Shadow mask

The shadow mask is one of the two technologies used in the manufacture of cathode-ray tube (CRT) televisions and computer monitors which produce clear, focused color images.

A shadow mask is a metal plate punched with tiny holes that separate the colored phosphors in the layer behind the front glass of the screen.

Shadow masks are made by photochemical machining, a technique that allows for the drilling of small holes on metal sheets.

This arrangement allows the three guns to address the individual dot colors on the screen, even though their beams are much too large and too poorly aimed to do so without the mask in place.

At the time, a number of systems were being proposed that used separate red, green and blue signals (RGB), broadcast in succession.

Most experimental systems broadcast entire frames in sequence, with a colored filter (or "gel") that rotated in front of an otherwise conventional black and white television tube.

This system did not directly encode or transmit the RGB signals; instead it combined these colors into one overall brightness figure, called the "luminance".

On a black and white television this extra information would be seen as a slight randomization of the image intensity, but the limited resolution of existing sets made this invisible in practice.

On color sets the extra information would be detected, filtered out and added to the luminance to re-create the original RGB for display.

[2] John Logie Baird, who made the first public color television broadcast using a semi-mechanical system on 4 February 1938, was already making progress on an all-electronic version.

In spite of their best efforts, the wide electron beams simply could not focus tightly enough to hit the individual dots, at least over the entirety of the screen.

Moreover, most of these devices were unwieldy; the arrangement of the electron guns around the outside of the screen resulted in a very large display with considerable "dead space".

Paramount Pictures worked long and hard on the Chromatron, which used a set of wires behind the screen as a secondary "gun", further focusing the beam and steering it towards the correct color.

Until 1968, every color television sold used the RCA shadow mask concept,[8] in the spring of that year Sony introduced their first Trinitron sets.

[9] 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.

[11] 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.

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.

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).

Signals that alternated between light and dark caused cycling that further increased the difficulty of keeping the mask from warping.

[citation needed] Wartime advances in electronics had opened up large swaths of high frequency transmission to practical use, and in 1948 the U.S. Federal Communications Commission (FCC) started a series of meetings on the use of what would become the UHF channels.

At the time there were very few television sets in use in the United States, so the stakeholder groups quickly settled on the idea of using UHF for a new, incompatible, color format.

[16] RCA had, by this time, produced experimental shadow mask sets that were an enormous leap in quality over any competitors.

The system was dim, complex, large, power hungry and expensive for all these reasons, but provided a usable color image, and most importantly, was compatible with existing B&W signals.

Bimetal springs may be used in CRTs used in TVs to compensate for warping that occurs as the electron beam heats the shadow mask, causing thermal expansion.

Invar and similar low-expansion alloys were introduced in the 1980s[27] These materials suffered from easy magnetization that can affect the colors, but this could be generally solved by including an automatic demagnetizing feature.

The Porta-Color used both of these advances and re-arranged the guns to lie beside each other instead of in a triangle, allowing the dots to be extended vertically into slots that covered much more of the screen surface.

[26] Grey-tinted faceplates dimmed the image, but provided better contrast, because ambient light was attenuated before it reached the phosphors, and a second time as it returned to the viewer.