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`Solid-State Light Sources Getting Smart
`E. Fred Schubert and Jong Kyu Kim
`Science , 1274 (2005);308
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`DOI: 10.1126/science.1108712
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`REVIEW
`
`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.
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`gases (CO2), emission of acid rain–causing
`SO2, and mercury pollution. Solid-state light-
`ing could cut the electricity used for lighting,
`currently at 22%, in half. Although tremen-
`dous energy savings have already materialized
`E
`e.g., traffic lights that use light-emitting diodes
`(LEDs) consume only one-tenth the power of
`^
`incandescent ones
`, there is a sobering possi-
`bility that energy savings may be offset by
`increased energy consumption: More waste-
`ful usage patterns, abundant use of displays,
`and an increase in accent and artistic lighting
`may keep the use of electricity for lighting at
`E
`its current level
`11% in private homes, 25%
`^
`in commercial use, and 22% overall (3)
`.
`Several promising strategies to create white
`light with the use of inorganic sources, organic
`sources, and phosphors are shown in Fig. 2,
`
`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 17 lm/W by the filament tem-
`
`T he history of lighting has taken several
`
`rapid and often unexpected turns (1).
`The first commercial technology for
`lighting was based on natural gas that served
`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 use 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.
`1 were 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 based
`on mercury vapor sources is limited to about
`90 lm/W by a fundamental factor: the loss of
`
`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.
`
`perature that has a maximum of about 3000 K,
`which results, as predicted by blackbody
`radiation theory, in the utter dominance of
`invisible infrared emission. In contrast, the
`present 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) discussed the huge poten-
`tial benefits of solid-state light sources,
`in
`particular reduced energy consumption, depen-
`dence on foreign oil, emission of greenhouse
`
`tri-, and tetrachromatic ap-
`including di-,
`proaches. These approaches differ in terms
`of their luminous efficiency (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
`B
`[
`source to show or
`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 selection, dichromatic sources
`have the highest possible luminous efficacy
`of radiation, as high as 425 lm/W, but they
`
`Department of Electrical, Computer, and Systems En-
`gineering and Department of Physics, Applied Physics,
`and Astronomy, Rensselaer Polytechnic Institute, Troy,
`NY 12180, USA.
`
`1274
`
`27 MAY 2005 VOL 308 SCIENCE www.sciencemag.org
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`R E V I E W
`
`B
`
`[
`[
`of an object
`true color
`Thus, the
`green.
`requires that we have a certain reference il-
`luminant in mind. Today, a procedure similar
`_
`to Palmer
`s is used: The apparent color of a
`set of sample objects is assessed (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 set 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.
`
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`ture of a blackbody radiator that has the same
`chromaticity as the white light source con-
`sidered. Figure 3 shows that a favorable wave-
`length combination is l 0
`450, 510, 560, and
`620 nm, giving a luminous efficacy of 300
`lm/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 decrease exponentially with tempera-
`ture; low-gap red LEDs are particularly sensi-
`tive to ambient temperature. As a result, the
`chromaticity point, correlated color tempera-
`ture, CRI, and efficiency of
`LED-based light sources
`drift as the ambient tempera-
`ture of the device increases.
`An example of the change
`in chromaticity point with
`junction temperature is shown
`in Fig. 4 for a trichromatic
`LED-based light source (5);
`the chromaticity changes by
`about 0.02 units, 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 84 to 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 use 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 because 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
`current performance closer 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
`
`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
`9
`luminous efficacies (
`300 lm/W).
`Figure 2 also shows several phosphor-
`based white light sources. Such sources use
`optically active rare-earth atoms embedded in
`an inorganic matrix. Cesium-doped yttrium-
`aluminum-garnet (YAG) is a common yellow
`phosphor. However, phosphor-based 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
`10 to 30% the overall effi-
`ciency of systems based on
`phosphors optically excited
`by LEDs. Such loss is not in-
`curred by white light sources
`based exclusively on semi-
`conductor LEDs. Further-
`more, phosphor-based sources
`do not allow for the exten-
`sive tunability afforded by
`LED-based sources, partic-
`ularly in terms of spectral
`composition and temporal
`modulation (YAG phospho-
`rescence radiative lifetime is
`in the millisecond range).
`The luminous efficien-
`cy of a light source is a key
`metric for energy savings
`considerations. 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 425
`lm/W and 320 lm/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 3 W.
`Besides luminous efficiency, color render-
`ing is an essential figure of merit for a light
`source used 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
`E
`_
`^
`source
`for Palmer
`s original paper, see (4)
`.
`Illuminating colored test samples with differ-
`B
`ent light sources, he found that
`red appears
`[
`B
`orange
`and, more strikingly,
`blue appears
`
`Although trichromatic sources already give
`very good CRI values, tetrachromatic sources
`give excellent CRI values suitable for essen-
`tially any application. The emission spectrum,
`luminous efficacy, and color rendering proper-
`ties of a tetrachromatic white LED-based
`source with color temperature of 6500 K are
`shown in Fig. 3. Color temperature may ap-
`pear to be a somewhat surprising quantity,
`as color and temperature would not seem 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-
`
`Fig. 2. LED-based and LED-plus-phosphor–based 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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`R E V I E W
`& 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 several important technical
`issues involved in meeting these challenges.
`Additional challenges and a roadmap with spe-
`cific goals were presented 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
`G
`efficient UV emitters (
`390 nm), green emit-
`ters (515 to 540 nm), yellow-green emitters
`(540 to 570 nm), and yellow emitters (570 to
`600 nm) is a major challenge. The internal
`quantum efficiency (photons created per
`electron injected) of some of these emitters,
`particularly in the deep UV, can be below
`
`package is complicated because 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 transverse electric–transverse
`magnetic (TE-TM) averaged mirror losses that
`are 1% those 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 diffuse reflective and transmissive surfaces.
`Chip and lamp power. Although substan-
`tial progress has been achieved in LED optical
`output power, an order of magnitude increase
`
`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 use 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 house-
`hold glue, offers mechanical flexibility (re-
`ducing stress) and great stability up to
`-
`temperatures of about 190
`C.
`
`Fig. 3. Spectrum (A) and contour plot (B) showing luminous efficiency of radiation and CRI of tetrachromatic LED-based white
`0
`5 kT (È125 meV), as is typical for
`light source with peak emission wavelength l1, l2, l3, and l4 and a spectral width of DE
`light-emitting active regions consisting of ternary alloy semiconductors. The power ratio is chosen to obtain a chromaticity
`location on the Planckian locus with a color temperature of 6500 K.
`
`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 pseudo-matched substrates and growth on
`nano-structured substrates (8, 9), will be re-
`quired to overcome these limitations.
`Phosphors. Hundreds of phosphors are
`available for excitation at 250 nm, the domi-
`nant emission band of Hg lamps. In solid-state
`lighting, however, the excitation wavelength
`is much longer, typically in the range 380 to
`480 nm. New high-efficiency phosphors, which
`can be efficiently excited at these 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
`
`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 because 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 increase
`in chip area is frequently accompanied by a
`reduced efficiency (scaling losses) due to ab-
`sorption losses of waveguided modes propa-
`gating sideways within the semiconductor.
`New scalable geometries and high-reflectivity
`omnidirectional reflectors are being developed
`by several research groups. Surface-emitting
`devices are generally more scalable, as they
`do not suffer from waveguide losses. Surface
`
`issues. In conventional pack-
`Thermal
`ages, LED chips driven at high currents
`quickly heat up. This is because the thermal
`resistance of ‘‘5-mm packages,’’ which have
`been around for decades, is greater than 200
`K/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 use 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
`5 K/W, nearly two orders of magnitude
`lower than conventional packages.
`Polarization control. Polarization control
`would be useful 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
`
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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
`far field. Flexible optical
`and control of
`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 precise
`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;
`
`of electricity to operate the lamp, would ap-
`pear most relevant, the lamp purchase price,
`measured in ‘‘$ per lumen,’’ is the cost that
`prominently appears on the price tag to the
`consumer. A high lamp purchase 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
`
`R E V I E W
`
`human eye, more than 150 years after the dis-
`covery of the rod cells and the red-, green-, and
`blue-sensitive 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-
`cause ganglion cells are most sensitive in the
`blue spectral range (460 to 500 nm, 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
`temperatures ranging from 6000 K under over-
`cast conditions to as high as 20,000 K under
`clear blue-sky conditions. However, in the eve-
`ning hours, the color temperature of the Sun
`decreases to only 2000 K. This periodic var-
`
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`Fig. 4. Change in chromaticity coordinate, correlated color temperature, and CRI of trichromatic LED light source for junction
`0
`-
`-
`-
`C represented in the (x, y) chromaticity diagram.
`temperatures of Tj
`20
`, 50
`, and 80
`
`photonic band gap structures and the use of
`reflectors will be required. Furthermore, spe-
`cific arrangements 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 focuses on
`the replacement of conventional sources, it is
`of lesser importance for smart lighting ap-
`plications. The benefits of smart lighting
`add another dimension to the economics of
`lighting, as these 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 purchase and cost
`
`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 promised by solid-state
`lighting, solid-state sources—in particular,
`LED-based 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
`
`iation of the color temperature of natural light
`synchronizes the human circadian rhythm.
`Figure 5 shows 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 upset 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 illumination
`
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`Fig. 5. CIE eye sensitivity function V(l) for the photopic vision regime mediated by retinal cone and rod cells. Also shown is the
`eye sensitivity function for the scotopic vision regime, V ¶(l), that applies to low ambient light levels, and the circadian efficacy
`curve C(l) derived from retinal ganglion cell photoresponse.
`
`source, given that we humans adapted to such
`a circadian source during evolution. Alterna-
`tively, we may want to influence and manip-
`ulate the human circadian rhythm: If circadian
`lights (e.g., blue automotive dashboard lights)
`could reduce driver fatigue, the number of
`traffic accidents and fatalities caused by this
`condition could be reduced as well.
`Another potential benefit of smart lighting
`originates in the ability to rapidly modulate
`the output power of LED-based light sources,
`thereby enabling communication features. New
`modes of communication based on room-light
`sources would help to reduce the overcrowding
`of the radio frequency bands. Of course, the
`visual appearance of such communicative light
`sources would be indistinguishable from conven-
`tional sources. In automotive communication
`applications, brake lights could communicate
`an emergency braking maneuver to a follow-
`ing car. Headlights could inform a red traffic
`light of an approaching car while fully main-
`taining their normal function as headlights.
`Smart road signs could flash warnings spe-
`cifically to drivers that approach a dangerous
`curve with excessive speed. Room lights could
`broadcast messages, alarms, and other types of
`information, without any noticeable change in
`the illumination quality. Modulation rates in
`the megahertz range are possible, with the
`limiting factor being the device resistance and
`capacitance (RC time) of high-power devices.
`The large-area junction capacitance means
`that such devices would be limited by RC time
`rather than by spontaneous lifetime.
`
`Smart lighting could be used in headlights
`that are spectrally and spatially dispersive, with
`peripheral regions having a spectrum different
`from that of the center. It is well known that
`the spectral sensitivity of the cone cell–rich
`central vision region of the retina is different
`from the rod cell–rich peripheral vision region
`of the retina. It is also well known that human
`vision has a photopic (daytime vision) regime
`with peak sensitivity at 555 nm and a scotopic
`(nighttime vision) regime with peak sensitivity
`at 505 nm. Although it is too early to guess
`at the magnitude of safety enhancements, ad-
`vances in automotive lighting enabled by solid-
`state sources would certainly reduce accidents.
`Plant growth in northern countries and dur-
`ing non-native seasons is already supported by
`artificial illumination. However, spectral dis-
`tributions are not yet optimized. Smart lighting
`would allow one to select the most efficacious
`spectral composition, thereby enabling plant
`growth in the most energy-efficient way.
`In microscopy applications, smart light-
`ing with infrared, visible-spectrum, and UV
`illumination sources with specific spectral com-
`positions, polarizations, and color temperatures
`(for white illumination) could render micro-
`scopic objects more clearly than a conventional
`light bulb could. Smart sources could enable
`real-time identification, counting, and sorting
`of biological cells. During surgical procedures,
`the real-time enhanced rendering of specific
`cells, tissues, and organs could be very helpful.
`Other applications are awaiting the arrival
`of smart sources for imaging, microscopy, and
`
`visualization. For television sets, computer
`monitors, and outdoor displays, smart light
`sources promise a huge color gamut, brilliant
`colors, and again, large energy savings. Solid-
`state light sources are already the type of source
`manufactured in the greatest numbers. They
`have enjoyed double-digit growth rates for
`more than a decade. The opportunities dis-
`cussed above will ensure that this trend will be
`sustained for years to come.
`
`References and Notes
`1. B. Bowers, Lengthening the Day (Oxford Univ. Press,
`Oxford, 1998).
`2. A. Bergh, G. Craford, A. Duggal, R. Haitz, Phys. Today
`54 (no. 12), 42 (2001).
`3. J. Kelso, Buildings Energy Databook (U.S. Department
`of Energy, January 2005 revision).
`4. D. L. MacAdam, Ed., Selected Papers on Colorimetry—
`Fundamentals, vol. 77 of SPIE Milestone Series (SPIE
`Press, Bellingham, WA, 1993).
`5. S. Chhajed, Y. Xi, Y.-L. Li, E. F. Schubert, J. Appl. Phys.
`97, 054506 (2005).
`6. J. Y. Tsao, IEEE Circuits & Devices 20 (no. 3), 28 (2004).
`7. L. S. Rohwer, A. M. Srivastava, Electrochemical Society
`Interface 12 (no. 2), 36 (2003).
`8. D. Zubia, S. D. Hersee, J. Appl. Phys. 85, 6492 (1999).
`9. X. Y. Sun et al., J. Appl. Phys. 84, 1450 (2004).
`10. G. C. Brainard et al., J. Neurosci. 21, 6405 (2001).
`11. D. M. Berson, F. A. Dunn, M. Takao, Science 295,
`1070 (2002).
`12. S. Hattar, H.-W. Liao, M. Takao, D. M. Berson, K.-W.
`Yau, Science 295, 1065 (2002).
`13. D. E. Blask, R. T. Dauchy, L. A. Sauer, J. A. Krause, G. C.
`Brainard, Breast Cancer Res. Treat. 79, 313 (2003).
`14. Supported by NSF grant 0401075, the U.S. Army Re-
`search Office, Samsung Advanced Institute of Tech-
`nology (Suwon, Korea), and Crystal IS Corporation
`(Watervliet, NY).
`
`10.1126/science.1108712
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`27 MAY 2005 VOL 308 SCIENCE www.sciencemag.org
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