Display Screens
RGB
How the eye sees colour
We can see things because our eyes can sense light beams that are reflected off objects.
White light is made up of a range of different colour hues mixed together. This can be seen by shining white light at an angle through a prism: each hue is refracted at a different angle, resulting in a colour spectrum.
Humans can normally see a range of hues from red through to violet.
This is because the eye has three colour receptors that are roughly tuned to red, green and blue hues. The ranges of each receptor overlap.
Our brain interprets the mixture of sensations received from the three receptors as a single colour:
When we see a red hue, only the red receptor tingles.
When we see a green hue, only the green receptor tingles.
When we see a yellow hue, both red and green receptors tingle.
When we see white light, all three receptors tingles.
If there is no light (i.e. black), no receptors tingle.
Tricking the eye
If we shine red and green light into the eye at the same time, the red and green receptors will tingle and the brain will THINK it is seeing yellow light.
By mixing red, green and blue light in varying proportions we can trick the brain into thinking it sees other colours. This is known as additive colour.
How computer displays show colour
Colour televisions and computer display screens mix red, green and blue light to produce the illusion of colour. Therefore they are known as RGB displays.
Small red, green and blue dots are placed in close proximity, usually in a line or forming a triangle; from a distance the brain cannot resolve the individual dots — it perceives the overall colour instead.
The size of the RGB cluster affects the quality of the image. One computer screens this is measured with dot pitch, the distance between clusters.
CRT
The Cathode Ray Tube (CRT) is the same technology used in oscilloscopes and traditional TV sets.
It consists of a vacuum tube enclosed in glass.
CAUTION: If a vacuum tube is smashed it can implode, sending glass fragments in all directions. CRT monitors should therefore be handled with care.
One end of the tube contains an electron gun assembly. When heated, the electron gun emits a stream of high-speed electrons — negatively charged particles — that are attracted to the other end of the tube by a positively-charged anode. This anode is typically charged to voltage of 25,000 V. Such voltages are referred to as Extra-High Tension (EHT).
WARNING: The high voltage levels in a CRT monitor (typically 25 kV) can be fatal; under this level of EHT pressure current can jump an air gap of approx. 1cm (depending on humidity). Large residual charges can be present inside the housing even up to a year after electrical power has been removed.
Along the way, a focus control and deflection coil steer the beam to a specific point on the screen. When struck by the beam, the phosphor glows.
The electron gun assembly contains three guns that illuminate red, green and blue phosphors respectively. This allows the RGB components of the picture to be individually tweaked.
A metal plate called a shadow mask is used to align the electron beams from the three guns. It has slots or holes that divide the phosphors into groups of three (one of each colour).
The shadow mask can become magnetised. This will result in distorted colours. To clear this magnetism a degaussing coil is wrapped around the screen. It is triggered when the CRT is switched on but it can also be triggered manually on most computer CRT monitors.
CAUTION: If magnets are placed near the screen they can bend or permanently magnetise the shadow mask.
The beam moves very quickly, sweeping the screen left-to-right in lines, moving from top-to-bottom in a pattern called a raster. The horizontal scan rate is the frequency at which the electron beam moves laterally across the screen.
During its sweep the beam intensity varies, producing different levels of brightness to draw the picture. Because the glow begins to fade almost immediately, the beam must refresh the screen many times per second.
Most current CRT displays have an ideal refresh rate (also called the vertical scan rate) of about 85 Hz. Refresh rates that are too low cause the screen to flicker, contributing to eyestrain. A higher refresh rate is better for your eyes but can also force the graphics card to work harder.
It is important that the refresh rates expected by your monitor match those produced by your video card. If you have mismatched rates, you will not see an image and can actually damage your monitor.
CAUTION: Setting a monitor to scan at too high a frequency can cause damage to its circuitry.
Adjustment
Older CRT device have analogue potentiometers ("pots") controls that can be used to adjust picture scanning. Modern digital monitors synthesize the scanning waveforms and can be controlled by buttons. These are commonly found on the front of computer monitors.
Flat screens
LCD & TFT technology
Liquid crystal is roughly transparent. If an electrical current is applied to the liquid crystal it twists light waves passing through.
By placing polarising filters (which remove horizontal or vertical components of the light wave) in front of and behind the liquid crystal, we can selectively pass or block light by applying a current. This is the principle upon which Liquid Crystal Displays (LCDs) work.
In order for LCDs to function as computer displays they usually need to be backlit.
Thin-Film Transistor (TFT) displays are a variant of LCD display that uses an active matrix. Traditional passive matrix displays scan across the pixels in a cycle, sending current to each in turn. An active matrix means that each pixel has its own micro-power supply controlled by a transistor.
TFT panels can suffer from defective transistors, meaning that individual pixels will fail to work correctly. This is expensive to correct so manufacturers usually give a warranty describing a specific ratio of defective pixels that will be considered grounds for replacement.
One of the key factors when choosing an LCD screen is that of response time, a measurement of the rate at which pixels can change colour. A slow response time will result in ghosting on rapidly-changing images.
KVM (keyboard, video & mouse) switches and video splitters can interfere with the video signal to a display screen, creating horizontal ghosting. On a CRT this is not always noticeable but the sharpness of LCD screens can make this readily apparent.
Plasma technology
Plasma displays use excited phosphors to create light. They can be made bigger than LCD screens and have a wider viewing angle. They also have faster response times.
However they are more fragile and have historically had problems with image burn-in. Large display screens can get very hot.
General information
Resolution
Resolution is the amount of detail a monitor can render, expressed as the number of horizontal and vertical pixels. The following list shows standard resolutions used in PC video adapters and the terms sometimes used to describe them:
| Resolution | Standard Designation | Recommended monitor size |
|---|---|---|
| 640 × 480 | VGA (Video Graphics Array) | 14" |
| 800 × 600 | SVGA (Super VGA) | 15" |
| 1024 × 768 | XGA (eXtended Graphics Array) | 17" |
| 1280 × 1024 | 19" or 21" | |
| 1600 × 1200 | 19" or 21" |
Connections
Standard VGA connector pins are illustrated below:
| pins | function | signal direction |
|---|---|---|
| 1 | Red video | out |
| 2 | Green video | out |
| 3 | Blue video | out |
| 4 | Not connected | |
| 5 | TTL ground | |
| 6 | Red ground | |
| 7 | Green ground | |
| 8 | Blue ground | |
| 9 | Key (plugged hole) | |
| 10 | Sync ground | |
| 11 | Not connected | |
| 12 | I²C data bus | in |
| 13 | Horizontal sync | out |
| 14 | Vertical sync | out |
| 15 | I²C data bus | in |