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DAYLIGHT AND THE CIE STANDARD ILLUMINANTS

Daylight and its spectrum

Standard illuminants and standard sources

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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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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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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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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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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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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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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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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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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.