2. BASIC PRINCIPLES
It would obviously be difficult
to design a system of colour measurement
that attempted to describe
• the colours that we see.
We simply have to think how we use words to describe
colours. Colour names such as
red, yellow, green and blue are reasonably well understood,
but names such as rose, salmon and cerise are
• less well standardised.
• What might be called rose by one person could be called pink
by another.
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3. BASIC PRICIPLES
Even for red there is a considerable range of colours
which any one person might accept as red.
This is not necessarily the same range as for a second person.
Experiments with spectrum colours have shown that if one person chooses the
wavelength considered closest to a true yellow,
a second person might well not agree and claim that the chosen
colour is too green or too orange.
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4. The aim of the CIE system
The aim of the CIE system is to tell us
how a colour might be reproduced
(by a mixture of three primary light sources)
rather than described.
The amounts of the three primaries required to match a particular colour
provide
a numerical specification of that colour.
A different colour would require different amounts of the primaries
and hence the specification would be different.
It turns out that in many applications this is all that is required.
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5. Colour is thr ee-dimensional,
Colour is three-dimensional, a property that is apparent in various ways.
Colour atlases arrange colours using three scales
(hue, value and chroma in the case of the Munsell system, for example).
If we apply a range of concentrations of, say, a yellow dye we can produce
a range of yellow samples,
but there will generally be many different yellows
that cannot be matched –
for example, those that are slightly redder than
our yellows.
With mixtures of our yellow with a red dye we will be able to produce a
range of
reds, oranges and yellows, but there will be many oranges that we cannot
match, including colours that are browner or greyer or less saturated than those
actually produced.
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6. M IXTURES OF 3 DYES
If we use mixtures of three dyes, however, such as our yellow and red
dyes together with a blue dye, we can match a wide range of colours:
yellows, oranges, reds and blues, and also browns, grays and so forth.
In fact, the only colours that we will not be able to match will be very
saturated or very pure colours, such as
a purer yellow
than that produced by our yellow dye.
By choosing particularly bright or pure dyes we
will reduce to a minimum the range of colours that cannot be matched.
(The widest range of colours can be matched if magenta, yellow and cyan are
used as the primaries.)
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7. System of Color Specifications
Ignoring the few colours that cannot be produced in this way, we could
imagine a system of colour specification
in which the concentrations of three specific dyes
Required on a particular substrate
to match a colour could be used
to specify that colour.
Of course we would need to specify the light source under which the colour
was seen, but the three concentrations would
give a numerical specification of the colour;
the specification would be different for different colours and it would be
possible for someone else to reproduce the colour.
Unfortunately there would be serious disadvantages to such a system.
The relationship between dye concentrations and any measurable
properties of a colour are complex, although definable with modern computers
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8. Moreover, dyes or pigments are
rarely pure,
and their precise colour depends on
the method of manufacture.
Even repeat batches from the same
plant will not match exactly,
and again properties of mixtures are
not completely predictable.
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9. Color ed Lights
In contrast, coloured lights are much easier to
define and reproduce.
Imagine a red light obtained by
isolating the wavelength 700 nm
from the spectrum.
Any laboratory
in the world capable of measuring wavelength
accurately (an objective physical measurement)
could produce the same red colour.
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10. Color ed light gr een
A green colour corresponding to 546.1 nm
could be produced even more readily,
since a mercury lamp emits light at
only four wavelengths in the visible region
(404.7, 435.8, 546.1 and 577.8 nm).
By filtering out the other three, the required green wavelength could be obtained.
The wavelengths 404.7 and 435.8 nm could be obtained in a similar manner.
Small variations in the operating conditions have no significant effect on the
wavelengths emitted by a mercury lamp.
Hence three primary light sources could be defined simply as appropriate
wavelengths and easily reproduced.
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11. M ixtur es of thr ee colour ed lights
Mixtures of three coloured lights can be produced in various ways,
but the simplest is to imagine
three spotlights
shining on the same area on a white screen (Figure 3.1).
The colour produced would be a mixture of the three colours, and a wide
range of colours could be produced
by varying the amounts of the three primaries.
The colours to be matched could include
surface colours illuminated by a particular light source.
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