Too Hot?
Too Cold?
The Goldilocks
Syndrome
Life was simple when the only choice we had
was between the different wattage on an
incandescent light bulb.
Nowadays we need to know about such
things as Colour temperature, Planckian
locus and CIE colour space diagrams.
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 Colour?
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” description
is used to describe the colour of light emitted by
a white light source.
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But every simplification introduces its own problems
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!
So why the difference?
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.
Before we go to Physics, lets have a history
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.
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
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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 two-dimension 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 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.
*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.
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
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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
Correlated.
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. 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
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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
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.
T: 01656 864618
[email protected]
www.lux-tsi.com
© Lux-Tsi. All rights reserved
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