Too Hot? Too Cold? The Goldilocks Syndrome - LUX-TSI

Too Hot?
Too Cold?
Life was simple when the only choice we
had was between the different wattage
on an incandescent light bulb.
The Goldilocks
Nowadays we need to know about such
Syndrome
locus and CIE colour space diagrams.
things as Colour temperature, Planckian
Before you start feeling blue, or indeed
seeing multiple shades of it, have a quick
word with the Doctor, who might be able to
help.
So why temperature to describe C olour?
When we in ‘the trade’ want to describe the
colour of light, and specifically when we
want to ensure nobody else really
understands what we’re going on about, we
talk about chromaticity coordinates xy, uv or
u’v’ or tristimulus values XYZ (so either a 2 or
3 number description). Yes our Christmas
parties can be quite a humdinger!
Clearly this is meaningless in the real world,
so a more intuitive “Colour temperature”
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description is used to describe the colour of
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light emitted by a white light source.
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Buteverysimplificationintroducesitsownproblems
It’s possible that light from different sources
(e.g. fluorescent lamps, HIDs or LED modules)
– with ostensibly the same ‘temperature’ are
noticeably different.
But nobody would ever know if it weren’t for
the fact that sooner or later, you would want
to source (say) a matching LED module from
another vendor!
Sowhythedifference?
Good question! … and to get to the bottom
of this issue, we do need to go back to some
basic physics and colour temperature
definitions.
True colour temperature is defined as the
colour of radiation emitted from a perfect
blackbody radiator held at a particular
temperature…….
…… and a blackbody radiator is a source that
emits radiation across a wide wavelength
range according to Plank’s law.
BeforewegotoPhysics,letshaveahistory
lesson:
Remember the humble light bulb? Edison*
discovered that when you pass an electric
current through a filament of tungsten wire,
the current encounters resistance. This
resistance creates heat and the tungsten wire
starts to glow – a process called
incandescence.
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With increasing current the temperature
increases and the light evolves from red to
white.
Another scientist by way of Max Planck
(1858-1947) discovered that a black
body’s (electromagnetic) radiation is
determined only by its temperature.
‘Modestly’ he called this Planck’s Radiation
Law. And in our example, the blackbody
radiator is the hot metal tungsten wires in
Edison’s light bulb!
So there we have it in simple terms: The
hotter the tungsten wires becomes, the
more the colour changes. All we needed
now was a way of correlating the exact
relationship between the two.
This was provided (in 1931 and updated
numerous times since) by The Commission
Internationale de l'Eclairage (CIE) a very
well respected International institution. The
CIE defined the link between physical pure
colors (i.e. wavelengths) in the
electromagnetic visible spectrum, and
physiological perceived colors in human
color vision.
They plotted blackbody radiation ranging
from temperatures between 1,000 Kelvin to
20,000 Kelvin on what is now known as a
Planckian locus. This locus is a twodimension graph with x,y coordinates
known as chromaticity coordinates.
Colours on this locus are considered to be
“white”. However at the lower end (2,000 K)
the light is considered reddish (or “warm”)
white”, whilst at 20,000 K the light is
considered bluish (or “cool”) white. So,
confusingly, the colder the black body the
warmer the colour temperature – as if nature
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*Edison - Ok I know that you know that
Edison did not discover the light bulb,
but was at the end of long line of
inventors that did all the hard work in
refining and perfecting it. It wasn’t even
Edison whose name was on the patent,
but his chief engineer- but never the less
he’s the one that made money out of it.
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was not cruel enough!
Most traditional (incandescent) light bulbs
emit light at a colour temperature of about
2,800 to 3,100 Kelvin – which as explained
earlier would put it in the “warm” white light
category as there is still a red (warm) hue to
the light.
Now here comes an interesting twist. Other
light sources, such as fluorescent or
discharge lamps, or LEDs emit light by a
process called electroluminescence and not
by heating up lumps of metal
(incandescence) so do not emit radiation
with the same distribution of wavelengths.
This means that the white light emitted by
that source will not (necessarily) fall directly
on the Planckian locus, so scientists had to
conjure up a ‘fix’. This fix called the
correlated colour temperature (CCT) is
designed to approximate the closest point
on the locus for the light being categorized.
The key word to note here is C orrelated.
But this fix is not always perfect and not
always well understood- it’s all in the
definition!
We’ve established the chromaticity
coordinates of a true blackbody source must
(by definition) fall exactly on the Planckian
locus - whereas the chromaticity coordinates
for other light sources will fall along a line
that intersects the blackbody locus at the
equivalent (true) colour temperature (this line
is called the “ISO-CCT” line).
So for example, for a standard incandescent
lamp (in this case a CIE “illuminant A”) with
a colour temperature of 2856 K, its x, y
chromaticity coordinates will be exactly 0.
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4476 and 0.4075 respectively.
However a light source with a CCT of 2,856
K can actual have chromaticity coordinates
which are not on this Planckian locus.
This is better explained in the picture shown
in CIE 1931 colour space (x,y, coordinates)
where the curved line is the Planckian locus
and the lines intersecting this are the lines of
constant correlated colour temperature
(CCT) thus showing a large range of
chromaticities which are described by the
same CCT. Above the line the light will be
more green in appearance and below the
line it will be more pink.
Now given the human eye perceives
differences with a variation of just ± 0.001 in
x or y, describing light colour using only CCT
permits deviations up to 20 times beyond
this perception threshold. Therefore CCT is
not a good way to specify light colour.
If you stick to defining the colour of your
white LEDs by their CCT, you’ll likely end up
with a smorgasbord of shades of greens and
pinks. This clearly is unsatisfactory when you
wish to produce a high quality lighting
product with a consistent colour.
The solution is obvious of course: Define the
colour of your white LED in terms of the CIE
chromaticity coordinates. Maybe not as
elegant but it will stop you seeing Red
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Measuring the colour, luminous efficacy and
brightness of LEDs, luminaires, lamps and displays is
what Lux-TSI does (amongst other light related
activity) –we’ll even do CCT readings.
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