Large-Area Solid State Intelligent Efficient Luminaires
O. Fernández 1, R. Ferrini 1, P. Evanschitzky 2 and Viviana Agudelo-Moreno 2
1
Centre Suisse d’Electronique et de Microtechnique, CSEM
2
Fraunhofer Institute for Integrated Systems and Device Technology IISB
Abstract
Light emitting diode (LED) technology for lighting applications has experienced a tremendous
increase in performance and price erosion in the recent years and is expected to dominate the lighting
market within few years from now. In addition to impressive efficiency values and its impact on energy
savings, LED technology has brought about new research areas namely Human Centric and Smart
Lighting. LEDs have also opened the door for new design concepts. In particular, LED-based, largearea thin luminaires for e.g. office lighting is a market worth millions and is receiving a considerable
amount of R&D efforts. Although already available commercially, most LED-based large-area
luminaires today do not fulfill the claimed specifications putting at risk a deep market penetration of
LEDs for this type of products. We believe that improvements on the system design as well as added
intelligence will greatly support LED-based luminaires to surpass even the most stringent
requirements and allow further energy savings. Here, we introduce a lighting system which comprises
a flexible LED-based light source, an innovative light management solution and integrated intelligence
for color and luminance tuning. All the technological packages are currently being developed in the
Framework of the EU-funded R&D project, LASSIE-FP7. Here, we provide the first results obtained on
the light management system. Optical simulations and preliminary experimental results indicate the
potential of this solution for efficient, large-area, low-cost modules with excellent light quality.
Introduction
The International Energy Agency (IEA) estimates that lighting represents almost 20% of global
electricity consumption [1], similar to the amount of electricity generated by nuclear power and
equivalent to 70% of the emissions from the world’s light passenger vehicles. Hence, the
development and implementation of more efficient lighting systems will undoubtedly make a
significant contribution to controlling global CO2 emissions. In addition, LEDs do not contain Mercury,
in contrast to high efficiency fluorescent lamps.
Commercially available white LEDs have crossed the 150lm/W barrier and 300lm/W has already been
demonstrated [2]. Such high efficacy values together with standard lifetime values in excess of 50’000
hours under continuous operation, makes LEDs the most convenient light source for energy savings.
In fact, LED technology is expected to conquer the lighting market in the coming years with a
penetration rate of 56% by 2016 [3].
On the other hand, soon after the beginning of this century, a new type of photoreceptors (ipRGCs;
intrinsically photosensitive retinal ganglion cells), were discovered which control many biological
functions, including the circadian rhythm [4][5]. This discovery triggered a new discipline in lighting,
the so-called Human Centric Lighting (HCL), which sets lighting at the core of those external stimuli
that affect human performance and well-being. Ever since the discovery of ipRGCs, a considerable
amount of experimental evidence has been collected that confirmed the strong link between luminous
excitation (spatial and angular distribution of the light intensity and primarily its spectral distribution),
and visual acuity, mood alteration, sleep quality and productivity to mention some examples. Contrary
to conventional incandescent or fluorescent light sources where the spectral light distribution is fixed,
a plethora of different spectra are possible with LEDs allowing improvements in human health and
well-being.
LED technology, as a digital-ready light source, is naturally compatible with electronic switching
circuitry integrated in the driving electronics. This so-called Intelligent or Smart Lighting represents the
shift of lighting from its most basic mandate to a much more valuable and useful asset, including HCL
and energy savings far beyond what is attainable with non-intelligent lighting systems.
For example, the first generation of intelligent lighting products will include relatively simple smartness
such as occupancy and daylight sensing as well as CCT/luminance tuning. In addition, controls can
be integrated that respond to pre-loaded user preferences.
This will likely be follow by a second generation with more advanced intelligence including more
sophisticated add-ons whose implementation will be only possible with SSL [6].
White emitting LEDs
White LED emission can be achieved using the so-called multichannel or RGB approach. This
approach uses a combination of red, green and blue LEDs (often 4 or 5 different “colors” are used).
When appropriately driven, such combination produces a white light of a predetermined correlated
temperature (CCT). Although compatible with CCT tuning, it suffers from low efficacy values at
medium to low CCT values due to the low efficacy of green/amber LEDs in the so-called green gap. In
addition, the spectral distribution of the resulting white light, formed by the superposition of few
relatively narrow spectra, results in a substantial number of missing wavelengths and the consequent
color misinterpretation, (poor color rendition) of objects with emittances around these wavelengths.
Finally, the differential aging of the red, green and blue LEDs must be properly compensated to avoid
undesired CCT shift during device operation.
Alternatively, in the so-called phosphor approach, a layer of a yellow phosphor-containing resin is
deposited on top of a blue LED chip. Part of the blue light passing through the phosphor layer
experiences Stokes down-conversion resulting in a white emission whose CCT is determined by the
composition of the phosphor layer and its thickness.
Phosphor-containing white LEDs give a much broader emission spectrum than CCT-equivalent RGB
LEDs and hence better color rendition. In fact, CRI values as high as 98 have been reported using a
combination of green, yellow and red phosphors [7][8]. Moreover, with phosphors near to 100%
quantum yields, phosphor white LEDs are also more efficient than RGB.
However, the phosphor approach also suffers from CCT shift and efficiency drop related to the
wavelength shift of the pumping blue LEDs during operation and more importantly, to the phosphor
degradation and emission quenching observed primarily at high temperatures. For example, a 10%
drop in down-conversion quantum efficiency or in the phosphor absorption coefficient (in the blue
region) can cause a color shift close to a MacAdam ellipse step size 4 [9]. High temperatures also
cause local delamination of the phosphor layer form the LED surface which results in a noticeable
CCT shift [10]. These temperature-driven detrimental effects can be highly alleviated by locating the
phosphor layer away from the emitting chip in what is known as remote phosphor approach.
One of the main drawbacks of the phosphor approach is that although the phosphor content and layer
thickness can be adjusted for any desired CCT, this cannot be dynamically tuned for a given
phosphor layer.
Here we propose a technology based on the joint use of the multichannel and remote phosphor
approaches. Color changing foils (CCF) manufactured by BASF (Ludwigshafen, Germany) are used
for down-conversion. BASF CCFs are flexible thin foils with embedded organic phosphors. Due to the
existence of unresolved molecular vibrational levels, light emission is broader from organic phosphors
than from inorganic. Thanks to the synergetic effect of the selected combination, broad spectral
emission and associated high color rendition as well as dynamic CCT and/or luminance tuning are
possible together with high efficacy, long lifetime and acceptably low cost.
Large-area SSL
Large-area light sources comprising a collection of individual LEDs spatially distributed represent the
most convenient option when large amounts of light are needed. They are widely used in a variety of
lighting segments including office, shop-retail, hospitality, industrial and architectural which are
predicted to worth 40 billion Euros in 2020 [11].
2
OLED, the organic counterparts of LEDs are flat, thin, areal emitting devices that have been seen as
the “next big thing” in large-area lighting and the natural successors of LEDs for large-area products.
However, several factors exist that are causing OLEDs to struggle in their way to the general lighting
market and that, according to many, will likely prevent this to ever happen.
On one hand, OLED technology lags well behind LED in terms of efficacy, lifetime, lumen output and
lumen-per-dollar. Indeed, the ever promised large-area, low-cost, solution-based R2R fabrication on
flexible substrates is not even close to become a reality in the next years. On the contrary,
commercially available OLEDs today are expensive, glass-based devices, manufactured using
several expensive evaporation processes under vacuum in a sheet-to-sheet process and with
2
emissive areas of typically 100cm or less (see Table 1).
Table 1. Performance comparison of current best commercially available OLEDs and standard
1
LED-based large-area lighting module
Highest Lum.
Largest
Highest
Large-Area LED
flux OLED
Area OLED
Efficacy OLED
Module
Model
2
Area (mm )
Thickness (mm)
Luminous flux (lm)
Efficacy (lm/W)
CCT (K)
CRI
3
LT70 (10 hours)
Cost (lm / $)
Philips FL300
LG N6SD30
LG N8SA30
LG Innotek ReflectA
Free
102 x 102
1
3.0
2
300
3
>50
4
3’000
>80
5
10
6
50
2.2
7
5
320 x 320
8
1.0
9
850
60
3’000
>90
100 x 100
12
1.97
75
80
3’000
>80
600x600
69.5
3’000
97
3’500
80
40
10, 11
15
1.25
13
16
50
0.91
~50
14
17
25.4
Another important aspect, that is generally overlooked, is the detrimental impact that temperature has
on OLED device lifetime. Although OLEDs operate under relatively low temperatures (because the
light is generated on the whole emissive area), they are considerably more sensitive to small
o
temperature changes than LEDs. For example, increasing the operating temperature from 45 C to the
o
moderate value of 65 C can reduce the lifetime by as much as two times as shown in Figure 1 [12].
On the other hand, non-transparent, scattering out-coupling films used to boost device efficacy and to
alleviate cavity-related angular color shift, are not compatible with appealing mirror-like or transparent
off-state appearance, a many-times cited advantage of OLED technology.
Finally, most of the mentioned unique selling points of OLED technology such as large-area, low-cost,
transparency and flexibility are continuously been eroded by LED-based technology.
2
Indeed, although LEDs are intense point-like light sources (a mid-power, 1mm phosphor white LED
can easily deliver luminance levels higher than 50Mio nits compared to ~5’000 for the brightest
2
100mm OLEDs; Phillips Brite FL300) there are solutions to engineer them into large-area (flexible
and/or transparent possible) luminaires.
1
Please note: 1 Including thermal backplate, housing and wiring; 2 At 340mA and 19V; 3 at 300 lumen ; 4 4000K planned; 5 at
300 lumen; 6 at 125 lumen; 7 for orders above 40 OLEDs ( http://www.oled-info.com/philips-launches-new-sales-campaign-seesoleds-reaching-mass-market-early-2017); 8 bare OLED9; 9 1700mA; 8.5V; 10 at 3’000 cd/m2; 11 using LG proprietary “Face Seal”
technology; 12 with housing and wiring; 13 initial luminance not specified; 14 https://www.maritex.com.pl/en/oled-lg-chem15
lighting/olp-n8sa30-i-34063-c-33634;
http://www.oled-info.com/lg-details-price-their-320x320-mm-and-truly-flexible-oledlighting-panels; 16 not quoted for the lamp, given figure is based on the lifetime information of the LED chips; 17
http://www.futurelightingsolutions.com/en/Technologies/Semiconductors/Lighting-Solutions/LED-LightModules/Pages/3029132-LLFML66-38K308A.aspx?ManufacturerName=LG-INNOTEK&isFLS=true&IM=0
3
Figure 1 OLED device lifetime as a function of initial luminance and temperature [12].
A common approach is to “hide” the LEDs behind a diffusing plate. In spite of its simplicity, large-area
spatially uniform luminance completely free of appreciable hot-spots requires a distance of few
centimeters between the LEDs and the diffuser, an approach incompatible with thin form-factors and
/or device flexibility. Although diffusers with large hiding factors can be used in closer proximity to the
sources, it comes at a substantial drop in efficacy as shown in Figure 2 Optical transmittance versus
half power angle (hiding power) for diffusing plates based on volumetric scattering manufactured by
Bayer. Modified from [18].
A way to overcome this issue is the so-called edge-lit technology where the LEDs are distributed
across at least one of the edges of a thin transparent plate. The light emitted by the LEDs is coupled
inside the waveguide and guided through total internal reflection (TIR). Light extraction features
replicated on the emissive surface/s of the waveguide disturb the TIR and allow the guided light to
escape.
4
Figure 2 Optical transmittance versus half power angle (hiding power) for diffusing plates
based on volumetric scattering manufactured by Bayer. Modified from [18].
In large-area edge-lit luminaires, large part of the emitted light has to be guided across long distances
which results in high optical losses even when relatively transparent materials such as e.g. PMMA are
-1
used (PMMA absorption coefficient of 0.0017 mm [13] leads to 17% optical losses on a 100mm
length). In addition, since the waveguide perimeter increases only as the square root of its area, high
LED densities are required as the emissive area increases which demand efficient thermal
management solutions to avoid excessive heat at the waveguide edges to prevent associated
detrimental effects such as waveguide deformation and/or material yellowing.
A different approach consists in embedding white LEDs inside the waveguide, evenly distributed
across its area [14]. This technology solves some, but not all the issues related to edge-lit. For
example, moderate-to-high temperatures around the LEDs over sufficiently long time periods may
degrade the waveguide material locally close to the LEDs. Therefore, unless an expensive, shortpitch LED configuration is adopted, the maximum attainable brightness of the luminaire is noticeably
limited. Also, non-standard side-emitting LEDs or precisely tilted standard surface-emitting LEDs are
required as otherwise most of the light will leave the waveguide without undergoing TIR, giving rise to
appreciable hot-spots. Moreover, as the LEDs must be fully embedded into the waveguide, a
minimum thickness of the latter is mandatory. Finally, access to the LEDs in e.g. the event of LED
failure seems rather complicated if at all possible.
Our light management technology (Patent pending) was designed and engineered to overcome the
mentioned shortcomings. By using diffractive nanostructures, light emitted around the normal to the
LED surface can be efficiently in-coupled even for standard, surface-emitting, LEDs located outside
the waveguide. The spatial separation of light emission and light guiding/out-coupling results in a
minimum temperature-driven degradation effects in the waveguide material and allows the use of
much thinner waveguides.
Transparent diffusers for large-area LED-based lighting
Here we describe in detail the lighting module that was outlined in the previous sections. A sketch of
the optical system cross-section (limited to only one LED, i.e. the system Unit Cell) is given in Figure
3.
5
Figure 3 Schematic cross-section of the light source and waveguide with replicated incoupling diffractive grating and out-coupling microstructures on the opposite surface of the
waveguide as well as the color conversion film (CCF).
The system comprises a series of blue LEDs bonded onto a flexible, R2R compatible film following a
highly spatial regular pattern. LED films with highly optical reflectance or transmittance respectively
must be used for single or double surface emission lighting modules. Heat management structures
are fabricated onto the foil surface/s in order to dissipate the heat generated by the light sources [15].
Several hundreds of microns above the LED foil, a thin (<1mm thickness) transparent (e.g.
Polycarbonate, PMMA, etc.) foil serves as the optical waveguide. Diffractive optical nanostructures
(gratings) concentric to the LEDs replicated on its bottom surface are responsible for light in-coupling
into the waveguide whereas replicated micro and/or nanostructures on its top surface allow light
escaping to the free space in a relatively well-controlled manner.
In-coupling efficiency: System optimization
In order to achieve a thin form-factor efficient R2R compatible system, its geometry must be
optimized. The interplay between waveguide thickness, t and its distance to the LED substrate, dLED
as well as the grating dimensions, Lx and Ly, determines the overall performance of the system.
For example, the use of very thin waveguides or too large gratings results in back out-coupling since,
due to optical reciprocity, any in-coupled ray hitting the grating from inside the waveguide after TIR at
its top surface will be partially out-coupled back to the LEDs substrate (see Figure 4) resulting in
optical losses and reduced control on the light distribution in the system. In particular, the maximum
thickness, t, is given (in one dimension) by:
Equation1
where θ (∼42 depending on the refractive index of the waveguide material) corresponds to the critical
angle for TIR (see Figure 4).
o
On the other hand, since R2R manufacturing is pursued, a maximum waveguide thickness exists,
dictated by the required flexibility for R2R processing.
Finally, the maximum possible light collection (amount of LED light that is collected by the grating) is
required for maximum amount of in-coupled light. Light collection for a given dLED increases with
increasing Lx and Ly.
These magnitudes were optimized using ray-tracing optical simulations. The calculated percentage of
the emitted power by the LED (OSRAM Topled Compact blue) that reaches the corresponding grating
is shown in Figure 5 as a function of the grating area, for different dLED values.
6
Figure 4 Schematic cross section of the light management system (CCF not shown) showing
the relevant geometrical parameters. Back out-coupling (blue line) is expected when the incoupling gratings extends beyond the second TIR.
A decent amount light collection can be achieved using gratings with areas similar to the LED emitting
surface located at a distance A above the LED emitting surface. According to Equation1, for
waveguides with submillimeter thicknesses, back out-coupling is prevented; a value compatible with
2R2 processing.
It is well known that grating diffraction efficiency depends substantially, for a given set of grating
parameters, on the angle of incidence. On the other hand, since the value of dLED is comparable to the
geometrical dimensions of the LED emitting chip (0.6x0.9mm for OSRAM Topled Compact), the latter
cannot be considered as a point source. As a result, different, non-trivial irradiance angular
distributions are expected at different x, y coordinates of the gratings.
Figure 5 Calculated percentage of emitted power reaching the in-coupling grating as a function
of the grating area and grating-LED distance, dLED. The grating area is calculated in units of the
area of the LED chip.
For this reason, in-coupling gratings with smoothly varying grating parameters across the x, y
coordinates according to the irradiance angular distribution at these coordinates are expected to give
the optimum performance. In order to keep ourselves manufacturing realistic, grating area was
segmented into 5 concentric zones (see Figure 6). The geometrical dimensions of each of the five
segments were selected so that each segment receives 25% of the optical power incident over the
complete grating. The angular distribution of the incident light calculated using again ray-tracing optics
and the real angular emission spectrum emission of the LED provided by the LED supplier. The
results for incident angles in the XZ and YZ planes are shown in Figure 7.
7
Figure 6 Sketched cross-section of the segmented in-coupling grating showing the 5
segments with respect to the LED source.
Figure 7 Angular distribution of the irradiance as a function of incident angle in the XZ (left)
and YZ (right) planes on the five zones of the grating for the selected LED (Topled Compact
from OSRAM) and selected geometrical parameters (dLED=0.25mm; Lx= 1.4mm; Ly=0.9mm).
Rigorous Coupled Wave Analysis (RCWA) simulation software coupled to an optimization algorithm
was used to optimize the grating parameters of the 5 segments for maximum light in-coupling. Wellknown manufacturability limits and the previously calculated irradiance patterns were used as
boundary conditions and light input respectively.
The in-coupling efficiency as a function of polar and azimuthal incident angles for gratings with
optimized parameters is shown in Figure 8. In-coupling efficiency values over 80% can be achieved
for a wide range of azimuthal incident angles and polar angles close to the grating normal. For the
real Lambert-type LED emission, a 54% in-coupling efficiency can be achieved. The remaining optical
power is distributed almost equally into directly transmitted and reflected light.
The light reflection can be greatly recovered by using (as mentioned above) highly reflecting
substrates. Flexible films with diffuse reflectance values well over 90% are currently commercially
available from different manufacturers (3M, BrightView, Bayer, FusionOptix, etc.)
8
Figure 8 Left) Calculated in-coupling efficiency as a function of polar and azimuthal incident
angles. Right) Definition of polar and azimuthal angles with respect to the grating orientation.
The spatial distribution of the electromagnetic field just after the first air/waveguide interface given as
a by-product of the optimization process can easily be converted into a collection of k-vectors or
geometrical rays. Such a collection of rays can be used as the light source in ray-tracing optics to
track the transmitted light and the propagation of the in-coupled light through the waveguide. As a
result, light out-coupling structures can be optimized according to the real distribution of light inside
the waveguide. The outcome of such optimization will be published elsewhere.
Fabrication and initial results
2
In-coupling gratings, with the optimum parameters were hot-embossed on a 0.5mm thick, 50x50mm
area Polycarbonate (PC) foil. As mentioned above, the replication of the grating must be restricted to
areas comparable to that of the emitting LED chip (see Figure 5).
Moreover, in order to better spatially distribute the light inside the PC foil, two grating orientations
where replicated, perpendicularly oriented with respect to each other as shown in Figure 9 and Figure
2
10. In particular, each grating pixel contains a large collection of 0.1x0.1mm subpixels with their
grating oriented perpendicularly with respect to each other.
Figure 9 Schematic view of the process for the replication of perpendicularly oriented incoupling gratings.
9
2
Figure 10 SEM picture of the pixelated gratings showing the 0.1x0.1mm grating pixels with the
grating grooves perpendicularly oriented with respect to one another.
The in-coupling capabilities of the fabricated foil can be readily seen in Figure 11. The fabricated foil
was illuminated with a 3x3 array of blue LEDs (λ=455nm). Considerable light in-coupling is observed
only when the grating pixels are well aligned with the LEDs (Figure 11 right).
It must be noted that a relatively efficient in-coupling was observed in two perpendicular directions.
Light guiding (and subsequent out-coupling) at relatively long distances from the light source can be
achieved by the use of out-coupling films.
We believe that the proposed diffractive transparent diffusers can be provided highly uniform light
emission over large area even in combination with long-pitch LED arrays. Thin-film, flexible color
converting foils (see below) will finally allow the fabrication of flexible thin homogeneous efficient
lighting modules.
Figure 11 3x3 array of pixelated in-coupling gratings replicated on a 0.5mm thick PC foil
located on top of a 9x9 array of blue LEDs (OSRAM Topled Compact). Left) No light in-coupling
observed due to the misalignment of the gratings and the LEDs. Right) Edge emission of the
light in-coupled by the well-aligned gratings.
Color changing flexible foils (CCF) for large-area SSL
Flexible phosphor-containing foils manufactured by BASF (Ludwigshafen, Germany) can be used to
convert the blue LED light into high-quality white light. BASF CCFs contain organic phosphors and
scattering particles extruded together into thin polymer foils such as e.g. PET, PMMA, PC, etc.
The composition and thickness of the foil was adjusted to give 3’500K CCT when pumped with a
450nm blue LED (such as OSRAM Topled Compact). For the initial assessment, the CCF was
10
2
laminated on a transparent (50x50mm ) blue LED foil and the integrated emission characteristics of
the system measured in an integrated sphere set-up. The CIE coordinates of the blue and the white
(blue + CCF) foils are presented in Figure 12 CIE coordinates of the blue and white (LED + CCF) LED
foils measured in an integrating sphere set-up at REGENT LIGHTING (Basel, Switzerland). The
Planckian locus represented by the thick black line. The white point lies perfectly on the Planckian
locus at a CCT of 3’546K, in excellent agreement with the target value. Moreover, CRI and CRI9
values of 94 and 78 were calculated from the measured integrated emission spectrum. Finally, a
relatively high efficacy above 100lm/W was measured considering that the system was not optimized
for efficient light recycling.
Figure 12 CIE coordinates of the blue and white (LED + CCF) LED foils measured in an
integrating sphere set-up at REGENT LIGHTING (Basel, Switzerland). The Planckian locus
represented by the thick black line.
Table 2 Integrated emission characteristics of the blue and white (LED + CCF) as measured in
an integrated sphere set-up at REGENT LIGHTING.
Integrated
Characteristics
Luminous
flux (lm)
Efficacy
(lm/W)
CIE-x
CIE-y
CCT (K)
CRI
CRI9
LED
-
26
0.1468
0.0336
-
-
-
103
0.4020
0.3876
3546
94
78
LED + CCF
128
2
Conclusions
A new approach has been presented for the fabrication of thin-film efficient LED-based lighting
modules compatible with R2R production. This approach overcomes the fundamental limitations of
current technologies for thin-film lighting modules and allows the fabrication of highly efficient, thin
SSL lighting modules with high luminance uniformity and light and excellent spectral properties
through proper engineering of the diffractive transparent diffusers and color converting foils, even with
long-pitch LED arrays.
2
Please note this corresponds to 512 lumen for a 10x10cm2 area; much higher than e.g. best OLED today; Philips Brite FL300.
11
Acknowledgments
The authors would like to thank S. Ivanovici at BASF (Ludwigshafen, Germany) for the supply of the
Color Changing Films and B. Koller at REGENT LIGHTING AG (Basel, Switzerland) for the
measurement of the blue and white LED foils.
The research leading to these results has received funding from the European Union Seventh
Framework Programme [FP7/2007-2013] under grant agreement n°619556, project LASSIE-FP7.
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