ASHRAE Journal - March 2013 - 80

EMERGING TECHNOLOGIES
What’s New

Solid State Lighting
By Mildred Hastbacka; John Dieckmann, Member ASHRAE; Antonio Bouza, Associate Member ASHRAE

olid state lighting technology continues to advance and will become a significant part of the building lighting picture in the near

future. There are two basic categories of solid state lighting: inorganic
light emitting diodes, commonly called LEDs, and organic light emitting
diodes, commonly called OLEDs. Both categories of LEDs convert electric
energy to light energy directly.
As free electrons ﬂow from the N-type
semiconductor material in the LED into
the holes in the P-type material, the electron drops into a lower energy state and
emits a photon whose energy equals the
drop in energy undergone by the electron.
The LED must be conﬁgured so that the
emitted photons have a path to escape
from the device. The direct conversion
in this sense is similar to gas discharge
lights, e.g., ﬂuorescent, sodium vapor,
metal halide, which use an electric current to excite the electrons in a gas to a
higher energy level, with photons being
emitted when the electrons fall back
down to their base energy level. This is
in sharp contrast to incandescent lights
that emit both light and a large amount
of heat from a heated ﬁlament.
Although LEDs and gas discharge
lighting technologies convert electric
energy directly to light energy, both types
of devices are subject to many losses, for
example, IR loss, light reabsorption, conversion loss of phosphors when used to
broaden the spectrum of the emitted light.
A major part of the promise of LED lighting is that a path is seen by researchers to
reduce these losses to much lower levels
than previously achieved over many decades of development of the various gas
discharge lighting technologies.

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In addition, LEDs turn on and off
instantly, are dimmable, have longer lifetimes, emit light in one direction, and do
not contain toxic materials like mercury.
LEDs are a high intensity point source
of light, whereas OLEDs are a diffuse
source of low intensity light over larger
areas. Development of OLEDs began
much more recently than the development of LEDs and cost-effective lighting
products based on OLED technology are
further in the future.
The efﬁciency of lighting products is
expressed as the lighting efﬁcacy, in units
of lumens per watt. In the production
and distribution of light by LEDs, that
efﬁcacy can be viewed at three levels: the
efﬁcacy of the LED package, of the light
emitted from the lamp that incorporates
the light source, and of the light actually
delivered from the luminaire that includes
the lamp.
For traditional sources (incandescent
and ﬂuorescent), efﬁcacy is normally
reported at two levels: lamp (or lampballast) efﬁcacy and luminaire efﬁcacy.
At each stage, as initially produced light
passes out of the lamp and then out of
the luminaire into the area being lighted,
losses occur, reducing the efﬁcacy.
For example, if light from the source
impinges on phosphors used to broaden
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the spectrum of the light produced by the
lamp, the phosphors will not be 100%
efﬁcient in converting the light from the
source to light emitted from the lamp
(the efﬁciency of phosphors is typically
80%). Another example is the loss of light
from the ﬂuorescent tube type lamp in a
typical troffer type ceiling luminaire. The
ﬂuorescent tube produces light uniformly
in all directions from the tube, but the
direction the light is needed is downward
to the illuminated space.
The luminaire includes a reflecting
surface above the tube, but much less than
100% of the light is reﬂected out of the
luminaire. Table 1, compiled from various
published data sources, summarizes the
efﬁcacy levels of current lighting technologies. The U.S. Department of Energy
Multi-Year Program Plan for Solid-State
Lighting Research and Development was
a source of efﬁcacy values for current
LED technology.1 These values represent the upper level of performance for
commercially available LED lighting
technologies.
We last covered the topic of solid-state
lighting in this column three years ago.2,3
At that time, LEDs were roughly comparable to ﬂuorescent lighting in efﬁcacy,
but were far higher in cost than ﬂuorescent lights. LEDs had made signiﬁcant
inroads into street lighting and trafﬁc
signal lighting, where the high cost of
replacing shorter lived lamps offset the
high initial cost. While interior lighting
products were coming onto the market,
they had yet to make an impact. Now
higher efﬁciency LED lighting products
are becoming available, and the prices
are falling.
On the research side, a variety of techniques are being pursued to raise the efMarch 2013

ASHRAE Journal - March 2013

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