32 NOVEMBER/DECEMBER2013 LEDsmagazine.com
optics | REFLECTORS AND LENSES
economical way. With this technique, lay-
ers of SiO2 and TiO2 will be deposited from
the gaseous phase alternately. By engineering thickness and the number of layers as
needed, the coating’s spectral characteristics are defined effectively. Reflectors perform optically as shown in Fig. 3a.
Besides lenses and reflectors, there are
solutions for optical problems that take
advantage of both worlds. Let’s call them
hybrid optics. A special form of such a hybrid
optic is the collimator, which enjoys great
popularity for LED applications. Such a
device exhibits lens-like features at the first
and the last optical surface and reflective
properties in between (Fig. 3b).
In most cases the reflectiveness is not due
to a coating but makes use of the TIR phenomenon. TIR is a consequence from the
underlying physical principles. When the
light rays hit the surface between media
coming from the optically denser side above
a certain critical angle (Fig. 4a). Prisms on
the outside can support this effect even if
the contour of the collimator itself would
not allo w for this critical condition. In principle, this effect is lossless. So collimators
may also be thought of as bulk lenses with
The concept of a reflector with lens features operates in a reverse manner. Here a
lens structure is added in the center region
of the LED to the reflector in order to gain
control over the direct emission from the
light source (Fig. 5).
Pros and cons
Lenses and reflectors show significant differences when comparing their advantages and
disadvantages in solving specific optical problems. That makes them more or less suitable
for different tasks. Let’s consider the facts.
Up to now the line between the optics was
drawn between lenses and reflectors. However, in LED applications, classical bulk
lenses are rarely used. Even the lighter-weight Fresnel lenses are only rarely seen.
Instead, the already described collimators
are typical and we will concentrate on them
for the time being.
In general, simple reflectors are limited
to only one interaction between light and
surface due to t he underlying optical principle. This has direct implications for pros
and cons with efficiency being a positive.
Since the light doesn’t have to enter t he substrate material but interacts directly at the
surface, coefficients of reflectivity of up to
98% can easily be obtained with sputtered
coatings. The very same holds true for the
described PICV D method of deploying highly
reflective dielectric coatings on substrates
such as glass or plastics.
The single interaction between light and
matter will also limit the influence of inevitable manufacturing tolerances (contour deviations, surface roughness) and allows for their
control or even compensation, respectively.
A nother important criterion for cost-effective production and quality of the optical element is its size and consequently the amount
of material being used for it. For a reflector
the glass is only the substrate material for
the coating and its thickness can therefore be
limited. A direct consequence of this fact is
the easy scalability of such a solution to comply with bigger light sources.
As many new LEDs feature a relatively
larger LES (light-emitting surface) of late,
scalability is an important advantage for
glass.Indeed,thetrendtowardchip-on-board (COB) LEDs makes reflector solutions
look even more attractive.
Obstacles for reflectors
Despite all the advantages there are also
drawbacks when using reflectors. While
tolerances, the one and only interaction
between light and surface limits the possibilities to alter the rays’ path. That means
many optical features have to be implemented in only one surface such as ray
deflection via the contour, controlled scattering via facets, and/or spectral filtering via
the coating, thus adding complexity.
The downside of the open and lightweight
construction of a reflector is its inability to
take control over light that is emitted from
the LED directly into forward direction. Only
the fraction of the light that will be emitted
laterally will interact with the surface of the
reflector and can be manipulated. This fact
is negatively affected by the LEDs’ characteristic emission pattern that has a strong bias
on forward light. Unpleasant light distribution characteristics are often seen as a consequence of this, leading to unwanted color
distribution effects or a background illumination called spill light. The latter effect is
especially noticeable with systems of smaller
beam angles such as lamps with a full width
half maximum (FWHM) angle less than 20°.
One possibility to counteract that behavior is to increase the depth-to-diameter ratio
of the reflector, limiting the solid angle of
FIG. 2. Ray paths for a bi-convex lens (a)
and a concave convex Fresnel lens (b).
FIG. 3. A reflector scatters rays
some what (a) while a collimator directs
the rays in a specific pattern (b).
LEDsmagazine.com NOVEMBER/DECEMBER2013 33
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optics | REFLECTORS AND LENSES
direct emission. As this is not always possible due to physical or manufacturing restrictions, a certain effect remains. The open
front of the reflector will also require additional encapsulation when a shielding of the
LED to environmental influence is desired.
Collimators, on the other hand, have the
inherent ability to protect the light source
from the surroundings when designed
accordingly. Its concept relies on a bulk
optical approach trying to capture most (if
not all) of the LED’s emission and gain con-troloverit.Formingacavityaroundthe
light source assures this and helps with the
shielding. So with this approach already t wo
surfaces are ready to take control over the
light (the entrance and the exit surface of the
collimator) by means of contour, micro-, or
The third place that is optically active is
the reflective boundary of the collimator.
Most commonly, TIR is used here to ensure
highly efficient ray deflection. All the surfaces need to be manufactured very precisely to allow for best performance without
unwanted light loss or distribution issues —
a potential disadvantage.
Moreover, the light has to enter a medium
with higher optical density and has to leave it
as well. Both processes exhibit Fresnel losses
that can only be suppressed by additional
antireflective coatings that are potentially
costly due to two separate surfaces that have
to be covered. The light will also have to travel
relatively long distances within the optical
material, giving rise to internal absorption
effects that have to be taken into account.
Efficiency-wise this is a drawback.
Enabling an optical element like a colli-
mator to efficiently collect all of the emitted
light requires its dimensions to be at least
as big as the light source. As already men-
tioned, collimators are bulk optics. This fact
together with the increasing size of modern
LEDs will result in even bigger optics that
will be increasingly hard to handle and will
drive costs as well as decrease performance.
Another issue worth mentioning is glare.
Since many lens optics are smal l, high lumi-
nance values will occur and lead to discom-
fort or even the inability to see. Reflectors,
on the other hand, can be easily designed
large enough to spread out the light flux and
hence to lo wer luminance values to pleas-
Present and future
Drawing on experience in actual SSL projects using reflectors and lenses, we can
share some observations of the present state
of optics design and the future. One major
goal of a project was the development of a
family of reflectors to be used with a Zhaga-compliant LED light source. This family of
reflectors was targeted at 10°, 15°, 25°, and
40° beam angles, respectively. The project
FIG. 4. The total internal reflection
phenomenon allo ws calculation of ray
paths through a medium.
FIG. 5. A hybrid lens uses collimator and