color physics

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
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 (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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