
s. A suggestion by Köhler led to the development of parfocal lenses which allow the specimen to remain in focus when changing objectives on a microscope.
Patents and publications
As a member of Zeiss, August Köhler filed at least 25 patent applications in Europe as well as at least ten patents in the USA. His patents include projection methods and illumination for kinematograph
Cinematograph or kinematograph is an early term for several types of motion picture film mechanisms. The name was used for movie cameras as well as film projectors, or for complete systems that also provided means to print films (such as the Cin ...
s, microscope applications, and light and dark field illumination, among others. He filed an application for a fixed-ocular microscope of his design in Germany on 16 April 1924, and with the United States Patent Office
The United States Patent and Trademark Office (USPTO) is an agency in the U.S. Department of Commerce that serves as the national patent office and trademark registration authority for the United States. The USPTO's headquarters are in Alexa ...
on 31 March 1925 (patent number 1649068).USPTO patent image database
/ref> His publications include essays on microscopy and projection systems, and in particular his specialty of microphotography. His contributions to biology include fine structure analyses of diatom
A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...
s.
References
{{DEFAULTSORT:Kohler, August
1866 births
1948 deaths
Microscopists
Engineers from Darmstadt
Carl Zeiss AG people
Businesspeople from Darmstadt