An electroluminescent (EL) lamps electrical characteristic is similar to the two conducting surfaces of a capacitor (light-emitting phosphor layer) with a dielectric layer between. EL lamps illuminate when powered with AC power. As voltage is applied to the conductive surfaces, an electric field is generated across the phosphor and dielectric layers. Twice during each cycle electrons are excited and emit light through the transparent front electrode.<\/p>\n
A typical electroluminescent lamp comprises a transparent conductive base. In general, polyester film having a thickness of 175-micrometer with an indium-tin (ITO) oxide coating is used for this purpose. These ITO films are manufactured by vacuum sputtering techniques. Screen printing the conductive paste is another method.<\/p><\/blockquote>\n
Advantages of EL<\/span><\/p>\nThe main advantages of using EL technology can be classified as:
\n\u2022 Very Thin – less than 0.010\u201d(0.254mm)
\n\u2022 Midnight hour back light solution
\n\u2022 Up to 90% uniformity
\n\u2022 Low current \u2013 10 to15 mA (with matched inverter circuit)
\n\u2022 Low tooling costs – @$1000
\n\u2022 Simple prototypes – cut to size prototype kits
\n\u2022 Performs well at temperature extremes
\n\u2022 Ready source of supply
\n\u2022 (Dupont) – Compatible materials chemistry
\n\u2022 Good durability
\n\u2022 Technology allows holes for indicator lamps, mounting keypad posts
\n\u2022 Micro-encapsulated phosphors & moisture resistant binders equals good shelf life
\n\u2022 Cell phones – LCD & keypad with one lamp<\/p><\/blockquote>\n
Disadvantages of EL<\/span><\/p>\nThe main disadvantages of EL technology are:
\n\u2022 Short \u00bd life
\n\u2022 Power considerations
\n\u2022 AC Device – typically requires inverter for power conversion
\n\u2022 Inverter circuits require @1\/4 square inch of PCB space
\n\u2022 High voltage and frequency (40-80 VAC & 600-1200Hz)
\n\u2022 Inverter and lamp emit EMI and RFI
\n\u2022 UV ages phosphor
\n2.3 General applications for EL
\nSome of the application areas where EL is suitable for the design, can be classified as:
\n\u2022 Limited or intermittent duty cycles
\n\u2022 @2000 hour \u00bd life (with inverter circuit)
\n\u2022 Shelf life is indefinite
\n\u2022 Initial brightness of 15 to 20 FL
\n\u2022 Fairly short lead times
\n\u2022 Mechanical design issues are minimal
\n\u2022 Made to any 2-dimensional shape<\/p><\/blockquote>\n
LED Backlights<\/span><\/p>\nLEDs are the most commonly used light source in small to mid size LCD displays backlights.
\nCurrent LED Based Technologies can be classified as follows:
\n\u2022 Fiber Optic
\n\u2022 LED Arrays
\n\u2022 LED Edge lit Light guide
\n\u2022 Small to Large Format TFT Backlighting<\/p><\/blockquote>\n
Recent trends in LEDs<\/span><\/p>\nDue to the advantages and changes in the LED technology, LEDs are said to be replacing light bulbs in the near future. We can attribute some of the recent trends in the technology to the following points:
\n\u2022 All LED colors are now readily available from multiple sources
\n\u2022 Small surface mount packages increase design flexibility and manufacturability
\n\u2022 Surface mount, tape and reel, tooth pick PCB & flex mounted LEDs
\n\u2022 LEDs are satisfactory for backlighting small and medium size color LCDs
\n\u2022 Competition driving costs down for white, blue and true-green LEDs (Indium Gallium Nitride (InGan) type)
\n\u2022 Achieving greater stability in color reproduction, green, white & blue
\n\u2022 Brightness and efficiency are continuously improving
\n\u2022 Piranha type packaging, process improvements, and larger chip sizes mean even brighter LEDs<\/p><\/blockquote>\n
Advantages of LEDs<\/span><\/p>\nThe main advantages of LEDs are:
\n\u2022 Long Service Life
\n\u2022 100,000 hours time to \u00bd life- YG, Red and Amber
\n\u2022 50,000 hours Green
\n\u2022 20,000 to 40,000 hours for Blue and White
\n\u2022 Good environmental performance including high UV, high temperature and high humidity resistance
\n\u2022 High MBTF
\n\u2022 Solid state chip embedded in epoxy- nothing to break or burn out
\n\u2022 LED consumption is increasing, driving costs down
\n\u2022 Low heat generation
\n\u2022 No EMI or RFI
\n\u2022 Low Power (5 to 30 mA at 3.6 or 2.2 Vdc)
\n\u2022 New narrower beam, light focusing packages that are better utilized in light guides
\n\u2022 Many color choices<\/p><\/blockquote>\n
Disadvantage of LEDs<\/span><\/p>\nThe main disadvantages of LEDs are:
\n\u2022 Sensitive to ESD
\n\u2022 Sensitive to voltage spikes
\n\u2022 Heat dissipation in some applications
\n\u2022 Not true full spectrum White LED (unless tricolor)<\/p><\/blockquote>\n
LEDs application notes<\/span><\/p>\nLEDs are current driven devices, so when the design requires the use of LEDs, the engineer must always consider the total current drain on the design or manage the power budget correctly to include the LEDs power consumption. Below are some design criteria to follow:
\n– Low voltage DC
\n– Constant current desirable
\n– Otherwise, use voltage plus resistor<\/p><\/blockquote>\n
<\/p>\n
Array LED Backlights<\/span><\/p>\nLED ARRAY
\n– Available in multiple LED colors including Yellow Green, Red, Amber and Blue
\n– White and Green typically too expensive to design in array type.
\n– LED chips Surface Mount in an evenly Spaced Matrix
\n– Use for Maximum display brightness<\/p><\/blockquote>\n
Advantages of array LEDs<\/span><\/p>\n– Highest LED Brightness – over 150 cd\/m2 (Light is directly from LED Chip)
\n– Highest uniformity – 80% +
\n– Few Development Issues
\n– Hi-Reliability and MBTF
\n– Low Development costs<\/p><\/blockquote>\n
Disadvantages of array LEDs<\/span><\/p>\n-Minimum thickness of around 4.5mm
\n– Higher Current draw compared to edge lit LED Light guides
\n– Potential Heat issues
\n– Mounting Features not possible as with Molded Light guide<\/p><\/blockquote>\n
Edge lit LED lightguides<\/span><\/p>\nLEDs can be built to the side of the plastic light guide, achieving low LEDs counts for the design and thinner overall construction. This is now one of the most common designs used in LCD display Backlighting.<\/p><\/blockquote>\n<\/div>\n
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Fiber Optic Backlights<\/span><\/p>\nFiber optic backlights use sheets of fiber optic cloth to create LCD backlights of custom shapes, sizes and configurations. In a typical design, there will be two or more layers of fiber optic cloth to form a rectangular light emitting area. The design is typically 0.068\u201d (1.7mm) thick and anywhere from 1 sq inch to over 100 sq inches. The fibers coming off one end are then bundled into a circular ferrule and coupled to one or more LED light sources.<\/p><\/blockquote>\n
Advantages of fiber optic backlights<\/span><\/p>\nThe main advantages in using a fiber optic backlights can be classified as:
\n\u2022 LED based so up to 100,000 hours of lifetime
\n\u2022 Low power (5 to 30 mA at 2.2 or 3.6 VDC)
\n\u2022 Lights large areas with 1 LED (up to 24 square inches or more)
\n\u2022 Available in small to mid range volumes
\n\u2022 Thin (starting at 0.013\u201d, 0.33mm)
\n\u2022 Explosion proof requirements
\n\u2022 Fast prototype turnaround (2 weeks or less)
\n\u2022 No heat at the panel
\n\u2022 No EMI
\n\u2022 Rugged construction
\n\u2022 Wide temperature performance
\n\u2022 Uniformity acceptable for most applications
\n\u2022 Flexible, pliable construction<\/p><\/blockquote>\n
Disadvantages of fiber optic backlights<\/span><\/p>\nThe main disadvantages can be classified as:
\n\u2022 Relatively high cost @$4 for a 3-4 square inch (woven) and $2-$3 including the LED
\n\u2022 All LED disadvantages
\n\u2022 Fiber bundle is difficult to package
\n\u2022 Uniformity not as good as with other technologies
\n\u2022 Standard is +\/-30% for panels up to 10 inches
\n\u2022 Requires through-hole LED (T 13\/4 package)
\n\u2022 Doesn\u2019t lend itself to automatic assembly techniques<\/p><\/blockquote>\n
General applications of fiber optic backlights<\/span><\/p>\nRecommended
\n\u2022 Low Power- Effective backlighting can be achieved with 1 LED
\n\u2022 Lights mid to large LCDs needing low power
\n\u2022 Low Volume
\n\u2022 (Low NRE)
\n\u2022 Mid-high piece price
\n\u2022 Applications with space
\n\u2022 Multiple Display colors needed
\n\u2022 Explosion proof requirements
\n\u2022 MRI equipment
\n\u2022 High brightness with an incandescent light source or High brightness LED<\/p>\n
Cold Cathode Fluorescent Backlights (CCFL)<\/span><\/p>\nCCFL backlight technology is usually implemented in medium size to large size LCD displays.<\/p>\n
In the cold cathode fluorescent lamp construction it typically includes a hollow glass cylinder that has been coated on the inside with a phosphor material composed of rare earth elements such as zinc silicate and various types of halophosphates.<\/p>\n
The tube is then sealed at both ends, each of which also contains a gettered, mercury-dispensing electrode and an iron-nickel cathode connected to copper sheathed iron alloy leads. Lamps normally contain 2 to 10 milligrams of mercury, and a mixture of gasses such as argon and neon.<\/p>\n
When high voltage is applied to the electrodes, ultraviolet energy at 254hm is produced as the mercury and the internal gasses are ionized. The resulting ultraviolet energy from the mercury discharge stimulates the phosphor lining inside the lamp producing visible light output in the 380 to 780nm range (also known as the photopic region).
\nSome features of CCFL backlight as follows:<\/p>\n
– Light source: CCFL (Cold Cathode Fluorescent Lamp)
\n– Long work-life: 25,000 to 50,000 Hours
\n– Light color: Full Spectrum White
\n– Brightness: 1800cd\/meter2
\n– Uniformity: 80% or more
\n– Applications: Photocopiers, PDAs, Industrial Equipment & Heavy Machinery<\/p><\/blockquote>\n
Advantages of CCFL<\/span><\/p>\nSome of the advantages in using CCFL technology are:
\n\u2022 High Brightness of 2000 to 5000 cd\/m2 (depending on technology)
\n\u2022 Full spectrum whites for good color balance
\n\u2022 Easy to achieve display uniformity
\n\u2022 Thin profiles (2 mm & less) available for tight packages
\n\u2022 Long service life (up to 50,000 hours)
\n\u2022 Power efficiency (1 to 5 watts depending on inverter and number of tubes)
\n\u2022 Efficacy- high power vs. light output ratio
\n\u2022 Infinitely dimmable between 20 and 100%
\n\u2022 Well established technology in many applications
\n\u2022 Works well with BEF and Microgroove technology<\/p><\/blockquote>\n
Disadvantages of CCFL<\/span><\/p>\nMajor disadvantages of CCFL technology can be classified as:
\n\u2022 High voltage and frequency
\n\u2022 Some systems -1000 Vac & 30-40KHz
\n\u2022 Glass tubes can require special handling & packaging
\n\u2022 Tube thickness can be an issue
\n\u2022 Not a good choice in small portables
\n\u2022 Thinner tubes have lower output (less phosphor surface means less light)
\n\u2022 Inverters require 25 X 100 mm of PCB space
\n\u2022 Flicker
\n\u2022 Cold starts & low temperature performance<\/p><\/blockquote>\n
General applications for CCFL<\/span><\/p>\nIdeal applications to consider CCFL backlights would include designs that:
\n\u2022 Requires high brightness
\n\u2022 mid to large format TFT\/AMLCD displays
\n\u2022 Surgical instruments
\n\u2022 High contrast
\n\u2022 High ambient lighting or sunlight readable applications
\n\u2022 High brightness portables with 2 mm tubes
\n\u2022 Displays with highly reflective transflectors<\/p><\/blockquote>\n<\/div>\n
\u00a0<\/span><\/div>\nConclusion about LCD backlights<\/span><\/p>\nThere are many choices in today\u2019s market for one to select the right technology and each technology has its own advantage and disadvantages. The choice of technology depends on the intended application; whether it is a small size portable application or a large sized home entertainment devices. The correct choice of backlight technology will help boost the optics of the display improving brightness, contrast, and color of the end product.
\nSo, when the question is \u201cWhat backlight technology do I choose for my display?\u201d, the following factors for the design should be considered:<\/p>\n
\u2022 Brightness
\n\u2022 Cost
\n\u2022 Schedule
\n\u2022 Mechanical constraints
\n\u2022 Power budget
\n\u2022 Color
\n\u2022 Uniformity
\n\u2022 Environment
\n\u2022 Product Lifetime<\/p><\/blockquote>\n
\u00a0<\/span><\/p>\n<\/div>\n<\/blockquote>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n