cathode ray tube inside the receiver set. The system was first demonstrated to the Federal Communications Commission (FCC) on August 29, 1940, and shown to the press on September 4. CBS began experimental color field tests using film as early as August 28, 1940, and live cameras by November 12. NBC (owned by RCA) made its first field test of color television on February 20, 1941. CBS began daily color field tests on June 1, 1941. These color systems were not compatible with existing black-and-white television sets, and as no color television sets were available to the public at this time, viewing of the color field tests was restricted to RCA and CBS engineers and the invited press. The War Production Board halted the manufacture of television and radio equipment for civilian use from April 22, 1942, to August 20, 1945, limiting any opportunity to introduce color television to the general public. Mexican inventor Guillermo González Camarena also experimented with hybrid field-sequential colour TV (known as telectroescopía at first). His efforts began in 1931 and led to a Mexican patent for the "trichromatic field sequential system" color television being filed in August 1940. As early as 1940 Baird had started work on a fully electronic system he called the "Telechrome". Early Telechrome devices used two electron guns aimed at either side of a phosphor plate. Using cyan and magenta phosphors, a reasonable limited-color image could be obtained. He also demonstrated the same system using monochrome signals to produce a 3D image (called "stereoscopic" at the time). A demonstration on August 16, 1944, was the first example of a practical color television system. Work on the Telechrome continued and plans were made to introduce a three-gun version for full color. This used a patterned version of the phosphor plate, with the guns aimed at ridges on one side of the plate. However, Baird's untimely death in 1946 ended development of the Telechrome system. Similar concepts were common through the 1940s and 1950s, differing primarily in the way they re-combined the colors generated by the three guns. The Geer tube was similar to Baird's concept, but used small pyramids with the phosphors deposited on their outside faces, instead of Baird's 3D patterning on a flat surface. The penetron used three layers of phosphor on top of each other and increased the power of the beam to reach the upper layers when drawing