advantage over the CBS system, and a modified version was selected by the NTSC as the new color standard in 1953. The major disadvantage was the difficulty in correctly focussing the beam on the correct color, a problem RCA solved with their shadow mask system. The shadow mask is a thin metal foil with small holes photoetched into it, positioned so the holes lie directly above one triplet of colored phosphor dots. Three separate electron guns are individually focussed on the mask, sweeping the screen as normal. When the beams pass over one of the holes, they travel through it, and since the guns are separated by a small distance from each other at the back of the tube, each beam has a slight angle as it travels through the hole. The phosphor dots are arranged on the screen such that the beams hit only their correct phosphor. To ensure the holes line up with the dots, the mask is used to create the dots using photosensitive material. The new broadcast system presented a serious problem for the penetron. The signal required the color to be selected at high speeds "on the fly" as the beam was being drawn across the screen. This meant the high voltage color selection grid had to be rapidly cycled, which presented numerous problems, notably high-frequency noise that filled the interior of the tube and interfered with the receiver electronics. Another modification was introduced to address this issue, using three separate guns, each fed with a different base voltage tuned to hit one of the layers. In this version no switching was required, eliminating the high-frequency noise. Producing such a system proved difficult in practice, and for home television use GE instead introduced their "Porta-Color" system, a dramatic improvement on RCA's shadow mask system. Other developers continued working with the basic system attempting to find ways of solving the high frequency switching issues, but none of these entered commercial production. Use in avionics For other uses, however, the advantages of the penetron remained. Although it was not well suited to the dot-sequential method of color broadcast, that was only important if one was receiving over-the-air broadcasts. For uses where the signal could be provided in any needed format, like in computer displays, the penetron remained useful. When a full color gamut was not needed, the complexity of the penetron was further reduced and it became