34 JULY/AUGUST 2017 LEDsmagazine.com
ssl design | OPTICAL MATERIALS
When selecting between acrylic and
polycarbonate, the designer does
not need to compromise and suffer
all the drawbacks of their selection.
Here are some examples of optional
modifications possible for acrylic
and polycarbonate polymers.
In the case of acrylic, you can
modify the mechanical properties. Specifically, the low-impact
property can be improved in two
ways. First, the lens or cover can
be designed with thicker walls. A
second option is to use high-im-pact PMMA grades. In that case,
beside the extra costs, there is some
tradeoff in terms of lower light
With polycarbonates, you can
achieve several types of modifi-
cations. Consider ultraviolet (UV)
sensitivity that is a problem for a
product exposed to sunlight. The
poor UV stability of polycarbon-
ates can be largely improved by the
introduction of UV absorbers (UVA)
into the bulk material. Different levels of
UVA will result in different levels of UV sta-
bility. For the case in which the cover is pro-
duced from a PC sheet, the UV protection
can be based on a co-extruded layer of PC
enriched with UVA that is much more effi-
cient and cost effective in comparison to the
option of addition of the UVA to the bulk.
Scratch resistance can also be addressed.
This property can be improved via special
coatings applied in a second-stage process.
This type of improvement is very popular in
the automotive industry in the production
of PC lamp covers.
And although polycarbonate is
by far better in terms of fire resistance, in some applications further
flame-retardant property improvement is required. This is especially
true when the product must comply with fire standards. A good
example is the European railway
norm (EN 45545-2) that covers all
parts in the train. For such applications, lighting suppliers must
use a special grade of PC that contains flame-resistant additives for
the manufacturing of LED covers
Measuring light diffusion
Now let’s move on to the task of
measuring light diffusion and characterizing optical performance.
For some of you, this may sound
very simple. However, this isn’t the
case. Manufacturers and developers employ different methods to
measure diffusion quality. There
is no harmonized method or norm
to express light diffusion performance. Due to the lack of uniformity, problems arise in describing light diffusion systems and in the comparison of one system
to the other.
Two parameters determine light diffusion performance: light transmission (
transparency) level and light diffusion level. It is
FIG. 2. The illustrations depict light distribution when
incident light is transmitted through an optic object. Wide-angle scattering shown at the top will result with high haze
and low clarity. Narrow-angle scattering show at the bottom
will result with low haze and high clarity.
Property comparison of polymers for light diffusers.
Properties Acrylic Polycarbonate
Impact strength Acrylic does not have the same strength and
resistance to impact as PC. If it is hit with
sufficient force, it will shatter.
Polycarbonate is well known for its strength and resistance to impact. When
it is hit with an object, it is almost impossible to break. This property makes
it ideal for machine guards and protective screens. It is one reason face
shields and protective goggles are often made from PC.
Weathering Excellent resistance to weathering. UV light
does very little damage to acrylic over time;
thus it is a good choice for outdoor use.
Very sensitive to UV solar radiation. When exposed to UV, PC is subject
to rapid yellowing and micro-cracking. It is possible to improve the PC
weathering resistance by introducing UV absorber into the bulk.
Heat stability Heat distortion temperature (under a load of
260 psi) is 200°F (93°C).
Heat distortion temperature (260 psi) is 264°F (129°C).
Scratch and abrasion
More resistance to scratches and abrasion
damage than PC.
Very sensitive to scratches and abrasion. This property is one of the well-known disadvantages of PC. In some cases, a hard coating should be
considered to overcome this weakness.
Flammability One of the most flammable polymers, with
no known solution to improve the flame
retardancy by additives.
Inherent fire resistance V0 UL-94 at 3. 2 mm. Range of options to improve
the flame retardancy properties without sacrificing transparency.
Optical Light transmission of about 92%. Refractive
Light transmission of about 88%. Refractive index 1.58.