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March 16, 1938: Katharine Blodgett patents anti-reflective coatings

Patenting anti‑reflective coatings
Photo:  AIP Emilio Segre Visual Archives, Physics Today Collection

Blodgett (center) demonstrating surface chemistry experiments for visitors at the opening of General Electric’s new Research Laboratory building
Many things became much clearer in 1938 after Katharine Blodgett developed anti-reflective coatings for glass. Uncoated glass is far from invisible. It reflects a significant amount of the light that hits it, creating distortion and glare.  Blodgett, a physicist at General Electric laboratories, developed a method of coating glass with a soapy film that would eliminate most of those reflections.  Her insight led to the practical ant-reflective coatings that now coat picture glass, windshields, eyeglasses, camera lenses and much more.  Blodgett received a patent for the process in March 1938, and GE announced the discovery in December of that year.

Katharine Burr Blodgett was born in Schenectady, NY, in 1898. Her father, a patent attorney at General Electric, died shortly before she was born, and the family moved several times during her childhood. She attended Bryn Mawr College, and during her senior year, Blodgett visited GE to tour the labs. Her tour guide was Irving Langmuir, who had known her father. He encouraged her to pursue further studies in chemistry and physics. After receiving a master’s degree from the University of Chicago in 1918, Blodgett returned to GE to work as Langmuir’s assistant. Langmuir, who went on to win the Nobel Prize in chemistry, was at the time opening up a new field of research by developing a way to produce uniform one-molecule thick oily films on a water surface.

After Blodgett had worked for a few years as his assistant, Langmuir encouraged her to continue her education still further. In 1924 she went to Cambridge University, where she worked with Ernest Rutherford and wrote a thesis on the topic of gaseous electronics. She was the first woman to receive a PhD in physics from Cambridge. She returned to the United States in 1926 and rejoined the staff of GE as a research scientist. At first, she collaborated with Langmuir on some improvements to light bulbs, and then in 1933 they began working again on surface films. The monolayer films were well understood by this time, so Blodgett and Langmuir began building up thicker films to study. Blodgett developed a way to transfer the soap film from a water surface to a solid surface such as metal or glass, and found that by repeating the process she could build up films of barium stearate layer by layer, up to about 3000 layers. These became known as Langmuir-Blodgett films.

Blodgett then began looking for some applications for the films. She noticed that even the clearest glass reflected as much as 10 percent of incident light, making it difficult to see through. Blodgett realized her soapy films could solve that problem. Since she could precisely control the thickness of the soapy films by building them up one molecular layer at a time, and could easily deposit the films on a glass surface, Blodgett figured out that she could develop a coating of just the right thickness to cancel out most reflections from the glass surface.  She built up a film with thickness equal to ¼ the average wavelength of visible light (about 1388 Angstroms). This way, any light that reflected off the glass surface would have traveled half a wavelength farther than light that had reflected off the film surface, so most of the reflections would cancel out. Blodgett also tweaked the chemical composition of the film to adjust its index of refraction to enhance the reflection-canceling, and she was able to eliminate almost all of the reflection, making the glass nearly invisible.

On March 16, 1938, Blodgett received US patent #2,220,660 for the “Film Structure and Method of Preparation.” This was one of six patents Blodgett received in her lifetime. She also published a paper titled “Use of Interference to Extinguish Reflection of Light from Glass,” in the Physical Review in 1939.

General Electric announced the discovery in December 1938, and it immediately caused a lot of excitement. Blodgett and her “invisible glass,” as the popular press called it, were featured in Time, Life, and The New York Times. The public’s fascination with Blodgett and her work may have been due in part to the fact that she was a woman in science, which was still very unusual at the time.  

During WWII, Blodgett contributed to the war research by developing poison gas absorbents, a method for de-icing aircraft wings, and improved smokescreens. She also developed a “color gauge” to measure the thickness of coatings, conducted more research on films, studied electrically conducting glass, and developed a way to use electrical discharges in gases to clean solid surfaces of impurities, a method that is used in semiconductor device fabrication. Blodgett retired in 1963 after a long career at GE. She died in 1979 at age 81, having received numerous awards for her work, though she was conspicuously left out of a 1953 article in Science celebrating 75 years of research at GE laboratories.

Blodgett’s original soap coatings were not useful for commercial products because they were too soft and could easily be wiped off the glass surface. Soon after her announcement, however, other groups developed durable coatings and improved methods of production. These coatings have been reducing reflections from eyeglasses, picture glass, camera lenses, microscopes, windshields, televisions and computer screens ever since.

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