Quality of light and the - Philips Lighting

iNtERViEW
it was, however, a bit of a shock to some to
discover that quality of LED light surfaced as one
of the major issues again. Colour shift problems
that had been overcome in conventional
lighting, surfaced again. a row of LED downlights
illuminating a corridor could appear to be
emitting greenish, blueish and pinkish white light
at random. LED manufacturers had to put a lot of
effort in to improving binning systems and solving
the colour shift problems that were evident.
Correlated colour temperature describes
whether white light appears warm, neutral or
cool to the observer. the aNsi chromaticity
standard for the definition of the colour
temperature of a light source makes allowances
for a small range of variation in chromaticity.
a 3000 K light source, for instance, is specified as
being 3045 K, plus or minus 175 K, so anywhere
between 2880 K to 3220 K. this means that
the observer could notice a small difference in
the colour of light between 2 light sources, with
technically the same
colour temperature.
the threshold at which
a colour difference
becomes perceptible is
defined by a Macadam
ellipse and the scale
of the Macadam
ellipse is defined by
the standard deviation of colour measuring, or
sDCM. Generally speaking the colour difference
with a LED light sources with a 2­3 sDCM step
is barely visible.
since the early days of white LED light for
general lighting a lot of improvements have been
made in manufacturing techniques and processes,
and the level of knowledge the market now
has of controlling the colour temperature
consistency of white LED light.
// Colour rendition
For a while now it has been a recognized fact
that the CRi system applied to conventional
light sources is less accurate and relevant for
predicting the colour rendition of LED light
sources. the age old CiE colour rendition system
of 8 test colours to determine the CRi of a light
source was designed around conventional light
source technology, favouring light sources that
mimic a black body radiator or daylight and is
not sufficient for LEDs. in many tests carried
out the conclusions are that observers tend to
rate the quality of light for LED light sources
as much higher than the calculated CRi scores
6 | Lightspec | issuE6/NoVEMBER2012
“The current CRI system is
less accurate and relevant
for LED light sources.”
would suggest. this is the difference between
the actual colour fidelity of a light source and
people’s liking or appreciation of it, which can
be entirely different things. in LED tests with low
colour renditions the LED light can still be rated
as visually appealing.
a colour rendition of CRi 40 with fluorescent
tubes leaves an office employee looking ghost like
and drained (bad colour fidelity) and is not liked.
yet a similar low colour rendition score for a LED
light source can still looks fresh and appealing,
as although the colours do not appear washed
out or faded, they may be distorted to different
shades (bad colour fidelity). Purple instead of
blue, pink instead of red, but a very acceptable
and saturated looking purple and pink.
the specific spectrum of a particular light source,
or the position of the colour points of a light
source in relation to the black body locus is a
more accurate way of determining the value of
the colour rendition,
but not many people in
the lighting market have
the knowledge to be
able to interpret these
figures from vector
diagrams.
the lighting industry
has accepted that
the current Ra8 system does not do LED light
sources justice and needs to be replaced. there
are several working groups looking at a new and
better method of measuring and rating colour
rendition for LED light sources. the CQs system,
for instance, rates colours against 15 test colours,
which more completely represent the range
of normal object colours and take the spectral
properties of LEDs in to account, also allowing
individual values, such as the R9 value for red
to be singled out and highlighted, as a sign of
light quality. this is not yet, however, a universally
accepted alternative to the current CRi system.
in the meantime it could still be a case of ‘seeing
is believing’.
// New levels of quality of light
Philips Lighting has been exploring how to take
LED light source technology for quality of light to
even higher levels than the current high quality
propositions. at the Light & Building Fair in april
2012 a colour quality demonstration was shown
to thousands of lighting professionals as an experiment
in colour perception, which generated
some new interesting insights.
in Retail lighting quality of light has always been
a very important factor in the choice of a lighting
installation. this is particularly true when material
or fabric with rich colours is being illuminated.
Retailers recognize the important role good
lighting can play in drawing customers in to their
stores and are very much aware of the choices
they can make for a light source with a particular
colour temperature and colour rendition.
Generally speaking, in Europe, warmer light
sources of around 3000 K with excellent colour
rendition have been the preferred choice for
many chains of shops, as they are welcoming
and invite the customer to come in and linger.
Philips Lighting has had years of experience of
discussing the effects and merits of different
light sources with major retailers in Europe and
recognizes the trend towards a warm ambient
lighting effect, giving the colours of materials,
clothes and other fabrics a warm, saturated glow.
But whereas the warm appearance is very much
appreciated for colours, there is still a preference
for white to be white and not necessarily offwhite,
creamy or yellowish in appearance.
a conventional light source with 3000 K and a
high colour rendition will make all the colours,
including white in the display seem warm. in
the same way a 4000 K light source will make
white and all the colours appear fresh and
cooler. yet what is often subconsciously wanted
is that unique combination of bright, cool white
together with warm, saturated colours.
using the extensive experience of colour
appearance and colour appreciation, Philips has
identified an intriguing way of optimizing the light
spectrum, which combined with the perfect beam
homogeneity gives a new colour experience.
this patented solution gives the appearance of
combining two fixed colour temperatures within
one light source. this same light source can make
white appear crisp and cool, whilst the other colours
in the goods on display will appear warm,
giving the effect of a completely natural scene.
the appearance of colour temperature contrast
in one lamp.
an ideal light source for Retail spot lighting?
Philips is considering applying this new
patented technology to the Fortimo LED sLM
modules in the near future.
www.philips.com/lightspec
issuE6/NoVEMBER2012 | Lightspec | 7