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Jan's Computer Basics:

Output: Screen Features

The device which displays computer output to us has various names:

Screen from "computer screen" or "display screen"
Monitor  from its use as a way to "monitor" the progress of a program
VDT = video display terminal  from early network terminals
CRT = cathode ray tube   from the physical mechanism used for the screen. 
VDU = visual display unit  to cover all the mechanisms from desktop CRTs to LCD flat screens on laptops to LED screen on palmtops

Making Colored Pictures

LCD screen

LCD MonitorLCD (Liquid Crystal Display) screen is very flat and thin. LCD displays are made of two layers of a polarizing material with a liquid crystal solution in between, divided into tiny cells. An electrical signal to a cell makes the crystals line up in a way that keeps light from going through entirely or just partly. When the screen is black, all the crystals lined up so that no light gets through. 

To make color an LCD screen uses 3 colored subcells for each cell: Red, Green, and Blue. This RGB system can create all the other colors by combining how much of each of these colors you see. The signal for a picture cleverly light ups just the right subcells in just the right strengths to show the desired color. Your eye blends the colors in the cells together and you see a picture. 

LCD screens used to be hard to see unless you were directly in front of the screen. Recent developments have fixed this issue.

CRT monitor (HP)CRT screen:

Old Type: A CRT monitor screen uses a 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.

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.

CRT screen with water droplet magnifying the phosphor dots for pink

Photo Credit: Julie Radalj

Here we see a photo of a water droplet acting as a magnifying glass on a CRT screen. Notice that three colors of dots create the overall pink color on the screen due to different strengths of each color and the brain working its magic. (Photo: Julie Radalj)

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.)

          The Big PictureLink to another web site about how the eye sees

          CRT screensLink to another web site 

          LCD screensLink to another web site

Screen Features

Size Desktop screens are usually 15 - 23 in. by diagonal measurement. (This is how TV screens are measured, too.) Larger sizes are available, at a significantly higher cost. Prices are dropping, but you still have to have the space for a bigger screen.
Resolution Determines how clear and detailed the image is.
Pictures on a screen are made up of tiny dots.
1 dot on screen = 1 pixel (from "picture element")
The more pixels per inch, the clearer and more detailed the picture.
One measure of this is the dot pitch, the distance between the dots that make up the picture on the screen. However, different manufacturers measure differently. Most measure from dot center to the center of the nearest same color dot. Some measure from the center of a dot to an imaginary vertical line through the center of the nearest dot of the same color, giving a smaller number for the same dots as the previous method. Some monitors use skinny rectangles instead of dots and so must use a different method altogether. So, dot pitch has become less useful as a measure of monitor quality. A dot pitch of .28 is very common and .26 should be good for nearly all purposes, however it is measured.
Refresh Rate CRT screens: How often the picture is redrawn on the monitor. If the rate is low, the picture will appear to flicker. Flicker is not only annoying but also causes eye strain and nausea. So, a high refresh rate is desirable. 60 times per second is tolerable at low resolutions for most people. 75 times per second or more is better and is necessary for high resolutions. (Does not apply to LCD screens)
Type Old types = CGA, EGA, VGA
Current type = super VGA

Determines what resolutions are available and how many colors can be displayed.
Type Stands for Resolution(s)
CGA Color Graphics Adapter 320 x 200
EGA Extended Graphics Adapter 640 x 350
VGA Video Graphics Adapter 640 x 480
SVGA Super VGA 800 x 600, 1024 x 768, or 1280 x 1024 etc.

New systems now come with super VGA with a picture size of 800 x 600 pixels (as a minimum) and 16 million colors 

Color Color Palette - 16 million colorsThe number of colors displayed can vary from 16 to 256 to 64 thousand to 16.7 million. The more colors, the smoother graphics appear, especially photos.

The number of colors available actually depends more on the video card used and on how much memory is devoted to the display. It takes 8 bits to describe 1 pixel when using 256 colors. It takes 24 bits per pixel when using 16 million colors. So a LOT of memory is needed to get those millions of colors. Video cards now come with extra memory chips on them to help handle the load.  

Reverse video   example: highlights selected text gif
Cursor/ Pointer The symbol showing where you are working on the screen, like:
   text cursor gif and window cursor gif
In the olden days of just DOS, there were few choices for the cursor. The invention of the blinking cursor was a tremendous event. Really! Under Windows there are a huge number of basic to fantasy cursors to choose from.
Scrolling Moving the lines displayed on the screen up or down one line at a time

Type of Screens

Monochrome One color text on single color background, i.e. green characters on black or white letters on blue

monochrome monitor display gif

Monitor with color

Color Various colors can be displayed. (This one is easy to understand!)

Cathode Ray Tube (CRT) CRT monitorFormerly the most common type of monitor, which uses a cathode ray tube. Bulky compared to modern flat screens.
Liquid Crystal Display 

Used in laptops esp.

Large LCD monitors are the most common type now.


Copyright (c) 123RF Stock Photos

Plasma Screens Used for very large screens and some laptops. Flat, good color, but much more expensive.
These have gone out of style.