He began working on the problem in 1940, shortly after he started working for the Minnesota Mining & Manufacturing Company (now 3M), doing research on pigments. For two or three years the problem was his "mental hobby." He kept trying to think of a quick, easy way of doing copy work

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When the idea on which he would build his copier finally came to him, it was not a bolt out of the blue. In fact, he had considered the general principle a number of times and passed right over it. His wife, Muriel, said he got the idea for his duplicating process from looking at a leaf on a snowbank on sunny days. The brown leaf absorbed the radiant heat from the sun, while the snow reflected the rays. The leaf melted a duplicate image into the snow. Many before Miller had noticed this common occurrence. But Miller's innovative genius led him to wonder if the same principle could be applied to a copying machine.

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While primarily involved in World War II governmental projects at 3M, Miller kept puttering with his copying gadget whenever he could find time. Early in 1944, Miller built a machine that was able to produce a copy image two inches square - a primitive prototype of his copying machine. He took the cellophane wrap from a box of candy sent to him by his mother, coated it with a heat-sensitive, mercury-based salt, stretched the coated cellophane over the image to be copied, and then taped it around a cardboard drum. The drum was exposed to a very bright light bulb, and in about five seconds an image was produced on the paper. The image on the original document absorbed the energy and generated heat, which caused the copy paper to darken, creating the copy.

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He showed this crude machine to 3M's director of research, who was impressed and assigned Miller an assistant. They began working on it full time. 3M initially allocated $10,000 for the project. But Miller needed nearly a decade of research and a million dollars in funding to develop a commercially viable product. During this long period of research and development, there were skeptics within 3M who wondered whether the world really needed a dry-copying process. After all, if an extra copy was needed, the secretary could be told to put an extra sheet of carbon paper into the typewriter.

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By 1949, Miller’s team of chemists and engineers had built a commercial copier about the size of a desk that cost $800, and used a specially coated, chemically treated, heat-sensitive paper also made and sold by 3M. The invention was named Thermo-Fax, also often called Thermofax (for thermal facsimile).

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It was an agency of the federal government, the CIA, that launched production of the machine. As a collector of every conceivable kind of information, the CIA had extraordinary clerical needs. A machine that could copy originals would be a tremendous help. After a 1949 demonstration in Washington, for which Miller provided on a hand-assembled prototype, the CIA placed an order and commercial production of the Thermo-Fax began.

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The 3M Thermo-Fax Duplicating Machine was commercially introduced in 1950 Miller and his team created the first desktop "console" model in 1951. Research continued, and by 1955, they had developed a smaller, low-priced easy-to-use dry copier.

The Thermo-Fax was a massive improvement for copying documents. Before it was invented, copies were made using a cumbersome process of wet copy duplicating (where the copy material was exposed, then run through a liquid tank, and then dried), using a mimeograph machine or a Ditto machine, typing documents using carbon paper, or hand copying documents.

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Not only did Miller invent the Thermo-Fax, he demonstrated and sold it around the country. "What really moved me was the expression on the faces of clerks and secretaries back in the early ‘50s the first time they fed an original and a buff copy sheet into a demonstrator and, after a moment, took a look at the results," said Miller in a 1974 interview.

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In a 2006 interview, Carl Kuhlmeyer, a development engineer who worked on the Thermo-Fax, explained that it was new concept for 3M to make the machine. Until then, 3M’s business was primarily abrasives, coatings, and related products. "The Thermo-Fax copy process included paper, which was coated on a big coater at high speeds. So that’s where the tie was to 3M," Kuhlmeyer said, "But the machine portion of the process was completely foreign to 3M. We never built anything mechanical other than a tape dispenser, which is very simple. You just mold them and you put a roll of tape in. We never had designed a machine that had motors and switches and transformers and lamps and all those things in the machine."

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The success of the Thermo-Fax proved the company could successfully make and sell a product combining hardware (a copying machine) and a consumable product (coated paper).

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By 1950, the office copier business had become crowded. More than 30 different copier machines were available. The biggest seller was the Thermo-Fax because it was fast and easy to use. In addition to copiers, 3M introduced the first overhead projector (the Thermo-Fax was the only copier on the market that also made transparencies). 3M was the undisputed world leader in copying from 1955 to 1970. Its success led 3M into other office-related businesses and accelerated 3M’s international growth.

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The easily portable Thermo-Fax machine had broad appeal inside and outside the United States. "Copying was a prime mover in helping us establish our foreign ventrure," recalled Maynard Patterson, a leading architect of 3M's global expansion. The first Thermo-Fax copier was introduced in Europe in 1955, only a few years after its United States debut. "The Thermo-Fax machine was aproduct that we could sell in almost any country or city of the world," said Kuhrmeyer. "After opening an office to sell copying products, 3M could then introduce tapes and abrasives."

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"3M was the market leader in tapes and abrasives in the United States, but in other countries there were local competitors that led the category. With copying machines, we won the business and the world beat a path to our door. That success generated cash flow 3M needed to invest in global growth," said Ron Mitsch, retired vice chairman of the board and executive vice president of 3M.

The thermographic copy technology developed by Miller had a profound impact on 3M's business performance. His invention and the line of duplicating products that it spawned produced major, sustained profits over decades, according to Don Kimble, a development specialist who worked with Miller for 31 years.

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The Thermo-Fax was popular until the 1970s, when xerography made plain-paper copying possible. As copying technology advanced, Thermo-Fax machines were marketed as a method of producing transparencies for overhead projector presentations. Thermo-Fax machines are also still widely used by artists. Tattoo artists use masters as tattoo stencils, to quickly and accurately mark the outlines of a tattoo on the skin of the person to be tattooed. Textile and printmaking artists use these machines for creating silk screen. These modern uses have kept up demand for many models of the Thermo-Fax machines.

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As for Miller, he became a member of 3M's eminent Carlton Society. He was an inspiration to a new generation of 3M researchers, especially the group that created the world's first color copier. He was issued 18 U.S. patents in the duplicating field, including both sole and joint inventions.

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A native of Canada, Miller became a naturalized U.S. citizen in 1948. An active member of the community, he served on the Board of Trustees of the Science Museum of Minnesota, and devoted many years in Science Fair program encouraging young people toward higher education goals. Along this line, his work with the Boy Scouts of America earned him the "Scouter of the Year Award" in 1967. He retired in 1978, after spending 38 years at 3M. He died in 1986, at age 73.