1. LED
Consistent light and color is demanded by major customer
brands, as well as specific colors, color palette and style guide;
however, meeting these demands with LED backlighting can be
a challenge. Adding LED lighting or any backlight to your proj-
ect adds another variable to understand.
It is generally acknowledged that LED lighting is the path for
backlighting (see your TV, computer, tablet or phone if you don’t
believe it). Problems emerge when backlighting is moved from
the realm of electronics to print and packaging.
Using light is a clear growth market in the signage and POP/
POS display community. But entering this market — and being
successful — takes more than just flipping a switch.
How Are You Going to Succeed? Through
Understanding
Understanding LED color specification and visual rendering is
the starting point.
Corrected Color Temperature (CCT) is now commonly speci-
fied and controlled, and the color temperature of any light
source or lighted panel dictates the perceived color and display
brightness. CCT is expressed in degrees Kelvin, abbreviated K.
The white LED CCT ranges from a warm yellow 2000K to a cold
blue 6000K, and most often, nearly pure white (5600K+) light
sources are selected for lighting signage. Major LED makers
are even offering white LEDs specifically tailored to market and
lighting locations within stores. The colors are given names
like Meat (2000K), Bread (2400K), Vegetables (5100K) and Fresh
(5400K).
Color temperature can be measured by using a handheld
meter, such as a Sekonic C500R meter. Readings can be taken
in seconds with the push of a button. This can be helpful with
verification and sample references. That said, meter readings
are affected by proximity to the light source, the surrounding
materials (reflective, color, etc.) and the ambient light condi-
tions. This generally prevents a highly repeatable result.
Color LEDs present a specific wavelength. For example,
green is 515 nanometers. This is a very narrow color, not a
Pantone/PMS defined color. Colored LEDs present a very tight
wavelength rather than a spectrum, so the use of colored LEDs
limits the available colors. The narrow light spectrum may pro-
duce unexpected results — this is why white LEDs are so impor-
tant to printers (see Figure 1).
Color Rendering Index
Color Rendering Index (CRI) is a more subjective and subtle
specification. CRI is a scale from 0 to 100. It indicates how
accurately a defined light source renders color when compared
to a halogen light source, and measures an LED's capacity for
producing the correct color appearance. The score is calculated
by measuring the average difference in chromaticities between
the LED and the CRI sample colors (See Figure 2). The lower the
average difference, the higher the CRI score. A high CRI number
is like a high grade point average for color quality.
An LED with a fixed 5600K CCT will produce a very different
print appearance with a low CRI light source. Many early LEDs
focused only on color and lumen output. The color rendering
was ignored and thus the appearance of lighted printed mate-
rial suffered.
V i s i t S G I A a t S G I A . o r g S G I A J o u r n a l ■ S e p t e m b e r / O c t o b e r 2 0 1 5 | 5 1
Marty Proctor,
Director of Business &
Market Development,
Lumvatech
Figure 2: Color samples used for creating a CRI score.
What Color is That? Not My Color!
Figure 1: The CIE 1960 Chromaticity diagram showing the range of LED colors
and the inset showing the specific K CCT values of white LEDs.
20000K
10000K
7000K
6000K
5000K
4000K
2500K
3000K
570
575
580
585
+ Duv
- Duv
Spectrum Locus
Blackbody Locus
Purple Boundary
550
580
600
700
400450
480
500
2. LED
Most LEDs score in the 80s (a solid B average). Fluorescent
has CRI down in the 70 range. There are also significant color
shift and brightness change issues over the life of a fluorescent
light source. This puts LEDs at a distinct advantage for longer
life uses.
Red is not only a very popular color with brands, but it is the
hardest color to render accurately. So much so that an extra set
of color samples (R9–R14, See Figure 2) are added to the color
evaluation, even though they do not impact the reported CRI.
Note that the use of these samples in any color creation, cali-
bration and photography is crucial.
So Where is Your Solution?
No LED manufacturer can produce uniform, exact and specific
colors in their white LEDs. Instead, they produce batches of
LEDs and sort the output into bins, which are specified by each
manufacturer. The light emitted by LEDs is characterized by
color and brightness and then sorted into tightly defined bins
that group finished LEDs of similar brightness (lumens or mcd)
and color (K) together to ensure visual consistency. Select an
LED supplier that reliably produces consistent product with a
very tight binning process. This is your best path to producing
repeatable results and correct color during production runs of
printed overlay. White chromaticity “xy” coordinates create
very tight color definitions, and top-quality LED manufacturers
use sorting bins based on CIE127-2007 specifications. Luminous
intensity binning is also done to this specification.
There are variables outside the control of the manufacturer
that also affect the light seen by the viewer. The physical tem-
perature of the LED and the amount of current applied will each
cause a shift in the light output and color. For example, the
same display mounted inside a freezer may look different from
one mounted as an aisle violator.
If the display is battery operated, the color of the LEDs will
change slightly as the batteries reach end of life. It does take
some time for a LED light source to stabilize, but this time is
generally quite fast and has significantly less change than other
lighting technologies. Fluorescent lighting will have significant
and uncontrolled color change and light output over their life.
Fluorescent lights also operate in a much narrower temperature
range with significantly shorter life. With LED lighting, there is
significantly less change over time. LEDs are rated for 50,000
hours (five-plus years) of continuous operation with only minor
color or light output degradation. They will continue to operate
for much longer.
What’s Coming?
As we try to produce beautiful displays and signage, the key
for all of us is the addition of a violet component to the exist-
ing blue/yellow spectrum used to create white. This addition
will make the current generation of LEDs for signage and print
applications obsolete. The CRI will exceed 95, allowing LED
light to mimic the broad spectrum color and even sunlight —
the best light for the human eye.
Additionally, Professor Shuji Nakamura, Global Energy and
Nobel Prize winner, has created a company named SORAA
that is dedicated to GaN-on-GaN diode production for LEDs.
This exciting new process will begin producing LED chips in a
couple of years. The expectation is a five- to 10-time increase in
brightness.
These future advances will further cement LED lighting and
backlighting onto the mercury-free and ever-challenging mar-
ketplace.
Showing Off
By understanding CCT, CRI rendering score, brightness and bin-
ning techniques employed by LED makers, your projects can
really shine.
Pictures and simulations cannot accurately represent the
response of the human eye to light due to the varied nature of
vision. Simulations, GIFs, point-and-shoot cameras or phones
cannot replicate the human visual experience. The true path to
understanding visual response to a lighting system is prototyp-
ing. Working with established LED lighting experts and using
stable suppliers allows your expertise to show through.
As printers and consumers, you already know that the print-
ing media, inks, print technology and techniques are vital ele-
ments to creating excellent solutions. Lighting brings forward
your knowledge in opacity, translucence, dead-front methods
and blocking light. The skill and art you bring to the technology
discussion for LED, or any backlighting project, will separate
you from the big-box print shop down the road or the internet-
based competitors. The excellent service you already offer will
make the needed proofs, prototypes and demonstrations pos-
sible, and adding the element of lighting to your skill set can
move you ahead of your current peers.
