A-level Physics Help / Advice - Television Screens and Computer Monitors

The Evolution of Television Screens and Computer Monitors

Aim:

The purpose of this report is to give a brief introduction of different stages of the evolution of television screens and computer monitors and then compare the strength and weaknesses of the various lighting systems. Then judging from the current trend and consumers’ requirements of an ideal light system, it offers a prediction of the future market.

Introduction:

I cannot emphasize strongly enough how quickly television screens and computer monitors have evolved in the past 80 years. Until about 15 years ago, chunky cathode ray tubes had monopolized the televisions and computer monitors markets for 65 years, but now they are so rare that they are treated as antiques and are placed on exhibition in the Science Museum in London. Within this short period of time, the rising light liquid crystal display (LCD) and plasma display panel (PDP) had replaced cathode ray tube as the new dominators of the market. Turn over the page if you want a more detailed analysis of the distribution of forces among the different lighting “empires” and finally help pick a winner among the candidates.

Cathode Ray Tube:

It was invented by German physicist Karl Ferdinand Braun in 1897 and first used for computer displays, video monitors, televisions, radar displays and oscilloscopes. Later, it was further developed from Philo Farnsworth's work in order to be applicable in television sets. After about 70 years of dominance in the relevant market, the CRT was finally replaced by plasma display and liquid crystal display. Despite its extinction, the CRT has given the television a nickname, "the tube" even when referring to non-CRT sets nowadays.

Theory:

An extremely high-voltage circuit is set up in a vacuum tube with a heated metal filament being the cathode and the inside surface of a fluorescent screen being the anode. The high voltage provide the electrons in the metal atoms sufficient kinetic energy to escape from the heated metal filament in the form of an electrical beam, which is named cathode ray. The emitted electrons are then attracted by the positive voltage difference applied across two electrical plates and accelerate into the deflection chamber. In this chamber, the electron beam is deflected by a magnetic field or an electric field (or both) to trace over the insider surface of the screen. The fluorescent screen then absorbs the kinetic energy from the electrons and emits photons of visible light when its electrons fall from an excited state back to a normal one. In television sets and computer monitors, the set up is similar but more sophisticated. The entire front area of the tube is scanned frequently in a fixed pattern called a raster. The period of scanning has to be smaller than the time for an image in one’s eye to fade completely, which is about 25ms, otherwise one will feel like watching a slide show of numerous pictures rather than a continuous TV drama. The TV set is installed with a set of wire coils driven by electronic circuits, which is an electromagnet changing with a received video signal. Also, the brightness of the image is obtained by modulating the intensity of the electron beam with the received video signal.

Strength:

1. It has the most combinations of colours.

2. It is relatively cheap because it is maturely developed throughout the years.

3. It is the easiest to achieve true darkness, which is essential in creating a theatre atmosphere when watching movies.

Weaknesses:

1. The fact that the metal filament takes time to warm up leads to a time lag between switching on the television or monitor and the first image displaying on the screen. The time lag ranges from 2 seconds to 15 seconds.

2. It is energy inefficient because of enormous theormal energy loss from the heated filament. More than half of the energy supplied by the voltage is lost in the form of heat.

3. The ions created in the imperfect vacuum are much heavier than electrons and thus bend less in a magnetic field. Therefore, they end up bombarding the phosphor in the centre of the screen, which is harmful to the screen. After a while, a brown patch is found at the centre of the screen, which is known as ion burn.

Plasma Display Panel (PDP):

It was invented at the University of Illunois at Urbana-Champaign by Donald L. Bitzer, H. Gene Slottow, and graduate student Robert Willson in 1964. In the early 1970s, it became more popular because the displays were rugged and needed neither memory nor circuitory to refresh the images. In the last 1970s, the sales of plasma displays decline because of an improvement of semiconductor in CRT displays boosted its price-attractiveness. Nonetheless, plasma displays were still widely used in high-profile placement such as lobbies and stock exchanges owing to its relatively large screen size and thin profile. Recently, the superior brightness, short response time, great color spectrum, and wide viewing angle of colour plasma video displays made them one of the most popular forms of display for High Definition Television Flat Planel.

Theory:

A plasma refes to a gas made up of free-flowing cations and delocalised electrons. Under normal conditions, a number of electrons are attracted by the same number of protons in the nuclei of gas molecules, which are neutral as a whole. If we apply a sufficiently high electrical voltage across the molecules, the free electrons knock off the outermost electrons of the atoms, resulting in a free-moving electrons and gasoues cations. Electrons accelerate toward the cathode, gaseous cations are pulled toward the anode. During this process, particles are constantly colliding with one another, which excite the gas atoms in the plasma. When the electrons falls back to its original energy state, it releases the energy in the form of a light photon. In a conventional plasma screen, noble gas xenon (Xe) and neon (Ne) atoms are pumped into hundreds of thousands of tiny cell, whose inside wall is coated with phosphor ,positioned between two glass plates. Noble gases are selected because they are chemically inert and release ultraviolet light photons when they are excited. The ultraviolet photons can in turn produce visible light photons. Thin and long electrodes are also sandwiched between the glass plates, on both sides of the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes, which are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer, are mounted above the cell, along the front glass plate. Both sets of electrodes form a basic grid that extends across the entire screen. The display electrodes are arranged in horizontal rows and the address electrodes are arranged in vertical columns. To ionise the noble gas in a particular cell, the plasma display's computer charges the electrodes that intersect at that cell. Then, an electric current flows through the gas in the cell, creating a rapid flow of charged particles, which stimulates the gas atoms to emit ultraviolet photons. The released ultraviolet photons hit a phosphor atom in the cell, one of the phosphor's electrons jumps to a higher energy level and then releases energy in the form of a visible light photon when falling back to its normal state. Every pixel is made up of three separate subpixel cells, which consists of red, green and blue coloured phosphors. These colours blend together to generate the overall colour of the pixel. By varying the pulses of current flowing through the different cells, the control system can change the intensity of each subpixel colour to create hundreds of different combinations of red, green and blue. In this way, the pixel can exhibit colours across the entire spectrum.

Strength:

1. Thin and light profile

2. Wide screen

3. Wide viewing angle

Weaknesses:

1. Medium price

2. High maintaining cost

3. Slightly hard to achieve true darkness, which is essential in creating a theatre atmosphere when watching movies

Liquid Crystal Display (LCD):

With a similar appreance with plasma display, Liquid Crystal Display is thin and flat but made up of numerous monochrome pixels arrayed in front of a light sourece or reflector. It is commonly used in battery-powered electronic devices, such as digital clocks and watches, microwave ovens, CD players and especially lap-tops, because of its relatively low power consumption and lower requirement of voltage.

Theory:

The idea of Liquid Crystal Display is based on four facts:

1. Light can be polarized.

2. Liquid crystals conduct electricity.

3. Liquid crystals transmit and rotate the direction of polarized light.

4. Electric currnet affects the structure of liquid crystals, which in turn determines how much the polarized light is rotated.

According to the temperture and particular nature of the substance, liquid crystals can be classified into several distinct phases, such as twisted nematic phase, which makes LCDs possible. Twisted nematics (TN) is naturally twisted but will untwist to varying degrees depending on the level of the electric voltage across the liquid crystal. They are used in LCD because they react predictably to electric current so that we can control light passage. To create an LCD, take two pieces of polarized glass. A special polymer that creates microscopic grooves in the surface is rubbed on the side of the glass that does not have the polarizing film on it. The grooves must be in the same direction as the polarizing film. Then add a coating of nematic liquid crystals to one of the filters. The grooves will cause the first layer of molecules to align with the filter's orientation. Then add the second piece of glass with the polarizing film at a right angle to the first piece. Each successive layer of TN molecules will gradually twist until the uppermost layer is at a 90-degree angle to the bottom, matching the polarized glass filters. As light strikes the first filter, it is polarized. The molecules in each layer then guide the light they receive to the next layer. As the light passes through the liquid crystal layers, the molecules also change the light's plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through. A colour LCD must have three subpixels with red, green and blue colour filters. Through careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. Therefore,

Number of combinations of colour = 256×256×256=16777216≈1.68×10^7

Strength:

1. Very thin and light profile

2. Extremely high native resolution, suitable for enhanced-definition television (EDTV), rapidly catching up with high-definition television (HDTV)

3. Low power consumption

Weaknesses:

1. Requires numerous transistors to manufacture one LCD display, which boosts the chance of a bad transistor, which breaks down a particular pixel. A bad pixel either cannot be switched on or constantly switched on.

e.g. For a lap top computer supports resolutions up to 1,024×768,

Number of transistors connected to the polarised glass = 1024×768×3= 2,359,296

2. Limited display size because of quality-control problems faced by manufacturers. To increase display size, manufacturers must add more pixels and transistors, which also increases the chance of including a bad transistor in a display. Manufacturers of existing large LCDs often reject about 40 percent of the finished panels.

3. Hard to achieve true darkness because some light inevitably escapes from the polarised glass.

Prediction of the Future Market:

It is obvious that Cathode ray tube (CRT) can no longer compete with the other two lighting systems because of its bulky size and weight as well as its high power consumption, but the fight between Plasma display panel (PDP) and Liquid crystal display (LCD) still remains equal-sided at the moment. Judging from the potential of the two types of technology, I would choose the latter because of recent enormous investment by various brands to improve their resolutions and viewing angles, which have already surpassed plasma display.

Reference:

Websites:

1. Wikipedia – Cathode ray tube

http://en.wikipedia.org/wiki/Cathode_ray_tube

2. Wikipedia – Plasma display

http://en.wikipedia.org/wiki/Plasma_display

3. Wikipedia – Liquid crystal display

http://en.wikipedia.org/wiki/Lcd

4. Wikipedia – Contrast ratio

http://en.wikipedia.org/wiki/Contrast_ratio

5. Home Theatre Projectors and Electronic Displays – the Contrast Ratio Game

http://www.practical-home-theater-guide.com/contrast-ratio.html