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A novel concept for large area light sources: Cavity-lit lightguides


Abstract and Figures

Many illumination tasks require large area light sources, e.g. LCD backlighting or general lighting. Depending on the application these sources have to fulfill criteria concerning uniformity, angular distribution and color of light output, brightness, efficiency, flatness and cost. After discussing established solutions with regard to these requirements, light guides which incorporate light sources like light emitting diodes or thin fluorescent lamps in cavity like recesses are introduced. The advantages of this scheme are flatness, scalability in area and reliance on established sources. This will be demonstrated for large area light sources utilizing high power LEDs or thin fluorescent lamps.
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A novel concept for large area light sources:
cavity-lit lightguides
Horst Greiner*
Philips Research Laboratories, Aachen
Many illumination tasks require large area light sources, e.g. LCD backlighting or general lighting. Depending on the
application these sources have to fulfill criteria concerning uniformity, angular distribution and colour of light output,
brightness, efficiency, flatness and cost. After discussing established solutions with regard to these requirements, light
guides which incorporate light sources like light emitting diodes or thin fluorescent lamps in cavity like recesses are
introduced. The advantages of this scheme are flatness, scalability in area and reliance on established sources. This will
be demonstrated for large area light sources utilizing high power LEDs or thin fluorescent lamps.
Keywords : large area light sources, flat luminaires, lightguides, edge-lit, direct-lit, backlighting, light tile, fluorescent
discharge lamps, light emitting diodes
1. Introduction
Light sources emitting over a large area in a uniform manner are required in many lighting tasks like display and
billboard backlighting, indoor and architectural lighting to name a few. Depending on the application the required area
can vary from square centimeters for backlighting small displays to square meters for large billboards and ceilings
(“artificial skies”). On the other hand the whole luminaire incorporating the source should have no extension in the
“third dimension”: the thickness of a backlight for a laptop LCD display should not exceed several millimeters whereas
for indoor and architectural lighting a few centimeters are probably acceptable. Ideally one would like to have truly 2D
light sources and recent developments in organic LEDs indicate that this may become possible in the not so distant
future. But in the meantime we have to make do with established light source technology which provides us with point-
like (light bulbs, high intensity discharges, inorganic LEDs) or linear sources (low pressure discharges). So the task
facing the designer of a large area light source is to evenly distribute the light emanating from these point like and linear
light sources in an efficient way over the required area using the minimum amount of space. In this regard light source
and luminaire should be hardly distinguishable.
Clearly our world abounds with large area light sources which contribute to our well-being (who has not enjoyed the
blue sky, the sun shining on a white wall or a movie). Shining the light of an (artificial) source on a diffusely reflecting
surface from some distance is one possibility to realize a large area light source which requires the third dimension and
is therefore not practical in many circumstances. To alleviate the problem of distance one can use many distributed light
sources like arrays of thin fluorescent lamps or LEDs which illuminate a partially transparent screen (light curtain) from
closely behind. If furthermore the sources are enclosed in a reflecting container of an appropriate shape to prevent light
from escaping in the wrong direction, one has a so-called “direct-lit” configuration. For applications where the
demands on depth and uniformity are not too stringent, this solution can be quite satisfactory. For applications where
uniformity and flatness are at a premium like in LCD backlighting so called “edge-lit” lightguides are used. Light
emanates from sources placed adjacent to the vertical edges of a thin rectangular lightguide panel made of transparent
plastic. After penetrating the guide it then propagates by total internal reflection at the top and bottom planes of the
panel. The amount of light leaving can then be controlled quite accurately by the use of appropriately designed
outcoupling elements like screen printed dots, prismatic grooves or other indentations. Good uniformity is achieved by
variation of the size and density of these structures. The achievable brightness is determined by the number of lamps,
which can be accomodated at its edges and the thickness of the lightguide which determines the incoupling efficiency.
To obtain reasonable values it should not be less than the lamp diameter. Contrary to the direct-lit case or “hollow”
lightguide case, “solid” lightguides do not require much thickness to distribute light in a nearly lossless fashion.
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Summarizing it can be stated that the main advantages of the direct-lit schemes are simplicity and high brightness even
for large areas as the number of lamps which can be used scales naturally with the area. Disadvantages are relatively
high depth which can only be traded against decreased efficiency as will be discussed below. The edge-lit designs offer
superior uniformity and flatness, but cannot be “upscaled” to large areas for the high brightness levels demanded by
applications like LCD-TV backlighting.
The object of this contribution is to introduce a novel scheme for large area sources which is still based on a solid
light guide for light distribution, but in which the lightsources (thin fluorescent lamps or LEDs) are accomodated in
recesses with vertical walls distributed across the light guide. If the top and bottom of these “cavities” are made
reflecting, light can only penetrate through the vertical cavity walls into the lightguide, where it propagates by total
internal reflection and can be coupled out in a controlled fashion. To prevent light from escaping at the sides, the
lightguide is tightly enclosed by a highly reflecting box. The main advantages of this “cavity-lit” configuration are good
uniformity and flatness and the possibility to upscale to large areas at high brightness.
The paper is organized as follows: the next section compares the established solutions for realizing large area light
sources in more technical and quantitative detail. We then introduce the “cavity-lit” configuration for fluorescent and
LED lamps and present the results of some realistic raytrace simulations and some performance figures. Then we
present the prototype of a light tile measuring 15 by 15cm in area and employing 4 by 4 High Power Luxeon LEDs in a
cavity-lit configuration. The results of some brightness and uniformity measurements are also given and compared to
calculated values. The paper is concluded with a comparative evaluation of the various approaches to realizing large
area light sources with an eye to applications.
2. Flat large area light sources
In this section we evaluate and compare the various concepts for flat large area light sources with particular emphasis
on direct- and sidelit configurations employing fluorescent or LED lamps.
2.1 Organic Leds
In a SID publication
of 1992 it was stated that “the ideal backlight for LCD displays would be a thin film that
efficiently converts low voltage direct current into white light with a uniform surface brightness up to 3000 cd/m
There is no such source”. With the advent of organic light emitting diodes during the last decade things have started to
change: they are truly two-dimensional sources with an emitting layer measuring only about 100nm in depth deposited
on a possibly flexible substrate allowing for low cost production. Total depth including packaging should not exceed a
few millimeters. During the last decade there has been considerable progress concerning their efficiency and
maintenance and for some applications (large areas at relatively modest brightness levels of about 1000cd/m
for indoor
lighting) they could contend with more classical solutions. For more details and references the reader should consult the
excellent review by A.Bergh et al.
2.2 Flat gas discharges
There have been a number of attempts to develop flat gas discharge lamps, either based on argon-mercury or dieelectric
barrier discharges
. These lamps offer high luminances at relatively modest efficiencies for a small area (5 inch
diagonal). The most promising flat gas discharge lamp to date is the Osram Planon lamp, a flat dielectric barrier
discharge with distributed micro electrodes which create a uniform, mercury-free UV discharge. The UV radiation is
then converted into visible light by a suitable phosphor. The whole assembly is not more than 1cm thick and can be
manufactured in large sizes and attain up to 7500cd/m
in brightness. For further information we recommend the
relevant SID publication
which also gives an interesting comparison between the PLANON, side-lit and direct-lit
configurations. In this evaluation the Planon and “multiple tubing” are credited a maximum luminance of 7500cd/m
whereas edge-lit only scores 4500cd/m
. Two remarks are in order: firstly the maximum brightness of multiple tubing
solutions is clearly not limited to the given value and secondly the brightness limitations of the edge-lit configuration
can be overcome by “multiple cavities” as we try to show in this paper.
2.3 Direct-lit configurations
A schematic drawing of a direct-lit configuration employing tubular fluorescent lamps is shown in fig. 1. Given a
regular spacing of the tubes light has to be spread evenly across an exit screen. A dimensionless measure assessing the
difficulty of this task is the “aspect ratio” defined by the lamp spacing and the depth of the “light box”
. To achieve an
even distribution one can shape the backwall into suitable reflectors around the back of each lamp and/or employ a
“light curtain” of variable reflectivity in front of them
. Furthermore a prismatic structure between the lamps and
the light curtain is beneficial
. The main design trade-offs are uniformity (with regard to all viewing directions),
efficiency and flatness. We have performed some model calculations to evaluate this design issue. To describe an
(infinite) periodic lamp array we have assumed lamp compartments with the following properties: a) the lamp phosphor
is a Lambertain emitter with an absorptance of 4%, a reflectance of 48% and a transmittance of 48% b) the back wall of
the rectangular light box is clad with a Lambertian reflector c) the perpendicular walls delineating the compartment are
perfectly reflecting d) the (Lambertian) diffusor screen possesses a varying reflectivity and a fixed absorption. e) no
reflectors or prismatic sheets are employed f) the absorption of the back wall cladding and the diffusor screen are equal.
For a given lamp spacing and depth expressed in lamp diameters and absorption of the back wall and light curtain we
have determined a reflectivity profile of the light curtain which produces an even intensity distribution on the exit
screen by an iterative procedure which was based on an evaluation of a given reflectivity profile by Monte Carlo ray
tracing. Depending on whether the calculated local intensity was below or above average the local reflectivity was
decreased or increased (allowing for some relaxation) and after a few iterations an even intensity distribution was
obtained. Fig. 2 gives the efficiencies as a function of aspect ratio for assumed absorptions of 3, 7 and 10%. In the
range of aspect ratios considered here, there was only slight dependence of plus minus 5% of the efficiencies on the
actual lamp diameter (we suppose that the lamp diameter is small compared to the depth). Clearly the efficiencies
decrease considerably with aspect ratio. Fig. 3 displays the corresponding reflectivity profiles across one lamp
compartment. For large aspect ratios the reflectivity in front of the lamps has high values approaching 100% and drops
off to low values at the compartment boundaries.
In summary the following conclusions can be drawn: efficient hollow lightguides require an aspect ratio not exceeding
5. With increasing aspect ratio the reflectivity profiles exhibit increasing modulation and reflectivity maxima approach
100% in front of the lamps. Profiles with considerable modulation and high reflectivities are difficult to manufacture in
the required precision and may lead to colour shifts due to increased multiple scattering. If we consider maximum
reflectivities of about 70-80% as a practical limit, we are again limited to aspect rations not exceeding 5. Modulation of
the reflectivity profile disappears if the depth reaches the order of the lamp spacing, i.e. with an aspect ratio of about
Fig.1: Sketch of the direct-lit configuration
Fig. 2: Efficiency vs. aspect ratio for various Fig. 3: Reflectivity profiles for aspect ratios of
Absorptances of the reflecting materials 1, 2, 3, 5, 10, 15, 30 (from bottom to top)
2.4 Edge-lit configurations
The overall efficiency of the side-lit arrangements is primarily limited by the incoupling efficiency into the solid light
guide panel
. For thin fluorescent lamps the ratio of the lamp diameter to the panel thickness is crucial, it should not
exceed unity. The lamp is tightly enclosed by some sheet whose reflectance is also important for the incoupling
efficiency (Fig.4). Simple raytracing simulations show for instance that increasing the sheet reflectance from 90 to 97%
increases the incoupling efficiency from 64 to 80% (panel thickness 5mm, lamp diameter 3mm, rectangular reflector
5mm high and 8mm deep). If the reflector compartment houses more than one lamp, efficiency decreases to values
between 50 to 70%, depending on the degree of mutual obstruction of the lamps with regard to the panel’s entry face.
Due to the directional nature of their light output, better incoupling efficiencies can be obtained with LED lamps
arranged along the lightguide edges. With state of the art high power LEDs linear flux densities exceeding cold cathode
fluorescent lamps can be achieved
Fig. 4. Sketch of edge-lit configuration
0.0 0.2 0.4 0.6 0.8 1.0
length lamp compartment
reflectivity (%)
1 5 10 20
aspect ratio
efficiency (%)
Distribution of the light by TIR inside the lightguide and subsequent outcoupling accounts for about 10 to 20% of light
loss, depending on the outcoupling mechanism. This is in contrast to the direct-lit case where the dominant losses arise
from the lateral distribution inside the “light-box”.
With increasing panel area and required brightness level it becomes however more and more difficult to provide the
necessary amount of light from the edges of the solid lightguide, in particular as the thickness cannot be increased for
reasons of weight and cost. To overcome this restriction it therefore appears natural to incorporate the light sources into
the lightguide panel. This novel concept will be introduced and discussed in the next section.
3. Cavity-lit lightguides
In this section we introduce and analyse cavity-lit lightguides for tubular fluorescent and led lamps. We also discuss the
practical realization of a cavity-lit lightguide employing high brightness LEDs.
3.1 Lightguides incorporating fluorescent lamps
Fig. 5 gives a perspective view of a “channel-lit” lightguide employing three thin fluorescent lamps. The lightguide is
tightly enclosed by a “white” box with highly reflecting walls to prevent light loss. The top and bottom of the
rectangular channels housing the lamps are covered by highly reflective mirrors so that light can enter the lightguide
only through the vertical channel walls, where it propagates by total internal reflection until it is coupled out by some
suitable structure on the top side of the panel. Figure 5 shows some typical “ray histories”. Incoupling efficiencies from
the fluorescent lamps into the lightguide attain about 90% as there is no reflector around the back of the lamp. Clearly
the reflectance of the top and bottom mirrors and the aspect ratio of the channel also strongly influence the incoupling
efficiency. Light travelling in the lightguide can however be absorbed by the phosphor of the lamps and the top and
bottom mirrors. Reflection at the side walls of the enclosing box is another loss mechanism.
To evaluate the concept we consider the following example: lightguide depth 12mm, width of channel 10mm, height of
channel 6mm, lamp diameter 3mm, lamp phosphor as in section 2.3, absorptance of reflecting elements 3%. For the
outcoupling we assume that the top side of the light guide is covered with a scattering layer such that an impinging ray
is either scattered with probability P
or reflected by TIR with probability 1-P
. If the ray is scattered, it is transmitted
or reflected with probabilities of 48%, the rest accounting for absorption. To achieve uniform output the outcoupling
probability P
has to be modulated especially above the lamp channels where the flow of light is
restricted by the lamp channels. But compared to the direct-lit case the required modulation of P
is much lower. Fig. 6
gives an example for the required modulation of the outcoupling structure. Fig. 7 gives the efficiency values for the
example lightguide for various lamp spacings which are expressed as aspect rations given by lamp spacing over
lightguide thickness to enable a better comparison with the direct-lit case. The analysis reveals that for flat luminaires
with aspect ratios larger than 5 to 10, channel-lit configuration are more efficient than direct-lit ones. Furthermore they
do not require an extreme modulation of the outcoupling structure.
Fig. 5. Sketch of channel-lit lightguide with fluorescent lamps
Fig. 6. Modulation of outcoupling structure above Fig. 7. Efficiencies of channel-lit lightguide vs.
the lamp channels indicated by dotted lines. aspect ratio for various absorptances
Finally we note that a scheme similar to ours has been described in the patent literature
. In contrast to our solution the
reflective layer shields above the lamps are allowed to be slightly transmissive to compensate for the dips in intensity
above the lamps. In our opinion this is not practical as the required reflectance well above 95% has to be very precisely
tuned (an error in reflectance of about 1% produces an error of 20% or more in light output).
3.2 Lightguides incorporating LED lamps
Cavity-lit lightguides can also be realized with LED lamps. In this case the LEDs are incorporated into cylindrical holes
with vertical walls. The top of the holes are again covered by a reflecting layer, so that light emanating from the leds
enters the lightguide panel only through the vertical cylinder walls. Fig. 8 gives a sketch.
The incoupling efficiency is excellent especially with diodes which emit preferentially to the sides. The geometrical
probability that a ray travelling inside the lightguide hits a source again is also much reduced compared to the
fluorescent tube case, so that there is little absorption in the lightguide. To achieve a uniform output the outcoupling
structures have to be modulated especially around the LEDs. The required degree of modulation is however moderate.
Fig. 8: sketch of cavity-lit LED lightguide Fig. 9: cross section through CAD drawing of prototype
Fig 10: Measured intensity distribution. The black Fig. 11: Calculated outcoupling “density” for
dots indicate the LED positions uniform ouput.
As a proof of principle we have built a demonstrator incorporating 4 by 4 white Luxeon leds
in a lightguide
measuring 15 by 15cm with a depth of 12mm. The cylindrical holes accomodating the LEDs measure 10mm in
diameter and 5mm in depth and are located in the center of the four by four squares into which the “light tile” can be
decomposed. Fig. 9 shows a cross section through a CAD drawing of the luminaire. The LEDs are mounted directly on
a metal plate which serves as a heat sink. Light outcoupling is accomplished by a thick homogeneous diffusive layer
applied to the top of the lightguide (we used hollow glass beads dissolved in a UV curing transparent lacquer). Fig. 10
gives the intensity distribution measured with a Radiant Imaging camera. The dark areas above the led cavities are quite
noticeable and to achieve a truly uniform output, the outcpoupling power would have to be modulated across the panel.
as shown in fig. 11. The modulation pattern was calculated by an iterative procedure similar to the one described in
section 2.3. The bright areas correspond to high outcoupling. The modulation has a wavelike appearance and its depth
amounts to about 25% of the average.
4. Conclusion
The comparative merits of various realizations of flat, large area light sources have been analyzed. The direct-lit
configuration can provide very high brightness by employing closely spaced lamp arrays. However, the lamp spacing
over depth ratio is limited to a factor of about 5. Edge-lit configuration can be very flat, but their brightness is limited
for large areas.The cavity-lit configuration allows for bright large area light sources with high aspect ratios. The total
lumen package of a flat luminaire can be “concentrated” into relatively few “high power” sources and does not have to
be “spread” across the luminaire into many “low power” sources. Clearly this has important repercussions on the cost of
a given lumen package and the complexity of the driving and wiring of the sources. In view of the recent developments
of high power LEDs, this seems to be particularly relevant for lighting based on inorganic LEDs
. We have
demonstrated the principle in theory and practice and future work will concentrate on the development of demonstrators
with a modulated outcoupling structure to achieve uniform light output at high efficiencies.
The contributions of Ronald Grieger, Tony Hammers (engineering and realization of the LED lightguide), Martin Jak,
Hugo Cornelissen (measurements) and Rainer Assent (software) are gratefully acknowledged.
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During the designing process for a luminaire with the light guiding plate, first it is necessary to analyze the issue of coupling light source with the light guiding plate, since only the luminous flux which is introduced into the light guiding plate might be utilized further on and become the useful flux. Theoretical calculations of the coupling index between the fluorescent lamp and the light guiding plate were made. The measurement's results confirmed the conclusions from theoretical calculations, namely that the best possible solution is placing the fluorescent lamp directly at the surface of the light guiding plate. Placing the lamp 2 millimeters from the plate's front surface causes the decrease by 20%, and further increase in the distance to 5 millimeters -- decrease by 50%.
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