LEDsmagazine.com JULY/AUGUST 2017 33
ssl design | OPTICAL MATERIALS
In recent years, lighting manufacturers and developers have moved away from traditional light sources and to LED-based solid-state lighting (SSL) technology. While LEDs offer many benefits such
as excellent energy efficiency, the point
sources also present some problems in gen-eral-illumination applications — glare, for
instance. So in LED lighting developments,
engineers must design lens systems and
covers that are optimized in terms of their
light diffusion properties to eliminate the
LED glare issue.
Glass and transparent plastics, especially
acrylic resins, have long been used in the
lighting industry for various aesthetic and
functional purposes in optics design. But as
the lighting market moves increasingly to
LED technology, requirements for high light
diffusion have spurred lens and cover manufacturers to devote much of their activity to
developing suitable plastic solutions.
In this, the first in a two-part article
series, we will discuss the issue of light diffusion technology in plastic. This article will
focus on selecting the optimal polymer as a
carrier for the light diffusion system and on
how to measure light diffusion properties. In
a subsequent article, we will focus on selecting the optimal light diffusion system.
Selecting the optimal polymer
When selecting the thermoplastic type for
a light diffusing cover or lens, there are few
candidates that can be considered. The
most essential feature is transparency. In its
natural form, the polymer must be highly
transparent to minimize LED efficiency
losses during the light diffusion action.
A short list of engineering polymers can
be technically considered as appropriate
solutions. But when cost is taken into consideration, only polycarbonate (PC) and
acrylic (polymethyl methacrylate, PMMA)
are left as possible options, with a basic
cost of $2–$3/kg. The price of other engineering or ultra-polymers can reach $20/kg
and even higher.
Polycarbonate or acrylic
When considering the choice of PC or
PMMA, there is no clear-cut answer. PC and
PMMA are very similar in price, processing
convenience, and optical properties. But
there the comparison ends and the choice
depends on application requirements. The
table compares properties that manufac-
turers and developers should consider when
selecting between the two polymers.
Manufacturers and developers need to
rank priorities of properties to make the
best decision. For example, if the plastic
part is designed for indoor service, the priority of mechanical properties can be low,
and acrylic is the best choice. If optical properties are extremely important and the illuminance level (lux units) is required to be
as high as possible, again acrylic might be
the right choice. On other hand, if the application requires fire resistance properties
because of building standards or risk of fire,
polycarbonate will be the only choice.
Plastic light diffusion systems
match LED lighting needs
GABI BAR and MOIRA NIR explain how to select thermoplastic compounds for SSL optical systems and
how to measure the important transmission and diffusion parameters.
GABI BAR is the global polycarbonate manager
and MOIRA NIR, PhD, works in the R&D
department at Tosaf Compounds Ltd. ( tosaf.com).
FIG. 1. The BYK-Gardner haze meter is widely used for appearance measurement
of transparent materials such as films, plaques, plastic products, lenses, or
windshields. The haze meter can measure light transmission, haze, and clarity.