1.85 combined absorption and scattering (kubelka–munk analysis)
1.4 daylight and the cie standard illuminants
1. Daylight and its spectrum
Standard illuminants and standard sources
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2. Daylight and its spectrum
Sunlight reaches the earth’s
surface only after a
significant amount of it has
been removed
○ by absorption
○ and scattering processes
as a result of encounters in
the atmosphere
○ with ozone, water vapour,
liquid water droplets, ice and
dust particles, as well as with
an increasing variety of
pollutant species.
The selective absorptions
(mainly by ozone and water
vapour) produce
○ the undulations in the SPD
curves of daylight shown in
Figure 1.8.
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3. Scattering
Scattering is most pronounced at the
low wavelengths; this leads to
○ the blue appearance of a clear sky
○ and the rapid fall-off of the energy in the UV
(400–300 nm region).
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4. daylight Variations
Figure 1.8 shows that the nature of
daylight depends on
○ the part of the sky that is viewed,
○ and on the viewing conditions.
Variations are also observed which
depend on:
– the latitude of the place of measurement
– the season of the year
– the local weather conditions
– the time of day
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5. daylight Variations
Despite these variations it is possible to assign colour
temperatures to the different
○ phases of daylight as shown in the legend to Figure 1.8.
Thus light from a clear north
skylight is distinctly blue and can have a
○ colour temperature from 7500 to 10 000 K, or even higher,
whilst that of average daylight (particularly when viewed
from indoors) is
in the region 5800 to 6500 K.
When sunlight is included with clear skylight the influence
of the blue scattered light is less and the colour
temperature drops to about 5500 K.
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6. Day Light regretted due to variability
In many colour technology applications daylight was
traditionally the preferred illuminant
○ for colour matching
○ and colour assessment operations.
However, with recognition of the above variability
and the trend towards tighter colour tolerances,
Most colour matching is now done under
○ artificial daylight illumination derived either from
○ fluorescent tubes or
○ from a filtered xenon arc lamp
○ conforming to the appropriate national
○ or international standards.
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7. variability of daylight
Although the variability of daylight
○ in the visible region is significant,
○ the much greater variability of the UV portion of
daylight (from 300 to 400 nm) is a more significant
problem in the visual assessment of
○ fluorescent whitening materials
○ and in the light-fastness testing of coloured
materials.
Light-fastness testing is increasingly being
carried out using
○ xenon arc sources.
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8. variability of daylight and
UV
Additionally, the characteristics of daylight, and
particularly the effect of ozone levels on the
proportion of UV in direct sunlight, has been of
recent concern
○ as it is the UV radiations that cause
○ skin damage, including
○ sunburn
○ and skin cancers, in humans.
For such studies the UV region is split into three
subregions:
○ UVA: 400–315 nm
○ UVB: 315–280 nm
○ UVC: less than 280 nm.
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9. variability of daylight and
UV
Glass, depending on its thickness, generally cuts off UV
radiation below 310 nm, so that exposure to daylight
○ behind glass involves mainly UVA radiation.
The DNA and the proteins in living cells
○ absorb below 300 nm,
○ and therefore it is exposure to UVB and UVC
○ which cause damage to the eyes
○ and give rise to the most serious types of sunburn damage (and
in the long term to skin cancer).
The current concerns over ozone ‘holes’ arise from the
increased transmission of UVB and particularly UVC
through the atmosphere
○ under conditions of depleted upper atmosphere ozone.
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10. standard method of test for light
fastness to daylight
In the standard method of test for light
fastness to daylight
○ samples are exposed behind glass,
○ so under these conditions only the visible and
UVA radiation are involved in light fading of
colorants.
In weathering tests, however,
○ the protective effect of glass is removed.
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