From pre prehistoric time man had a need of flat surface to display his imaginations and thoughts, cavemen used their own walls for the purpose. Still their descendants have this tendency to write on walls in their early childhood.
Along with humans, their screens and displays also evolved from stone to silicon. Second half of 20Th century and early 21st century witnessed increased rate in this evolution from CRT monitors to VRD (Virtual Retinal Display; in which human eye retina work as a projector screen).
Basic technology and mechanism of different electronic screens & displays is explained here. Will try to go chronologically so the first pick would be CRT monitors.
Some Important Concepts To Understand
Atoms and Molecules: smallest fundamental unit of any material or matter.
Electron: Sub-atomic particle revolving around the atom or molecule. Having negative charge and negligible mass compared to protons.
Electron orbit: Electrons in an atom are at different positions revolving around it. When it absorb or emit energy, position is changed accordingly. These positions which depict their energy level is known as electron orbit and the energy electron posses is called orbital energy.
Electromagnetic radiation: A form of energy which is generally released when electrons jump from higher orbit (energy level) to lower orbit (energy level). This radiation exhibits characteristics of wave and particle both.
Light: Type of electromagnetic radiation which is detectable with our eyes.
Colors of light: When electron jumps from higher to lower energy orbit, the difference between two energy decides the wavelength of the radiation(light). violet, indigo, blue, green, yellow, orange and red; this is the sequence of wavelengths of light from shorter to longer. Light of shortest wavelength is of violet color and longest wavelength is red. Shorter the wavelength of radiation higher the energy, Shorter than violet and longer than red are not visible to human eyes although they exist and are detectable by other artificial and natural devices.
X-ray: wavelengths shorter than violet are ultraviolet and even shorter waves fall in x-ray category. Shorter the wavelength more energy they carry.
Phosphor: A substance, when x-ray radiation or high speed electron falls on it, visible light is emitted back from the substance.
Pixel: A smallest unit on any electronic screen which can produce light of a particular color. The color of this unit is controlled by electronic signal. and all surrounding pixels can independently produce all possible colors depending on the signal they receive individually. smaller the unit or more units per inch makes picture more sharp and crisp.
Sub-pixel: Sub-pixel is one of three segments in a pixel. Generally a pixel consist of three sub-pixels, each one is responsible for a particular color of light. Red, blue & green are the colors produced by three sub-pixels respectively. when red color is needed in a pixel only red sub-pixel gets a signal while blue and green remain off. when yellow color is needed in a pixel green and red both sub-pixel will get signal and blue will remain off. in above description of colors you would notice that white is not mentioned. In-fact white is perception of our eyes for presence of all colors in the radiation. So when a pixel needs to emit white light all sub-pixel will be illuminated and eyes will perceive it as white color, it’s an illusion!. In the same way thousands of colors are possible with combination of these three colors. Intensity of light in one sub-pixel play important role in overall color of pixel, so intensity of light is achieved by intensity of signal in that particular sub-pixel.
Polarization: Light exhibit characteristics of transverse waves, that mean its energy oscillates "up & down" while travelling ahead, in other words 'oscillation of energy field is perpendicular to direction of radiation'. Just to understand, direction of light & oscillation of energy could be compared with arrow & its fletchings. Fletchings on the arrow will always remain perpendicular to the length of arrow but at the same time orientation of fletching could be rotated to any degree in the air.
In the same manner oscillation of energy in the light is randomly positioned at any angle around the ray but perpendicular to the radiation.
Polarization is a process in which only particularly positioned or oriented rays of light are filtered and rest are discarded. This filtration needs a physical filter which is called polarization filter. It is like to insert coin in piggy bank, coin will only be able to enter the box if the orientation of coin is aligned with the hole made on the box.
Pixels Visible on the Typical TV Screen
How Polarizing Filters Work?
CRT monitors are available in 'black & white' and 'colored' both categories. Mechanism of 'Black & white' monitor is simple as compared to 'colored'. complete inner screen of black & white CRT is covered or coated with perfectly mixed powder of 2-3 types of phosphors. When electrons from the gun hit a point on it all types of phosphors emit different lights and Their combination gives the illusion of white color.
CRT stands for ‘Cathode Ray Tube’. It is actually a bulky closed and vacuum glass container of pyramid shape. Quadrilateral base of this pyramid is used as screen. Reverse side of this screen is an extrusion, mounted with a gun within it. The gun shoots out electrons towards the inner flat surface of the tube. The flat surface is covered with phosphor, so when the electrons from the gun hit this surface with great speed the phosphor compound emits photons or light. The intensity of light directly depends on the intensity of electron’s speed which varies according to the electrical signal. The gun shoots electrons continuously, simultaneously this stream of electrons sweeps through the screen horizontally from one side to other, row by row. The screen has around 450 to 1000 horizontal imaginary lines on which electron gun move row by row. Depending on the image to portray, each point on every horizontal line is bombarded with different intensity to create different shades of gray and white. Scooting of electrons all over the screen in fraction of seconds create an illusion of image on the screen. Images are reproduced 30 times in each second which generate the illusion of moving image.
Contrary to black & white monitor, color monitor have three photon guns one for each primary color. Pixels on the screen are made-up of sub-pixels, each sub pixel is coated with different phosphor which emit different primary colors when bombarded with electrons, blue, green and red respectively. when all 3 phosphors(sub pixels) are bombarded with same intensity then the pixel seems to be white to our eyes. Different colors are generated in a pixel by differentiating 'electron bombardment intensity' of one or more electron guns.
In this mechanism it is very essential that the gun for red color always hits the red sub-pixels, in the same way two other guns are supposed to work. To ensure that the gun doesn't hit wrong sub-pixel a metal grid with tiny holes is placed in between gun and screen. This grid shadows the electrons from hitting wrong sub-pixels therefore it is called shadow-mask.
Illusion of color and picture in TV
For better understanding of LCD screen it is necessary to understand its basic units i.e. PICture ELements 'PIXEL'. Around 300,000 tiny pixels are arranged in typical 14” LCD screen(640 x 480).
LCD stands for Liquid Crystal Display; the flat screen of LCD has actually several layers of different materials behind it, a key component of the whole sandwich is the liquid with special molecules or crystals. Two distinct features of this liquid crystal are:
- Crystals have capability to change the orientation of polarized light entering into it.
- Configuration of crystals could be controlled by electrical or magnetic fields. that means first property of the molecule could be controlled by its second property!
- Manipulating crystal configuration with electrical signal in each sub-pixel individually is basic of this technology.
Generally when two polarizing filters are placed back to back such that; one polarizes ‘vertical rays’ and other ‘horizontal rays’ then final result is a black-out. That isn’t the case here, because in between two polarizing filters resides the special liquid. This liquid crystals are squeezed between two transparent electrodes and they in-turn are crammed between two polarizing filters (one vertical and other horizontal).
Path Of White Light In A Sub-Pixel From Source to Viewer
White Light Source►Vertical Polarizing Filter►Transparent Electrode►Liquid Crystals►Transparent Electrode►Horizontal Polarizing Filter►Color Filter►Transparent Medium towards the viewer.
- White CCFL light is placed behind the screen.
- White back-light pass through first polarizing filter which allows only vertical rays through it.
- Polarized white-light pass through transparent electrode.
- If electrode is ON in the sub-pixel then it will deform the liquid crystal placed between electrodes. This will fail to change the orientation of the polarized light by 90° and light will be blocked later-on by the second filter.
- If electrode is OFF in the sub-pixel then the liquid crystal placed between electrodes will change the orientation of the polarized light by 90° and light will be able to pass through the second filter.
- After passing second polarizing filter color filter will allow only particular color rays to pass (red,blue or green). Eventually this technology can control each and every sub-pixel (red,blue and green) at any given time independently.
So illumination of each and every sub-pixel depends on the signal of electrodes present in it. Signal controls the orientation of liquid crystals and crystals control orientation of polarized light. Orientation of light will eventually decide whether it will be able to pass 'second horizontal polarization filter' or not. Different amount of light intensity of sub-pixels results in final color of the containing pixel. It’s like mixing primary colors to produce required shade.
You may clarify your concepts of; 'how signals are transmitted to subpixels in modern screens', by visiting this link.
Working Diagram of LCD Sub-pixel
LCD & LED display doesn't have much difference in the technology. The pixels of both screen can't produce light by themselves, they need backlight to get pixels illuminated.
- LCD have CCFL lamps in the backlight which are very bulky and its brightness can't be controlled for different areas of the screen.
- LED screen have LED lamp series in the backlight, LEDs are thin and lightweight, even individual lamps could be controlled independently for better distribution of light. Pixel and sub-pixel signal technology is same as LCD.
Back-light in both screens leak out from pixels, and this leakage is more in LCD compared to LED. It means black pixels in LED are more black and they are greyish in LCD because of more backlight leakage.
OLED has technological advantage over both LCD & LED.
- Unlike LCD, OLED pixel can produce its own light and doesn't require backlight.
It is a well known fact that electricity(electron) flows through conductors. 'Special conductors' known as semiconductors could be transformed into two different types of conductors.
- N-type with extra electrons among its molecules.
- P-type with deficiency of electrons i.e. extra holes among its molecules.
when these two types of semiconductor are connected or joined, it forms an unusual junction. Both conducting components i.e. electrons and holes attract each other and replace each other at the junction site. This replacement of electron-hole at the junction, resist further replacements beyond junction.
Resistance created by the junction could be overcome by specific minimum DC voltage in appropriate (forward) direction. when particular DC voltage is applied electrons jumps from N-type conductor to P-type conductor and acquire lower energy. When electron and holes combine electron release a specific amount of energy which depends on which materials are used in p-type and N-type conductor. This released energy is electromagnetic radiation (light).
This technology is exploited to its fullest by OLED display screens. All individual sub-pixels in the screen are LED in itself. Signals are transmitted to each and every sub-pixel in a form of specific voltage. This signal turn ON the sub-pixels. An array of signals decide which sub-pixel is to be turned ON and which not, eventually producing an image on the screen. Furthermore intensity of electric current in the signal decide the color intensity of particular sub-pixel. Hence intensity of the current is responsible for thousands of color possible in a pixel.
P-type and N-type conductors used in display screen LEDs are transparent & organic (compound containing carbon atoms), this makes the screen Organic LED (OLED).
Simple Working of OLED Pixel
- How OLEDs (organic LEDs) work - Explain that Stuff
Easy and detailed understanding of OLED with animation.
Gases under high pressure and voltage are able to exhibit some special characteristics. this form of gas is known as plasma. To better understand the working of plasma read 'How Light is Produced'.
Screen of plasma TV is made-up of small minuscule glass tubes in place of each sub-pixel. These micro glass tubes contain:
- Plasma (noble gases & mercury vapor at high pressure)
- Signal Electrode
- Phosphor Coating inside the tube walls
Electric signals in each micro tube electrify the mercury vapor in the tube, consequently this vapor emits UV(ultraviolet) radiation. Each sub-pixel in a pixel is coated with different phosphors, one with red, other with blue and third with green. When ultraviolet rays produced by mercury vapor strikes to phosphor inside the tube it emits light. Three different kind of phosphor results in three different color of light emission. Intensity of current in a tube decide the intensity of color, hence thousands of color are possible with combination of three subpixels and varying current.
- Inside a Plasma Display | HowStuffWorks
Take a look inside a plasma display and learn about the gases and components that produce images.
Plasma TV Screen Pixel Explanation
VRD Virtual Retinal Dispaly
until now we have seen that image is formed on the screen, and from there light enters our eyes so we can see the image. In VRD the case is simpler. a Device on our eye directly throw a laser beam to our retina, this makes our tiny light receptor cells in the eye as pixels. This is like electron beam from electron gun hitting phosphor in CRT monitor. Here laser (photon) directly hit our cells one by one.
- Virtual Retinal Display
Detailed explanation of VRD
Your Eyes as a Screen
E-paper or E-Ink is very unique modern display technology; it doesn't require to produce light! Underneath the screen, whole Screen is covered with transparent micro tubes or capsules. They are so small that a bunch of these capsule make a pixel. These capsules are filled with very tiny balls. Tip of our hair is wider than the width of these balls.
Balls in the capsules are of two colors, black and white. Black are negatively charged and white positive. All capsules are connected with their signal electrode beneath them. when negative signal is transmitted to an electrode, black balls of that particular pixel will be repelled to top and white balls will be attracted to bottom. The capsule will be visible black on the screen, and vice-versa when positive signal is transmitted to the electrode.
Fundamentally, physical balls are used to turn pixels black and white and no light is produced. This makes the content of the screen more visible in bright light and less power consuming. Color screens are also possible to make by just adding 3 different filters on a pixel and transmitting three different signals to a pixel.
There are lots of other methods by which E-ink or e-paper could function but above one is more popular and known as electrophoretic display.