Backlight Technology Overview
18Pages

Backlight Technology Overview This white paper was developed to provide an overview on backlight technologies. It is intended to help the reader better understand the different technologies along with the advantages and disadvantages. The paper begins with an introduction addressing the importance backlight technology plays in the design of Liquid Crystal Displays and then takes you through the various types of backlights. The technologies included are Electroluminescent (EL) lighting, LEDs, Fiber Optic, Array LEDs with Edge lit LED Lightguides, and Cold Cathode Fluorescent (CCFL) backlights. For each technology, the theory, structure, advantages and disadvantages are discussed. Author: Michael Yu Date: February 16, 2006 DENSITRON TECHNOLOGIES plc. – PROPRIETARY DATA – ALL RIGHTS RESERVED

4.0 Array LED Backlights………………………………………………………………. 9 4.1 Advantages of Array LEDs…………………………...…….……………….……. 9 4.2 Disadvantage of Array LEDs………………………...………….…………….….. 9 4.3 Edge lit LED lightguides.……………………………..…………..…………….… 10 5.0 Fiber Optic Backlights.………………………………………………………….….. 13 5.1 Advantages of fiber optic backlights.……………..………………………….…... 13 5.2 Disadvantages of fiber optic backlights…….…..………………….……….……. 14 5.3 General applications of fiber optic backlights. ……………………..…….……… 14 6.0 Cold Cathode Fluorescent Backlights (CCFL)……………………………………... 15 6.1 Advantages of...

1.0 Introduction Backlights are used for electronic devices with flat panel displays that require illumination from the back and includes devices as small as hand held PCs or as large as big screen TVs. A typical backlight consist of a light source such as a Cold Cathode Fluorescent (CCFL) or Light Emitting Diodes (LED) and a rectangular light guide, which is also referred to as light pipe. The light is usually located at one edge of the light guide to minimize the thickness of the display. Edge lightning typically uses total internal reflection (TIR) to propagate light along the length of...

2.0 EL Lighting 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. A typical electroluminescent lamp comprises a transparent conductive base. In general, polyester film having a thickness of...

2.1 Advantages of EL The main advantages of using EL technology can be classified as: • • • • • • • • • • • • • Very Thin - less than 0.010”(0.254mm) Midnight hour backlight solution Up to 90% uniformity Low current – 10 to15 mA (with matched inverter circuit) Low tooling costs - @$1000 Simple prototypes - cut to size prototype kits Performs well at temperature extremes Ready source of supply (Dupont) - Compatible materials chemistry Good durability Technology allows holes for indicator lamps, mounting keypad posts Microencapsulated phosphors & moisture resistant binders equals good shelf...

3.0 LED Backlights LEDs are the most commonly used light source in small to mid size LCD displays backlights. Current LED Based Technologies can be classified as follows: • • • • Fiber Optic LED Arrays LED Edge lit Lightguide Small to Large Format TFT Backlighting 3.1 Recent trends in LEDs Due 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: • • • • • • • • All LED colors are now readily available from multiple sources Small surface mount...

3.3 Disadvantage of LEDs The main disadvantages of LEDs are: • • • • Sensitive to ESD Sensitive to voltage spikes Heat dissipation in some applications Not true full spectrum White LED (unless tri-color) 3.4 LEDs application notes LEDs 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: - Low voltage DC - Constant current desirable - Otherwise, use voltage plus resistor 7 DENSITRON...

V = I x R or R = V / I • Assume a 5 Vdc input • *B/G/W LEDs consume 3.6 Vdc • 5 – 3.6 = 1.4 Vdc for the resistor • R = 1.4V / 0.025A = 56 ohms *Blue/Green or White LEDs Scotopic Response (Nighttime) R e l a t i v e L u m i n o u s Photopic Response Blue light at 470 versus… Green light at 555 is 1/10th as bright So…@1mW of power a Green LED is 10X as bright as a Blue LED Daytime application? Use Green! Scotopic Response (Nighttime) R e l a t i v e L u m i n o u s If an application will be used mostly at night, consider blue or white! 8 DENSITRON TECHNOLOGIES plc. – PROPRIETARY DATA – ALL...