Signed into law in January, the Energy Independence and
Security Act of 2007 directs the U.S. Department of Energy
(DOE) to establish the “Bright Tomorrow Lighting Prizes”
(L Prize) competition. This contest is designed to spur the
development of ultra-efficient, solid-state lighting products to
replace the common light bulb.
Specifically, the DOE hopes to replace the 60-W incandescent
lamp and the PAR 38 halogen lamp. It also calls for a “21st
Century Lamp” that delivers more than 150 lm/W. The competition
will award significant cash prizes, plus opportunities
for federal purchasing agreements, utility programs, and other
incentives for winning products (see the table).
Prizes, which aren’t funded yet, will total as much as $10
million for the 60-W incandescent replacement lamp and $5
million for the PAR 38 halogen incandescent replacement
lamp. Specs and prizes for the 21st Century Lamp will be
determined later. Program details are available at www.lightingprize.org/pdfs/LPrizeCompetition.pdf.
Whether or not Congress funds the contest, the fact that
light bulbs snuck into a bill that primarily addresses fueleconomy
standards for cars and trucks is significant for design
engineers. In fact, there’s more important news.
Energy Star criteria established last September added quality
specifications to simple energy efficiency for lighting. The criteria
are based on a proposed American National Standards Institute
(ANSI) chromaticity standard, a now-approved “Luminaire
Efficacy” standard, and a “Lumen Maintenance” standard.
These criteria require indoor fixtures to have a minimum
color rendering index (CRI) of 75, zero off-state power draw
for the fixture, and a power factor no worse than 0.7 for residential
use and no worse than 0.9 in commercial use. Lumen
maintenance must be better than 25,000 hours for indoor use or 35,000 hours for outdoor use. And, luminaire efficacy must
be 20 to 35 lm/W, with the prospect of higher efficacy requirements
looking forward.
WHITE LIGHT
Most EEs can run a string of LEDs from a dc source. For
more subtle issues like dimming and strobing, a plethora of ICs
hails from Analog Devices, Linear Technology, Maxim Integrated
Products, National Semiconductor, ON Semiconductor,
Texas Instruments, and others. (If you don’t want white light, see
“Designing Multichannel HBLED Systems,”) Yet when it
comes to lighting, we tend to be less well versed in the nuances
of color selection than we’d like to be. What, for example, is
that “
color rendering index” in the L Prize table?
Part of this explanation comes from a long talk with Mark
McClear, director of business development at Cree Inc., and
part
is abstracted from an article called “The Color White” in
Architectural Lighting by James R. Benya (www.archlighting.com/industry-news.asp?sectionID=1341&articleID=460610).
As
McClear explains it, there are two kinds of “white” LEDs.
The
ones used for “mood” lighting and in some LCD backlighting
devices are triads of red, blue, and green LEDs. Those
used
for general illumination have only a single diode. Blue light from its junction excites a
phosphor system on the inside
of the glass bubble over the
junction, causing the phosphor
to emit polychromatic light
across the visible light spectrum,
though not uniformly.
A “cool” (6000 K) white
LED emits a combined spectrum
like the blue curve in Figure
1. A pronounced blue peak
develops at about 460 nm from
the LED, along with the spectral
response of the phosphor
for the other wavelengths. Different
phosphor systems produce
different relative amounts
of green (555 nm) and red (620
nm) as well as the other colors.
The red curve in the figure
shows a much stronger
response toward the red end
of the spectrum. You’d call that
a “warm” white LED. Warm
white LEDs closely match the
colors typically demanded by
indoor lighting situations. On
the other hand, “cool” white
LEDs are about 30% more
efficient and lend themselves to outdoor
applications like street lights.
The human eye is extraordinarily sensitive,
so small process variations in chip
wavelength make a big difference. So do
phosphor thickness, concentration, and
composition, as well as deposition. In any
batch of LEDs, there’s enough variation
in chip wavelength and phosphor system
behavior to have a noticeable effect. This
leads LED makers to bin for color, and
understanding that kind of
binning is the trickiest part
of designing lighting systems
with white LEDs.
The international standard
for describing the color of light
is the CIE Chromaticity chart
(Fig. 2). On that chart, it is
possible to draw a curve called
the Planckian black-body locus
(BBL) that describes the color
of light emitted by a theoretical
non-oxidizing object as it’s
heated. At about 1000 K, the
object will radiate a dull reddish
light. As the temperature increases, the object’s light becomes warmish
white, then coolish white and then blue
above 10,000 K. A measure called the correlated
color temperature (CCT) defines
specific points along the BBL.
In the real world, only the sun and
incandescent light bulbs closely match the
BBL, and such bulbs can only do so until
their filament melts. To describe the light
from other sources, such as high-intensity
discharge (HID) lamps, fluorescents, and LEDs, we have the CRI. In
this 0-to-100 scale, 100 represents
color rendering equivalent
to a black body. Not even
sunlight is 100 CRI all the
time, as clouds and even window
glass act as filters. (CCT,
however, remains high.)
As a practical matter, people
who design lighting fixtures
insist on a CRI of at least 80,
and this has been the basic
challenge for LED manufacturers.
The L Prize, of course
makes it even tougher.
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