Editing Shadow mask (section)

===Shadow mask===
In 1938 German inventor [[Werner Flechsig]] first patented (received 1941, France) the seemingly simple concept of placing a sheet of metal just behind the front of the tube, and punching small holes in it. The holes would be used to focus the beam just before it hit the screen. Independently, Al Schroeder at RCA worked on a similar arrangement, but using three electron guns as well. When the lab leader explained the possibilities of the design to his superiors, he was promised unlimited manpower and funds to get it working.<ref name=color40>Abramson & Sterling, p. 40.</ref> Over a period of only a few months, several prototype color televisions using the system were produced.<ref name=color41>Abramson & Sterling, p. 41.</ref>

The guns, arranged in a delta pattern at the back of the tube, were aimed to focus on the metal plate and scanned it as normal. For much of the time during the scan, the beams would hit the back of the plate and be stopped. However, when the beams passed a hole they would continue to the phosphor in front of the plate. In this way, the plate ensured that the beams were perfectly aligned with the colored phosphor dots. This still left the problem of focusing on the correct colored dot. Normally the beams from the three guns would each be large enough to light up all three colored dots on the screen. The mask helped by mechanically attenuating the beam to a small size just before it hit the screen.<ref name=g81>Gilmore, p. 81.</ref>

But the real genius of the idea is that the beams approached the metal plate from different angles. After being cut off by the mask, the beams would continue forward at slightly different angles, hitting the screens at slightly different locations. The spread was a function of the distance between the guns at the back of the tube, and the distance between the mask plate and the screen. By painting the colored dots at the correct locations on the screen, and leaving some room between them to avoid interactions, the guns would be guaranteed to hit the right colored spot.<ref name=g81/>

Although the system was simple, it had a number of serious practical problems.

As the beam swept the mask, the vast majority of its energy was deposited on the mask, not the screen in front of it. A typical mask of the era might have only 15% of its surface open. To produce an image as bright as the one on a traditional B&W television, the electron guns in this hypothetical shadow mask system would have to be five times more powerful. Additionally, the dots on the screen were deliberately separated in order to avoid being hit by the wrong gun, so much of the screen was black.<ref name=g178>Gilmore, p. 178.</ref> This required even more power in order to light up the resulting image. And as the power was divided up among three of these much more powerful guns, the cost of implementation was much higher than for a similar B&W set.<ref name=g83>Gilmore, p. 83.</ref>

The amount of power deposited on the color screen was so great that thermal loading was a serious problem. The energy the shadow mask absorbs from the electron gun in normal operation causes it to heat up and expand, which leads to blurred or discolored images (see [[doming (television)|doming]]). Signals that alternated between light and dark caused cycling that further increased the difficulty of keeping the mask from warping.

Furthermore, the geometry required complex systems to keep the three beams properly positioned across the screen. If you consider the beam when it is sweeping across the middle area of the screen, the beams from the individual guns are each traveling the same distance and meet the holes in the mask at equal angles. In the corners of the screen some beams have to travel farther and all of them meet the hole at a different angle than at the middle of the screen. These issues required additional electronics and adjustments to maintain correct beam positioning.