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Computer Monitors - CRT, LCD, LED, Plasma & OLED Display Monitors

Written By: 

Ashok Sharma

 

We all are familiar with the computer monitors. We spend time sitting in front of them for hours working, gaming or watching movies. A monitor is used to display the output of any computer system. A good display makes all the difference and no doubt enhances the user experience. The innovation in the display technologies has improved the quality of the display devices including monitors. Now the desktop computers are available with a variety of displays ranging from technologically obsolete CRT monitors to latest slim LCD, LED or OLED monitors.
 

Fig. 1: A Representational Image of Latest Slim LCD Monitor

A computer monitor, technically termed as visual display unit is an output device that presents the information from the CPU on the screen working as an interface between CPU and the user. A cable connects the monitor to a video adaptor or video card which is set up on the motherboard of the computer. The CPU (Central Processing Unit) sends instruction to the video adaptor telling what needs to be displayed on the screen. The video adaptor converts the instructions into a set of corresponding signals and sends to the monitor. Monitor contains a circuitry that generates the picture on the screen from the set of signals.
 
The major parameters that measure the performance of a monitor are luminance, contrast ratio, resolution, dot pitch, response time, refresh rate and power consumption. The common problem that arises in monitors is dead pixels, blurred screen, phosphor-burn, etc.
 
History
The first monitor dates long back in history. In the early stages of its evolution they were known as Terminals, An Image of First Monochrome Monitor known as Video Display Terminals

Fig. 2: An Image of First Monochrome Monitor known as Video Display Terminals

which were the boxy Video Display Terminals (VDTs). VDTs were monochrome monitors which used CRT (Cathode Ray Tube) technology. They were capable of working with any type of computer by connecting through a serial interface.
 
IBM’s CRT - IBM launched its first computer also known as a ‘three piece computer’ in 1981. It had three different units – CPU, monitor and keyboard separately. By 1984, IBM introduced the new CRT monitor with enhanced Color Graphics Adaptor (CGA) with 16 colors and a resolution of 640 x 350 pixels. In 1987 IBM started offering the Video Graphics Array as part of its new PCs which allowed 256 different colors and a resolution of 640 x 480 pixels.
 
XGA and UXGA – A new technology named Enhanced Graphics Array or XGA was introduced in 1990 which allowed 16.8 million colors with a resolution of 800 x 600 pixels. The new monitors were now offering true colors that matched the human eye (human eye can detect 10 million different colors). Later the technology extended as UXGA, Ultra Extended Graphics Array which allowed 1600 x 1200 pixels.
 
In the 90s the LCD monitors came in the scene and gradually started competing with the CRT monitors. By the end of the 20th century, the CRT era was declining with the increasing popularity of Liquid Crystal Technology (LCD). This technology produces sharper images than the CRT monitors and the LCD monitors are significantly thinner having lower radiation emissions.
 
Few years’ back, LED displays came in the scene and they are gradually making its space in the market. LED technology has various advantages over LCD technology like better image quality, low power consumption, etc.
 
Display Technologies
Since the beginning of computer era, there have been a number of technologies used for the display of output. The major technologies are CRT, LCD, Plasma, LED and OLED displays.
 
1.      Cathode Ray Tube (CRT) Monitors
These monitors employ the CRT technology to create a display. The CRT (also known as picture tube) receives the A Diagram Illustrating CRT Technology Used in Monitors to Create Display

Fig. 3: A Diagram Illustrating CRT Technology Used in Monitors to Create Display

signals through a cable and the signal is decoded by the display controller which finally appears on a phosphor screen. The detailed working is as following:
 
As shown in the image CRTs have a conical shape and there is an electron gun or cathode ray gun at the back end of the monitor and a phosphor screen in the front. The electron gun fires a stream of electrons towards the display screen through a vacuum tube. This stream of electrons is also known as cathode rays. At the middle of the monitor, there are magnetic anodes which are magnetized in accordance with the instruction from the display controller. When electrons (cathode rays) pass through the magnetic anodes, they are pushed or pulled in one direction or other depending on the magnetic field on the anodes. This directs the electrons towards the correct part of phosphor coating inside the display glass. When electrons strikes the phosphor coated screen passing through a mesh (shadow mask or aperture grill), the phosphor lights up making a displayable dot on the computer screen. There are three different colored phosphors (Red, Green and Blue) for each pixel and the color of the pixel depends on the phosphor on which the electrons strike.

This image shows the color combination schematic for phosphor particles. The monitor that has a single electron gun An Image Showing Color Combination Schematic For Phosphor Particles

Fig. 4: An Image Showing Color Combination Schematic For Phosphor Particles

has three different phosphors for each pixel. A cathode ray strikes to one or more of these phosphors and the corresponding colored pixel appear on the screen. However high quality monitors use individual electron gun for each color which improves the image quality. Distance for two same colored phosphors (for single electron gun monitors) is known as dot pitch. Lesser the dot pitch higher is the quality of monitors.
 
Aperture Grille v/s Shadow Mask
CRTs incorporate a metal sheet behind the display screen which affects the pixels on the screen as well as An Image Showing Shadow Mask and Aperture Grill Incorporated in Cathode Ray Tube Monitors

Fig. 5: An Image Showing Shadow Mask and Aperture Grill Incorporated in Cathode Ray Tube Monitors

brightness on the screen. Shadow mask is an obsolete technology in which there is a metal sheet with millions of holes to pass electrons in order to hit the phosphor coating. The shadow mask covers the entire screen thereby protecting the phosphors from stray ions (due to vacuum) and also limits the strength of the rays reducing the brightness on the monitor.
Aperture grille is a mesh of wires rather than any metal sheet with holes in it. Although the grill is fragile, it allows a brighter display.
 
What is the resolution of the screen?Resolution of a monitor tells how densely pixels are arranged on the screen. A combination of dot pitch and the viewable image area defines the maximum resolution of the screen. For example if a 21 inch monitor screen with a viewable area of 401mm x 298mm has a dot pitch of 0.26 mm, then its resolution is 1843 x 1370 pixels derived from a formula.
 

LCD Monitor

LCD Monitors
LCD, Liquid Crystal Display or also known as Liquid Crystal Diode is one of the most popular display technologies A Representational Image of LCD or Liquid Crystal Diode Monitor

Fig. 6: A Representational Image of LCD or Liquid Crystal Diode Monitor

 
currently. LCD monitors are lightweight, compact, occupy less space, consume low power and are available in a reasonable price. Currently there are two types of LCD technology in use – Active matrix LCD technology or TFT and Passive matrix technology. The TFT technology is more reliable with better image quality while the passive matrix technology has a slower response and gradually becoming outdated.
 
As the name indicates, liquid crystals are the key elements of the display screen. By manipulating the crystal we can change the way they interacts with the light. There is a display controller in the monitor which receives the display signals from the video adaptor in the motherboard. The display controller controls two things – the electric signals to the liquid crystals and the back light. Structure of an LCD is shown in the below images (Also see how LCD works).
A Diagram Showing Structure and Working of a LCD
 

Fig. 7: A Diagram Showing Structure and Working of a LCD

The liquid crystals used in the LCD are Twisted Nemantic (TN), a type of liquid crystals that are twisted at 90o with the surface. In this state, crystals allow the light to pass through the polarizer but on applying a voltage, they get untwisted and block the light to passing through the polarizer. The display controller starts the backlight that passes through the first piece of the glass. At the same time the display controller also send the electrical currents to the liquid crystal molecules to align and allowing the varying level of light to pass through the second piece of glass, forming the desired picture on the screen. In color monitors, each pixel is made of three liquid crystal cells fronted with red, green and blue filters. The light passing through the filtered screen forms the color what you see on the monitor. A wide range of colors are formed by varying the intensity of colored pixels.
 
The backlight is made of cathodes, and depending on the quality of the monitor, there may be a single cathode at the top or one at the top and one at the bottom, or two at the top and two at the bottom to improve the brightness and clarity of the monitor. These cathodes are diffused through a layer of plastic and diffusing materials.
 
Resolution - Unlike the CRT monitors there is no complex equation for the dot pitch and the resolution. The resolution of a monitor is simply the number of pixels contained in the matrix. Typically a 17 inch monitor has a resolution of 1280 x 1024 pixels.
 
In the below video Bill Hammack explains how a TFT monitor works, how it uses liquid crystals, thin film transistors and polarizers to display information.
 
 

LED Monitors

LED Monitors
In the previous decade, the display technology has changed significantly. LED displays are one of the latest developments.A Diagram Showing Working of the Plasma Technology

Fig. 8 : A Representational Image of a LED Monitor

In this field. LED monitors use light emitting diodes that acts as a performance booster in the monitors. Basically LED monitors are the LCD monitors with a LED backlight to power up the LCD panel. It means that LEDs are placed behind or around the LCD panel to enhance the luminosity and video definition of the monitor screen.
As we have seen in the above section of LCD monitors, they use a cold cathode light as backlight. In the LED monitors all the concepts are same except this backlight, which is replaced by LEDs.
There are three different types of LED monitors available based on the manner how the diodes are arranges in the monitor. These are – Direct LEDs, Edge LEDs and RGB LEDs. Both Edge and Direct LED display monitors use white diodes that are used to illuminate the LCD panel to produce the improved picture quality. The arrangement of LEDs in the monitor is shown in the below image:

An Image Showing Comparison of Direct LED and Edge LED Technology

Fig. 9: An Image Showing Comparison of Direct LED and Edge LED Technology


In the Direct LEDs display, white diodes are placed all over the panel to produce higher quality image while the Edge LEDs display uses LEDs only on the borders of the LCD panel. Direct LEDs are generally used in the production of high definition TV whereas the Edge LEDs is mainly used in the production of computer screens. RGB LEDs display is better among the three types of LED monitors as it uses red, green and blue diodes to produce the lifelike images with amazing contrast ratio.

 
LCD v/s LED Monitors / Why LED displays are better than the LCD displays:
Both types of monitors work on the same technology. LED monitors are LCD monitors with replaced cold cathode backlight to LED backlight. Here are the differences that make the LED displays better than the LCDs
 Contrast and Black level of the LED screen is better than the LCD screens because the liquid crystals cannot stop 100% of the backlight from cold cathode backlight and hence when the black screen is to be shown on the monitor, it is not completely black (as shown in the below image). But Edge LED screens perfectly show the black screen as there is no backlight at all. 
 
An Image Showing Comparison of LCD and LED Monitor Contrast and Picture Quality

Fig. 10: An Image Showing Comparison of LCD and LED Monitor Contrast and Picture Quality

 
·         Color accuracy for direct and edge LED displays and LCD displays are almost same but the RGB LEDs display has quite better color accuracy.
·         When comparing the LED and LCD monitors with respect to viewing angle, they are same as backlight has nothing to do with viewing angles.
·         LED displays consume less power. It is reported that they consume up to 40% less power than the LCD displays.
·         LED displays do not use mercury (used in cathode lamps in LCD backlight) so they are environment friendly.
·         The size of Edge and RGB monitors is slight thinner than the LCD monitors while prices are slight higher.
 

Plasma Monitors

Plasma Monitors
Plasma technology is another technology used in display devices. The basic idea behind the plasma technology is to A Representational Image of Plasma Monitors Based on Plasma Technology
 

Fig. 11: A Representational Image of Plasma Monitors Based on Plasma Technology

 
illuminate tiny colored fluorescent lights to create image pixels. Each pixel is made of three such fluorescent lights – red, green and blue lights. To create a wide range of colors, intensity of these lights is varied accordingly.
 
The heart of plasma displays is plasma which is basically a gas (generally Xenon and Neon) made up of free flowing electrons and ions. When the electrical current flows through the plasma, negatively charged particles move towards the positively charged area of the plasma and vice versa. This makes collisions which resultantly excite the gas atoms in the plasma and then release the energy as photons of light. A Diagram Showing Working of the Plasma Technology
 

Fig. 12: A Diagram Showing Working of the Plasma Technology

 
There are millions of tiny cells filled with the gas like xenon and neon. They are positioned between two plates of glass known as front plate glass and rear plate glass. Two transparent electrodes covered by an insulating dielectric material and a magnesium oxide protective layer are also sandwiched between the glass plates on both sides of the cells on the entire screen.
 
When the CPU sends the signals to the Plasma monitor, the corresponding electrodes are charged which ionizes the gas in the intersecting cells by passing an electric current. Due to the collisions between the gas ions they release energy in the form of the photons of light which illuminate the respective cells. This process occurs thousands of times in a small fraction of second making the display faster. The released ultraviolet photons strike the phosphor material coated on the inner wall of the cell and hence phosphor electrons jump to the higher energy level. When the electron falls back to its normal state, it releases the energy as a visible light photon. Every pixel on the screen is made of three different colored phosphors – red, green and blue.

OLED Monitors

OLED Monitors
OLED, short for Organic Light Emitting Diode is the latest technology for display devices. As the name suggests there  A Representational Image of a LED Monitor
 

Fig. 13: A Representational Image of OLED or Organic Light Emitting Diode Monitors

 
are some organic material (containing carbon, like wood, plastic or polymers.) that is used to convert the electric current into light. Since the LEDs are capable of producing different colored light, they are directly used to produce the correct color and there is no need of a backlight which saves power and space. With fast response time, wide viewing angles, outstanding contrast levels and perfect brightness, OLED displays are surely better than the existing other display technologies. 

A Diagram Illustrating Structure and Working of a OLED Display

 

Fig. 14: A Diagram Illustrating Structure and Working of a OLED Display

The heart of the OLED display is a stack of thin organic layers which is sandwiched between two conductors - a transparent anode and a metallic cathode, which in turn are sandwiched between two glass plates known as seal and substrate. The organic layer consists of a hole-injection layer, a hole-transport layer, an emissive layer and an electron-transport layer. When an appropriate voltage is applied, an electric current flows from cathode to anode through the organic layers. The cathode give electrons to the emissive layer of organic molecules while the anode takes equivalent electrons from the conducting layer of organic molecules. At the boundary of emissive and conductive layers, electrons and the holes are gathered. Here electrons are recombined with the holes by releasing energy in the form of photon of light. Hence the organic layer emits the light to produce the display. The color of the light depends on the type of organic molecules while the brightness depends on the amount of the current applied. By maximizing the recombination process in the emissive layer the output light can be improved in OLED devices. Thus the emissive layer is slightly doped with highly fluorescent molecules to enhance the electro-luminescent efficiency and control of color.
 
Advantage over conventional display technologies:
·         The fabrication process is easy and the display devices are thinner than the conventional display devices.An Image Showing Emissive Nature of OLED Monitors
 

Fig. 15: An Image Showing Emissive Nature of OLED Monitors

 

·        Comparing it with the LCD devices, OLED displays can be viewed from different angles as they are “emissive” devices i.e. they emit light rather than modulating transmitted or reflected light.
·         They do not use backlight.
·         The driving voltage and total power consumption is low as comparing to other display technologies.
·         The material used are eco friendly and do not use lead or other such material.

Comments

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