LEDs Magazine - 2006 - 08.pdf

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NEWS & ANALYSISNokia predicts increasing LED usage in

mobile phonesDOE, IESNA work together

on solid-state lighting standardsCree sets

new standard with 131lm/W white LED

Toyoda Gosei calls for caution regarding its

LED patentsand more p3

PATENTSPatents: are they a blessing orcurse for the LED industry? p12

LIGHTINGLighting industry still learninghow to apply advanced LEDs p17

CHIP TECHNOLOGYPhotonic quasicrystalsboost LED emissioncharacteristicsp21

OPTICAL DESIGNOptical design softwareimproves design and validationof LED-based systems p25

DISPLAYSFull-color video LED billboardsfeel the impact of alternative

technologies p29

MEDICALHigh-power LEDs provide

illumination and treatment inmedical applications p33

WEBSITE ARTICLESOptical and thermal designs hold key to

Schefenackers LED headlamp

developmentLED modules offer a bright

future for aviation lightingMotion capture

cameras surrounded by LEDs p27

Photonic crystals enhance the extraction efficiency of LEDs and place the light where its required. p21

Optical design software accelerates the design

process for LED-based systems. p27

Editor: Tim Whitaker [email protected] Tel. +44 (0)117 930 1233 Advertising sales: [email protected] Tel. +44 (0)117 930 1028

LEDs Magazine is published by IOP Publishing Ltd and Cabot

Media Ltd. Contact address: Institute of Physics Publishing,

Dirac House, Temple Back, Bristol BS1 6BE, UK.

Copyright 2006 IOP Publishing and Cabot Media Ltd.All rights reserved.

www.ledsmagazine.com Technology and applications of light emitting diodes

LEDSM A G A Z I N E

LEDs MagazineReviewIssue 8 August 2006

Advertising billboard networks are using LED

displays, as well as competing technologies. p29
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3/35ledsmagazine.com August 2006 Copyright 2006 IOP Publishing Ltd www.iop.organd Cabot Media Ltd. All rights reserved

N E W S & A N A L Y S I S LEDS

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MOBILE APPLIANCES

Nokia predicts increasing LEDusage in mobile phones

The number of LEDs that are used in mobile phones is likely to increaseas more functionality is added, says Barry Rowland, a technology spe-cialist for Nokia in the field of LEDs and LED power management.Speaking at PIRA-Intertechs conference, The Future of LEDs, inlate June, Rowland said that camera flash using LEDs is going to be a

big market, and that added functionality using LEDs is on the increase.Handsets are likely to become more user-configurable, said

Rowland. As one example, RGB LEDs can allow the user to changethe keypad backlight color. Another example is using different LEDcolors to indicate the arrival of SMS or email messages, or to differ-entiate between callers.

Other speakers including Asif Anwar of market research firmStrategy Analytics said that the overall market for LEDs in phones will

be roughly flat, mainly owing to declining prices counteracting thegrowth in segments such as flash. Rowland did not disagree with theseviews, saying, As a major buyer, Nokia is pushing costs down. Thecost of using LEDs takes into account drivers, light-guiding and manyother factors, as well as the LED price. However, said Rowland, lowerLED pricing means that Nokia can use more LEDs for new functions.

Power consumption is always a key issue in mobile handsets.Improvements in LED brightness, which have continued despite pre-dictions that they would level off, translate to lower power consump-tion for a given display brightness. The introduction of blue LEDs withforward voltages as low as 3 V means that these devices can be runfrom a battery without a boost converter.

Rowland estimated that LEDs will continue to become more effi-

cient, and that lm/Wvalues have increased by a factor of 10 in the last10years. This helps reduce the power drain and ease thermal man-agement issues. Since none of the functions are 100% efficient, wealways have to consider how to get the heat out, he said.

Display backlighting could benefit from using RGB LEDs toincrease the color gamut and allow dynamic adjustment of the white

point , for example under different ambient lighting condi tions.However, there are significant challenges in realizing accurate white-

point control, for example the wide variation in red LED performancewith temperature.

For keypad backlighting, a major trend is the introduction of verysmall and thin LEDs measuring 10.5 mm. In a few yearstime wewill see paper-thinLEDs with dimensions of 0.1 mm, said Rowland.Well be able to put them in places we never thought of before. Aswell as leading to more placement and light-guide construction options,this will cancel out the main advantage of electroluminescence tech-nology for keypads, i.e. extreme thinness.

In the flash area, Rowland forecast a move from sub-2W LEDs anddrivers to the 4 W level. I agree this is going to be a big market [forLEDs], he said. Customers have higher and higher expectationsfrom their high-pixel-count cameras. LED improvements and moresophisticated optics are resulting in large increases in delivered luxlevels. However, said Rowland, the drive levels may become lim-ited by battery technology and could require the use of supercapaci-tors. Xenon flash is viewed as providing better light output and colorrendering, but there are packaging issues. Another advantage of LEDsis that they can also operate in continuous mode for video filming.

MEDICAL

Trumpfs adjustable light helpssurgeons control the contrast

Trumpf Medizin Syst-eme has developed asurgical light that con-tains 184 white and col-ored LEDs, allowing thesurgical team to adjustthe lights color temp-erature and to maintaingood contrast under dif-ferent conditions. More details:www.ledsmagazine.com/articles/news/

3/8/4

INDUSTRY ORGANIZATIONS

DOE, IESNA work together onsolid-state lighting standards

The US Department of Energy (DOE) and the IlluminatingEngineering Society of North America (www.iesna.org)are workingtogether to develop industry metrics and standards for solid-state light-ing (SSL) technology. Such metrics will be one of the foundations for

bringing SSLinto Energy Star, a voluntary labeling program designedto identify and promote energy-efficient products (see below).

The agreement outlines five broad goals for joint DOEIESNAefforts, including developing DOE efficiency standards and IESNAstandards and procedures; developing guides and procedures to assistthe lighting community in the photometric measurement of SSLdevices, and providing consistency and uniformity in photometricreports; and developing standards that include a focus on energy-conservation strategies to benefit design professionals and users.

IESNAis a recognized technical authority on illumination, and com-municates information on all aspects of good lighting practice to itsmembers, the lighting community and consumers. IESNA corre-lates research, investigations, and discussions to develop consensus-

based lighting recommendations.The relationship with IESNAis part of a wider commitment by the

DOE to develop industry standards. ADOE workshop in March 2006provided a forum for greater co-operation and coordination amongstandards organizations, including IESNA, National Institute ofStandards and Technology (NIST), National Electrical ManufacturersAssociation (NEMA), American National Standards Institute (ANSI),Underwriters Laboratories (UL), International ElectrotechnicalCommission (IEC), International Commission on Illumination (CIE)and Canadian Standards Association (CSA). The DOE continues towork with these organizations to align their individual priorities andschedules, and maintain a master roadmap of development activities.

SSL partnership collaborates on draft Energy Star criteria

The DOE is also working closely with the Next Generation LightingIndustry Alliance (NGLIA) to develop draft Energy Star criteria for white
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MAGAZ INE

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AC LEDS

AC supply drives AmericanBright LED lighting module

American Bright Optoelectronics has introduced a new LED lightingmodule powered by a custom AC driver from Lynk Labs. The BWCLseries can accept an AC input of 31000V, and American Bright saysthat this affords the designer the greatest possible freedom to designfixtures bases on desired aesthetics rather than packaging of compat-ible drivers. The light engine comprises four NovaBrite series LEDscoupled to an innovative sandwich of PCBs, with standard FR4material bonded to a 2 mm aluminum heatsink PCB. The individual

NovaBrite modules are then mechanically bonded through both the

FR4 and the metal-core PCB to ensure maximum thermal conductiv-ity and efficiency. More details: www.ledsmagazine.com/articles/news/3/7/26

DEVICES

Cree sets new standard with131lm/W white LED at 20 mA

Cree, a leading US-based LED manufacturer, has set a new bench-mark for white LED efficacy with an R&D result of 131 lm/W. Thewhite phosphor-converted device was built around a small-area LEDchip (less than 0.3mm square) and was driven at 20mA. The corre-lated color temperature was 6027 K and the results were confirmed

by the US National Institute of Standards and Technology (NIST) inGaithersburg, Maryland.

This is an R&D result for a prototype device and does not reflect theperformance of current production devices, although it certainly pointsthe way to commercial LEDs in the near future with efficacies exceed-ing 100lm/W. Both Cree and Nichia have previously announced R&Dresults for white LEDs of 100lm/W.

Fritz Morgan, CTO of Color Kinetics, a Cree customer, said:Technical advancements at the component level are critical to grow-ing the emerging white LED lighting space. Crees results speak tothe exciting developments underway that will enable new white lightapplications and subsequently facilitate market adoption.

The prototype white LEDs were fabricated using Cree EZBright

LED chips operating at 20mA. EZBright chips, typically measuringless than 0.30.3mm, are designed to be incorporated into white LEDsthat are used as backlights for LCD screens in mobile phones, as wellas many other applications. Cree sells EZBright chips to external cus-tomers, rather than using them to manufacture its own white LEDs.

Crees high-power XLamp packaged LEDs incorporate larger chips(11mm). These devices are designed to operate at 350mA, and con-sequently have lower efficacy than the smaller-chip devices. Creescommercial white XLamp product produces 57lm at 350mA, with47 lm/Wefficacy, although the company is planning to release a newiteration with an efficacy exceeding 60 lm/W. More details: www.ledsmagazine.com/articles/news/3/6/19

PATENTS

Toyoda Gosei calls for cautionregarding its LED patents

Toyoda Gosei Co Ltd, one of the worlds leading LED manufacturers,has surprised the community by issuing a press release advising thefullest caution to avoid infringement of its patents on GaN-based LEDs.

Toyodas rivals Nichia and Osram Opto Semiconductors have beenactive in recent years in pursuing companies that they believe infringetheir patents, and have also publicly announced licensing agreementsfor their technology (see www.ledsmagazine.com/articles/features/1/8/21). However, Toyoda has kept relatively quiet, despite owning a

patent portfolio that is second to none and arguably forms the basisfor the entire GaN-based LED industry.

Toyoda has signed two major agreements with its rivals; an agree-ment with Nichia dating back to September 2002 and a more recentagreement with Philips Lumileds signed in April 2006. Toyoda says

that these separate agreements allow the parties to utilize each othersLED-related patents, and also says that it licenses the use of particu-lar LEDs as defined in individual agreements.

The Toyoda press release goes on to say that, because the manu-facture, sale or use of GaN-based semiconductor LEDs by any entityother than these may constitute infringement of Toyoda Goseis

patents, Toyoda Gosei advises the fullest caution to avoid infringe-ment. The company says that it has filed more than 2000 patent appli-cations related to GaN-based semiconductor LEDs, and has acquired

patent rights on some 600 of these.Details of the agreement between Nichia and Toyoda have never

been made public, but there is a strong indication that Toyoda emergedin by far the strongest position, given that it had won almost all thecourt cases between the two companies prior to the settlement date.One suggestion is that Toyoda has the right to use Nichias GaN-basedLED patents up until the settlement date, but that the reverse is not true.

White LED-related patents

Toyodas press release also discusses the silicate-based phosphor tech-nology patented by Toyoda in partnership with Tridonic OptoelectronicsGmbH (Austria), Leuchtstoffwerk Breitungen GmbH (Germany) andLitec GbR (Germany).

The patents cover technology for combining blue LEDs with sili-cate phosphors. Patent rights have been granted in Austria, the US(patents 6,809,347 and 6,943,380), Korea, Taiwan and Russia, andChina will be soon added to the list.

Toyoda says that it has about 20 licensees for these patents world-
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Long-lasting, reliable lighting. Accessible in themost inaccessible locations imaginable. Thatsthe promise of LEDs. And thanks to NuSil, high-

powered versions will soon be available fromKaohsiung to Copenhagen to Kodiak, Alaska.

While our advanced packaging materials are helpinghigh-brightness LEDs fulfill their potential, yourneeds might be very different. From LEDs to fiberoptics, large batches to small, our Lightspan brandof products deliver precise, custom formulationsand the most complete line of high-refractive indexmatching adhesives, encapsulants and thermosetsavailable. All backed by more than 25 years ofengineering materials expertise.

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MAGAZ INE

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wide, but it now believes that companies other than licensees havebeen manufacturing and selling white LEDs using silicate phosphors.Again, the company is advising the fullest caution to avoid anyinfringement that might be taking place. More details: www.ledsmagazine.com/articles/news/3/8/10

Our article on patents on page 12 discusses the rights conferred bya patent license on the licensee, and many other related issues.

REPLACEMENT LAMPS

LSG, Osram Opto develop LED-based MR-16 and R-30 lamps

Lighting Science Group Corporation (LSG), an LED-fixture manu-facturer, is building its new LED-based MR-16 and R-30 lamps usingOstar Lighting LEDs from Osram Opto Semiconductors. Prototypelamps were displayed on Osrams booth at Lightfair in June. LSGsnew energy-efficient lamps offer a direct alternative to the highly pop-ular MR-16, or metalized reflector halogen lamp, currently used inthe marketplace.

The company says that significant energy savings are possible whenthese lamps are used in industrial and commercial applications, espe-cially in shopping malls, kiosks, museums and office complexes, wherethe benefit of replacing conventional lighting over hundreds of thou-sands of square feet can add up very quickly. The MR-16 lamp con-tains a single 6-chip Ostar while the R-30 version (pictured) containsthree such LEDs; the hexagonal shape of the Ostars board makes iteasier to pack the LEDs close together. More details: www.ledsmagazine.com/articles/news/3/7/14

NATIONAL PROGRAMS

China invests millions in itssolid-state lighting program

By Robert Steele, Strategies Unlimited

Saving energy and stimulating domestic LED production are key goalsbehind Chinas decision to invest around $44million in its nationalsolid-state lighting program. China expects to be the largest market

for LEDs and solid-state lighting. Anational solid-state lighting pro-gram was formally inaugurated in February 2006 as part of Chinas11th Five Year Plan. Dr Ma Songde, vice-minister of science and tech-nology, says that the growth of the hi-tech sector is key to Chinas futuredevelopment, and that LEDs are seen as an indispensable part of that

sector. Dr Ma, chair of the Governing Committee for China Solid-StateLighting, was speaking at the third annual China International Forumon Solid-State Lighting (CIFSSL) in Shenzhen in July.

Only 10% of Chinas GDP comes from the hi-tech sector, and 88%of that results from foreign companies located in China or from jointventures with Chinese companies. Therefore, one goal of the nationalSSL program is to stimulate domestic production through public

private partnerships. Another key goal is energy savings; China con-sumes the most energy after the US, and its energy consumption perdollar of GDPis much higher than Japan, Europe or the US. The coun-try has ambitious plans to quadruple its total GDPby 2015, but to onlyincrease energy consumption by a factor of two. Lighting is a key ele-ment of energy-demand growth, especially as 14 million rural Chinese

move to cities each year. The adoption of SSL will be an importantelement in reducing the growth of energy demand.To address the R&D needs for SSL, China has budgeted

RMB350 million ($44 million) during the 11th Five Year Plan.Participation will include more than 15 research institutions and uni-versity research labs, as well as more than 2500 companies involvedin LED wafers, chips, packaging and applications. Protection of IP isa significant concern and a major element of the SSLprogram. Chinesecompanies are encouraged to file patents to protect their IP and to lookfor patent gaps where there are areas of SSLthat have not yet beenaddressed by foreign companies, primarily in the downstream areasof packaging and applications. See www.ledsmagazine.com/articles/features/3/7/6 for the full ver-

sion of this article, including a detailed discussion of patent issues.

DEVICES

Avago joins high-power marketwith one-watt InGaN LEDs

Avago Technologies has belatedlyentered the high-power LED mar-ket with a new product range ofInGaN-based, one-watt classLEDs. High-power LEDs are usu-ally classified as devices operatingfrom a power level of more than0.5 W, with a drive current of morethan 150 mA. Typical one-wattclass LEDs operate at 350 mA

(where the actual dissipated power is around 1.21.4 W) and the chipsize is generally around 1mm 1mm. Meanwhile, BridgeLux has justcommenced production of a 1.5 mm chip, while Osram Opto is fill-ing a gap in its product line-up with a mid-range LED operating ataround 140mA.

Avago launches one-watt LED

In June, Avago finally entered the high-power LED market with a fam-ily of InGaN-based (blue, green, white) LEDs targeting solid-statelighting (SSL) applications. The company, which was previously
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LEDSMAGAZ INE

ledsmagazine.com August 2006 Copyright 2006 IOP Publishing Ltd www.iop.organd Cabot Media Ltd. All rights reserved

N E W S & A N A L Y S I S

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Agilents semiconductor products division, was until recently restrictedto servicing low-power LED segments (< 200 mW). LumiledsLighting, the joint venture set up by Agilent and Philips and now owned

by the latter company, focused on the high-power LED market space.Now, as Avago, there are no restrictions, says Patrick Trueson,Avagos product marketing manager. The high-power LED marketis the key focus segment moving forward.

Trueson acknowledges that although Avago is quite late to a mar-ket with plenty of competition, the market has seen a rapid expansionin recent years. It absolutely makes sense for Avago to have these

products and to focus on solid-state lighting, he argues. We have alow-profile, reliable and easy to use package, and we hope to cap-ture 10% of the addressable market in Europe. Avagos estimates forthis market are shown in the graph above.

Avago has also recently released new products in its illuminationand color management (ICM) family, including RGB color sensorsand controllers capable of maintaining a fixed RGB ratio over varia-tions in time, temperature and drive conditions. Currently these aretargeted at the display backlighting market. However, in future, Truesonsays that Avago will combine new state-of-the-art high-power (20 W)RGB light sources with its ICM products to offer closed-loop systemsthat can maintain precise color temperatures in SSL applications.

The current offering includes InGaN-based devices only, but AlGaInPproducts will be available around the end of this year. Were currentlyworking closely with our strategic partners to qualify the industrys

best AlInGaPchips into our PowerLED packages, says Trueson.Avagos package uses silicone encapsulation and has a low thermal

resistance of 10C/W, both factors that extend the LED lifetime. Goodthermal management and silicone encapsulation, as well as using high-quality dies, leads to a better lifetime than other approaches. Avagoquotes a lifetime of 50,000 hours to 30% degradation at 350mAat anambient temperature of 25C.

The package is compatible with IR reflow soldering and standardSMTprocess lines, says Trueson, and has a moisture sensitivity level(MSL) of 2a, which means that the LED can last for four weeks afterthe sealed package is opened on the factory floor. Some LEDs haveto be soldered after 72 hours, or a week, otherwise they have to be

re-baked. We see this as a big advantage. Handling of the packagesis also made easier by the in-built zener diode that provides 16kVelectrostatic discharge (ESD) protection (class 3b). The devices arealso ROHS compliant and Pb-free. In terms of output, the LEDs havea typical total luminous flux of 40 lm in green, 10 lm in blue and 35lm

in white at 350mAforward current. More details:www.ledsmagazine.com/press/12713

Osram Optos Advanced Power TopLED

Osram Opto Semiconductorsnew Advanced Power TopLED sits inwhat some describe as the high-current regime, above standardHB-LEDs operating at around 20 mAbut below high-power devicesin terms of drive current and light output. At 140mA, the device hasan output of 19lm in green and amber, 15 lm in white and red, 14 lmin yellow and 5 lm in blue. This is 50% more than previous versions.The surface-mount package is designed for applications such as illu-minated signage and effect lighting. More details:www.ledsmagazine.com/press/13026

BridgeLux unveils 1.5 mm LED chips

BridgeLux, a US-based fables supplier of high-power LED chips, hasintroduced the KO family of LEDs, including what it describes as theonly 60-mil (1.5 mm) chip available in production volumes today.BridgeLux says that the 60-mil KO chip, available in blue, cyan andgreen, can support up to 1.2 A drive current. When packaged with

phosphors, the resulting white LED should generate 140 lm, depend-ing upon packaging design. Smaller blue chips are also available. Theentire family is designed to stand up to 150C junction temperaturesand 40% higher drive currents than commonly found for similarlysized devices.

The development objective with the KO series was to reduce theinput voltage while increasing the overall light extraction, saidBridgeLux CEO Robert Walker. Weve nicknamed the 60-mil KO thebulb bustersince we have something here that can really begin to chal-lenge traditional lighting solutions in general-purpose illumination. More details: www.ledsmagazine.com/articles/news/3/7/8

250

300

200

150

100

50

02006 2007 2008 2009

standard brightnesshigh-power AlGaInPhigh-power InGaN

marketvalue($millio

n)

Avagos estimates for the LED market for solid-state lighting in

Europe. The new Avago products address the high-power InGaN

segment, while existing standard-brightness products are suitablefor certain mood lighting and other applications.
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LEDSMAGAZ INE


P A T E N T S

12

Patents are they a blessing or

a curse for the LED industry?

When patent disputes occur, such as in the LED industry recently,questions arise about whether patenting helps or hurts individual busi-nesses and the industry as a whole. To form educated answers on issuesinvolving patents and patent litigation, an understanding of themechanics of obtaining and enforcing patents is helpful.

What is a patent?

Patents are one form of protection for intellectual property that includescompositions of matter (e.g. products such as chemicals), methods(e.g. how to produce a lighting effect, business methods), apparatus(e.g. lights, winches) and computer software.

United States patents are documents issued by the US governmentgranting exclusive rights to inventors over their inventions. Patentshave three basic parts: the specification, which describes what theinvention is, and how to make and use it; the drawings, if required,which depict one or more embodiments, or variations, of the inven-tion; and the claims, which set the boundaries of what is protected, i.e.define the claimed invention.

This distinction is important because some undue concern is gen-erated surrounding the misconception that the items described in thespecification are covered by the patent. However, it is the claimsthat determine what subject matter is protected. Unless subject mat-ter is covered in the claims of an issued patent, it is not protected.

While the specification supports and explains the claims, the exclu-sive rights in the invention are defined by the claims. Parts of thespecification not appearing as claimed elements cannot be enforced.

Claims are generally written in two ways: independent form ordependent form. An independent claim is just that, it stands on its ownand defines a complete invention having a discrete set of elements.Dependent claims refer to a previous claim, either an independentclaim or another dependent claim, and are a short-hand way of add-ing additional elements. The elements covered in a dependent claiminclude the elements listed in that claim plus the elements in the claimfrom which the dependent claim refers.

What is a patentable invention?

Claims are patentable if they delineate an invention that is novel, use-ful and non-obvious based on the claim elements considered as awhole. To satisfy the novelty criterion, there must be no public dis-closure, publication, presentation, use, sale or offer to sell, by the inven-tor or others, of the invention as delineated in the elements of theclaims. (There are some exceptions and grace periods.)

A claim is novel if there is no prior art that discloses the inven-tion. (Prior art includes publications, patents and so forth that precedean invention.) Even one novel element in a claim should satisfy the

novelty requirement. Most inventions are combinations of existingelements (bolts, screws, gears, etc.) arranged in a novel way.

Improvements to an existing product, method or apparatus may bepatentable even over existing patents. Asimple example is a patentedpencil having the elements of (a) a graphite core and (b) a protective

sheath positioned about the graphite core. Apatentable improvementcould be adding element (c) an eraser, to one end of the pencil, assum-ing that no one had done that before. Despite the new pencil/erasercombination being patentable, it would still infringe the first patentthat required only a graphitecore and a protective sheath,

because the new pencil/erasercombination has each ofthe claimed elements of thefirst patent.

In such cases, there isincentive to cross-license.For instance, using the pencil

example, the inventor of theimproved pencil wouldhave an incentive to seek alicense from the owner of the

prior-art patent so the inventorcould market his new pencil.The owner of the prior-art

patent, seeing the improvement and the likelihood that his customerswill want the improved pencil, would have an incentive to seek alicense from the inventor on the patented improvement so that he cancontinue in the pencil market.

Some other patentability requirements include enablement thespecification must teach how to practice the invention without undueexperimentation and written description proving the inventorpossessed the invention. To satisfy the enablement and wri ttendescription requirements, a patent must contain a complete descrip-tion of the invention, and how to make and use it. The patent appli-cation should include ways to make and use the invention in everyconceivable variation in the elements of the invention, including theway the inventor believes is best.

Can a combination of popular technologies be patented?

Yes, if the patent office decides that the combination is not obvious(US) or lacks an inventive step (non-US) and rejects claims to thecombination. This hurdle is almost always raised against patent appli-cations, but can be overcome in debates with the examiner during pros-ecution of the patent application (patent prosecution refers to dialogues

The patent system is intended to encourage invention and innovation, and should benefit

the LED industry, provided that the need for litigation can be alleviated by licensing and

avoiding infringement, sayAlice Martin andJohn Wappel of Barnes & Thornburg, LLP.

The patent system

strikes a balance

by granting

exclusive rights to

inventors for a

limited period of

time in exchange

for the inventors

knowledge and

technical

expertise.
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LEDSMAGAZ INE

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with the examiner to get a patent issued).Before concluding that something obvious

has been patented, examine the claims allowedto issue a patent. The title, abstract and descrip-tion in the specification may make the claim

seem unreasonably broad and obvious.However the actual claims, which define the

boundaries of the patent protection, may bevery limited and include a non-obvious ele-ment or combination.

To overcome an obviousness rejection, anapplicant must show that there was no teach-ing, motivation or suggestion known to thosein the technical field (those of skill in the art)to combine the technologies, and/or that evenif there was such teaching, motivation or suggestion to combine thetechnologies, the combination would not equal all of the elements inthe claims of the patent application. For example, just because paints,

canvas and brushes were known, the Mona Lisa was not obvious.Another way to overcome an obviousness rejection is to show thecombination constituting the invention yields surprising and unex-

pected results. If putting a device A that creates a fog together withdevice B that produces a pink light results in a blue beam then this isan unexpected result. Paper and adhesives may each be known, but acombination of paper with an adhesive border on one side that is sticky,

but readily peels off may not be obviousbecause an adhesive with such a function wasnot known in the art.

In hindsight, many inventions may seemobvious, but the courts have rejected

impermissible hindsight, developed byknowledge of the patent application as supportfor patent claim rejections. It is easy to thinkanyone could have made that combination,

but the examiner must use legal criteria to makethat call. To find out how a patent escaped orovercame obvious rejections, it is necessaryto examine the prosecution history (filehistory) that relates to the debate between theexaminer and the applicant to obtain a patent,

from the filing of the patent application in a patent office to thepatent issuing.

Reviewing the prosecution history may reveal how an obviousness

rejection (which is almost always made during patent prosecution)was overcome by an applicants arguments. Other informationobtained from the prosecution history includes the meaning of claimterms (claim interpretation and limitations), which is based on lit-eral meaning in view of explanations in the specification and com-ments in the prosecution history; and what prior art was considered

before the patent issued that was not found by the examiner to destroy

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Unless subject matter

is covered in the

claims of an issued

patent, it is not

protected by the

patent. Parts of the

specification not

appearing as claimed

elements cannot be

enforced.
http://ledsmagazine.com/http://www.iop.org/http://www.iop.org/http://www.carclo-optics.com/mailto:[email protected]://ledsmagazine.com/mailto:[email protected]://www.carclo-optics.com/http://www.iop.org/


14/35

LEDSMAGAZ INE

ledsmagazine.com August 2006 Copyright 2006 IOP Publishing Ltd www.iop.organd Cabot Media Ltd. All rights reserved14

novelty or make the invention obvious.There are procedures (re-examination) for the public to bring prior

art to the examiners attention. Prosecution histories are alwaysreviewed if there is litigation.

Also, the public is able to access the US Patent Office records atportal.uspto.gov/external/portal/pairafter a patent application is pub-lished (18 months after its priority date) or is issued as a patent. Forforeign (PCT) patent applications, the International Search Reportlisting prior art and the International Patentability Report are pub-lished and available to the public about 18 months after the PCTappli-

cation is filed. Sometimes the US or PCTpublications are mistakenfor actual patents. The publications provide notice to the public andmay be used as prior art against future applications, but these publi-cations do not confer any rights to exclude others from practicingthe invention. Although patents and patent application publicationslook similar, one must check the first page to determine if the docu-ment is a patent or merely a published application.

How can there be a series of patents going back in time to

a small number of parent (original) patents?

The first application(s) filed on an invention establish a priority datefor what is disclosed in the application. In the US, a priority date may

be established from a provisional (which is not examined but holds

that date for one year), utility or design application.Routinely the examiner might decide there is more than one inven-tion and divide claims into restriction groups. This forces applicantsto elect one group and file divisional applications if they want toobtain patents on the other groups of claims.

Applicants may also file continuations of the parent or other co-pending application to prosecute claims not originally filed. To do this,there must be support for the claim elements in the specification of the

previous patent application. For example, if the patent examiner hasissued a final rejection of the claims in an application, the inventormay want to file a continuation application narrowing the claims byadding additional disclosed elements in combination.

Another way to extend a chain (or tree) of patents is to file a con-tinuation-in-part (CIP) application. This application adds new mater-ial to the original specification and usually incorporates the newmaterial into claims. Going back to our pencil example, assume thesame person invented the pencil and the pencil/eraser combinationdiscussed above. While the first application for the pencil was pend-ing, the inventor thought of adding an eraser to one end of the pen-cil. Because the inventor did not include an eraser in his original patentapplication, he cannot claim an eraser. Therefore, the inventor wouldfile a CIP application with the new disclosure of the eraser and draftclaims covering the pencil/eraser combination.

In a CIP, the old material should get the priority date of the originallyfiled case and the new material should get as priority the date thatthe CIP was filed. In reality, priority issues are more complex. Also,where in the past priorities usually became an issue only at litiga-

tion, recently examiners have been determined to challenge prioritydates on the basis of alleged lack of support.

To avoid issuing multiple patents on the same invention by differentinventors, the patent office initiates an interference (US) or opposit-ion (foreign countries), which is a form of litigation. Priority dates

and proof of dates of invention are important in this outcome. Onlyone patent for one claimed invention will survive this procedure.

The further back in time a priority date extends, the more prior artcan be cut off because it is not prior and the more likely a patenteeor applicant is to prevail in a contest where others claim the same inven-tion. However, a disadvantage of creating a patent tree is that patentterms are 20 years from the priority date. So the further back in timethe priority date is, the shorter the patent term, i.e. the period duringwhich others may be excluded from making, using or selling the

patented product or method.

What does a patent do for the patentee?

Patents do not give patentees the right to practice the invention. Patents

give the right to exclude others from making, using, offering for sale,selling or importing the invention as claimed. If those who are excludedmake, use, offer for sale or sell the invention as defined in the claims,they infringe the patent and can be sued. A patent is only as useful asthe claims that appear at the end of it and these are sometimes writ-ten in cryptic legalese.

How to tell what a patent excludes others from doing

Claims define what an inventor is allowed to prevent others from doing.Claims are identified by a heading including the word claims andare numbered sequentially. Depending on what source is used to printa patent, claims will be either at the beginning or end of the patent.

The first part of each claim states whether a composition, an appa-

ratus or a method is claimed.The remainder of the claimlists elements of the inven-tion. No-one may make, use,sell, or offer for sale a prod-uct that has all of the ele-ments listed in a patent claimwithout the permission of the

patent owner. If the accusedproduct has each and everyelement of a patent claim, theclaim is said to read on theaccused product and the

product infringes the claim.To literally infringe, each

claim element must be pre-sent in the method, productsor apparatus being chal-lenged. There are other forms of infringement, if litera l infringe-ment is not found. It is important to see to what the claims relate to

because only the same type or category of subject matter can infringea claim, that is, a light bulb can infringe a claim for a light bulb, but alight bulb cannot generally infringe a method of making a light bulb.

Litigation offensive and defensive

If a patent is infringed, the owner or exclusive licensee enforces theirrights by, for example, warning the infringer and/or seeking a license

Another way to overcome an

obviousness rejection is to show

the combination constituting the

invention yields surprising and

unexpected results.

Patents do not givepatentees the right

to practice the

invention they

give the right to

exclude others

from making,

using, offering for

sale, selling or

importing theinvention as

claimed.
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15/35

agreement to obtain royalties from the infringer, and/or suing theinfringer. Litigation is very expensive, so avoiding it by agreementsis preferred. Remedies include injunctions and monetary damages. Ifit can be proven that infringement was willful, awards of damagesmay be increased by as much as three times the actual damages.

Defenses to infringement include proving the method or productdoes not have all of the claimed elements in the patent. Also, the patent

validity is challenged by determining whether criteria for patentabil-ity were satisfied and determining if there was any impropriety on thepart of the patentee, e.g. hiding art from the patent examiner.

Are patents a blessing or a curse?

The patent system was instituted based on the belief that the benefitsof patents were to reward inventors and stimulate invention. The goalwas to discourage trade secrets as a means of protection. Thus, the

patent system strikes a balance by granting exclusive rights to inven-tors for a limited period of time in exchange for the inventors knowl-edge and technical expertise. Disclosing intellectual property to the

public was to encourage further invention and innovation, using patentdisclosures as a building block for further innovations.

Although the US patent system provides protection for inventorsfor a limited time, competitors are free to design around the inventionprotected by a patent and build a better mousetrap of their own. Thisenergy should benefit an industry. Litigation is expensive, risky andtime-consuming, and the need for litigation can be alleviated by licens-ing and avoiding infringement.

Ultimately, the incentive allowing inventors to reap the rewardsfrom their inventions encourages technological advances that bene-fit society as a whole, as well as industry and the inventors.

About the authors

Alice O Martin is a partner in the Chicago office of Barnes &

Thornburg LLP and is co-chair of the Life Sciences Practice Group.

Her practice covers genetics, genes and gene products and control,

molecular genetic methods, treatments and assays for various

diseases, surgical techniques, tissue culture methods, and waste

management and monitoring. John P Wappel is an associate in the

Intellectual Property Department of the Chicago office of Barnes &

Thornburg LLP. His background is in mechanical engineering and

he focuses his practice on patent and trademark prosecution and

litigation and related matters.

On our website: Patents&LicensingChannel:

www.ledsmagazine.com/articles/features/1/4/8

Links

Litigation is very expensive,

so avoiding it by agreements is

preferred...If it can be proven

that infringement was willful,

awards of damages may be

increased by as much as three

times the actual damages.


LEDSMAGAZINE

15

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16/35

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17/35

Lighting industry still learning

how to apply advanced LEDs

White light is boring color is the only reason we get excited aboutLEDs, announced Dominic Meyrick, Lighting Principal of HoareLea, a specialist lighting design consultancy, in an impassioned talkat the Applying Advanced LEDs conference, held on 28 June in

London, UK. However, there were many other speakers and audiencemembers who could appreciate the benefits of using white LEDs indifferent applications, while at the same time acknowledging themyriad of challenges still to be faced.

Industry standards

The lack of industry standards is a recurring issue. In a panel session,Kevin Dowling, vice-president strategy and technology at ColorKinetics, listed several organizations that are developing standards,

procedures and metrics for the solid-state lighting industry. There israpid activity but its never quick enough, he said. Standards needto make sense to manufacturers, specifiers and the end users.

The industry has to decide what characteristics to measure, the meth-

ods of measurement and also identify accredited laboratories to per-form the measurements, said Dowling. Required metrics includeluminous flux, efficacy, lifetime, color rendering index (or an updatedreplacement), light distribution, and binning.

Among the organizations involved in addressing these issues arelighting standards bodies (e.g. IESNA, ANSI, IEC, LTEC), color sci-ence and photometry organizations (e.g. CIE, CORM), research orga-nizations (e.g. NIST, NPL, Lighting Research Center) and industrygroups (e.g. NEMA, NGLIA). Dowling said that we are likely to seereleased review documents this summer.

In late July, the US Department of Energy's solid-state lightingprogram announced a collaboration with the Illuminating EngineeringSociety of North America (IESNA) to develop standards for solid-state lighting (for further information see www.ledsmagazine.com/articles/news/3/7/23).

LEDs in building design

Orri Petursson, senior designer with lighting designers Speirs andMajors Associates, said that there are many claims surrounding LEDsand it is hard to distinguish truth from hype. On the majority of thecompanys current schemes, LED-based products are part of a lightsource pallet. In most cases, they take the role of feature lighting, notfunctional lighting, he said, highlighting three main reasons for this:white light LEDs tend to change color of their own accord and the cost

per lumen is too high to be able to compete with conventional lightsources. Also, the quality of white light from LEDs is sub-standard

compared with what we are used to and accept. However, for RGB,Petursson agreed with Hoare Leas Meyrick that LEDs offer a far bettersystem than conventional methods of color mixing.

Most LED products are custom built around a project, rather thanoff-the-shelf. However, lighting designers want more from their sup-

pliers. Wheres the unique application, wheres the unique fitting,wheres the blue-sky thinking? said Meyrick. We want unique prod-

Some lighting designers view LEDs as a troublesome technology that is only useful for color-

changing applications, but the LED community is now moving to address the concerns of

the lighting industry.Tim Whitakerreports from the Applying Advanced LEDs conference.


L I G H T I N G LEDS

MAGAZ INE

17

RGB LEDs for architecture: the Sports City Tower, a 1000 ft (300m)

structure being built in the Gulf state of Qatar, will be covered in a

state-of-the-art lighting skin composed of around 4000 individually

addressable LED elements. The tower will support the Olympic

flame for the Asian Games in November 2006. Kevan Shaw Lighting

Design is responsible for the towers lighting scheme, which

features a network of RGB LED elements. Each custom-made unit

contains six 1W LEDs, two of each color, and has custom optics to

provide a predominant distribution below the horizontal, said

design director Kevan Shaw.
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18/35

ucts from the LED industry dont ram LEDs into boring fixtures.The majority of LED manufacturers get away with not supplying

photometric data, said Petursson, and this is against the norm. Forother sources, we wouldnt let the manufacturers through the doorwithout for example seeing the .iesfiles, he said.

Petursson also highlighted the issue of not being able to find replace-ment LED products when the original LED-based luminaire fails,given the speed of developments. Will the new product match theother 99 working versions that have been burned in for 20,000 hours?he said. Can you ever expect to specify the same product twice?

Petursson commented on cost benefits and lifetime savings in archi-

tectural projects, pointing out that hitting the original budget is themost important factor for his company. The building budget and main-tenance budget come from two totally different pockets, he said.

Quality of white light

After a morning of discussing LED technology, Ken Kane, vice-president product and market development of Lighting Services Inc(LSI) put things into context: LED developments are important butnot to my customers, he said. Whats important to specifiers and theirclients is visual effect the quality of the light that theyre receiving.

During the same session, Color KineticsKevin Dowling said thatcustomers want easier and more flux control, fun and interesting

lighting, and the ability to controland enhance moods,environmentsand ambiance. In terms of lumi-naires, lighting designers andarchitects want more variation,more control and more uniquelooks, with the ability to paint withlayers of light, he explained.

Luminaires can be used inreal applications today, saidKane. LEDs are just one compo-nent of the luminaire, Imagineif we had to go to a store and buya filament, a bulb enclosure and

a screw-in socket, then put them all together to make an incandescentlamp, he said.

For LSI, which has licensed TIRs Lexel technology for use inits LumeLEX luminaire, the focus is on interior lighting and thekey factors are high quality, directional light, as well as accuracy andconsistency. The ability to easily adjust color temperature is akey feature of LSIs luminaire. Kane toldLEDs Magazine about ameeting with museum and gallery curators, who could choose the

best color temperature (around 4000K) to display their artifacts, whilestaying within the maximum illuminance level of about 50 lxmandated by conservation requirements. The problem was that atsuch low light levels, the halogen system that the LumeLEX was

being compared with looked muddy, and did not render the colors aswell, said Kane.

UV content is also very important for museum and gallery lighting,and is one aspect of photometric information that should be supplied,ideally in the same format as for a traditional source. Crucially, thisinformation has to be at the system level. Users dont care what thelight output and distribution of a CFL is like, until its inside the lumi-naire, said Kane. The same applies to LEDs.

LEDSMAGAZ INE


dissipatedpower

heat current

forwardvoltage

lightoutput

increase decrease

Fig. 1. The usable light pentacle: this complex interaction model

shows how changes in different parameters affect the LED light

output. Courtesy of Future Lighting Solutions.

Another conference in London in June also discussed architectural

lighting. One of the highlights of the Future of LEDs event was a

talk by Rogier van der Heide of ARUP, who won the 2005 IALD

Radiance award for his lighting design work on the Galleria shoppingmall in Seoul, Korea (see www.ledsmagazine.com/articles/

features/2/1/4).The Galleria exterior, arguably the worlds largest

TV imaging system at ultra-low resolution, has become an attraction

in its own right, as well as pulling in many more shoppers.

Lighting provides a meaningful addition to the built

environment, said Van der Heide, suggesting that lighting

decisions should be driven at a strategic level rather than being

an afterthought. We are interested in the value that we can add

as designers.

Lars Bylund, a professor at the Bergen School of Architecture

in Norway, highlighted a number of barriers for LEDs. These include

a lack of definitions and standards, as well as numerous

performance, price and technical issues. For a project that willbe built in two years time, we are required to plan and specify

the lighting now, said Bylund. This makes it very difficult for us

to use LEDs.

Bylund also questioned the use of existing infrastructure. Should

we build LED installations in the same way as we build with

fluorescents? he asked. Another problem is the use of colored

LEDs for city beautification projects, which can add to overall light

pollution levels and risks making some cities looking like miniature

versions of Las Vegas, said Bylund.

But there are many positives relating to energy savings and

maintenance. Bylund was involved in the project to install LEDs

into the Turning Torso building in Malm, Sweden (see

www.ledsmagazine.com/articles/features/3/4/4).White LEDswere built to match the interior curved surfaces and reduced the

load by 60% compared with fluorescents, while removing

maintenance concerns.

LEDs in architecture: positives and negatives

There is rapidactivity but itsnever quick

enough.Standards need

to make sense tomanufacturers,

specifiers andthe end users.
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Kane spoke about lifetime, acknowledging that the rules for tradi-tional lighting dont readily apply to LEDs. Even so, customers shouldexpect a system warranty from a luminaire manufacturer; Kane gavethe following warning: If you dont see system output data run!

LED success factors

Volker Mertens, head of LED marketing for Osram Opto Semicon-ductors, discussed some of the success factors associated with usingLEDs. To achieve a similar lighting function as for other light sources,he said, the cost of LED systems is often a disadvantage. Of course,

customers are concerned about the overall system cost, includingoptics, drivers and heat sinks, as well as the LEDs. High costs can beoffset in part by the development of standard LED products at the mod-ule and subsystem level, and this is what the new Osram LED systemsdivision hopes to achieve.

Other advantages include fully exploiting the technical benefitsof LEDs, for example in color-changing or to simulate human

biorhythms. Retrofits with standard sockets can get LEDs faster tomarket, said Mertens, but this is not a good long-term solution dueto heat management issues.

Francois Mirand, European technical manager for FutureLighting Solutions, admitted that comparing different LEDs can

be a problem. However, it is possible if the key performance factorsare understood and the usable light is determined for eachapplication (see figure 1). Key factors includethe luminous flux, which is usually specifiedat a junction temperature of 25 C and atypical drive current; the packaging thermalresistance; the extent to which the fluxand forward voltage vary with temperature;and the effect of changing the drive currentup to its maximum level. Not only is themaximum LED junction temperature im-

portant, but also the operat ing condi tionsat which the stated lumen maintenance can

be achieved.

Drivers and optics

Gordon Routledge, managing director of Dialight-Lumidrives, dis-cussed LED technology trends such as moves to higher drive currentsup to 1500mA and higher maximum junction temperatures of up to185C. While this can make some applications much easier, notablyautomotive lighting, Routledge says that clients are also becomingmuch more demanding, which presents even more challenges.

Built-in drivers (i.e. drivers incorporated within the LED fixture)represent an opportunity to reduce system costs and increase systemefficiency while adding more features, but this could happen at the

expense of increased form factors and thermal challenges. Often, elec-trolytic capacitors determine the life of the driver.Lumidrives has built modules with on-board drivers for low-voltage

(1224 VAC or DC input) applications, which eliminate electrolyticcapacitors and have on-board thermal feedback (see figure 2). Theseoperate at 350mA, and 700 and 1000mAversions with the same formfactor are planned for later in 2006. Mains voltage is more problem-atic, partly due to the need to ensure EMC compliance.

Tomi Kuntze, managing director of Ledil Oy, said that the challengeof integrating optics in LED-based luminaires is to serve money throughintegration without endangering the quality and differentiation ofend products. Optics save a number of useful purposes, such as redir-ecting and reshaping light output and smoothing color variations, allof which help to cut down the system price of the finished luminaire.

The first step is to integrate the optics with theLEDs, using mechanical methods, glue, heator adhesive tape. Next, it is possible to integ-rate mechanical and heat transfer elements,which could involve the use of interchange-able elements (lenses or optics), or buildingoptics with multiple elements, or combininga heat sink and a reflector as a single part (seefigure 3). The next step is to consider ways tointegrate decorative and protective elementsand finally to incorporate control electronicsinto the finished luminaire.


LEDSMAGAZ INE

19

Fig. 3. Integrating optics: a precision castingallows a combined heat sink and reflector.

LED developments

are important butnot to my customers.

Whats important tospecifiers and their

clients is visual effect the quality of light

that theyre getting.

Fig. 2. Dialight-Lumidrives has built modules with on-board drivers for low-voltageapplications that eliminate electrolytic capacitors and incorporate thermal feedback.


20/35

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21/35

Photonic quasicrystals boost

LED emission characteristicsCustom photonic quasicrystal structures can help to efficiently extract light from LEDs and

place it into a desired emission profile, according to Majd Zoorob and Gregory Flinn.

The case for high-brightness LED adoption lies increasingly with thepotential cost savings made possible by reduced servicing, longer life-times and higher overall efficiency, as well as better technical func-tionality for many lighting applications. These cost benefits for theend-user are at present balanced by considerable performance chal-

lenges still facing the manufacturing industry, including providingsufficient luminous flux for projection applications. To help achievethese goals, the use of photonic quasicrystal structures can bothsignificantly enhance the luminous flux emitted by current LED tech-nology and also place the light where it is needed.

Improved light output

High-brightness LEDs represent a multi-billion dollar market that ispredicted to grow to $8.2 billion by 2010. Strong future growth is antic-ipated, in particular for ultra-high-brightness devices. In moving fromniche applications to true mass adoption, potential illumination mar-kets include room and desk lighting, projection lighting for the auto-motive and display markets, and task lighting. Also, development of

existing flat screen backlighting applications is expected to evolvebeyond the demand associated with mobile phone handset displays.Nonetheless, before all these lighting applications can be addressed

adequately, the industry still faces considerable performance-relatedchallenges. Supplementary to color matching and color balance issuesover the lifetime of a device, the tota l and specific luminous fluxcurrently available is still only borderline for some tasks, despite theincreased efficacy achieved by modern LEDs. White light projectiontasks such as automotive headlighting and projection displays are twoexamples of applications needing exceptionally highly efficient, aswell as tailored, light output.

To help meet these demands, the effective light output of an LEDcan be optimized by both improving photon extraction efficiency andeffectively shaping the intensity distribution for the task at hand, uni-form or not. Planar photonic crystals, and in particular photonic quasi-crystals, are a cost-effective and flexible way of achieving this goal.

Photonic crystals in optics

The term photonic crystal (PC) simply refers to a repeating structurein an optical medium, the characteristic dimensions of the structure

being comparable to optical wavelengths (hence photonic). Thesestructures can be 1D, usually as repeating layers of differing materi-als; 2D, as will be discussed here; or even 3D, these structures beingthe most difficult to prepare and being the focus of much research. Theterm crystal refers to the repeating structure in one, two, or all threedimensions, in direct analogy with the structure of true crystals found

in nature. Similarly, this repetition is also critical to their function.Instances of indirect application of PCs have been present in optics

for many years, a well documented but somewhat simplistic examplebeing the Bragg reflectors used in semiconductor vertical-cavity sur-face-emitting lasers or in some LED structures.

Planar PCs, i.e. those credited with the more comprehensive pho-ton management qualities often highlighted in the photonics press,can be generated by etching 2D arrays of shaped structures into adielectric surface (see figure 1). Although by no means confined tothe optical regime, when fabricated with appropriate dimensions thesearrays form the optical analogy of a semiconductor material, produc-ing a range of allowed or disallowed photon states of the PC lattice.Just as for a charge carrier moving through a semiconductor, the con-tinued propagation of photons incident on the PC is subject to the prop-erties of the lattice in relation to the direction of travel.

Suitable choice of symmetry for the array, pitch (generally sub-micron, /2 to 2), depth (100200 nm) and shape of the etched


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Fig. 1. Planar photonic crystals etched into a dielectric surface.

Modern nanoimprint lithography enables both high accuracy

of features of the array (a), as well as high repeatability (b).

Courtesy of Molecular Imprints Inc.


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features are all factors that influence the specific optical properties of

the structure. Further fine-tuning can be accomplished by filling thestructure with a material of differing refractive index.In simple terms, planar PCs represent a range of tools that can be

used to control the emission and propagation of light on a scale smallenough to permit their use in integrated optical systems. More specif-ically, this includes short-range spatial beam steering, spectral fil-tering, polarization control, optical time delay and beam division. Forexample, a planar PC structure introduced into the output facet of aVCSELprovides increased lateral control of the lasing mode profile,enabling these top-emitting laser diodes to be driven at higher out-

put powers without sacrificing the coupling efficiency into single-mode telecommunications fibers (see, for example, www.alight.dk).

LED manufacturing

The increased interest in the use of planar PC structures for more com-plex optical tasks is due to the recently acquired ability to create intri-cate 2D structures using nanoimprint lithographic techniques. In theinstance of LED manufacturing, the PC is imprinted into the topmostsurface of the LED, namely into the semiconductor material coveringthe active layer.

Critically, nanoimprinting lends itself to the cost-effective manu-facture of custom photonic crystal patterns in large volume and is thusan important enabler for the LED industry. As we shall see, it is theability to reliably produce structures with seemingly peculiar sym-metries that allows the realization of LED devices with both enhancedlight output and tailored illumination profiles.

Emission enhancement with photonic crystals

The problem with LEDs is the high refractive index of the semicon-ductor material sandwiching the active quantum well layers. As thesemiconductor interfaces directly to air at the output facet, a signifi-cant amount of the light generated inside the active region remainstrapped within the capping layer (see figure 2).

Extraction efficiency can be improved by something as simple asroughening the emitting surface, or by adding features like microlensesto the surface of the LED to help channel the emission. However, earlyPC-patterned LED structures have already been shown to increase thelight extraction efficiency several-fold and now newer, optimizeddesigns are responsible for generating still higher extraction effi-

ciencies. The PC structure improves emission extraction by provid-ing preferred leakage paths out of the LED structure for photonstrapped in the capping layer.

Emission management with photonic quasicrystals

In spite of the gain in extraction efficiency, the problem with regularPC structures is that they are anisotropic, i.e. the regularity of the struc-ture translates into a gain in the light extraction that is directionallydependent. More exactly, there is an interplay between the gain inoverall extraction efficiency, its variation into different emission direc-tions and the overall illumination homogeneity of the emission cone.This interplay is decided by the design parameters chosen for the PC,with the result that not all of these aspects can be optimized indepen-dently of one another. For example, while the overall intensity canincrease compared with a Lambertian emitter, this can be at the expenseof the spatial distribution of the light flux.

The most common PC geometries are based on naturally occurringsymmetries, like square or triangular, but similar structures can be

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Fig. 3. SEM images of fabricated pinwheel (a) and sunflower (b)

photonic quasicrystals.

GaN

air

substrate

air

4.35%

67.8%

23.5%

4.35%

Fig. 2. Light emission from the active layer in an LED is hindered by

the high refractive index step between the semiconductor layer and

air. Figures show the percentage of light in each layer.
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made with unusually high levels of symmetry. Structures exhibitingartificial symmetries are termed photonic quasicrystals (PQCs) andseem to lack order when examined over a short range (see figure 3).

The advantages of PQCs over regular PC structures lie in the broa-der design space available to the optical engineer. That is, PQCs can

be designed to perform similarly to regular PC structures, but at thesame time allow more flexibility, greater design tolerance and lessdirectional dependence. The reason is that although the overall opti-cal properties of any PC are by and large determined by its long rangerepetition, the lack of short range order exhibited by a PQC acts to blurthe dependencies of these properties.

Although figure 4 shows only a limited number of PC and PQC para-meter combinations, it is illustrative of a general effect seen with PQCstructures, namely that the far-field emission pattern is flatter. An addi-tional result is that the profile is rotational symmetric, a propertythat is not the case for lower order PC structures.

For many illumination applications the emission in far-field is ideallyflat. Projection display applications are a good example of where costly

optical design is necessary to remove variations in the projected inten-sity from conventional illumination technologies.

Conversely, international automotive headlight standards call fora wide field of illumination containing specific intensity maxima andminima, as well as the sharpness of the cut-off (for further informa-tion seeen.wikipedia.org/wiki/Headlight).In this instance, a tailoredquasicrystal structure may be preferable, as both the light extractionand illumination placement can be optimized to a higher degree thanis possible with regular PCs.

Working the numbers

Table 1 compares light extraction efficiencies for four different typesof top-emitting LEDs; a simple flat surface, a roughened device anddies with PC and PQC structures. Values are given both for the totalemission and for emission into a 30 cone (a value of 24 is a com-mon criteria for digital light projection applications).

While the values given for flat and roughened LED surfaces arebased on calculations given in reports in the literature, for both PC and


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Fig. 4. Comparison of simulated PC and PQC LED far-field emission patterns for different lattice parameter combinations, normalized to a

Lambertian source (top left). The profiles illustrate the optical power emitted into different angles: emission profiles generated by PQC

structures show better illumination homogeneity.

Standard Roughened LED die with optimized LED die with photonic

LED die LED die regular photonic crystal quasicrystal (predicted)

LED die: light in 30 cone 7.65 12.4 34 >37.5

LED die: total line output 11 22 35.2 ~38.5

LED die: percentage light in 30 cone 69% 56.6% 60%

Packaged LED: extraction efficiency 1 1.62 4.4 >4.9

enhancement in 30 cone

Table 1. Enhanced performance with PQC structures

Comparison of emission characteristics for a variety of LED emission surfaces. Best overall performance comes from PQC structures,

providing significantly better light extraction and directional control.
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PQC LEDs the values given represent simulations using optimizedparameters for typical flip-chip GaN structures. In the case of a reg-ular PC, this means choosing parameters that maximize the emissionintensity into the designated cone.

Both the PC and the PQC structures improve total emission and the

amount of light channeled into the 30 cone. The PC structure showsconsiderable intensity variation in the illumination profile para-meters could be re-optimized to improve the profile, but this would

be at the expense of extraction efficiency. On the other hand, the PQCstructure actually improves illumination homogeneity and overallemission simultaneously, pushing almost five times more light intoa 30 cone. In terms of brightness close to the central axis, luminousflux is increased by a factor of 10 compared with an unroughened LED.For the improvement in total light extraction, an un-roughened LEDdie that is encapsulated can extract 20.9% of the total light from thetop surface, while an encapsulated PQC LED would extract 47.4%.

The future for PQC LEDs

The need to efficiently extract the light generated inside the activelayer of an LED out into a desired illumination profile is a majorhurdle facing LED development. The beauty of the PQC approach, inaddition to the aspects already highlighted and contrary to some othersurface treatments, is that the enhancement can be easily applied tothe total emitting area, even for larger LED devices. The enhancedluminous flux thus scales linearly with increasing LED mesa size.

Design-specific aspects, like custom illumination profiles and fab-rication tolerant designs, can also be readily included. Integration ofthe nanoimprinting step into the production process thus allows man-

ufacturers to maximize LED functionality for a range of lighting taskswhile minimizing supplementary manufacturing costs. Moreover, theneed for complex beam-shaping optics in the packaging is reduced,thus simplifying overall LED design and improving cost-effective-ness still further. Significant work has already been undertaken with

leaders in the LED industry to closely examine these aspects.Notably, the simulations highlighted in the text only account for the

enhancement to the light extraction from the LED surface, all with-out invoking the Purcell effect. That is, additional benefits are expectedwhen the PC or PQC lies close enough to the active region to be ableto influence intrinsic emission directly. Although work is in progresswith the industry to develop optimized LED die templates, initial sim-ulations for devices indicate that wall-plug efficiency for LEDs couldtop 75%. Should this development come about, the prospects for con-ventional lighting technology should dim considerably in compari-son with the bright future anticipated for solid-state devices.

About the authors

Majd Zoorob is the chief technology officer of Mesophotonics Ltd,Southampton, UK. Tel. +44 (0)23 8076 3752. E-mail: majd.zoorob@

mesophotonics.com. Gregory Flinn heads Putting Photonics into

Context, Munich, Germany. E-mail: [email protected].

Mesophotonics Ltd (www.mesophotonics.com)specializes in the

development of PC and in particular PQC structures, has substantial

experience and IP in their application to photonics in general and is

currently working with partners to bring this technology to the market.

Mesophotonics has a joint marketing agreement with Molecular Imprints

Inc to address volume manufacturing using nanoimprint lithography.

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mailto:[email protected]:[email protected]:[email protected]://www.mesophotonics.com/http://ledsmagazine.com/http://www.iop.org/http://www.iop.org/http://www.enfis.com/mailto:[email protected]:[email protected]://ledsmagazine.com/http://www.iop.org/http://www.mesophotonics.com/mailto:[email protected]


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Optical design software improves

LED system design and validation

Simulation software can significantly improve the design process ofLED- and OLED-based systems, and virtual validation without phys-ical prototyping allows engineers to save time and money.

The first step is to evaluate the capabilities of the software that willbe used for the design. Agood lighting design including LEDs requiresvery accurate simulation and analysis tools. Thus optical design, pho-

tometry, colorimetry and photo-realism capabilities are necessary inthe design process of LED-based systems.For instance, photometry allows designers to simulate and analyze

the intensity, illuminance and luminance of the 3D system. The moreaccurately these physical results can be simulated, the better optimizedthe final LED-based system will be. Moreover, very accurate algo-rithms (full spectral propagation) based on real physical laws are anecessary condition for improving the design process. These reasonsexplain why very advanced software in optics is the necessary firststep in dramatically improving the design process.

Device catalog

The second step is the LED or OLED catalog availability within the

software. Ideally, a designer using LEDs to build a lighting systemneeds to find the best component to optimize and to get the best light-ing result. To make this possible, optical design and lighting softwarehas to provide the largest-possible LED catalog to develop lightingindustrial systems. In other words, with such a library integrated inthe software, the designer can very easily choose a component froma long list, and is able to try several LED configurations.

Overall, two points are extremely significant in this library. Firstly,all the LEDs in the catalog must be commercially available and thelibrary must include components from all the major LED manufac-turers (see figure 1). Moreover all the components in the catalog haveto be accurately referenced. Thus, industrial reality is imported in thesoftware and gives much added value by hugely increasing the real-ism and the relevance of the simulation.

Secondly, the way in which the LED is modeled is also fundamen-tal in order to obtain very good results that can be virtually validated andthus exert a real influence on the design process. The best way to do thatis to first model the mechanical geometry of the component and then tomeasure its light-emitting properties with a very accurate optical bench.Only after that does it become possible to obtain a realistic model basedon a market-available LED that can be used in the simulation.

Design validation

To increase accuracy, geometric modeling is important because lightcan interact with the LED structure as well, influencing the propa-gation. This explains why ray files are not sufficient for obtaining high-

level results. Exactly the same method is used for OLED modeling;OLED libraries can be made as easily as LED libraries. Optis suppliessoftware with algorithms based on accurate physical laws and withlibraries made up of measured LEDs. These online catalogs are widelyused for a range of different applications such as displays, light guidesfor cellphones, headlamps, tail-lamps, yacht interior design, archi-tecture and many more.

Algorithms and catalogs are two necessary factors for improvingthe process of LED-based systems design. When these two elementsare present it is possible to save time and cost, to improve system per-formance and to optimize quality.

One of the goals of software is to virtually validate a design to reducethe number of physical prototypes. This requires a very high level ofrealism in the software and significant gains can be achieved when thisis realized. Algorithms and libraries are necessary but not sufficient.

The third important factor is the analysis capability of the software.The aim is to fully understand the results obtained with a mechani-cal design using LEDs from a catalog, enabling adjustment of para-meters such as position, power and color of the LED. This adjustmentwill in turn reduce ghost images, stray light and

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