COLOR PSYCHOMETRICS
Much of the drive behind this
sort of engineering came out
of fluorescent lighting in the
1950s. The science tends to be
a little touchy-feely, though,
with a lot of studies dealing
with “warm” (reddish) and
“cold” (bluish) light sources
and diurnal rhythms and
melatonin production.
Whether the results are
hard-edged enough or not,
two things emerge: good correlation exists
among individuals in groups that share a
common culture, and people who design
lighting systems for offices, restaurants,
hotels, and schools take it quite seriously.
As a result, LED makers and designers
pay attention. Indeed, touchy-feely notions
sometimes turn out to have solid roots.
“From the 1960s to around 1990, an
unexplained phenomenon called ‘visual
clarity’ was a common discussion topic
among lighting practitioners.
When viewed under the light
of high CCT, high CRI lamps,
many tasks appeared easier to
see, yet the cause could not be
explained by the vision scientists
of the era. Starting in
the late 1980s, important new
work by Dr. Sam Berman and
his colleagues at Lawrence
Berkeley National Laboratory
finally provided an explanation,”
Benya wrote in “The
Color White.”
“An abundance of shorter
wavelength blue light causes the pupil of the human eye to contract,
and through increased depth of field and
reduced visual noise, visual performance is
enhanced. Further experiments by Berman,
Navaab, and others have also demonstrated
that high CCT light appears brighter than
lower CCT light at the same footcandle
levels,” he continued.
“Based on these findings, large-scale
experiments retrofitting complete buildings
with high CCT lamps, but lowerthan-
normal footcandle levels, have been
conducted by Pacific Gas and Electric with
some success. [Although] whether this is
an acceptable practice is still being debated
among vision scientists,” Banya wrote.
McClear also starts with the chromaticity
chart of Figure 2, adding some notations
of his own. Down in the left corner,
the blue LED emits photons with a wavelength
of approximately 460 nm. To produce
white light, McClear draws a heavy
black line across the color model, through
the locus of white light, to find the equivalent
wavelength of the phosphor system
that will balance the blue of the diode.
To indicate the uncertainty of the blue
LED wavelength, it’s a fat line. McClear
also draws several dotted paths to indicate
the uncertainty of the phosphor wavelength.
All of the variations in chip-process
parameters and variations in phosphor
chemistry contribute to variations in color
produced by the LED.
Figure 3 is the same color model, but
now we’re tightly zoomed into the white
area. The magenta line is the black line
McClear drew on the color space between
the LED emitter wavelength and the phosphor
wavelength in Figure 2. This figure
also includes the BBL, which would have
been hard to show at the scale of the previous
figure. The colors around the edges of
the “white” area bleed in from the more
saturated areas of the color model. If you
get far enough off in LED or phosphor
wavelength, the nominally white LED
light output takes on tinges of those colors.
BINNING
The boxes around the BBL represent possible
bins of LEDs. In fact, these particular
boxes represent Cree’s bins. Other LED
makers will bin differently. Cree’s engineers
work with customers to help them
determine which bin or bins fit their needs.
The lighting system designer provides the
basic input and makes a judgment based
on the criteria Benya wrote about.
Gas stations, restrooms, and hotel lobbies
all demand different kinds of “white”
light, and it matters whether you’re in
Trondheim or Tahiti. What is critical is
that “care be taken to match the bins above
and below the black-body locus. Equal
numbers of LEDs from the bins above
and below the BBL will color mix in most
applications to produce a white chromaticity
that appears to be right on the BBL. To
help the mixing, diffusing films can also be
added,” says McClear.
In other words, it takes a lot of work to
get the CRI as close to 100 as possible.
That’s why it’s not going to be a walk in
the park to meet the 90+ CRI requirement
to qualify for an L Prize.
ENERGY STAR, REV 1
But if the L Prize is still a chimera, Energy
Star is real. It affects how many governments
around the world manage their procurement
and how many ordinary people
make buying decisions. Under the new
standards, quality of light matters a lot.
Three brand-new or ready-to-bereleased
standards matter. The ANSI chromaticity
standard was approved in March.
The IESNA (Illuminating Engineering
Society of North America) LM79 luminaire
efficacy standard was approved in
May. And, the LM80 lumen maintenance
(how long the LEDs last) standard is still
in draft form. Power factor, also an issue, is
part of the ac adapter design.