Background
The 1930's saw the standardization of color models by the International Commission on Illumination. There are five major color models: CIE, RGB, YUV, HSL/HSV, and CMYK.
During the 1930's, the use of Steganography - the practice of concealing messages or information within other non-secret text or data, was booming!
In 1938, Mr. Martin J. Weber, a contemporary of Mr. Rockwell, filed a patent for a photographic process called "Posterization." Mr. Weber's invention causes two-dimensional photographs to appear as three-dimensional images that spring from the page.
Norman Rockwell was a photography enthusiast who understood the technical aspects of the complex process of posterization. He was exposed to Steganography during his work for the US Government during World War II. His in-depth knowledge of the mathematical basis for the differences between the color models gave him an idea!
Mr. Rockwell developed a process that combined the "high technology" innovations of the 1930's to create his anti-forgery technique. It was a blend of posterization and Steganography. He never disclosed this technique to anybody before his passing in 1978.
In 1999, Robert Stupack purchased a purported Norman Rockwell original painting. It depicted Mr. Rockwell's art studio in West Arlington, Vermont, which burned in 1943. Several attempts to authenticate the painting failed. Then in 2016, Mr. Stupack noticed the initials NR hidden on the left side of the bridge in the latticework. The bridge is the focal point of the piece.
It took nine months of research and extensive testing to determine how Mr. Rockwell created a "hi-tech" method to prevent forgery of his works and, more importantly, what steps were necessary to reveal the hidden messages. In November 2019, the US Patent and Trademark Organization issued the first patent of its kind to Mr. Stupack for Authentication of Norman Rockwell Paintings.
The authentication process requires a large format photograph, a laptop or desktop computer, and commonly used software programs. The easy to learn process reveals the initials NR (or reversed as RN) plus several additional anti-forgery marks hidden by Mr. Rockwell in every painting he completed after 1942.
NONE OF THE ANTI-FORGERY MARKS IS VISIBLE TO THE HUMAN EYE.