Jan's Computer Basics:5-4 Screen Features

A standard monitor screen is a CRT (cathode ray tube). The screen is coated on the inside surface with dots of chemicals called phosphors. When a beam of electrons hits a dot, the dot will glow.

On a color monitor these phosphor dots are in groups of three: Red, Green, and Blue. This RGB system can create all the other colors by combining what dots are aglow.

There are 3 signals that control the 3 electron beams in the monitor, one for each RGB color. Each beam only touches the dots that the signal tells it to light. All the glowing dots together make the picture that you see. The human eye blends the dots to "see" all the different colors.

A shadow mask blocks the path of the beams in a way that lets each beam only light its assigned color dots. (Very cool trick!)

LCD screen

LCD (Liquid Crystal Display) screens use an entirely different technique. The screen is still made of dots but is quite flat. LCD displays are made of two layers of a polarizing material with a liquid crystal solution in between. An electrical signal makes the crystals line up in a way that keeps light from going through entirely or just partly. A black screen has all the crystals lined up so that no light gets through.

A color LCD screen uses groups of 3 color cells instead of 3 phosphor dots. The signal for a picture cleverly lets just the right spots show their colors. Your eye does the rest.

Scan Pattern

There are two patterns used by CRT monitors to cover the whole screen. Both scan across the screen, in a row 1 pixel high, from left to right, drop down and scan back left. (LCD screens do not use these methods but display the whole screen at once.)

The non-interlaced pattern scans each row of pixels in turn, from top to bottom. This type is more prone to flicker if the scan has not started over by the time the phosphor dots have quit glowing from the last scan. This can make your eyes hurt or even make you nauseous.

The interlaced pattern scans every other row of pixels. So the odd rows are done, then the even rows, in the same left to right to left way. But since the rows of pixels are very close together, the human eye doesn't notice as easily if a row has gone dim before it is rescanned. Much friendlier to your eyes and stomach.

Light vs. Ink

Colors created by glowing dots are not quite the same as those created by ink on the printer. Screens use the RGB system described above. Inks use the CMYK system using the colors Cyan (a kind of blue), Magenta (a kind of red), Yellow, and blacK. This is why what you see on your screen is not quite the same color when you print.

Physics Lesson:

Color from mixing pigments: Ink and paint make colors by the colors that they reflect. The other colors are absorbed, or subtracted, from the light hitting the object. The primary colors for inks and paints are traditionally said to be red, yellow, and blue. It is more accurate to say magenta, yellow, and cyan. These cannot be created by mixing other colors, but mixing them does produce all other colors.

Color from mixing lights: Lights show the colors that the light source sends out (emits). The colors from different light sources are added together to make the color that you see. A computer screen uses this process. The primary colors for lights are red, green, and blue-violet. Mixed together, they can produce all the other colors.

Color from optical mixing: The illusion of color can be created by tricking the eye. Artists of the Impressionist period created paintings using only dots of color. Newspaper photos are made of dots, also. The human eye blends the colors to "see" shapes and colors that were not actually drawn with lines, just suggested by the dots.

Fun site! - Learn more from the University of Colorado, using interactive Java applets. (Java applets take a while to load. You will be happier if you have a fast computer and a fast Internet connection.)

Making Colored Pictures

CRT screen:

LCD screen

Scan Pattern

Light vs. Ink

Screen Features

Type of Screens