Quality of light and the - Philips Lighting

iNtERViEW
a Question of Colour
Eric senders,
Marketing Manager,
Philips Lumileds
Five ways to evaluate colour quality and
consistency of LEDs
“LEDs are energy efficient and long lasting, but they are inconsistent
in terms of colour quality”: it’s a common complaint from lighting
specifiers and consumers who doubt that LED light sources can
match traditional sources like incandescent and halogen in terms
of colour quality and consistency. it’s also a myth. oftentimes these
assumptions are based on colour rendering index (CRi) value
comparisons of LED products and traditional sources. the problem
is that CRi isn’t the only method of determining colour quality. in
fact, especially with LEDs, additional information exists that may
reveal even more about a source’s colour quality and consistency.
From our conversations with oEMs, lighting specifiers, and others
involved with the selection and implementation of LED lighting
systems, Philips Lumileds has identified the following five factors to
consider when evaluating the colour quality and consistency of LEDs.
// 1. Colour Rendering
Colour rendering is about how accurately
the different colors of an object show up
under light. to ensure a good quality of light,
lighting designers are demanding a CRi that
is greater than 80 for indoor applications. But
CRi isn’t the only metric that matters when
determining the colour rendering ability of an
LED light source. Colour rendering also consists
of different R­values, where certain R­values
represent a certain colour. the right R­value will
be determined by the application. For instance,
in Retail settings a high R9 value is important
because it is the red index that will help to
render the vivid red colour that is present in
most Retail objects. From a colour rendering
perspective, Philips Lumileds offers different
LuXEoN LEDs that deliver the right R­value
for the intended application.
// 2. Colour consistency over temperature
since LEDs have different colors at different
temperatures, the colour of an LED depends
on the junction temperature of its system.
this is important to consider when evaluating
LED light sources since many LED manufacturers
list a 25 o C operating temperate on their data
sheets, while real application temperatures
often are around 85 o C. also, LED products
from different manufacturers may shift colour
in different directions. the good news is that
some leading LED manufacturers, including
Philips Lumileds, now specify their LEDs at real
application temperatures (85 o C), so that the
colour customers order is the exact colour that
they receive.
// 3. Colour consistency between LEDs
Binning is a huge topic of discussion surrounding
LEDs. and for good reason. it’s not uncommon
for a specifier to select
LED products that are
seemingly from the same
bin, only to find significant
visible differences
between those products
after they are installed.
to help end such mishaps,
aNsi introduced new
standards in 2008
that mandate LED
manufacturers to bin their LEDs according to a
7­step Macadam ellipse. (the smallest ellipse is
a 1­step ellipse, which will give the best colour
consistency.) While ensuring that bins fall within
7 steps of this center point was a good first step
towards eliminating colour inconsistency, there
is still additional work to be done. according
“CRI isn’t the only
metric that matters
when determining the
colour rendering ability
of an LED light source.”
www.philips.com/lightspec
to Macadam’s research, people may detect a
difference in colour between two sources if
they are farther apart than a 3­step Macadam
ellipse. Philips Lumileds was the first one to
implement products that completely fall within
a single 3­step Macadam ellipse. Even now,
LED manufacturers are continuously improving
the phosphor technologies to achieve a tighter
distribution of LEDs in the production process.
// 4. Colour consistency in the beam
When designing an LED system, it is important
that the final system design has a uniform colour
in the beam. the downlight with a halo in the
beam, for instance, is not ideal. While this beam’s
colour consistency can be improved with a good
reflector or secondary lens design, it helps when
the LED source is itself consistent.
this will enable customers who work with
individual optics on LEDs to bring products to
market faster and without incurring additional
costs associated with cumbersome secondary
lensing. Most LEDs in the market have a
difference in colour over angle of about 20­25
points (delta u’v’). New LED technologies,
however, prove that it is possible to stay within
10 points delta u’v’ difference in the beam.
Examples of this are Philips Lumileds
LuXEoN a and LuXEoN s products.
LuXEoN a used in Philips Lightolier spot lights
to illuminate the splendid store in New york.
// 5. Colour consistency over time
Perhaps specifiers’ biggest concern about LEDs is
reliability. Everybody knows a manufacturer that
promises a 50,000­hour lifetime for its products,
but the larger question is: What does the light
source look like at 50,000 hours? How much
light is still produced? and have the colors shifted
since the initial installation?
since most illuminationgrade
LEDs have not
yet been installed for
50 years, it is difficult to
determine this; however,
LED manufacturers have
developed sophisticated
stress models that predict
the behavior of LEDs
over lifetime. Guidelines
like iEs LM­80 or iEs tM­21 can help guide LED
manufacturers through the process of measuring
LED maintenance and colour shift over time.
it is essential that manufacturers make their
test results and data publically available, and we
suggest that all oEMs proactively ask for these
data from their LED manufacturers.
ultimately, if oEMs take these five main tenets of quality of light into consideration, we believe that it
will improve LED colour quality and increase the adoption of this great new technology.
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