Cathode Ray Tube monitor (CRT)

A black and white monitor can display different tones (shades of gray) by varying the intensity of the flow.

For colour monitors, three electron beams (coming from three different cathodes) each strike a point with a specific colour: red, green, and blue (RGB).

Three points of colour are called a triad (or dot trio).

Blue phosphors use zinc sulfide, while green ones use zinc sulfide and cadmium sulfide. The red ones are hard to create, and are made from a mixture of yttrium and europium, or gadolinium oxide.

However, these phosphors are so close together than the eye cannot separate them enough to tell them apart; it sees a single colour made up of these three colours. Try flicking a tiny drop of water onto the glass of your monitor: It will magnify the phosphors so that you can see them.

What's more, to avoid smearing (such as an electron meant to strike a green phosphor colliding with blue instead), a metallic grid called the shadow mask is placed in front of the phosphoric layer to guide the electron flow.

There are several different categories of CRT monitors, which are set apart by the mask used:

• FST-Invar (Flat Square Tube), whose phosphors are round. These monitors use a grid called a shadow mask. They give the right colours overall, but have the disadvantage of distorting and darkening the image at the corners.

  

• Mitsubishi's Diamondtron tubes and Sony's Trinitron, whose masks are made up of vertical slots (called an aperture grille or tension mask), which lets through more electrons and therefore gives a brighter image.

  

• Nec's Cromaclear tubes, whose mask is uses of a hybrid system with dimpled slots, is the best technology of the three.

  

Technical specifications

The specifications for CRT monitors include:

• The definition: The number of pixels that the screen can display. This number is generally between 640x480 (640 pixels long, 480 pixels wide) and 1600x1200, but higher resolutions are technically possible.
• The size: This is calculated by measuring the diagonal of the screen, and is expressed in inches (an inch is about 2.54 cm). Be careful not to confuse a screen's definition with its size. After all, a screen of a given size can display different definitions, although in general screens which are larger in size have a higher definition.
• The dot pitch: This represents the distance which separates two phosphors of the same colour. The lower the dot pitch, the better the image quality. A dot pitch equal to or less than 0.25 mm will be comfortable to use, while monitors with a dot pitch equal to or greater than 0.28 mm should be avoided.
• The resolution: This determines the number of pixels per surface unit (given in linear inches). This is abbreviated DPI, for Dots Per Inch. A resolution of 300 dpi means 300 columns and 300 rows of pixels per square inch, which means that there are 90,000 pixels per square inch. By comparison, a resolution of 72 dpi means that one pixel is 1"/72 (one inch divided by 72) or 0.353 mm, which corresponds to one pica (a typographical unit). The terms "resolution" and "definition" are often confused in the media.
• The refresh rate: This represents the number of images which are displayed per second, or more precisely the number of times the image is refreshed each second. Also called the vertical scan rate, it is expressed in Hertz. The higher this value is, the better the visual comfort (the image does not appear to flicker), so it must be much higher than 67 Hz (any lower than that, and the image appears to "blink"). Most people do not notice the flicker effect at 70 Hz or higher, so a value equal to or greater than 75 Hz is generally suitable.