1.4 daylight and the cie standard illuminants


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1.4 daylight and the cie standard illuminants

  1. 1. Daylight and its spectrumStandard illuminants and standard sources COMPILED BY TANVEER AHMED 1
  2. 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. COMPILED BY TANVEER AHMED 2
  3. 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). COMPILED BY TANVEER AHMED 3
  4. 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 COMPILED BY TANVEER AHMED 4
  5. 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. COMPILED BY TANVEER AHMED 5
  6. 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. COMPILED BY TANVEER AHMED 6
  7. 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. COMPILED BY TANVEER AHMED 7
  8. 8. variability of daylight andUV 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. COMPILED BY TANVEER AHMED 8
  9. 9. variability of daylight andUV 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. COMPILED BY TANVEER AHMED 9
  10. 10. standard method of test for lightfastness 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. COMPILED BY TANVEER AHMED 10