4
400
0
20
40
60
80
100
450
500
Wavelength (nm)
Relative Radiant Power (%)
550
600
650
700
10000
6000
4000 3000
2500
2000
1500
580 nm
590 nm
400
0
20
40
60
80
100
450
500
Wavelengh (nm)
Relative Radiant Power (%)
550
600
650
700
Warm White
3000K
Warm White
2700K
Natural White
4000K
Cool White
6000K
6000K
4000K
3000K
2700K
Cool White 6000K
Natural White 4000K
Warm White 3000K
Warm White 2700K
3 Step
MacAdam ellipse
Planckian
locus
5 Step
MacAdam ellipse
7 Step
MacAdam ellipse
0.42
0.43
0.44
0.45
0.38
0.39
0.40
0.41
0.42
0.41
3000K Colour temperature area (AINSI C78.377) =
Standard selection for the LED industry
LED TECHNOLOGY
In order to get a white LED, nowadays just
as 20 years ago, one is made which emits a
light blue known as royal blue, and it is given
a coating by mixing phosphorus types, that
is a mix of components which are excited
by the wave length of Royal Blue and can
emit white light, the color temperature
(CCT) depends on the specifi cs of the mix.
Diode technology underlies LEDs, a diode
being an electronic component whose
functioning depends on the behavior of two
regions next to each other with different
chemical characteristics. On the basis of
the various twinned regions, a LED will be
able to emit a precise wave length, in other
words generate a specifi c color; as white is
not a color but a mix of all of them together,
it is clear why conversions of some kind
cannot be avoided. Given the growth in
the availability of different selections of
CCT, and the market’s desire for quality,
a universal and standardized method
of cataloging was soon codifi ed as ANSI
78.377-2008.
It was necessary to defi ne a space on which
the light emitted by a LED could be defi ned,
and within this space a series of regions
could be delimited in order to defi ne their
different color temperatures. The space of
the colors C.I.E. xyz was used, one of the
fi rst to be defi ned mathematically (1931),
while to delimit the different color regions,
it was natural to extend the Plankian place
(in other words, the group of various shades
of white which a Black Body assumes if
taken to various temperatures), and thus
a series of rhombuses was created, each
of which was next to another and centered
on the black body curve known as the
ANSI BIN. By means of the mathematics of
conversion of the same color space and the
spectral distribution of the light emitted by
a LED, it can be inserted into the graph and
then described precisely. While in so-called
cold LEDs the blue contribution is greater
(obviously on the 460nm characteristics
of the LED on which the white has been
constructed), as the temperature is reduced
if a warm LED is observed it is clear that the
phosphorus have a more important role
to play in lowering the blue component
and gene rating a new component at the
same time, which becomes predominant
around 600nm (typical of yellow). For these
purposes the percentage of ‘Rare Lands’ in
the phosphorus mix is altered.
As is normal for any Energy transformation
when the conversion work is increased the
effi ciency decreases, and this explains
why warm white LEDs are less effi cient
than cold ones. This is also true for LEDs
with a high color rendering index (CRI).
The CRI, as its name suggests, is an
index and it expresses how a light source,
when compared to a reference sample,
can reproduce colors. By defi nition the
reference sample should be Plank’s Black
Body, obviously in the same CCT conditions.
In any event, for practical purposes it is
possible to use an incandescent lamp
which by defi nition and construction
is
very
similar
to
the
BlackBody.
There are also different color tables
which can be used for the comparison and
increasing the number of ‘color samples’
obviously improves the defi nition of the
CRI. As well as the standard, nowadays
the LED leads the main manufacturers to
compare themselves to each other and
improve the light effi ciency and quality on
a daily basis. Two different strategies are
becoming important for the selection of
the BINs. The simpler, which has been more
commonly used for some time, divides the
ANSI BINs into narrower and narrower
sub-regions in order to obtain portions
which are 1/16 the size of the standard.
This gives companies assembling LEDs a
refi ned degree of selection for the fi nished
product. The second possibility undertaken
by manufacturers was that of reintroducing
an old model based on a more favorable
geometry, namely MacAdam’s Ellipses. The
advantage of this proposal is the geometry
involved which, with the same surface area
as ANSI automatically excludes the points
which are further from the geometric
center, that is the tops of the rhombuses
which as a result of their construction
are the regions with more variation in
chromatic homogeneity. Clearly this is not
suffi cient to guarantee that the selections
have a high degree of precision. In fact the
quality of this system depends once again
on the dimensions of the geometric places
themselves and to complete the model
different ellipses must be type-tested, all
of them centered on precise chromatic
coordinates (and so always referable to
C.I.E. 1931), more and more reduced in
surface area. For these purposes they
are defi ned as MacAdam’s Steps. Step3
is currently important in the marketplace
since, if directly compared to the ANSI
Standard, it is equal to half an ANSI BIN.
