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General Electric's Porta-Color was the first "portable" color television introduced in the United States. The Porta-Color set introduced a new display system that improved image brightness, allowing the set to operate at lower power than competing systems based on the almost universal shadow mask display introduced by RCA. The original Porta-Color set remained in production from its introduction in 1966 until at least 1978, without changing its original tube-based chassis or making other major changes.
The name has been variously written, even in GE's literature as "Porta Color", "Porta-Color" and "Porta-color". The name may also refer to the specific television model, or less commonly, the style of television tube it used.
A conventional black and white television (B&W) uses a tube that is uniformly coated with a phosphor on the inside face. When excited by high-speed electrons, the phosphor gives off light, typically white but other colors are also used in certain circumstances. An electron gun at the back of the tube provides a beam of high-speed electrons, and a set of electromagnets arranged near the gun allow the beam to be moved about the display. The television signal is sent as a series of stripes, each one of which is displayed as a separate line on the display. The strength of the signal increases or decreases the current in the beam, producing bright or dark points on the display as the beam sweeps across the tube.
In a color display the uniform coating of white phosphor is replaced by dots or lines of three colored phosphors, producing red, green or blue light (RGB) when excited. When excited in the same fashion as a B&W tube, the three phosphors give off different amount of these primary colors, which mix in the human eye to produce a single apparent color. In order to produce the same resolution as the B&W display, a color screen needs to have three times the resolution. This presents a problem for conventional electron guns, which cannot be focussed or positioned accurately enough to hit these much smaller individual patterns.
The conventional solution to this problem was introduced by RCA in 1950, with their shadow mask system. The shadow mask is a thin metal foil with small holes cut 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 hits only their correct phosphor.
The primary problem with the shadow mask system is that the vast majority of the beam energy, typically 85%, is lost "lighting up" the mask itself as the beam passes over the opaque sections between the holes. This means that the beams have to be greatly increased in power in order to produce an acceptable brightness when they do pass through the holes.
General Electric (GE) had been working on a variety of systems that would allow them to introduce color sets that did not rely on the shadow mask patents. Through the 1950s they had put considerable effort into the Penetron concept, but were never able to make it work as a basic color television, and started looking for alternate arrangements. GE eventually improved on the basic shadow mask system with a simple change to layout.
Instead of arranging the guns, and phosphors, in a triangle, their system arranged them side-by-side. This meant that the phosphors did not have to be displaced from each other in two directions, only one, which allowed them to be extended vertically to form short stripes instead of small dots. This simple change dramatically increased the painted area of the screen; a contemporary shadow mask design might have 25% of the screen surface covered by phosphor, but in the Porta-Color layout this was improved to about 50%. This doubled the brightness of the system with no other changes.
This change allowed GE to reduce the power of the guns, which led to reductions in size of the entire chassis. GE used the small size of their system as the primary selling feature. The original 28 pound set used a 11" tube and sold for $249, which was very inexpensive for a color set at that time. Introduced in 1966, the Porta-Color was extremely successful and led to a rush by other companies to introduce similar systems. By 1968 there was a wide variety of sets in the $200-$300 range, which led to the popularization of color television after over a decade of being largely ignored in the market.
GE produced the basic Porta-Color design well into the 1970s, even after most companies had moved to solid state designs when transistors with the required power capabilities were introduced. The Porta Color II was their attempt at a solid state version, but did not see widespread sales. The basic technology, however, was copied into GE's entire lineup as product refresh cycles allowed. By the early 1970s most companies had introduced these "slot-mask" designs, including RCA.
In a conventional shadow mask television design the electron guns at the back of the tube are arranged in a triangle. They are individually focussed, with some difficulty, so that the three beams meet at a spot when they reach the shadow mask. The mask cuts off any unfocussed portions of the beams, which then continue through the holes towards the screen. Since the beams approach the mask at an angle, they separate again on the far side of the mask. This allows the beams to address the individual phosphor spots on the back of the screen.
GE's design modified this layout by arranging the electron guns in a side-by-side line instead of a triangle. This meant that after they passed through the mask they separated horizontally only, hitting phosphors that were also arranged side-by-side. Since the phosphors were no longer arranged around each other, they could be extended vertically and made much larger. The mask was similarly modified with slot-shaped holes instead of round ones, and was sometimes referred to as a "slot-mask".
Sony's Trinitron system is similar in concept, but introduced two additional improvements. Unlike the Porta-Color, Trinitron used a single gun with three cathodes, which simplified the arrangement and eliminated focussing issues. Additionally, Sony extended the slots until they were the same vertical size as the entire screen, further improving the painted area, and thus the brightness to power figures.
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