October 2014
6
C
OVER
S
TORY
Methods of achieving high Color
Rendering Index with LEDs
By Marc Dyble,
Osram Opto Semiconductors and
Lynnette Reese, Mouser Electronics
With standard LEDs,
it is not possible to realize
LED luminaires and lamps with both
high efficacies and high CRI.
This article introduces the Brilliant Mix
concept which now allows
to effectively realizing both high
CRI and high luminaire efficiencies.
The proper use of light is both a science and
an art form. Lighting can be mere illumination
or can create an effect, but new LED technol-
ogies can help to make dull spaces come alive.
Everyone’s color vision is different, therefore
interpreting color can be very subjective, but
specifications can provide a basis for objective
comparison. One major specification used in
the lighting industry is the Color Rendering
Index (CRI). “Color rendering” describes how
an object appears to the human eye based
upon an ideal or natural light source.
An artist showing their paintings would
most likely want their work lit so that rich,
vibrant colors are noticed. A discussion using
attributes of CRI can quickly get artist and
gallery manager “on the same page” regard-
ing lighting. CRI is used to compare how true
objects are rendered. Presently, this is the only
recognized measure of color rendering in the
lighting industry since being introduced in
the early 1960s. Light sources cause subtle
variations in how colors are rendered.
CRI is rated on a scale from 0 to 100. Lighting
with a CRI of 85 to 90 is typically very good
at rendering the color of objects while light
sources with a CRI of 90 or greater are excel-
lent and are used for tasks that require very
accurate color discrimination. With respect
to LEDs, typically the lower the CRI value, the
higher the luminous efficacy (lumens per watt.)
CRI is also an indicator of how natural an
object color appears when illuminated (Fig-
ure 1.) These are the highest achievable CRI
values for common light sources. The lighting
community tends to use CRI as an indicator of
quality, or preference, even though it was not
necessarily intended for this purpose. Light
sources with dramatically different spectral
power distributions can have identical color
points yet render colors very differently.
At the centerpiece of the LED is a highly effi-
cient semiconductor chip, which is affixed to
a lead frame that has an Anode (positive side)
and Cathode (negative side). All are assem-
bled in a thermally optimized IC-type package.
Phosphor is either coated over the die, placed
on top as a layer, or mixed within and encap-
sulated to protect the diode. Several phos-
phors are used; the OSLONSSL LED family, for
example, is available in four different phosphor
blends based on the target application. Out-
door lighting may require cool or neutral color
temperatures (4000-6500K) with a CRI of 70.
For general interior lighting, a CRI of 82 may
be sufficient for office related tasks. Retail,
museum, and other color-critical applications
may require yet another phosphor blend for
high CRI requirements. Producing white light
with LED sources is a challenging task. Two
methods are commonly used to create white
light with LEDs. The first approach is through
the combination of multiple colors, usually red,
green, and blue, in multi-chip packages or LED
clusters. This is the same principle used in back-
lighting LEDLCDTVs: mixing the three colors
in various proportions results in an entire spec-
trum of colors. The second approach is to com-
bine a semiconductor chip (blue or UV) with
converter materials (phosphors) through lumi-
nescence conversion within a single package. A
third approach involves an innovative combi-
nation of the best of both methods, called the
Brilliant Mix concept.
A phosphor-based high CRI approach is a
method that includes an additional stable
red phosphor. The latest developments using
enhanced phosphors increase the CRI value to
95, but the trade-off is lower efficacy. LEDs are
particularly efficient when color coordinates
are closer to blue light, and the phosphor does
not have to shift the blue coordinates quite
so far. To achieve a warm white light, several
luminescent substances must be combined,
although this reduces the LED efficacy. There
is a limitation; with standard LEDs, it is not
possible to realize LED luminaires and lamps
with both high efficacies and high CRI. How-
ever, with the Brilliant Mix concept by Osram,
both high CRI and high luminaire efficiencies
can be effectively realized. This approach fuses
