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LCD Displays with LED Backlights

Posted by Barbara Dutra on

An introduction into LEDs

This article about LED Backlights is the third in a series written by Barbara Dutra, an exchange engineering student from Brazil, currently attending Arizona State University, college of electrical engineering. She is currently an intern at Focus Display Solutions.

LEDs (Light-Emitting Diode) are a simple technology, manufactured with low-cost, semiconductor materials that dissipate excess energy in the form of light. Unlike the case with other materials used for lighting that lose a large percentage of their energy in the form of heat. Most other lighting technologies are very inefficient compared to Light emitting Diodes.

LED Backlights

LED lights have been used in LCD displays for over 25 years, but only recently was it possible to produce the colors: yellow and red in mass production.

The colors are used in various devices such as car headlights, traffic lights, and residential lamps, among other applications. LEDs have several advantages such as: low power consumption, very high durability, ability to operate in a wide range of temperatures and they are free of heavy metals. Their construction contributes very little to pollution, making them one of the first green technologies used in the manufacturing of Liquid Crystal Modules.

LED technology has been used extensively in the lighting industry. The cost of LED lamps is a little more expensive than that of conventional bulbs, but the energy savings and the durability provided easily justify this investment. The half-life of LEDs are rated at 70,000 hours while the fluorescent models last on average 10,000 hours.

LED Half-life

Half-life is the amount of time a LED can be lit before its brightness is half as bright as when it was first activated. This is not the amount of time before the LED will burn out. Each time a LED reaches its half-life, the brightness drops to 50% of when it first started.

Excessively high temperatures can greatly reduce the amount of half-life of many LED applications.

LEDs are energy efficient

The LED lamp is extremely efficient and in terms of lighting technology: it is a solid state lite source. This means that each LED has a block of semiconductor material that does the actual work of producing the light.

Solid state devices mean that there are no moving or vibrating components that could wear out or break. This is different than incandescent and fluorescent lamps that emit light by electron excitation in other elements, filaments or mercury vapor, which vibrate when current (amperage) passes through them.

By eliminating the vibration, LEDs have achieved a longer half-life. This is one reason why LED technology is used in the majority of LCD applications that need some type of backlighting. LEDs operate well in high vibration environments. The shaking of the solid state component does not shorten its half-life.

Vibration was one major issue that led to the elimination of CCFL (Cold Cathode Fluorescent Light) backlights in larger LCD technologies such as character and graphic displays.

In the past, LEDs were not used in larger LCDs since they tended to produce hot and cold spots. But advances in diffuser technology have eliminated most issues with uneven lighting.

LED backlights produce less heat, noise and radiation

The LED uses the majority of its energy in the production of light and not in heat or noise.

Older lighting technology produce a great deal of heat, even to the point that the light cannot be touched while in operation and may need to be off for a few minutes before it is safe to remove.

Also, LEDs do not produce noise pollution such as older neon lighting technology.

Some LED products claim to save as must as 60% more energy than older technologies while still producing the same amount of light.

Some older lamps can produce infrared and ultraviolet radiation that attracts insects and fades certain surfaces such as paintings. LEDs are free of all these negative effects.

LED backlights do not generate EM noise

Older lighting technologies, such as EL (Electroluminescence) and CCFL require an AC (Alternating Current) signal to operate. This AC signal can interfere with neighboring circuits and requires the product to contain an inverter to convert the DC (Direct Current) current to AC.

EL and CCFL also suffer from a shorter half-life of 3K hours compared to 70K hours of a LED.

Composition of LEDS

The LED is a simple semiconductor typically composed of aluminum gallium arsenide (AlGaAs) which is doped with other materials to define colors.

When phosphorus is doped, the emission may be red or yellow according to the concentration. Using gallium phosphide with nitrogen doping, the emitted light can be green or yellow.

Note: Yellow/Green LED backlights are the most popular color incorporated with character LCD Modules, although this color combination is losing ground to blue and white.

With the use of other materials added to the mix, the results are LEDs that emit light blue, violet and ultraviolet colors. There are also white LED backlights, but these are blue-emitting LEDs coated with a phosphor layer which absorbs blue light and emits white light.

Note: Blue and White LED backlights are a newer technology and have grown in popularity with LCDs that operate in negative mode.

The emitted light is monochrome (single color) and is therefore dependent on the crystal and doping impurity with which the component is manufactured.

Use of LEDs in LCD displays

A common use of current LED technology is in building such products as TV sets, cell phones, watches, monitors, calculators and everything else that requires information to be displayed to the end user.

In the case of TVs, the advantages of LCD displays compared to nearly extinct CRT (cathode ray tube which is a vacuum tube containing one or more electron guns, and a used to view images) monitors are numerous.

Note: CRTs use a vacuum tube that contains one or multiple electron guns to project on a fluorescent screen. The images are displayed on the screen and can sometimes be visible for a few seconds after the CRT is turned off.  CRT’s also had the habit of ‘burning’ the screen. This is where an image, if left on the screen too long, would burn into the glass. The image could be seen when the power was off and would sometimes distort the live image on the screen. One solution to eliminate the CRT burn was to use a screen saver that changed the images every few seconds.

The size difference (thickness) between CRT and LCD is most noticeable since even in the early development of the technology, an LCD panel rarely exceeded 3.5 cm thick while CRT ray guns required large and heavy enclosures. Sometimes eighteen inches to two feet thick.

During the operation of the LCD, there is no emission of any harmful electromagnetic wave to create health concerns. And the average half-life can reach 70,000 hours of use - the equivalent of 2500 days or just over six years running.

If you manage to reach the half-life of your LCD TV, you need to get out and get a life.

Note: Books have no half-life and require ultra-lower power to operate.

In LED TVs, the operation is quite simple. They use as a light source from a board containing hundreds of RGB (Red, Green, Blue) LEDs. The RGB LEDs are modulated to produce a wide range of colors.

Many TFT displays can generate up to 64M colors from the RGB combination.

Some TVs use one of two types of LED backlights, which are called Edge-lit (side-lit) and local dimming (Lighting spot reduction). Edge-lit has LEDs in continuous strips across monitor area, which produces much thinner televisions and achieves 30% lower power consumption.

Newer LED TVs are much brighter, with higher contrast compared to the traditional LCD model.

Plasma screens, LCD screens and LED screens

In Plasma TV technology, the screen that you see is a compound constructed with two thin glass plates. Between these plates are a number of electrodes that receive the decoded video signals and display accurately.

As in Plasma screen technology, the LCD screen uses two thin glass plates placed side by side. The space between them is filled with a very specific liquid crystal solution which aims to function as a kind of curtain that allows the regulating of manner in which light is diffused through it. The formation of an image starts when an electric pulse is applied to each individual pixel composed of liquid crystals. At the same time, a non-polarized light is emitted towards the bottom panel and passes through the liquid fluid which produces several distinct colors.

How to choose your backlight for a custom LCD

NEW LCD Designs:

All new LCD designs requiring a backlight use LEDs. If you need a display that is as thin (in the Z direction) as an LCD with and EL backlight, it is possible to use LEDs arranged as a side-lit (or edge lit). The diffuser will allow a nice even light flow.

Replacement displays:

In the last few years, several LCD suppliers and display manufactures have ceased to exist or have stopped  production on LCDs. Suppliers include: Ocular LCDs and Standish Displays.

Focus Displays (Focus LCDs) is able to design and supply many LCDs to replace your discontinued LCD supplier.

If needed, Focus Displays can still build a replacement display containing an EL backlight, but there is a MOQ (Minimum order Quantity) of 500 to 1K displays. If you do require an EL backlight, we will need to know if you need us to supply the inverter or if this will be on your PCB.

Standard in-stock LCDs:

Focus Displays does offer a wide selection of low-cost, standard LCD displays with inventory located in Chandler, Arizona. For help in choosing the best technology, use Focus LCD technology selector tool.

If you need answers ASAP, contact us or call us at 480-503-4295.