CRT monitors

Like televisions, the first computer monitors used cathode ray tubes (CRTs). A CRT develops an image by sweeping an electron beam (cathode ray) across a screen coated with a phosphor (a material that glows when hit with an electron beam).^[1]

Cutaway side view of a CRT.

Color perception and color CRT monitors

Most humans have three types of color receptors called cones.^[2] These cones are sensitive to different wavelengths of light. The colors we perceive depend on how the different cones are stimulated. For example, light with a wavelength of about 575 nanometers stimulates the three types of cones such that we perceive the color yellow. However, a mix of two or more colors stimulates the cones the same as a different single color. For example, an equal mix of red and green light stimulates the cones by the same amount as yellow light. Therefore, we perceive an equal mix of red and green light as the color yellow. By exposing our eyes to various amounts of red, green, and blue light, we can perceive any color of the visible spectrum.

Red, green, and blue lights show how overlapping additive primary colors create cyan, yellow, magenta, and white.

This is only limited by the purity of these additive primary colors.^[3] We can also perceive colors that don't exist in the visible spectrum. For example, The color purple doesn't exist in the visible spectrum. However, we perceive a particular mix of red and blue as purple.^[4]

Color monitors have phosphor material that glows either red, blue, or green, arranged in a pattern of tiny dots.^[5] The colored phosphors are arranged in thousands of groups of three called triads. Between the electron guns and the colored phosphors is a screen with tiny holes called the shadow mask.

A color CRT shadow mask.

A closeup of the shadow mask.

The CRT has three electron guns with the beams angled such that each beam, shining through the shadow mask, only strikes the dots that glow in the appropriate color; there are red, blue, and green electron beams striking red, blue, and green phosphors.^[6]

An oblique view cutaway diagram of a color CRT. The inset shows the three electron beams passing through the shadow mask, each striking only the appropriately-colored phosphors. The numbers indicate essential elements of the color CRT, which are: 1, the electron guns; 2, the electron beams; 3, the focusing coil; 4, the deflection coils; 5, the high-voltage anode connection; 6, the shadow mask; 7, the phosphor-coated screen; 8, a colored phosphor (blue).

 Each hole in the shadow mask is aligned at the center of one triad, one hole per triad. As the three beams sweep across the face of the CRT, each beam is angled to strike only the appropriately colored phosphors. The electron beams are not precisely aimed through the holes in the shadow mask. This is unnecessary as long as the angles are precisely maintained.

Here, the red electron beam is not perfectly aimed at the rows of triads and thus only partially illuminates the phosphors. However, the red beam still only illuminates red phosphors, etc. Also, despite the misaiming, the same amount of phosphor is illuminated as with good alming. The viewer will not perceive any difference. Therefore, although precise alignment of the angles must be maintained, precise aiming at the holes in the shadow mask is not required.

The above describes CRTs that use circular holes in the shadow mask. Many CRTs had rectangular holes and thus rectangular subtriads.

The arrangement of color phosphors with a slot mask color CRT.

These were uncommon as computer monitors because they sacrificed dot pitch for a brighter screen. Most CRT computer monitors used the traditional circular dot shadow mask.

The Trinitron CRT, developed by Sony, had a mask with vertical slots cut from top to bottom called an aperture grill. The Trinitron used a single electron gun with three cathodes, producing three electron beams originating from three different points on a horizontal plane. The phosphors were arranged in vertical stripes to match the aperture grill. The three electron beams were aligned to strike the appropriate phosphor stripes after passing through the aperture grill. The aperture grill elements were kept in alignment by one or two fine horizontal wires attached to the aperture grill. These wires resulted in narrow horizontal black lines in the image.

Shadow mask-type color CRTs blocked the electron beam except where the holes existed in the shadow mask. This resulted in a much darker image than monochrome CRTs. Slot masks were an improvement. However, by not blocking the beam in the vertical dimension, the trinitron produced a brighter image than traditional CRT displays.

Degaussing

Magnetic fields will deflect the electron beams. This can cause the electron beams to illuminate the wrong colored phosphors. Magnets near color CRT monitors can cause color shifts on the screen. However, the shadow mask is made of an iron alloy^[7] and can become magnetized. Therefore, a magnetized shadow mask can cause color anomalies on the screen.

The washed-out color in the upper left of this monitor is due to a nearby magnet or residual magnetism on the shadow mask.[8]

Color CRT monitors have a coil of wire around the front of the screen. When the monitor is turned on, an AC current briefly passes through this coil to remove any magnetism from the shadow mask. This process is called degaussing. Many color CRT monitors have a degaussing button on the front panel to demagnetize the shadow mask whenever necessary. This can be necessary after merely adjusting the position of the monitor. The Earth's magnetic field can magnetize the shadow mask enough to change the colors. Moving the monitor changes its orientation to the Earth's magnetic field and can require degaussing the shadow mask.

Dot pitch and resolution

The dot pitch of a CRT monitor is the distance between the centers of adjacent color triads. Early color CRT monitors had a typical dot pitch of 0.39 mm. High-quality monitors had a dot pitch of around 0.28 mm. Shortly before CRT technology died out, a dot pitch of 0.25 mm was typical. Large monitors often had a dot pitch of 0.22 mm.

As mentioned above, the coverage of the electron beams does not have to match the triads. Therefore, the dot pitch does not necessarily match the footprint of the electron beams. However, an electron beam footprint that differs substantially from the size of the triads is counterproductive.

However, dot pitch does set an upper limit of resolution. If we look again at the illustration of a CGA image at 160 x 100 resolution on page 15, we see that each pixel covers many triads.^[9] As resolution increases, each pixel covers fewer triads. If the resolution is high enough that each pixel covers less than a triad, unpredictable color artifacts are introduced. As graphics quality evolved rapidly, it was not uncommon for graphics resolution to be smaller than a monitor's dot pitch. This caused difficult-to-read characters in text modes, and in graphics modes, it also caused color anomalies.

Interlacing vs. non-interlacing monitors

When computer video broke away from television standards, non-interlacing monitors were introduced.

Interlacing was developed to create a television signal that presented 30 images (frames) per second while scanning the screen 60 times per second. Scanning the screen, top to bottom, 30 times per second caused unacceptable flicker. Presenting 60 images per second reduced the flicker significantly but required television channels with twice the bandwidth. Engineers developed a system to scan the odd lines in one pass and the even lines in a second pass. This scanned the screen 60 times per second while only presenting 30 images per second. Unlike broadcast television, computer video didn't need to limit the signal's bandwidth, so progressive scanning (odd and even lines in one pass) was introduced.

When non-interlaced video was introduced, there was a lot of marketing nonsense saying that interlacing was cheap and low-quality. It was neither. It was a good solution to a problem computers didn't need to solve. The truth is that it took longer to scan the screen at higher resolutions, resulting in flicker. Therefore, the system still interlaced at high resolutions.

This is no longer an issue since LCD monitors don't scan the screen the same way as CRT monitors.

High voltage

A CRT uses dangerously high voltage to accelerate the electron beam. This high voltage is created by a device called a flyback transformer. The high voltage connection is recognized by its thick wire and well-insulated connection to the CRT.

Don't open a CRT monitor's enclosure if you aren't trained to work on them. People have been killed by CRT televisions and monitors. If the voltage discharge resistor fails, a CRT monitor can be deadly even when turned off and unplugged.