3.3 Screens

3.3.a Traditional Screens

Principle: Screen shows bit maps.

Every pixel consists of three sub-pixels: Red, Green and Blue. Controlling the amount of light from each pixel creates a full range of color.

Cathode Ray Technology
Liquid-Crystal Display
StructureFor detail, see more in
* A1 Physics Practical: Cathode Ray Oscilloscope.
* And A1 Physics Theory: Electric Field.
For detail, see more in A2 Physics: Polarization of light.
How it worksThere are no individual pixels.

Pixels dots are created by controlling the direction of the electron beam fired.

When the beam is directed to a spot on the screen, the charge causes phosphor on screen to radiate. This emits light.
The pixels do not emit light. Light comes from the backlight, from LED tubes.

The alignment of liquid crystal is affected when a voltage is applied. This changes the polarization of light when it pass through.

This affects the final display.

Organic Light-emitting diode (OLED) screens is.a new technology. OLED can act as a light source, so there is no need for a back lighting.

3.3.b Touch Screens

Touch screens are both input and output device. There are two types of them: Capacitive and Resistive.

(The syllabus is much easier than the IGCSE!)

Resistive Touch ScreenCapacitive Touch Screen
Structure* Two layers, separated by thin space.
* Screen is rigid.
* Conductive layer beneath the screen.
* Rigid surface.
How it worksWhen the two layers meet, a voltage divider is produced, (Horizontally and vertically).

When touched, an electrical state (capacitance) change in circuit component happens beneath the surface.

Supports multi-touch.
Next page: 3.4 Printers and Scanners


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