1. Standard illuminants
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COMMISSION INTERNATIONALE DE L’ÉCLAIRAGE (CIE) IN
1931 IT WAS RECOGNISED THAT
STANDARDISED SOURCES OF ILLUMINATION
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2. Standard illuminants
TABLE OF CONTENT
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1. Commission Internationale de l’Éclairage (CIE)
2. Standard illuminant A
3. Standard illuminant B
4. Standard illuminant C
5. Standard illuminant D
6. standard illuminant D Hunt Suggestions
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3. Commission Internationale de l’Éclairage
(CIE)
When the international system of colour measurement and
specification was set up by
the Commission Internationale de l’Éclairage (CIE) in 1931
it was recognised that standardised sources of illumination
would have to be defined,
and three such sources
(CIE Standard sources A, B and C) were adopted at that time as
approximations to three common illumination conditions.
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4. Commission Internationale de l’Éclairage (CIE)
Although these sources were defined in such a
way that they could be
physically realised (a standardised tungsten lamp
in combination with suitable blue-coloured solution filters)
the opportunity was also taken to define
a set of numerical values
representing the relative SPD of the appropriate standard
illuminant
at 10 nm intervals across the visible spectrum (380–770 nm).
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5. It is thus important to distinguish between standard illuminants,
which are defined 5
in terms of spectral power distributions,
and standard sources,
which are defined as
physically realizable emitters of radiant energy
and have SPDs that only approximate to those of the
corresponding illuminants.
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6. Standard illuminant A
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was designed in 1931 to
be representative
of indoor artificial
(tungsten lamp) illumination
and is defined as an
illuminant having the same
SPD as a
Planckian radiator
at a temperature of about
2856 K.
An actual source
corresponding to this
illuminant is readily
achieved,
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7. Standard illuminant A
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and calibrated standard tungsten lamps are available
from standardising bodies in each country; in the UK this is
the National Physical Laboratory (NPL).
Such an illuminant is relatively yellowish in colour as it is
deficient in power in the blue end of the visible spectrum
and rich in the red wavelengths
(Figure 1.9).
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8. SPDs of CIE standard illuminants
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9. Standard illuminant B
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with a correlated colour temperature (CCT)
of about 4870 K,
was supposed to represent
daylight plus sunlight,
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10. standard illuminant C
10
(CCT = 6770 K)
was intended to
represent average
daylight;
both are now
largely redundant
in favour of the D
illuminants
introduced
subsequently.
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11. standard illuminant D
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In 1963, therefore, the CIE
recommended several new
standard illuminants (the D
illuminants) by defining
spectral distributions across
the UV,
visible
and near-IR (300–830 nm)
to represent various phases of
daylight.
CIE illuminant D65, with an
approximate CCT of 6500 K,
is now accepted by the CIE as
a standard illuminant (CIE
1986).
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12. standard illuminant D
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Its SPD is a good approximation
• of average daylight,
taking into account the following types of
variation:
– from early morning to late evening
– from a blue sky to completely overcast conditions
– at different latitudes.
The SPDs of the standard D illuminants were originally
defined
at 10 nm intervals
but values at 5 nm and 1 nm intervals
have been obtained by interpolation and are now
available (CIE 1971 and 1986).
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13. standard illuminant D
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Figure 1.9
compares the SPD
of
standard
illuminant D65
with the CIE
illuminants A, B
and C;
the higher UV
content of D65
compared with
CIE illuminants B
and C is clearly
evident.
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14. standard illuminant D
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The CIE recognised that a single distribution such as
D65 would be unlikely to satisfy all colour users
and suggested others, such as D50 and D55
with CCTs of 5000 and 5500 K for use where yellower phases of
daylight than average were desirable
(D50 is favoured by the graphic art trade for illumination of colour
prints and photographs).
D75, with a CCT of 7500 K, is popular in some parts
of America for colour assessment
where a bluer phase of daylight is preferred.
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15. standard illuminant D
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Although the CIE has published a method for
assessing the quality of daylight simulators
• for colorimetry
problems have been encountered in attempts to
manufacture
• practical sources
• that simulate the illuminant D curves,
particularly the undulations present naturally and
accentuated by the interpolation procedures.
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16. standard illuminant D
Hunt has suggested
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Recently Hunt has suggested that the CIE needs to accept
that
• only approximations to the standard D illuminant curves are ever going
to be possible.
He advocates that practical D sources be carefully
specified and adopted for use as
• the best approximations to the D illuminants achievable
and suggests these could be distinguished as:
source DT: a tungsten–halogen lamp with a blue glass
filter
source DX: a filtered xenon arc
source DF: a fluorescent lamp with a suitable CCT.
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17. Studies of daylight and its spectrum in the 1950s
and early 1960s
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Studies of daylight and its spectrum in the 1950s and
early 1960s confirmed that
standard illuminants B and C had too little power in
the UV region to be of value in assessing
fluorescent brightening agents
(or optical brighteners as they were then termed),
and the SPDs
and the colour coordinates
also deviated from those of the natural daylight
conditions they were supposed to represent.
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18. Studies of daylight
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The demand for a practical and standard D source is now
substantial,
both in connection with the comparison of the quality of
instrumentally measured and visually assessed colour
matches
and also, more particularly, in the measurement and
assessment of fluorescent samples
(where the UV content of the source is critical).
The situation has been conveniently summarized
recently by McCamy
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