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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