Television sets have seen huge developments and advancement since the days they were first invented. The oldest type was CRT television with big cathode ray tubes which made them thick and heavy. They had been around since 1922 but their sales have declined considerably in the last decade since more portable and light weight TV sets have become available. Introduced in 1997 by Fujitsu, plasma TVs have a lot of advantages over CRT. They are thinner, lighter and have flat screens that prevent distortion at the edge of the screen. But plasma TVs could not compete with LCD TVs on pricing. LCD TVs are far more energy efficient and their prices have come down since they were first introduced in spite of being available in large screen sizes. LCD TVs are the most widely sold TVs today and have taken over the high definition market. After plasma and LCD TVs, LED TVs come next in the story of evolution of television sets. LED TVs are an improved version of LCD TVs with many advantages but are more expensive. A discussion on LCD and LED TVs follows next.
What are LCD TVs
An LCD TV as the name suggests is a TV that has a liquid crystal display (LCD) as its screen. LCD TVs are slim and flat; and advances in technology now allow for wider viewing angles and higher-quality video images. LCD TVs can be mounted almost anywhere – on a wall, under a cabinet, or on a desktop.
Liquid Crystals
Liquid crystals are a specialized state of matter that consists of rod shaped molecules having properties between that of liquids and solids. They can flow like liquids and crystallize like solids.
Working of LCD TVs
Firstly, it must be understood that liquid crystals or liquid crystal display (LCD) itself does not create or generate any light. A light source, in this case fluorescent tubes, is placed behind the screen which shines light through the LCD. This is the reason why a liquid crystal display is also called as a 'transmissive' display. A white diffusion panel is used to scatter light from fluorescent tubes to LCD evenly to create a uniform image.
An LCD consists of two polarizing electrodes with liquid crystals between them. The screen's front layer of glass is etched on the inside surface to form a grid pattern for the liquid crystals. The alignment of liquid crystals can be controlled by passing electricity through it. Each crystal can align such that either light is allowed to pass through or it is blocked. The pattern thus generated by the liquid crystals along with the backlighting forms the imagery.
LCD TV is made of the following layers:
- Backlighting: The source used for backlighting in an LCD TV is fluorescent lamps.
- Diffusers: Diffusers are used to scatter light uniformly across the panel.
- Polarizers: The function of polarizers is to ensure proper alignment of images.
- LCD Panel: This is the display panel which contains the liquid crystal solution.
- Antiglare coating: This makes the picture look brighter.
- Display screen
In an LCD TV, each pixel consists of three sub-pixels with green, blue and red color filters. In order to get better picture quality and 
performance, it is required that the LCD's pixels can be rapidly switched on and off. For this LCD TVs make use of 'active-matrix' LCD, an advanced type of LCD which is based on thin film transistors (TFT). TFTs are a combination of tiny switching transistors and capacitors that are arranged in a matrix on a glass substrate and which control the switching on and off of pixels.
The speed with which an individual pixel's color can change without blurring is defined as pixel response time. This time must be fast enough to avoid the phenomenon of 'motion lag' or 'ghosting' which may be seen in the case of fast-moving objects on screen.
Benefits of LCD TVs:
- Extremely energy efficient, they consume 60% less power than tube-type TVs.
- LCD TVs don't use phosphor as in the case of Plasma TVs. They use filters instead. This avoids the problem of image burn-in.
- hey are lighter and more durable than plasma TVs.
- They can be used as PC monitors as well if they have additional input options such as standard analog VGA or DVI digital input.
Limitations of LCD TVS:
- Fluorescent tubes used can be space consuming, heavy and inefficient in color quality.
- Fluorescent tubes used may contain mercury which is toxic.
What are LED TVs
LED TVs are just an improved version of LCD TVs. In fact, LED TVs have exact same layers as that explained for LCD TVs. The only difference is in the source used for backlighting. LED TVs use an array of LEDs to light up the LCD screen instead of fluorescent tubes. Use of LEDs gives better picture quality. In an LED TV, even purest blacks and whites are clearer.
Light Emitting Diode (LED)
LEDs are special kinds of diodes which emit light. When electricity is passed through an LED, a current is generated by the movement of electrons and holes. When electrons and holes recombine, a photon is emitted. The photon emitted may or may not be in the visible region depending on the material used in the diode, for example silicon diodes emit radiation in the infrared region and not in the visible region. Diodes made from materials that emit photons in the visible region are called light emitting diodes. The color of light emitted depends on the material and the energy band gap between valence and conducting band.
Working of LED TVs
As in the case of LCD TVs, the screen used in an LED TV is still an LCD screen only. The difference is in the backlighting; here LEDs are used for backlighting. That's why LED TVs are also called as LED-backlit LCD TVs. Electricity supplied to the LED TV controls the alignment of the liquid crystals and it is this alignment that controls how the LEDs' backlighting is transmitted on to the screen generating on-screen imagery. A technology called 'twisting crystals' is used in LED TVs to avoid burn-in.
LED TVs are of two types: Full-Array LED TVs and Edge-lit LED TVs.
- Full-Array LED TVs: LEDs are placed at the back of the television panel and cover the entire screen in rows and columns. Back-lit LED TVs use 'local dimming' to selectively dim specific groups of LEDs. This is why full array LED TVs have better contrast ratio which translates into better overall picture quality. But they are heavier and costlier than Edge-lit LED TVs.
- Edge-lit LED TVs: LEDs are placed along the perimeter of the television sets. A light-diffusing panel is used to spread light from the outer edge of the set to the middle of the screen. They are thinner and more power efficient than back-lit LED TVs. The main advantage of edge-lit TVs over Full-Array TVs is that Edge-lit TVs are relatively less expensive than full array LED TVs. On the other hand, the edge areas of the TV screen tend to be brighter than center of the screen giving rise to "spotlighting" in the corners of the screen or "white blotches" scattered across the screen. This effect is more pronounced when dark scenes are viewed on TV. A feature called 'micro dimming' has been developed to counter this effect, for example, Samsung's UND8000 series use micro dimming. Sony uses the name Dynamic LED instead of micro dimming for their products. Even micro dimming used in edge lit LED TVs is less precise than local dimming used in Full-Array LED TVs and hence the best picture quality can be obtained from full array LED TVs only.
Benefits of LED TVs:
- LED TVs produce better picture quality than LCD TVs because they have higher contrast levels.
- Since the materials used in an LED TV can be recycled, they are considered to be environment friendly.
- Their power consumption is 20-30% less than that of LCD TVs. This is because LEDs are low power and energy efficient devices.
- They have additional features such as digital TV Tuner and digital picture process.
Limitations of LED TVs:
LED TVS are expensive. They can cost as much as 20% more than LCD TVs.
Difference between LCD TV and LED TV
The table below summarizes the differences between LCD and LED TVs:
|
|
LCD TV |
LED TV |
|
Technology |
Liquid crystal display screens backlit by fluorescent tubes |
Liquid crystal display screens backlit by LEDs |
|
Yearly Electricity Cost* |
$26.30 |
$17.57 |
|
Thickness |
Average thickness of 3.6” |
Edge lit TVs can be less than 1” thick |
|
Power Consumption* |
131W |
88W |
|
Advantages |
Can be used as a PC monitor Lighter and more durable than plasma TVs |
Energy efficient Environment friendly Higher contrast levels |
|
Disadvantages |
Fluorescent tubes can be space consuming, heavy and inefficient |
More expensive than LCD TVs |
*45" screen when used on an average for 5hrs a day and the cost of electricity $0.11/kWh