Zbigniew TURLEJ
Electrotechnical Institute
Efficiency and colors in LEDs light sources
Abstract. The light gains from LEDs continue to grow, doubling a about every two years. It gives real hope for the LEDs solving problem of
efficiency in the lighting. This paper presents review some problems connected with efficiency and colors inorganic LEDs technologies, also gives
some new perspectives for development based on organic LED and plasmonics.
Streszczenie. Światło emitowane przez z
ródła LED podwaja swą skuteczność świetlną co dwa lata. To stwarza poważną nadzieje na rozwiązanie
problemu efektywności energetycznej w oświetleniu. W referacie zarysowano historię i perspektywy rozwoju efektywności i barwy w technologii LED
ze szczególnym uwzględnieniem materiałów nieorganicznych, organicznych i efektów plazmoniki. (Barwy i efektywność z
ródeł światła LED).
Keywords: energy efficiency, LED colors and technology, light and health, illuminating technology.
Słowa kluczowe: efektywność energetyczna, barwa i technologie LED, światło i zdrowie, technika świetlna.
Lighting and energy efficiency Lighting gobbles up 20% of the world s electricity, or the
Population and economic growth threaten to keep equivalent of roughly 600.000 tons of coal a day. Forty
energy demand and carbon emissions growing, too. But the percent of that powers old-fashioned incandescent light
new long-range forecasts (fig.1) produced by energy bulbs  a 19th-century technology that wastes most of the
experts show that in many key areas, increased efficiency power it consumes on unwanted heat. Light emitting diode
offers real hope for solving the problem. (LED) lamps, not only use less electricity then incandescent
bulbs to generate the same amount of light, but they also
last 50 times longer. In December 2006, Dutch electronic
firm Philips became the first major bulb manufactures to
announce a gradual phase out of the production of
incandescent bulbs. Now exist an opportunity to have
lighting systems that modulate their intensity to supplement
natural light. These systems will require the integration of
LEDs with photovoltaic (PV) and architectural transparent
materials (fig.2).
The unique properties
Light sources should be as small as possible, produce
light efficiently and have a long life. Until now, however, no
filament or discharge lamp has combined all three
properties. Only light emitting diodes (LEDs) achieves this.
No other lamp possesses comparably small dimensions.
The miniature form requires small optical systems and
creates new demands for light guidance. The light optical
Fig.1. The increasing efficiency offers the cutting carbon emissions
systems are mode from synthetic materials with light
[1]
refractive indices and replace the classic metal reflector.
The light gains from light diodes continue to grow, doubling
about every two years. It is not unrealistic to assume that in
ten to fifteen years LEDs will become the most efficient light
sources (fig. 3).
Fig.3. The light yield from LEDs is reaching ever higher vales [2]
With 50.000 operational hours light diodes have a very
long life. This results in a new conceptual approach to the
design and development of lighting. There is no longer a
need for equipment for changing the light source. LEDs and
luminaire grow old jointly and both are changed together
when the lamp has reached the end of its lifespan.
Fig.2. The integration of LEDs with photovoltaic (PV) and
architectural transparent materials
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The light production
In conventional lamps visible light arises as a by-product
of the warming of metal helix, or by a gas discharge or by
the conversion of a proportion of the ultraviolet radiation
produced in such a discharge. In light diodes the production
of light takes place in a semiconductor crystal which is
electrically excited to elektroluminescence (fig. 4, table 1).
Fig.5. The three possible approaches to white-light production [3]
Wavelength Conversion Approach
The first approach for transforming narrowband
emission into broadband white light involves using UV LEDs
to excite phosphors that emit light at down-converted
Fig.4. LED functional principles. The light comes from
wavelengths. In general, this approach is likely to be the
semiconductor crystal (LED chip). It is electrically excited to
produce light: two areas exist within the crystal, a n- conducting lowest cost, because of its low system complexity (only a
area with a surplus of electrons and a p- conducting area with a
single LED chip, and since the colors are created already
deficit of electrons. In the transitional area, called pn- transition or
blended, lamp-level optical and color engineering is
depletion layer, light is produced in a recombination process of the
minimized). It is also likely to be the least efficient, because
electron with the atom with the deficit of an electron when current is
of the power-conversion loss associated with the
applied to the crystal [2].
wavelength down-conversion; and the least flexible, since
the colors are  preset at the factory.
Table 1. History of light production by LED
Hence, a general challenge will be the development of UV
1907 Henry Joseph Round (1881- 1966) discovers the
(340-380 nm) LEDs with high (>70%) external power-
physical effect of elektroluminescence.
1962 The first red luminescent diode of type GaAsP conversion efficiency and input power density, and
comes onto the market. The industrially produced
multicolor phosphor blends with high (>85%) quantum
LED is barn.
efficiency.
1971 From the beginning of the seventies LEDs are
available in further colours: green, orange, yellow.
Color Mixing Approach
Performance and effectiveness is continually being
The second approach for transforming narrowband
improved in all LEDs.
emission into broadband white light is to combine light from
1980s to early 1990s High performance LEDs (LED
multiple LEDs of different colors. In general, this approach
modules) in red, later red/orange, yellow and
is likely to be the most efficient, as there are no power-
green become available.
1995 The first LED producing white light by conversion losses associated with wavelength down-
luminescence conversion is introduced.
conversion. It is also likely to be the most flexible, since the
1997 White LEDs come onto the market.
hue of the light can be controlled by varying the mix of
primary colors, either in the lamp, or in the luminaire.
However, it is also likely to be the most expensive, because
As protection against environmental influences the
of its high system complexity (multiple LED chips, mixing of
semiconductor crystal is set into a housing. This is
light from separate sources, and drive electronics that must
constructed so that the light radiates in a semicircle of
accommodate differences in voltage, luminous output,
almost 180 degrees. Guidance of the light is thus easier
element life and thermal characteristics among the
then in filament or discharge lamps, which generally radiate
individual LEDs). Hence, a general challenge will be the
light in all directions.
development of red, green and blue LEDs with high (>50%)
external power-conversion efficiencies and input power
The monochromatic and the white colores
density, and low-cost optics and control strategies for
According to the type and composition of the
spatially uniform, programmable color-mixing either in the
semiconductor crystal the light diode has different
lamp or in the luminaire.
monochromatic colors. Today there are blue, green, yellow,
orange, red and amber, together with nuances of these
Hybrid Approach
colors. The white light can be generated by three general
The third approach for transforming narrowband
approaches, illustrated in figure 5. The first is the
emission into broadband white light is a hybrid approach.
wavelength-conversion approach; the second is the color
The present generation of white LEDs, with luminous
mixing approach; and the third is a hybrid between
efficacies of 25 lm/W, is based on this approach. Primary
the two. light from a blue (460 nm) InGaN-based LED is mixed with
blue-LED-excited secondary light from a pale-yellow
YAG:Ce3+-based inorganic phosphor. The secondary light is
centred at about 580 nm with a full-width-at-half-maximum
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line width of 160 nm. The combination of partially factory. The compensation factors are permanently stored
transmitted blue and reemitted yellow light gives the in the control gear.
appearance of white light at a color temperature of 8000 K
and a luminous efficacy of about 25 lm/W. This combination Blue LED and health
of colors is similar to that used in black-and-white television Circadian phototransduction is a term used to describe
screens  for which a low-quality white intended for  direct how the retina converts light into neural signals that
rather than  indirect viewing  is acceptable. Other regulate rhythms such as sleep, body temperature and
variations of this approach are possible. The simplest hormone production, and has been a topic of interest in
extension would be to mix blue LED light with light from a many laboratories around the world. We now know that the
blue-LED excited green and red duo-color phosphor circadian system is maximally sensitive to short-wavelength
blend25  this variation is likely to be give the best balance light and that a combination of classical photoreceptors and
between efficiency, color quality, cost and system newly-discovered retinal neurons, which respond directly to
complexity. A more complex but perhaps more efficient light exposure (called intrinsically-photosensitive retinal
extension of this approach would be to mix blue and red ganglion cells or ipRGCs), participate in circadian
LED light with light from a blue-LED excited green phototransduction. Much of what we do in lighting rests
phosphor. In general, this approach is intermediate upon a quantitative foundation for the specification of light
amongst the three approaches in efficiency, complexity and sources and light levels for vision. The model of circadian
cost. It is likely to be intermediate in efficiency, as power- phototransduction is the first attempt to establish a parallel
conversion losses from wavelength down-conversion are foundation for the circadian system. Much like we want to
less from the blue than from the UV, but still greater than no know many lumens per watt a light source produces for the
power-conversion losses. It is likely to be intermediate in visual system, it is now possible to calculate circadian
cost and system complexity, as only one (or at most two) stimulus per watt. Table 2 shows values of circadian
LEDs is used, but light from the LED must still be color- stimulus per watt for several commercially available light
mixed with light from the phosphor. Hence, a general sources.
challenge will be the development of blue LEDs with high
Table 2. Photopic lumens per watt and circadian stimulus per watt
(>60%) external power-conversion efficiencies and input
for various light sources [3]
power density, blue-excitable duocolor phosphor blends
with high (>80%) quantum efficiency, and low-cost optics for
spatially uniform color-mixing in the lamp.
Light source Photopic Circadian
lumens/watt stimulus/watt
Fluorescent 100 lm/W 74 CS/W
3000K
a)
Fluorescent 100 lm/W 157 CS/W
7500K
Incandescent 12 lm/W 12 CS/W
D65 70 lm/W 133 CS/W
Clear Mercury 45 lm/W 18 CS/W
Blue LED 8 lm/W 223 CS/W
(470nm) 15 lm/W 418 CS/W
As we can see in Table 2 the blue LED light source (470
nm) is the most effective in suppressing melatonin than
others. Figure 7 shows the fixture for a melatonin regulation
in workplace.
b)
Fig. 6. The human eye registers even the slightest deviation in hue
(a) such as coloured wall washing (b) [2]
Problems with colors
One of the key characteristics of LEDs is their light color
saturation. Because of the manufacturing process we con
have deviations in the light colors of two of same LED
modules. The human eye registers even the slightest
deviation in hue (fig.6). Semiconductor producers classify
Fig.7. The blue LED (470 nm) fixture for a effect melatonin
each LED into different categories, known as  binnings
regulation in workplace designed by Electrotechnical Institute [4]
using the values actually measured. But even with the most
stringent selection, deviations still have to be accepted. To
Looking a long way into the future, it is easy to imagine
ensure consistent colour Erco has introduced a colour
that new standards will be adopted, new light sources
compensation system. Every colour compensated LED
circadian systems, we may all end up in a healthier built
modules is individually measured and adjusted in the
environment.
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LEDs - the new horizons with the aid of conversion principle, exactly as with
Korean company has released a single-die white inorganic LEDs. If white OLEDs, which are constructed in
inorganic LED that can emit up to 240 lm at its maximum this way, then the light source is not transparent when
drive current of 1A. The new P4 emitter is also claimed to switched off. OLEDs, which are constructed from single,
offer the word s highest luminous efficacy, coming at 100 individually controllable points, offer maximum flexibility in
lm/W at 350 mA drive current that is required for general the production of color and in dimming, however at very
illumination applications. Company says that the high high cost. In future solutions to this problem information
luminosity was reached through its proprietary phosphor could, for example, be presented on illuminating surfaces.
and packaging techniques, and further improvements are in Recently, however, scientist have been working on a new
the pipeline. A 135 lm/W source is due to emerge this year, technique for transmitting light through nanoscale interface
and more incremental improvements are expected to lead structures made of a metal and a dielectric. Under the right
to 145 lm/W performance early in 2008. Another circumstances, we have a resonant interaction between the
revolutionary means of lighting for the future is organic waves and the mobile electrons at the surface of the metal.
LEDs (OLEDs).Today they illuminate displays, but they will The result is the generation of surface plasmons  density
soon open up other types of lighting. Research on materials waves of electrons that propagate along the interface like
has discovered a series of systems in which light can be the ripples that spread across the surface of a pond after
produced. The results reveal two groups: sm-OLEDs with you throw a stone into the water. Plasmonic materials may
small molecules and p-OLEDs with polymers. They are revolutionize the lighting industry by making LEDs brighter.
mainly differentiated by the number of materials necessary It has become evident that this type of field enhancement
to construct the light producing layers (fig 8). can also dramatically raise the emissions rates of dots and
quantum wells  tiny semiconductors structures that absorb
and emit light  thus increasing the efficiency and
brightness of solid-state LEDs. In 2004 at Japan s Nichia
Corporation was demonstration that coating the surface of a
gallium nitride LED with dense arrays of plasmonic
nanoparticles (made of silver, gold or aluminum) could
increase intensity of the emitted light 14-fold.
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[1] Gl ai n S., Kashi wagi A., Krovati n Q., Seeing the
scenarios, Davos Special Report, Newsweek, (2007), n.4, 44-
45,
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Gute Licht, (2006)
[3] Light Emitting Diodes (LEDs) for General Illumination, OIDA,
(2002)
Fig.8. Schematic representation of the functional principles of
[ 4 ] T u r l e j Z., Czynnik hormonalny w oświetleniu wnętrza, Prace
OLEDs  the organic layer of sm-OLEDs consists of four coatings.
Instytutu Elektrotechniki, (2006), n.228, 297-306
The same functionality can be achieved in p-OLEDs with two
[5] Briefings, Lighting, (2007), n.2, 8-14
coatings [5].
[ 6 ] A t w a t e r H., The Promise of Plasmonics, Scientific American,
(2007), n.4, 38-45,
Using a method of light mixture in this organic layers,
[7] Fi guero M., Research matters, LD+A, (2006), n.5, 24-26,
white and colored OLEDs which are completely transparent [8] S c h i e l k e T., Color compensation: ERCO technology for trude-
color varychrome LED luminaires, ERCO Leuchten GmbH, (2006),
when switched off, can be manufactured. The production of
25
these is simple but their light can, however, only be dimmed
_____________________
and not changed in color. The mixture for white light makes
Author: dr inż. Turlej Zbigniew, Electrotechnical Institute,
it possible to adjust color temperature because distinct
Pożaryskiego 28, 04-703 Warsaw, Poland, e-mail:
organic layers are used to produce the three basic colors.
[e-mail];
Such solutions hence offer possibilities for the design of
color sequences. Alternatively white light can be produced
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