Maarg Manthan · Topic 14.6

Display Technologies: Cathode Ray, LCD, LED, Plasma, OLED

Science & Technology › IT, Telecommunications and Computers · Topic 14.6

A display is the part of a device that shows information as pictures and text. In a cathode ray tube, CRT, an electron gun fires a beam that strikes a phosphor-coated glass screen and makes it glow, which gave bulky but cheap televisions and monitors. A liquid crystal display, LCD, uses a layer of liquid crystals that twist to let light from a backlight pass through colour filters. An LED display or LED television is usually an LCD lit from behind by light emitting diodes. A plasma display uses tiny cells of gas that glow when charged. An organic light emitting diode display, OLED, has pixels that make their own light, so it does not need a backlight and can show true black, be very thin and be made flexible.

5 min read · UPSC Prelims · Science & Technology · IT, Telecommunications and Computers

At a Glance

CRT Electron beam on phosphor
LCD Liquid crystals plus backlight
LED TV LCD with LED backlight
Plasma Glowing gas cells
OLED Self-emitting organic pixels
Exam link UPSC Prelims, Science & Technology: IT, Telecommunications and Computers

Where Does It Fit?

This topic follows the account of computer components. It describes the output devices that people see every day, and the physics behind each technology. Previous topic: Types and Components of Computer Systems. Next topic: Evolution of Telecommunication and Mobile Generations (3G, 4G, VoLTE, 5G).

How Does Each Display Technology Work?

Technology	Working and features
Cathode ray tube	An electron gun in a vacuum tube fires beams, steered by magnetic coils, onto a screen coated with red, green and blue phosphors. Deep and heavy, uses much power, but gives good colour and viewing angles
Liquid crystal display	Liquid crystals change orientation under voltage and block or pass polarised light from a backlight. Thin and low power but the black is not perfectly dark
LED backlit LCD	Marketed as LED TV. The backlight uses light emitting diodes instead of fluorescent lamps, giving a thinner, brighter and more efficient screen
Plasma display	Each pixel is a small cell containing a noble gas that becomes plasma when a voltage is applied and emits ultraviolet light that excites phosphors. Good for large screens, but heavy, power hungry and now phased out
OLED	Thin layers of organic material emit light when current flows. Each pixel lights independently, so black pixels are fully off. High contrast, wide viewing angle, flexible
QLED and Mini-LED	LCD with quantum dot films for purer colour, and with thousands of small LEDs for better local dimming
MicroLED	Tiny inorganic LEDs as pixels, offering brightness and long life but difficult to make

Which Terms and Facts Matter?

Pixel: the smallest element of a picture, with red, green and blue subpixels
Resolution: the number of pixels, for example HD 1280 by 720, Full HD 1920 by 1080, 4K 3840 by 2160
Pixel density: pixels per inch, which decides sharpness
Refresh rate: how many times per second the image is redrawn, in hertz
Contrast ratio: ratio of brightest white to darkest black, highest in OLED
Response time: how fast a pixel changes colour, important for gaming
Touch screens: capacitive screens sense the electric charge of a finger, resistive screens sense pressure
E-ink: electronic paper display used in e-readers, which uses power only when the image changes

What Else Should You Remember?

Blue LED: Isamu Akasaki, Hiroshi Amano and Shuji Nakamura won the 2014 Nobel Prize in Physics for inventing efficient blue light emitting diodes, which made white LED lighting and bright displays possible
Liquid crystal discovery: Friedrich Reinitzer found liquid crystals in 1888
LCD in calculators: early LCDs appeared in watches and calculators in the 1970s
Energy efficiency: LEDs use much less electricity than older bulbs, and India’s UJALA scheme, launched in 2015, distributed LED bulbs
Organic electronics: OLED research also led to flexible and foldable phones
Display industry in India: India has begun to build display and component manufacturing under the electronics component schemes. Check the latest status
Eye comfort: blue light and flicker are debated, and many devices give night modes
E-waste: old CRTs contain lead, so disposal is regulated under the E-Waste Management Rules
Projectors and AR: micro displays are used in projectors and in augmented and virtual reality headsets, as in Biometrics and Virtual Reality
Smartphone screens: most phones use LCD or OLED, as in Smartphone Technology and Related Concepts
Brightness: measured in nits, and HDR content needs high brightness
Caution: terms such as LED TV mean an LCD panel with LED backlight, not a screen made of LEDs

Why Does It Matter for the Exam?

Principles: phosphor, liquid crystal, plasma, organic emission
Differences: LCD versus OLED
Terms: pixel, resolution, refresh rate
Current affairs link: display manufacturing and foldable screens
Common errors: calling LED TV an emissive LED display or saying plasma uses liquid crystals

As an analytical point, display choice affects power use, picture quality and thickness. The shift from CRT to LED and OLED saved energy, and India’s electronics policy now looks to build display parts locally.

Why Is This Topic Important?

Principles Each type works differently
Terms Resolution
Nobel Blue LED

What Came Next?

The next topic covers the evolution of telecommunication and mobile generations. Next topic: Evolution of Telecommunication and Mobile Generations (3G, 4G, VoLTE, 5G).

Key People and Terms

Phosphor Material that glows when hit by electrons
Backlight Light behind an LCD
OLED Organic light emitting diode
Pixel Picture element
Refresh rate Image updates per second
Contrast Bright to dark ratio

Exam Corner

Points to Remember

LED TV = LCD with LED backlight.
OLED pixels emit their own light.
Full HD: 1920 x 1080.
Blue LED Nobel: 2014.
Liquid crystals found: 1888.

Do Not Confuse With

LED and OLED Backlight diodes versus self-emitting organic pixels
Plasma and LCD Gas cells versus liquid crystal shutters
CRT and LCD Electron beam versus light valve

Memory Hook

Tube shoots, crystal twists, gas glows, organic lights itself; LED TV is just an LCD with new lamps.

Mains Angle

Questions ask for principles and differences. These points help.

Evolution CRT to OLED
Benefits Energy and quality
Industry Local production

A Question You May Face

An original practice question, not a past paper question.

“Compare the working of CRT, LCD, plasma and OLED displays and discuss the energy and industrial implications of the shift to LED technology.”

How to Answer

Principles Each type.
Compare Contrast, power.
Energy LED saving.
Industry Local display.

GS Relevance

Prelims: working principle and difference of each display, resolution, blue LED Nobel. GS Paper 3: science and technology, electronics.

Frequently Asked Questions

What is the difference between LED and OLED displays?

An LED television is an LCD panel lit from behind by light emitting diodes, so the pixels only filter the light. An OLED display has organic pixels that each produce their own light, so a black pixel can be completely off. This gives OLED better contrast, thinner panels and flexible designs, but it costs more.

How does a liquid crystal display work?

A liquid crystal display has a layer of liquid crystal molecules between polarising filters, lit from behind by a backlight. When a voltage is applied to a pixel, the crystals twist and change how much polarised light passes through coloured filters, producing the colours seen on the screen.

What was a cathode ray tube?

A cathode ray tube is a vacuum tube in which an electron gun fires a beam of electrons that is steered by magnetic coils to scan across a phosphor coated screen, making it glow. CRTs were used in old televisions and computer monitors, but they have been replaced by flat panels because they are heavy and use more power.

Why was the blue LED important?

Red and green LEDs existed in the 1960s, but efficient blue LEDs were created in the early 1990s by Isamu Akasaki, Hiroshi Amano and Shuji Nakamura, who won the 2014 Nobel Prize in Physics. Blue light allowed bright white LED lamps and full colour displays, which greatly cut energy use in lighting.

What does 4K resolution mean?

4K resolution, also called Ultra HD, means a screen with 3840 by 2160 pixels, which is about four times the pixel count of Full HD at 1920 by 1080. More pixels give finer detail on large screens, though the benefit depends on screen size, viewing distance and the quality of the content.

PYQ Practice — Statement Analysis

1 An LED television is generally an LCD panel with an LED backlight.

True

Not self-emitting.

2 OLED displays require a backlight to produce light.

False

Their pixels emit light.

3 Plasma displays use liquid crystals.