PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008 167
Sławomir ZALEWSKI
Warsaw University of Technology
Light distribution forming in LED luminaires
Abstract. Electroluminescent diodes perfectly cooperate with monolithic optical elements based on total internal reflection such as massive
collimators and massive planar optical fibers. LED light should be inducted into luminaire elements in a way that internal reflections have appeared.
Suitable selection of dimension and shape of individual luminaire elements allows forming light distribution of the luminaire. Light leading technology
has wide capabilities of appliance.
Streszczenie. Diody elektroluminescencyjne doskonale współpracują z monolitycznymi elementami optycznymi bazującymi na całkowitym
wewnętrznym odbiciu, takimi jak kolimatory masywne I masywne światłowody planarne. Światło diody należy wprowadzić do elementu oprawy w taki
sposób, by nastąpiły odbicia wewnętrzne. Właściwy dobór wymiarów i kształtu poszczególnych elementów oprawy pozwala na kształtowanie rozsyłu
strumienia świetnego oprawy. Technologia elementów światłowodzacych ma bardzo szerokie możliwości zastosowań. (Kształtowanie rozsyłu
strumienia świetlnego w oprawach oświetleniowych z LED-ami).
Keywords: luminaire, electroluminescent diode, total internal reflection, planar optical fiber.
Słowa kluczowe: oprawa oświetleniowa, dioda elektroluminescencyjna, całkowite wewnętrzne odbicie, światłowód planarny.
LEDs used for lighting
Diodes, in spite of continuous technological progress,
will remain low power sources of light. Even considerable
increase in luminous efficiency will not cause achievement
nominal luminous flux to level of traditional gas discharge
sources. So, diodes are applied in lighting in micro and mini
scale in appliances such as torches, lamps of local
illumination, signal lights. LEDs can compete with traditional
lighting equipment only in multi-source applications [1].
Fig. 1. Light distribution curves of power LEDs: a) Lambertian, b)
batwing, c) side emitting [2].
Fig. 2. LED luminance distribution (Foto A. Kotowicz).
Only power LEDs are applied in lighting technology.
Their power is up to 5W by one chip and light distribution is
given as one from three base curves: Lambertian shown in
figure 1 in part a), batwing shown in figure 1 in part b) and
side emitting shown in figure 1 in part c).
LEDs are high luminance light sources. Luminance of
chip reaches level 10
6
–10
7
cd/m
2
, at its small size.
Exemplary luminance distribution is presented in figure 2.
This specific features cause proper approach to design
luminaires based on electroluminescent diodes.
Diodes, from the point of view of their small size, can be
treated as quasi point sources. Due to it, they could be used
in the areas inaccessible to other sources. In cooperation
with diodes monolithic optical elements such as collimators
and planar optical fibers work the best.
Monolithic elements applicable in luminaires with LEDs,
because of long optical way which light overcome in
material, must be made of proper clarity and clearness
materials. However, refractive index of the material should
be low because of luminous flux losses at the boundary
between environments during crossing light from one to
another.
Massive collimators in luminaires
Massive collimators are used for converging light of
diodes. One collimator works with one diode and give
narrow angle beam of rotary symmetry. Special profile of
collimator output surface makes beam wider and can
change its symmetry.
Multisource LEDs luminaires are made as a matrix of
collimators and diodes. They are properly individually
rotated and their optical axes are aimed. Resultant light
distribution of whole luminaire is obtained as a sum of all
matrix element light distributions. Methods of massive
collimators designing have been already given [3].
Planar light fibers in luminaires
Light leading plates (high dimension planar fibers) are
applied in luminaires with wide light distribution. One plate
simultaneously cooperate with many diodes. That plates are
multimode optical fiber. It means that light rays can
propagate under different angles to main axe of fiber.
Therefore during designing luminaires with light leading
plates, propagation of each mode should be taken into
consideration.
In the light leading plate some zone can be detailed.
Each one is responsible for another period of resultant light
distribution forming. The first is zone of introduction light to
the plate where light is put into the fiber in angle allowing for
total internal reflections. The second is mixing zone where
modes are mixed. The third is output zone where light is
finally emitted to environment.
168
PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008
Fig. 3. Introduction of light to planar optical fiber by flat side
surface.
Fig. 4. Lambertian diode light distribution and a its light distribution
after crossing the bondage of material of n = 1,5.
Fig. 5. Introduction of light to planar optical fiber by nest at main
surface.
Fig. 6. Introduction of light to planar optical fiber by nest for two
diodes at opposite main surfaces.
Fig. 7. Output elements: trenches and channels.
Diode light is introduced into fiber by input surface.
Shape of it and primary light distribution of LED are jointly
responsible for angular distribution of light inside the plate
(inherence of modes). This surface shape should be proper
for used diode light distribution. Flat side surfaces of plate,
other surfaces perpendicular to planar optical fiber main
surface or proper diode nests located on side or main
surfaces can be used as input surfaces. Surfaces of nests
depending on diode light distribution can be flat or
converging.
The simplest manner of introducing light into the plate is
presented in figure 3 illumination at straight edge of plate.
Light rays after crossing bondage of material change their
directions in order to refraction law. Diode light distribution
inside material is different to primary distribution. Figure 4
shows Lambertian diode light distribution and its change
after crossing the bondage of material of n=1.5.
Changed light distribution is given by transformation:
(1)
)
sin(
)
sin(
)
(
)
(
'
β
α
α
β
⋅
=
⋅
=
n
I
n
I
where:
I’(β) – luminous intensity in direction β inside material of
refractive index n,
I(α) – luminous intensity in direction outside material,
α – direction in air,
β – direction in material,
n – refractive index of the material.
In this equation partial reflection of luminous flux at
bondage of material has been neglected.
Light distribution curve is also possible to appoint
graphically in simple way.
Diode light can be introduced through a nest placed on
lateral edge of plate. It is shown in figure 5. The nest in this
case is formed according to massive collimators design
principles. This shape of input surface is able to give the
narrowest beam of light inside plate that means the smallest
number of modes.
The nest can be placed on the main surface of the plate.
In this case diode should have side emitting light
distribution. Nest depth can be equal or smaller than
thickness of plate. In case the nest is over whole thickness
of the plate two diodes at opposite sides of plate can be
located in the one nest as shown in figure 6.
Light which enters the interior of optical fiber is
transmitted along its straight section. Modes are mixed
there. Getting several modes to every point of the plate
depends on mutual proportion of thickness and length of the
fibre and dimensions of diode. If length of the fibre straight
section is long over certain critical minimal value with
satisfactory accuracy it can be told that all modes are
blended enough. In other case modes occurrence analysis
in every point is necessary.
A plate with properly short mix section can be used for
separation of modes and controlled emission out of the
luminaire. However, it requires proper accommodation of
output elements along the plate.
Emission of light as it is presented in figure 7 mostly
takes place through flat main surface of the plate after
reflection on elements changing directions of rays like
trenches or channels. Trenches are hollows on planar fiber
surfaces. Mostly they are a linear structure, however they
can be point structure too. Channels are inside the plate
and because capabilities of making are rectilinear. Inclined
surfaces of trenches and channels change direction of rays
propagation. Beams reflected on this elements falls on the
opposite main surface of the plate. Angle of incidence is
smaller than critical angle that light is refracted and emitted
out of luminaire. The best solution is when elements shape
causes small angle of emission. It means that rays are
about perpendicular to exit surface. Bigger angles are
needed when beams should be additionally refracted.
PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008 169
Fig. 9. Luminaire with central light leading pane.
Fig. 10. Luminaire with central diode module and two panes.
Fig. 11. LED illumination luminaire.
Proper assortment of trenches or channels dimensions
and suitable distance between them cause controlled
modes radiation. Elements surfaces inclination determines
beam direction.
Minimal distance between neighbouring elements is
given by full employing of elements surface and equal
(2)
)
(
min
gr
tg
h
α
⋅
=
Δ
where: h – height of element, α
gr
– critical angle of total
internal reflection.
Practically, optimal distance is bigger and depends on
modes especially maximum light coincidence angle in
discussed point at plate main surface. Active surface
inclination angle is approximated for 45º and depends on
angular distribution of light reaching to the element and
direction of output light.
Edges of the plate are potential places of uncontrolled
losses of luminous flux. Without places formed for
introducing diodes light to planar fiber they should be
formed to turned back light inside of the plate or, if it is more
advantageous, to radiate light in assigned direction.
Forming of edge for turning back light is made as symmetric
cutting with 45º angle.
Applications
LED luminaires equipped with light leading plates can
find a lot of different applications. Each luminaire consists of
group of diodes covered by casing and of one or more
plates made of transparent material properly formed. Such
luminaire is characterised by very small thickness, about
some centimetres. Other dimensions are unlimited also
approximate to luminaires for fluorescent lamps. The casing
is diodes radiator too.
General indoor lighting luminaires made in LED
technology can be in one of all class I to V for all mounting
systems in or under ceiling. Their dimensions can be
suitable for typical ceiling units or fitted for individual needs
as well. Shape of luminaires can be square, rectangular,
round or oval with diodes located in central part or on
luminaire periphery. Rectangle luminaries can be made as
one plate with two side diode modules as shown in figure 9
or with central diode module and two side plates as
presented in figure 10. All of such luminaires can have
ornamental value.
Illumination luminaires of type “linear lighting” made in
LED technology as shown in figure 11 can have suitable
length for individual use. They are thinner and less viewable
than traditional luminaires for this application.
Emergency and evacuation luminaires with proper signs
can be made in this technology too. Due to application of
two light colours diodes, for example white for pictogram
and green for background, it is possible to reduce power
needs and made information longer visible in emergency
supplying lack.
Light leading elements could be also applied in many
other fields such as advertising and signal lights. It is
possible to create lights mounted on high buildings and
lights for trucks. Back signal lights in vehicles made in LED
technology can be shown at back window-pane.
REFERENCES
[1] Żagan W., „Rzetelnie i rozważnie o LED-ach – ocena
obecnych i prognoza przyszłych aplikacji oświetleniowych diod
elektroluminescencyjnych” Przegląd Elektrotechniczny, 1/2008
[2] http://www.lumileds.com/
[3] Zalewski S., Projektowanie wtórnych układów optycznych do
LED-ów, Przegląd Elektrotechniczny 5/2007
Author: dr inż. Sławomir Zalewski, Politechnika Warszawska,
Instytut Elektroenergetyki, Zakład Techniki Świetlnej ul. Koszykowa
75, 00-662 Warszawa, E-mail:
slawomir.zalewski@ien.pw.edu.pl