Module 5: Digital Techniques and Electronic Instrument Systems

5.11 Electronic Displays

CRT Cathode Ray Tube

Monochrome: A heated cathode emits

electron beams, accelerated by magnetic fields and collide in a phosphor coating screen. The collision emits light.

Color: Three cathodes emit one

electron beam for each color (RGB). Each pixel of the screen contains 1 phosphor dot for each color. Each beam collides only with the corresponding phosphor dot of each pixel.

The CRT contains: A heated cathode Accelerating magnetic field A phosphor coating

Monochrome CRT

The input of each plate is a combination of the input signal and a sawtooth wave.

The refresh rate is around 70Hz.

Color CRT By controlling the

intensity of each beam, various colors can be obtained.

LCD Liquid Crystal Displays.

The polarization of light: Liquid crystals are organic

compounds that exhibit both solid and liquid properties.

A property of liquid crystals is that they can be reoriented by an electric field.

2 polarization filters are placed with 90 degrees angle with each other.

The polarized light that exits the first polarization filter is reoriented by the liquid crystals by 90 degrees and passes through the second polarization filter.

If an electric filed is applied to the liquid crystals, the light is not reoriented and does not pass the second filter.

Color LCD Each pixel is

divided to three sub-pixels: RGB.

A color filter is placed in front of each sub-pixel.

TFT LCD Screens Thin-film-

transistor LED technology (1 transistor switching each sub-pixel) enables higher resolutions and fast response time.

(O)LED displays (Organic) LED

displays use the LCD technology, but the backlight provided by a LED panel.

Each LED emits light when supplied with 1.7 to 3V.

CRTs vs. LCDs.

CRTs: Low resolutions Generate magnetic fields Heavy, require depth behind the

front panel Heating problems Excellent viewing angle Response time: 1msec.

LCDs: Higher resolutions and image quality Power efficient Thinner, lighter than CRTs Poor viewing angle, but better in

newer models and excellent in OLEDs.

Response time: 1-8msec for LED and sub-ms for OLED. Sluggish in low temperatures.