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LCD Backlighting


LCDs create their display with the manipulation if visible ambient light. In the absence of this light, we must add backlighting to make these LCD displays visible.  There are many choices to consider when backlighting an LCD. Once again the choice comes down to appearance vs. cost vs. features. Each approach has its advantages and disadvantages, and no one method is right for all applications. The data below will only give the highlights of each technology with general comments. For our LCD modules we integrate most of these types of backlights into our displays.  If you wish to add your own backlight to a LCD Glass application, suppliers of backlights can be found on our Links page. These vendors can supply catalogs and up to date technical information for use in your design

Here are the 5 most common methods of backlight and information on how to drive LED backlighting.

  LED Backlighting

LCD Backlighting - LEDLight Emitting Diode, or LED, backlight is the most popular backlighting for small and medium LCDs.  The advantages of LED backlighting are its low cost, long life, immunity to vibration, low operational voltage, and precise control over its intensity. The main drawback is it does require more power that most of the other methods, and this is a major drawback if the LCD size is large enough.  LED backlights come in a variety of colors, with yellow-green being the most common, and now white is becoming cost effective and very popular. LED backlights offer a longer operating life - 50,000 hours minimum - and are brighter than ELPs. Being a solid state device, they are configured to operate with typically a +5VDC power (and optionally 12VDC power), so they do not require an inverter. The LED backlight has two basic configurations; Array and edge lit.  In both types the LEDs are the light source that are focused into a diffuser that distributes the light evenly behind the viewing area. In Array lit configuration there are many LEDs mounted uniformly behind the display, it offers more uniform and brighter lighting and consumes more power.  In Edge lit configuration, the LEDs are mounted to on side (typically the top) focused edge on into the diffuser, it offers a thinner package and consumes less power.

LED Backlighting - Module cross-sections

 

  Electroluminescence Panel (ELP) Backlighting

LCD Backlighting - EL panelElectroluminescence Panel, or ELP, is a solid state phenomenon which uses colored phosphors, not heat, to generate light. EL backlights are very thin, lightweight and provide an even light. They are available in a variety of colors, with white being the most popular for use with LCDs. While their power consumption is fairly low, they require voltages of 100 VAC @ 400Hz. This is supplied by an inverter that converts a 5, 12 or 24 VDC input to the AC output. Information about these inverters can be found in the Power Supply section of our website. ELPs also have a limited life of 3,000 to 5,000 hours to half brightness. The biggest drawbacks to an EL panel is that it requires an inverter to generate the 100VAC, consistent brightness, and limited life. .
 

EL Panel Backlighting - Module cross-section

 

  Cold Cathode Fluorescent Lamp (CCFL) Backlighting

LCD Backlighting - CCFLCold Cathode Fluorescent Lamp, or CCFL, backlights offer low power consumption and a very bright white light. The primary CCFL configuration used in LCD backlighting is edge lighting. A cold cathode fluorescent lamp is the light source with a diffuser distributing the light evenly across the viewing area. CCFLs require an inverter to supply the 270 to 300 VAC @ 35KHz used by the CCFL tube. Information about these inverters can be found in the Power Supply section of our website.  They are used primarily in graphic LCDs and have a longer life - 10,000 to 20,000 hours - than ELPs do.  Their biggest drawbacks are: cold weather will reduce the light output by as much as 60% (see graph below), they require an inverter to generate the 350VAC (please note that the inverters do not function well at low temperatures), the light intensity cannot be varied (it is either on or off), and vibration can reduce the life expectancy of up to 50%.

 

CCFL Backlighting - Temperature Preformance & Module Cross-section

 

  Woven Fiber Backlighting

LCD Backlighting - Woven FiberWoven Fiber Optic Mesh backlighting provide extremely uniform backlight, without the need for an inverter. The lifetime is dependent on the type of bulb used, with halogen (which generate high heat) or LED sources providing up to 100,000 hours. The bulbs themselves are usually mounted away from the LCD, where they can be easily replaced when necessary. Woven fiber optic panels tend to be somewhat expensive, but the uniformity and brightness are worth the extra cost for some applications.  Woven fiber backlighting is not offered directly by Pacific Display Devices.





 

  Incandescent Backlighting

LCD Backlighting - IncandescentIncandescent Lamp backlighting is only used where cost is a major factor. While Incandescent lights are very bright, they are not uniform, generate a significant amount of heat (which can cause problems at high temperatures), have short life spans, and use significant power for the brightness achieved. They can provide very white light, but the color can change with changing supply voltages, and they can be sensitive to shock and vibration.






 

  PWM - LED Backlight Driving Method

LED backlights on LCD modules are generally driven with a dc voltage through a current limiting resistor. This simple approach is perfectly acceptable for most applications. When the primary consideration is an extra bright display, the lowest possible power consumption, or a back light that can be controlled over a very wide brightness range another method is needed.
By using a Pulse Width Modulation (PWM) scheme several advantages can be realized over the simple dc voltage method, the main advantage is in efficiency in all schemes.

Maximum Backlight Brightness
Let's consider a LED backlight module where the nominal LED driving current for this display is 120ma which produces a typical brightness of 50 NIT. If, instead of a dc or constant current, we apply 5 times the current, 600ma, for 1/5 of the time, the average current is the same, 120ma.

PWM Duty Cycle

The average brightness of the LED would also be the same if measured electronically. The difference is in the brightness perceived. The human eye has a certain amount of persistence. If exposed to a bright light the eye will "remember" the light for a short period of time. This allows us to view a motion picture or TV screen as a steady image when in fact it is flickering at 24 to 30 times a second. When the LED is flashed on brightly for a short time and then turned off the eye "remembers" the light at the high brightness level. The result is that the perceived brightness of the back light is closer to the high pulsed brightness than to the lower average dc brightness.

This effect can be used to advantage in several ways. If the brightest possible backlight is needed the display can be pulsed at a 1:4 on/off ratio with 5 times the typical current. The pulse repetition frequency should be greater than 100Hz so the flickering is
not perceptible to the eye but not greater than about 1kHz.

Lower Power for Normal Backlight Brightness
This technique can also be used to give a "normal" looking brightness level to the display but at a Lower Average Current to save power. The average power can be cut by a factor of at least 30% to produce a given perceived brightness level. This can be a big advantage in battery operated equipment.

Controlling Backlight Brightness
One can also very the LED backlight brightness is by simply varying the DC current to the LED's, but at low current the individual LED emitters start to become visible resulting in an uneven looking back light. The third use of the PWM method is to facilitate a wide range of brightness control for the LED backlight, without an uneven looking back light. By varying the on/off ratio of the controlling PWM waveform, a very wide range of brightness can be achieved while maintaining a very even appearing back light.
 


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