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308 Science E. Fred Schubert and Jong Kyu Kim Solid-State Light Sources Getting Smart
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Solid-State Light Sources Getting Smart
E.Fred Schubert and Jong Kyu Kim
More than a century after the introduction of incandescent lighting and half a century after the introduction of fluorescent lighting,solid-state light sources are revolutioniz-ing an increasing number of applications.Whereas the efficiency of conventional incandescent and fluorescent lights is limited by fundamental factors that cannot be overcome,the efficiency of solid-state sources is limited only by human creativity and imagination.The high efficiency of solid-state sources already provides energy savings and environmental benefits in a number of applications.However,solid-state sources also offer controllability of their spectral power distribution,spatial distribution,color temperature,temporal modulation,and polarization properties.Such ‘‘smart’’light sources can adjust to specific environments and requirements,a property that could result in tremendous benefits in lighting,automobiles,transportation,communication,imaging,agriculture,and medicine.
T
he history of lighting has taken veral
rapid and often unexpected turns (1).
The first commercial technology for
经典英文歌曲试听
lighting was bad on natural gas that rved thousands of streets,of-fices,and homes at the end of the 19th century.As a result of the com-petition from Edison _s incandescent lamp,gas-lights were strongly im-proved by the u of mantles soaked with the rare-earth compound tho-rium oxide,which con-verted the gas flame _s heat energy and ultravio-let (UV)radiation into visible radiation.Ulti-mately,however,the gaslights shown in Fig.1were displaced by in-candescent light bulbs first demonstrated in
1879.Fluorescent tubes and compact fluorescent lamps became widely
available in the 1950s and early 1990s,respec-tively.Along with high-intensity discharge lamps,they offer a longer life and lower power
consumption than incandescent sources,and have become the mainstream lighting tech-nology in homes,offices,and public places.The efficiency of fluorescent lamps bad
on mercury vapor sources is limited to about 90lm/W by a fundamental factor:the loss of
energy incurred when converting a 250-nm UV photon to a photon of the visible spectrum.The efficiency of incandescent lamps is limited to about 17lm/W by the filament tem-perature that has a
工程档案管理制度maximum of about 3000K,which results,as predicted by blackbody radiation theory,in the utter dominance of invisible infrared emission.In contrast,the prent efficiency of solid-state light sources is not limited by fundamental factors but rather by the imagination and creativity of engineers and scientists who,in a worldwide concerted effort,are longing to create the most efficient light source possible.Bergh et al .(2)discusd the huge poten-tial benefits of solid-state light sources,in particular reduced energy consumption,depen-dence on foreign oil,emission of greenhou gas (CO 2),emission of acid rain–causing SO 2,and mercury pollution.Solid-state light-ing could cut the electricity ud for lighting,currently at 22%,in half.Although tremen-dous energy savings have already materialized ,traffic lights that u light-emitting diodes (LEDs)consume only one-tenth the power of incandescent ones ^,there is a sobering possi-bility that energy savings may be offt by incread energy consumption:More waste-ful usage patterns,abundant u of displays,and an increa in accent and artistic lighting may keep the u of electricity for lighting at its current level E 11%in private homes,25%in commercial u,and 22%overall (3)^.Several promising strategies to create white light with the u of inorganic sources,organic sources,and phosphors are shown in Fig.2,
including di-,tri-,and tetrachromatic ap-proaches.The approaches differ in terms of their luminous e
fficiency (luminous flux or visible light output power per unit electrical input power),color stability,and color ren-dering capability (i.e.,the ability of a light source to show or B render [the true colors of an object).It is well known that there is a fundamental tradeoff between color rendering and luminous efficacy of radiation (luminous flux per unit optical power).For optimized
wavelength lection,dichromatic sources have the highest possible luminous efficacy of radiation,as high as 425lm/W,but they R EVIEW
Department of Electrical,Computer,and Systems En-gineering and Department of Physics,Applied Physics,
and Astronomy,Renslaer Polytechnic Institute,Troy,
NY 12180,
USA.
bib
Fig.1.(A )1880s illustration of the nightly illumination of a gaslight with a thorium oxide–soaked mantle.(B )Replica of Edison’s lamp.(C )Contemporary compact fluorescent lamp.(D )High-pressure sodium lamp.27MAY 2005VOL 308SCIENCE www.sciencemag
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poorly render the colors of objects when
illuminated by the dichromatic source.Tetra-chromatic sources have excellent color ren-dering capabilities but have a lower luminous
efficacy than dichromatic or trichromatic
sources.Trichromatic sources can have both
good color rendering properties and high
luminous efficacies (9300lm/W).
Figure 2also shows veral phosphor-bad white light sources.Such sources u
optically active rare-earth atoms embedded in
an inorganic matrix.Cesium-doped yttrium-aluminum-garnet (YAG)is a common yellow
phosphor.However,phosphor-bad white
light sources suffer from an unavoidable Stokes
energy loss due to the conversion of short-wavelength photons to long-wavelength photons.This
energy loss can reduce by
10to 30%the overall effi-ciency of systems bad on
phosphors optically excited
by LEDs.Such loss is not in-curred by white light sources
bad exclusively on mi-conductor LEDs.Further-more,phosphor-bad sources
do not allow for the exten-sive tunability afforded by
LED-bad sources,partic-ularly in terms of spectral
成绩英语composition and temporal
modulation (YAG phospho-rescence radiative lifetime is
in the millicond range).
The luminous efficien-cy of a light source is a key
metric for energy savings
elishaconsiderations.It gives the
luminous flux in lumens
(light power as perceived by
the human eye)per unit of
electrical input power.Lu-minous efficiencies of 425lm/W and 320lm/W could potentially be achieved with dichromatic and trichromat-ic sources,respectively,if solid-state sources with per-fect characteristics could be fabricated.Perfect materials and devices would allow us to gen-erate the optical flux of a 60-W incandescent bulb with an electrical input power of 3W.Besides luminous efficiency,color render-ing is an esntial figure of merit for a light source ud in illumination applications.It is a very common misconception that the color of an object depends only on the properties of the object.However,as George Palmer first found in 1777,the perceived color of an object equally strongly depends on the illumination source E for Palmer _s original paper,e (4)^.Illuminating colored test samples with differ-ent light sources,he found that B red appears orange [and,more strikingly,B blue appears green.[Thus,the B true color [of an object requires that we have a certain reference il-luminant in mind.Today,a procedure similar to Palmer _s is ud:The a
pparent color of a t of sample objects is assd (quantitative-ly in terms of chromaticity coordinates,no longer just qualitatively as Palmer did)under illumination by the test light source and then by the reference light source.The color dif-ferences of a t of eight standardized color samples are added.The sum,weighted by a prefactor,is then subtracted from 100.This gives the color rendering index (CRI),a key metric for light sources.A high CRI value indicates that a light source will accurately render the colors of an object.Although trichromatic sources already give
very good CRI values,tetrachromatic sources
give excellent CRI values suitable for esn-tially any application.The emission spectrum,luminous efficacy,and color rendering proper-ties of a tetrachromatic white LED-bad
source with color temperature of 6500K are shown in Fig.3.Color temperature may ap-pear to be a somewhat surprising quantity,as color and temperature would not em to have a direct relationship with each other.However,the relationship is derived from Planck _s blackbody radiator;at increasing temperatures it glows in the red,orange,
yellowish white,white,and ultimately bluish white.The color temperature is the tempera-ture of a blackbody radiator that has the same chromaticity as the white light source con-sidered.Figure 3sho
ws that a favorable wave-length combination is l 0450,510,560,and 620nm,giving a luminous efficacy of 300lm/W and a CRI of 95.Such a CRI makes tetrachromatic light sources suitable for prac-tically any application.However,the emission power,peak wave-length,and spectral width of inorganic LEDs vary with temperature,a major difference from conventional lighting sources.LED emission powers decrea exponentially with tempera-ture;low-gap red LEDs are particularly nsi-tive to ambient temperature.As a result,the chromaticity point,correlated color tempera-ture,CRI,and efficiency of LED-bad light sources drift as the ambient tempera-ture of the device increas.An example of the change in chromaticity point with junction temperature is shown in Fig.4for a trichromatic LED-bad light source (5);the chromaticity changes by about 0.02units,thereby ex-ceeding the 0.01-unit limit that is considered the maxi-mum tolerable change by the lighting industry.Fur-thermore,the CRI changes from 84to 72.To avoid this change,corrective action must be taken by tuning the rela-tive electrical input powers of the LEDs.Energy-efficient adaptive drive electronics with integrated temperature compensation are already under development.White sources that u phosphor,particularly UV-pumped phos-phor sources,have great col-or stability and do not suffer from the strong change in chromaticity and color ren-dering.This is becau the intra–rare-earth atomic tran-sitions occurring in phosphors do not depend on temperature.Technological Challenges What specific advances will be required to move solid-state light sources from their curre
nt performance clor to their funda-mental limits?What are the ‘‘bottlenecks’’that will need to be overcome to enable specific types of control for smart lighting systems?The major technical challenges in solid-state lighting can be categorized into three groups:&Epitaxial and bulk crystal growth;materials including nanomaterials and sub-strates;
phosphors Fig.2.LED-bad and LED-plus-phosphor–bad approaches for white light sources implemented as di-,tri-,and tetrachromatic sources.Highest luminous source efficiency and best color rendering are obtained with dichromatic and tetrachromatic approaches,respectively.Trichromatic approaches can provide very good color rendering and luminous source efficiency.
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&Device physics;device design and architecture;low-cost processing and fabri-cation technologies
&Packaging;integration of components into lamps and luminaires;smart lighting systems We next discuss veral important technical issues involved in meeting the challenges.Additional challenges and a roadmap with spe-cific goals were prented by Tsao (6)and Rohwer and Srivastava (7).Here,we empha-size inorganic materials and devices,which at this time are more advanced in terms of lu-minance and reliability than organic devices.Internal efficiency.The development of efficient UV emitters (G 390nm),green emit-ters (515to 540nm),yellow-green emitters (540to 570nm),and yellow emitters (570to 600nm)is a major challenge.The internal quantum efficiency (photons created per electron injected)of some of the emitters,particularly in the deep UV,can be below
1%.A better understanding of the materials physics—in particular,defects,dislocations,and impurities—will be required to attain ef-ficient emitters in this wavelength range.Novel epitaxial growth approaches,including growth on pudo-matched substrates and growth on nano-structured substrat
es (8,9),will be re-quired to overcome the limitations.
Phosphors.Hundreds of phosphors are available for excitation at 250nm,the domi-nant emission band of Hg lamps.In solid-state lighting,however,the excitation wavelength is much longer,typically in the range 380to 480nm.New high-efficiency phosphors,which can be efficiently excited at the wavelengths,are now being developed.Whereas high-efficiency yellow phosphors are readily avail-able (e.g.,cesium-doped YAG phosphors),the efficiency of red phosphors still lags.
Extraction efficiency.The efficient extrac-tion of light out of the LED chip and the package is complicated becau this light tends to be generated near metallic ohmic contacts that have low reflectivity and are partially absorbing.Either totally reflective or totally transparent structures are desirable.This in-sight has driven the replacement of absorbing GaAs substrates with transparent GaP sub-strates,and it has also spurred the develop-ment of new omnidirectional reflectors with angle-integrated transver electric–transver magnetic (TE-TM)averaged mirror loss that are 1%tho of metal reflectors.Sophisticated chip shapes and photonic crystal structures are becoming commonplace.Another fruitful strat-egy is to reduce deterministic optical modes trapped in the chip and the package by intro-ducing indeterministic optical elements such as diffu reflective and transmissive surfaces.Chip and lamp power.Although substan-tial progress has been achieved in LE
D optical output power,an order of magnitude increa
in power per package is still required.Several strategies are being pursued simultaneously,including (i)scaling up the chip area,(ii)scaling up the current density,and (iii)increasing the maximum allowable operating temperature.Scaling of the chip area is particularly interesting becau it reminds us of the scal-ing in Si microelectronics technology that for decades has been governed by Moore’s law.Whereas feature sizes are shrinking in Si technology,die sizes are growing in solid-state lighting devices.However,the increa in chip area is frequently accompanied by a reduced efficiency (scaling loss)due to ab-sorption loss of waveguided modes propa-gating sideways within the miconductor.New scalable geometries and high-reflectivity omnidirectional reflectors are being developed by veral rearch groups.Surface-emitting devices are generally more scalable,as they do not suffer from waveguide loss.Surface
标准韩国语第一册emission can be accomplished by micromir-rors that redirect waveguided modes toward the surface-normal direction of the chip.The scaling of the current density requires strong confinement of carriers to the active region.Such confinement reduces carrier es-cape out of the active region and carrier overflow.Changes in device design will be required,including the u of electron and hole blocking layers that prevent carriers from escaping from the active region.
Semiconductors with band gap energies corresponding to the visible spectral range,in particular wide-gap III-V nitrides,exhibit great temperature stability.However,com-mon epoxy encapsulants limit the maximum temperature of operation to about 120-C.Silicone,mostly known as a common hou-hold glue,offers mechanical flexibility (re-ducing stress)and great stability up to temperatures of about 190-C.
Thermal issues.In conventional pack-ages,LED chips driven at high currents quickly heat up.This is becau the thermal resistance of ‘‘5-mm packages,’’which have been around for decades,is greater than 200K/W.Active cooling (with a fan or thermo-electric device)is not an option for most applications,as such cooling reduces the power efficiency.Advanced packaging meth-ods u a direct thermal path:a metallic slug that extends from the LED chip through the package to a larger heat sink (such as a printed circuit board)that spreads the heat.Such packages will have thermal resistances G 5K/W,nearly two orders of magnitude lower than conventional packages.
Polarization control.Polarization control would be uful for a number of applica-tions.For example,a backlighting power saving of up to 50%in liquid crystal display applications would result from the ability
to
Fig.3.Spectrum (A )and contour plot (B )showing luminous efficiency of radiation and CRI of tetrachromatic LED-bad white light source with peak emission wavelength l 1,l 2,l 3,and l 4and a spectral width of D E 05kT (È125meV),as is typical for light-emitting active regions consisting of ternary alloy miconductors.The power ratio is chon to obtain a chromaticity location on the Planckian locus with a color temperature of 6500K.
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control polarization.Photonic crystal struc-tures,which can have a photonic gap for only one polarization,offer a unique capability for achieving this goal.Superluminescent struc-tures offer an alternative way to enhance one polarization.
High-luminance/high-radiance devices and control of far field.Flexible optical designs require high-luminance devices with small,very bright surfaces (high luminance and radiance).Such high-radiance point sources can be imaged with greater precision and en-able flexible optical designs with preci steering of beams.LEDs emitting through all side surfaces and the top surface are not well suited for point-source applications.New structures that completely lack side emission will need to be developed for such applications;would like的用法
photonic band gap structures and the u of reflectors will be required.Furthermore,spe-cific arrange
ments of phosphors will allow for chromatically dispersive emission patterns (i.e.,patterns that exhibit a correlation be-tween emission color and direction).
Cost.Although cost is a conditio sine qua non from a point of view that focus on the replacement of conventional sources,it is of lesr importance for smart lighting ap-plications.The benefits of smart lighting add another dimension to the economics of lighting,as the benefits derive from the possibility of temporal,spatial,spectral,and polarization control,a feature that conven-tional lighting technologies are unable to of-fer.Whereas the ‘‘cost of ownership,’’which includes the cost of lamp purcha and cost
of electricity to operate the lamp,would ap-pear most relevant,the lamp purcha price,measured in ‘‘$per lumen,’’is the cost that prominently appears on the price tag to the consumer.A high lamp purcha price is a barrier for the broad adoption of solid-state lighting.
Substantial cost reductions are to be ex-pected mostly through scaling of LED chips,lamps,and packages.In silicon technology,scaling of integrated circuits has reduced the cost of a logic gate by more than six orders of magnitude.Similarly,the scaling up of the LED chip size (analogous to geometric scaling in Si integrated circuits)and of the current density (analogous to current-density scaling in Si integrated circuits)will enable substantial cost reductions that,in the years to come,will
bring LEDs into offices,homes,and maybe even the chandeliers of dining rooms.
Smart Lighting
In addition to the energy savings and positive environmental effects promid by solid-state lighting,solid-state sources—in particular,LED-bad sources—offer what was incon-ceivable with conventional sources:controlla-bility of their spectral,spatial,temporal,and polarization properties as well as their color temperature.Technologies currently emerging are expected to enable tremendous benefits in lighting,automobiles,transportation,commu-nication,imaging,agriculture,and medicine.Recently,a remarkable discovery was made:A fifth type of photoreceptor had first been postulated and then identified in the retina of the
human eye,more than 150years after the dis-covery of the rod cells and the red-,green-,and blue-nsitive cone cells (10–12).The fifth type of photoreceptor,the ganglion cell,had been believed to be merely a nerve interconnection and transmitter cell.Such cells are now be-lieved to be instrumental in the regulation of the human circadian (wake-sleep)rhythm.Be-cau ganglion cells are most nsitive in the blue spectral range (460to 500nm,Fig.5),they act as a ‘‘blue-sky receptor,’’that is,as a high-color-temperature receptor.Indeed,dur-ing midday periods natural daylight has color temperatur
es ranging from 6000K under over-cast conditions to as high as 20,000K under clear blue-sky conditions.However,in the eve-ning hours,the color temperature of the Sun decreas to only 2000K.This periodic var-
iation of the color temperature of natural light synchronizes the human circadian rhythm.Figure 5shows that the circadian and visual efficacies are vastly different (orders of mag-nitude),particularly in the red spectral range.Inappropriate lighting conditions were shown in mammals to upt the body chemistry and to lead to deleterious health effects,includ-ing cancer (13).Thus,circadian light sources with tunability of color temperature would be beneficial to human health,well-being,and productivity.Furthermore,such circadian lights could lead to a reduced dependence on sleep-inducing pharmaceuticals.For this reason,sources replicating the Sun’s high color tem-perature during the midday period and low color temperatures during early morning and at night would be a wonderful
txu
illumination
Fig.4.Change in chromaticity coordinate,correlated color temperature,and CRI of trichromatic LED light source for junction temperatures of T j 020-,50-,and 80-C reprented in the (x ,y )chromaticity
diagram.
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