Energy-Efficient Lighting by LED
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Helmut F.O. Mueller , Francesco Sasso
1-Green Building R&D, Duesseldorf, Germany,
Email:* helmut.mueller@greenbuilding-rd.com
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Email : francesco.sasso@aol.com
Abstract:
Light Emitting Diodes (LED) is the light source of the future because of energy efficiency,
performance, and life-time. Due to their small dimension and high flux many innovative solutions for
non-glare and controlled light distribution are developed for various applications. This paper deals with
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a joint R&D project for edge lit and highly transparent light guiding plates with controlled single-sided
light emission. The transparency allows for looking through the plate and is not affected by the specific
micro-structure for light emission on one surface of the light guide.
The work plan, including optimization of microstructures by ray-tracing simulation, development of
manufacturing technology for large scale production, and system design for different applications, is
carried out by an integrated team from university and industry. The main objectives are economic
manufacturing technologies, high quality and low energy lighting, and architectural system integration.
Keywords: LED, Light Guide, Micro Structure, System Design, Applications.
Topic: 3 Sustainable & low energy architecture
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Controlled light emission from transparent panes by micro structures
The functional principle in Figure 1a) shows the LED edge lighting of a highly transparent, light guiding
pane, e.g. made of PMMA. The light emission by only one surface of the guide is generated by a
specific micro structure. Form, geometry and position of the micro structure determine the angles of
light distribution and whether the light is emitted by one or two surfaces. The task of this project is a
one-sided emission in controlled angles, i.e. perpendicular or inclined, in order to achieve transparent
building components with defined light distribution curves for various applications. Figure 1b) shows
exemplary simulation and measurement results of a specific cylindrical structure given in Figure 1a.
The coincidence of calculated and measured values is relative high and demonstrates, that one-sided
emission can be achieved to nearly 100 % [1].
a)
b)
Figure 1: a) Principle of an edge lighting a transparent plate by total internal reflection (TIR) and onesided light emission by a plane cylindrical micro structure.
Parameters: Cylinder-diameter d, -height h, -axial distance D.
b) Measured (blue) und simulated (red) light distribution curves, cylindrical structure, diameter d 50
µm, distance D 150 µm. (A. Neyer, TU Dortmund [2]).
The objective of high transparency is solved by flat micro structures with lateral dimensions near to the
resolution limit of the human eye. Simulations showed, that optical structures of cylinders with flat top
covers, diameter 50 to 100 µm and the ratio of pitch (D) to diameter (d) not smaller than 3, are best
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R&D project, funded 2012-14 by AiF : Entwicklung flacher, durchsichtiger und gleichzeitig selbst-leuchtender
Elemente nach dem Prinzip der Lichtleitung mit gezielter Lichtauskopplung durch Mikrostrukturen. Research
team: Green Building R&D, JO GmbH & CO. KG., RIF Institut für Forschung und Transfer, Temicon GmbH, TU
Dortmund Faculty of Electro- and Information Technology
suited for the task of transparency and one-side light emission [1]. The transparent pane can be
looked through from both sides, with one exception: If the eye is exposed to light from the emitting
surface and if the luminance of this surface is higher than that of the background, it´s not possible to
look through. Figure 2 shows the sample of a transparent pane, light emitted away from the eye to the
background.
a)
b)
c)
Figure 2: a) Sample of transparent pane, light emitted away from the eye to the background
b) Sample with background picture, LED switched out, c) LED switched on.
(A. Neyer, TU Dortmund [2]).
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Manufacturing
The manufacturing of light guiding plates (LGP) with optical micro structures for extracting the light is
state of the art for TV and monitor backlight units (BLU) [3]. But these manufacturing techniques, e.g.
by burning ceramic colour dots into glass [4], cannot be used for products with one-sided light
extraction and distortion free view perpendicular to the plate.
First prototypes of 100 mm x 100 mm with 50 µm high cylindrical structures were produced by silicon
casting (PDMS), the master being manufactured by photolithographic technology [1]. For larger
structures up to 500 µm moulds were produced by cutting in brass.
The transfer to large-scale manufacturing can be realized by two different technologies, hot embossing
and UV embossing (Figure 3). Hot embossing uses flat form tools of metal, which are heated, to
manufacture structures > 500 µm in thermoplastic materials like PMMA. The pulsed plate to plate
process is used for flat panes or sheets.
UV embossing uses cylindrical tools of metal or synthetic films with metal layers to form a thin lacquer
film, which is dried by UV radiation. The substrate of the lacquer can flat plates or films, thus allowing
for a continuous plate to plate or roll to roll production process.
Masterfilm
UV
LED
UVlacquer
Rigid substrate, e.g. glass,
PMMA
a)
b)
Figure 3: Large-scale manufacturing of micro structures: a) Hot embossing b) UV embossing
First experiences show, that cylindrical structures are difficult to manufacture by hot embossing in the
necessary accuracy of shape and optical surfaces, even with cylinder shafts inclined up to 10°, while
cones and pyramids are suited well for this embossing technique.
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Architectural integration
The thin and transparent light guiding plates are predestined for integration in building elements, like
windows, skylights, transparent partitions and balustrades. Additionally they can be used as
extremely thin and transparent luminaires or be applied for exhibition purposes in glass
show-cases or lighting safety glass for pieces of fine art.
Table 1 gives a systematic survey of possible applications, tabulated according to direction of
light emission, and area of application. The overview demonstrates the wide functional and
creative variety of the LED lighting system.
Table 1: Areas of application of transparent LED lighting systems
The integration of artificial lighting in daylight openings like windows or glazed areas of
partitions allows for similar lighting conditions by day and night, thus increasing the range of
utilizing and furnishing rooms. Last not least a combination of innovative sunlight redirecting
systems [5] and LED lighting can be realized in clerestory windows and skylights in order to
increase lighting and cost efficiency.
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Case studies for office lighting
The lighting of typical office rooms with core and window zones was designed in order to
compare the advanced LED lighting with standard solutions. Figure 4 shows the location of
transparent LED light guide panels in the upper areas of windows and partitions. The
resulting illuminance in task level is shown by false colours in the plan.
For comparison a reference lighting system with fluorescent lamps in suspended luminaires
was chosen, which generates a similar mean illuminance of 500 lx as the LED systems.
Figure 5 shows the significant difference of brightness distribution on room surfaces
(luminance) by the two lighting systems. The uniformity of luminance of the LED system
.
Figure 4: Vertical section with location of transparent LED light guide systems in upper areas
of windows and partitions and plan with illuminance in task level. Simulation by RELUX
Figure 5: Comparison of luminance distribution on room surfaces for transparent LED light
guides (left) and reference system with suspended luminaires and fluorescent lamps (right).
is striking in comparison to the reference system with significantly higher contrasts. The
transparent light guides, emitting the LED light in an angle of about 40° downwards, can be
looked through.
An additional case was developed with LED task lights complementing the general room
illumination by LED light guides in clerestories (Figure 6). The room has a basic illuminatin of
300 lx and the task area of 500 lx. This specific lighting system has ergonomic advantages,
as direct or indirect glare are avoided, e.g. by light sources or contrasts on monitors or room
surfaces. The transparent task luminaire is located directly above the monitor, distributes the
light evenly on task area, and can be looked through.
Figure 6: Luminance distribution by a combined LED lighting system of task luminaires above
monitors and integrated luminaires in upper areas of windows and partitions.
All lighting simulations were carried out by RELUX. In contrast to standard luminaires there
still is a demand of specification for the advanced LED luminaires by technical data in the
programme library. The most significant data of luminaires and room illumination are given in
Table 2, comparing the LED lighting system with micro structures and the conventional
reference system. Given approximately the same mean illuminance, the uniformity of
luminance and the energy efficiency (specific wattage per room) are better for the LED light
guide system than for the reference one with fluorescent lamps. The higher energy efficiency
is mainly caused by the higher luminous efficacy of the LED and the luminaire (no losses by
total internal reflection TIR in light guide), and the higher maintenance factor (long lifetime of
LED, low degree of pollution due to enclosure of LED). Consequently the operating costs for
energy and maintenance are reduced.
Table 2: Characteristics of LED lighting system in comparison to reference system
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Conclusions
The development of micro structured LED lighting systems yields manifold solutions of architectural
integration and interior design. Transparent building components can additionally perform as
controlled light emitters without glare and conventional luminaires can become redundant. Hybrid
systems for artificial- and day-lighting provide advantages of multifunctional configuration. The new
product development has passed the phases of prototype manufacturing and testing successfully and
principals of large-scale manufacturing are resolved. The main advantages of LED lighting by
transparent light guides in comparison to luminaires with fluorescent lamps as well as OLEDS are
obvious: Economy as to investment and operation costs, long lifetime, and high quality of room
illumination.
Literature
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[2]
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[4]
[5]
[6]
[7]
Tengler; Florian-Carsten, Jakubowski, Michael, Neyer, Andreas:
High Transparent Light Guiding Plate for Single-Sided Light Emission,
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gleichzeitig selbst-leuchtender Elemente nach dem Prinzip der Lichtleitung mit
gezielter Lichtauskopplung durch Mikrostrukturen. RIF, TU Dortmund, 2012-2014.
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Tageslichtlenksystem mit hoher Effizienz: Licht 2012, Berlin.
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Master Thesis, A.B.I.T.A., Universität Florenz, 2014.
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xia intelligente architektur, 87, Alexander Koch GmbH, Leinfelden-Echterdingen, 05-2014.