February 2, 2004 page 1 of 9

LEDs, New Light Sources for Display Backlighting

Application Note

Introduction

Because of their low intensity, the use oflight emitting diodes (LEDs) as a light sourcefor backlighting was previously limited to usein small displays.The rapid advancement of semiconductortechnology together with new concepts inpackaging design has led to a significantincrease in LED brightness, so that the useof light emitting diodes in backlightingapplications has gained increasingimportance. Due to their increasedefficiency, LEDs are being used in manynew application areas ranging from everlarger display panels to television screens.The following provides a general overviewand comparison of light sources which arecurrently available for use in backlightingapplications, with a focus on the differencesbetween cold cathode fluorescent lamps(CCFL) and LEDs.Afterwards, the principle construction ofbacklighting is explained.In addition, various types of LEDs areshown, along with their possibilities for useand associated challenges relating to the

development and implementation ofbacklighting technology.

Application Areas

Displays, particularly liquid crystal displays(LCDs), come in various sizes from smalldisplays for use in consumer applications upto large LCD displays used in televisionscreens, and have meanwhile establishedthemselves as a standard product forvisualization.In addition to advances in development ofLCD panels regarding display size andresolution, one must also concentrate on theother components of the display module(color filter, light medium, lightguides etc.) inorder to achieve improvement andoptimization of the display.The primary focus is display backlighting,with emphasis on the light source. Withincreased display size and informationdensity, the requirements and performanceof backlighting also increase; the trend in thearea of visualization is generally tendingtowards photorealistic images (Figure 1).

Figure 1: Backlighting performance in relation to information density / display size

February 2, 2004 page 2 of 9

Light Sources

At present, fluorescent lamps (CCFLs)prevail as the state of the art for backlightinglarge displays (larger than 3 inches) whencompared to other light sources such asincandescent lamps or electroluminescentfilm, and offer a cost-effective light sourcedue to their high intensity, long life andavailability in various sizes.

Figure 2: Overview and comparison oflight sources

CCFLs exhibit two considerableweaknesses, however. First of all, the redcomponent of radiated light is fairly low(Figure 3).

Figure 3: CCFL Spectrum and LCD ColorFilter

This means that the display is only able toshow a limited color spectrum (color gamut)(Figure 4), and the brilliance of the colors islow.

Figure 4: Comparison of color spectrums(NTSC: National Television SystemCommittee)

Secondly, due to the materials andconstruction employed, fluorescent lampsexhibit a high sensitivity to mechanicalstress and vibration.For these reasons, their use in manyapplication areas such as the automobileindustry or air and space technology(avionics) is limited or they cannot be usedat all.In comparison, a significant improvementcan be achieved with the use of LEDs, sincethey can produce a specific light within anarrow spectrum and exhibit a considerablyhigher level of mechanical stability.In addition, their lifetime is as great as orgreater than that of CCFLs and they can bedriven with much simpler control circuitryand at low voltage levels. An additionaladvantage of LEDs is their short switchingtime which lies in the range of nanoseconds.

February 2, 2004 page 3 of 9

By using a pulsed light source, it is possibleto considerably improve the display ofmoving images on LCDs, since the smearingeffect is minimized.

Backlighting

In general, backlighting modules areconstructed according to a commonscheme.The module principally consists of one ormore light sources, a layer to diffuse the lightevenly across the surface, and a micro-prismatic foil which serves to homogenizeand increase the brightness.

Diffusion of the light is predominantlyaccomplished by surface-structured light-guides of plastic (PMMA, Plexiglas, acrylicglass etc.), whereby the light is introduced atthe sides and is reflected by the divertingsurfaces of the lightguide. The advantage ofthis variant is the inexpensive, compact andvery flat modular construction of thebacklight (Figure 5).

Figure 5: Modular construction withlightguide

Another method of diffusing the light is theuse of a reflector box (Figure 6). Thisapproach is primarily driven by higherbrightness requirements, in which the use ofmultiple light sources is unavoidable.

With this method, the light sources aredirectly built into the reflector box, andradiate the light in all directions.The reflector box serves to mix andhomogenize the light. The advantage of this

approach is the significantly higherbrightness of the backlight. This can only beaccomplished with a greater module depth,however.

Figure 6: Modular construction withreflector box

Depending on brightness requirements andconditions, a combination of both variantsmay be employed.

LEDs for Backlighting

For use as a light source for backlighting,LEDs roughly fall into three categories –single-color, multi-color and white.

Figure 7: LED groups with examples

For displays with a monochromaticbacklight, all of the LEDs in the three groupsare equally well suited, and show noparticular specialty.

For backlighting of multi-colored displays,however, the three groups exhibit individual

February 2, 2004 page 4 of 9

advantages and disadvantages with respectto each other and also when compared toCCFLs (Figure 8).

The most significant advantage of single andmulti-color LEDs as opposed to white LEDsand CCFLs is the decidedly larger color

gamut available for the display and theindividually selectable white point, as well asproviding the potential for sequentialbacklighting.

Figure 8: Advantages and disadvantages of the three LED groups

Backlighting (RGB) with single-color LEDspresents a particular disadvantage,however.Because of the alternating arrangement ofthe individual colors, additional space isrequired for color mixing, where the size ofthe mixing area is dependent on thedistance between two LEDs of the samecolor and their emission angle (Figure 9).

Beyond this area, homogeneous, mixed lightis available for illuminating the display.

For modules with lightguides, this meansthat the surface of the lightguide must beenlarged, or for reflector boxes, the depthmust be increased.

Figure 9: Estimation of the mixing area

With an LED spacing of 10 mm and a colorsequence of "blue-green-red-green-blue", an

February 2, 2004 page 5 of 9

additional mixing area of around 28 mm isrequired (Figure 10).

Figure 10: Mixing area for single-colorLEDs with an LED spacing of 10 mm.

In contrast, the color mixing already takesplace within the package for multi-color andwhite LEDs.A much smaller area is necessary tocompensate for the emission characteristicsin this case. For the same LED spacing, onlyan area of around 7 mm is required (Figure11).

Figure 11: Compensation area for multi-color and white LEDs (LED spacing 10mm)

In general, white LEDs are well suited forbacklighting applications.In part, this is due to the availability ofvarious packaging styles ranging frommicrocomponents to high power LEDs, andis also due to the relative simplicity of drivingthe LEDs at low voltage levels.

Based on the principle used for creatingwhite light (Figure 12), a display with alimited color gamut and specific white pointcan be achieved, as is the case with CCFLs.

The principle is based on luminescenceconversion, where a portion of the blue lightof the semiconductor chip is converted intoyellow light by means of a converter(phosphor).

Figure 12: Principle for creating whitelight

By mixing blue and yellow light, white lightcan be obtained; the color coordinates(white point) are determined by the ratio ofblue to yellow.

Generally speaking, among the threegroups, the multi-colored LEDs emerge asthe best choice for color display backlighting.Because several different coloredsemiconductor chips are incorporated into asingle housing, the optical as well as design-related mechanical preferences of the otherLED groups can be combined together.

In spite of the more complex drive circuitryrequired, multi-colored LEDs are therecommend light source for backlighting full-color displays.

Construction of LED Backlighting

The challenge when creating LEDbacklighting is to obtain a homogeneouslyilluminated surface, in spite of the fact thatLEDs act as a point light source with aconical radiation pattern.

Similar to the backlighting of CCFLs, LEDmodules are based on two principlearrangements regarding the LC panel.

In the first variant – direct backlighting, theLEDs are arranged such that their light

February 2, 2004 page 6 of 9

directly illuminates the back surface of theLC panel (Figure 13).

Figure 13: Direct backlighting scheme

The homogeneous illumination of the displaysurface is achieved by the regular spacing ofLEDs and the corresponding depth of thereflector box. The depth of the reflector boxis primarily dependent on the LED spacingand the emission angle of the LEDs.

For an initial estimate of the reflector depthfor SMT-LEDs without lenses, 1.5 – 2 timesthe LED spacing can usually be used. Thisdepth provides a good homogeneousillumination.Further homogenization can be achieved byuse of various diffusers and/or specialbrightness enhancement foils (BEFs). With aclever combination of the various foils, thedepth can be reduced.

Typically, direct backlighting is preferred formodules with high brightness requirements(>500 cd/m²), medium size and limitedspace characteristics (x/y direction).

With the second method – indirectbacklighting, the light is deflected orreflected onto the back surface of the panel,usually in combination with a lightguide(Figure 14).

Figure 14: Indirect backlighting scheme

The uniform distribution of light is achievedby special microstructures (microlenses, V-shaped grooves) or dispersion points on thesurface of the lightguide. The arrangementof the structure conforms to the position andoffset of the LEDs and must be individuallycalculated for each configuration.

With the use of lightguides, many designsare possible, from the simplest version witha planar lightguide and one-sided LEDarrangement (Figure 15), to a lightguide withangled deflector surfaces (Figure 16),angled input coupling areas (Figure 17) andindividual LED arrangements.

Figure 15: Planar lightguide withmicrostructure and compensation area,structure becomes thicker from left toright, LED positioned at right

February 2, 2004 page 7 of 9

Figure 16: Lightguide with structure,2 LEDs located under the angleddeflector surfaces

Figure 17: Lightguide with structure,6 LEDs located at the angled inputcoupling areas

LED Backlighting for a 10.4” TFC-LC Display

The requirements for the model constructionof a white backlight for a 10.4 inch TFTdisplay are twofold: first of all, a brightnessof 600-1000 cd/m² on the display surfacewith a transmission of 7.5 % is required.Secondly, backlight should not be largerthan the size of the display itself (x/ydirection).

Furthermore, the backlight should not besensitive to vibration and have a lifetimegreater than 10,000 hours, for possible usein industrial and aerospace applications.Because of the high brightness requirementsand display size, direct backlighting with areflector box offers the only solution.

The resulting module (Figure 18) consists of336 LEDs based on the whitePowerTOPLED with a backgroundbrightness of 710 mcd at a current of 30 mA.Since an LED spacing of 10 mm wasselected, the resulting reflector height is18 mm. The height of the entire module(reflector and PCB) is 20 mm.

Figure 18: Backlighting module for a10.4" backlight

The module circuitry was designed to beoperated at 20 V and 2.5 A.

February 2, 2004 page 8 of 9

LED Backlighting for a 17” TFT-LCDisplay

For demonstration purposes and in thecourse of a feasibility study, a 17-inch TFT-LCD monitor was retrofitted with RGB LEDbacklighting, using the original monitorhousing.In addition, a typical brightness in the rangeof 200-250 cd/m² was assumed.The principle construction of the LEDbacklight (Figure 19) is based on indirectlighting.

Figure 19: Principle LED backlighting of a17" LCD

The multi-color LEDs (2x50 pieces, 6-leadMULTILED LATB G66x) were side-mountedwith direct contact to the upper and lowersides of a lightguide.To provide better heat dissipation, the LEDswere mounted on a combination consistingof a flexible PCB and an aluminum baseplate. By spacing the LEDs close together,the compensation area could be reduced toa few millimeters, so that the original LCDframe could still be used.

The brightness of the modified monitor(Figure 20) was measured with a lightdensity camera and amounted to 220 cd/m²at a light intensity of 620 lm for the LEDsand a specified white point of 0.33/0.33.

Figure 20: 17” TFT-LCD with RGB backlighting

February 2, 2004 page 9 of 9

Summary:

New generation LEDs exhibit highperformance regarding their use in LCdisplay backlighting.Through the combination of new chiptechnology and various packaging designsfrom miniature to high-power packages,LEDs can be used in many applicationareas, and fulfill the associated brightnessrequirements.

In the area of color management and thedisplayable color space (~120% NTSC),LEDs clearly surpass the customarily usedCCFLs and in addition, are insensitive tomechanical stress and vibration.

The many advantages of LEDs and theircontinual further development in the areas ofefficiency and output intensity make themeven more attractive for use as a lightsource for backlighting; in many applicationareas, they have already establishedthemselves as a standard.

Author: Andreas StichAbout Osram Opto Semiconductors

Osram Opto Semiconductors GmbH, Regensburg, is a wholly owned subsidiary of Osram GmbH,one of the world’s three largest lamp manufacturers, and offers its customers a range of solutionsbased on semiconductor technology for lighting, sensor and visualisation applications. Thecompany operates facilities in Regensburg (Germany), San José (USA) and Penang (Malaysia).Further information is available at www.osram-os.com.

All information contained in this document has been checked with the greatest care. OSRAM OptoSemiconductors GmbH can however, not be made liable for any damage that occurs in connectionwith the use of these contents.