Editing Sunlight (section)

== Surface illumination and spectrum==
{{see also|Diffuse sky radiation}}
[[File:Sun_over_Lake_Hawea,_New_Zealand.jpg|thumb|upright=1.25|Sunlight shining through [[cloud]]s, giving rise to [[crepuscular rays]]]]
The spectrum of surface illumination depends upon solar elevation due to atmospheric effects, with the blue spectral component dominating during twilight before and after sunrise and sunset, respectively, and red dominating during sunrise and sunset. These effects are apparent in natural light [[photography]] where the principal source of illumination is sunlight as mediated by the atmosphere.

While the color of the sky is usually determined by [[Rayleigh scattering]], an exception occurs at sunset and twilight. "Preferential absorption of sunlight by ozone over long horizon paths gives the zenith sky its blueness when the sun is near the horizon".<ref>{{cite web|url=http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf|title=Atmospheric Optics|author=Craig Bohren|author-link=Craig Bohren|url-status=live|archive-url=https://web.archive.org/web/20131206175627/http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf|archive-date=2013-12-06}}</ref>

=== Spectral composition of sunlight at Earth's surface ===
The Sun may be said to [[Lighting|illuminate]], which is a measure of the light within a specific sensitivity range. Many animals (including humans) have a sensitivity range of approximately 400–700&nbsp;nm,<ref name="BuserImbert1992">{{cite book|last1=Buser|first1=Pierre A.|last2=Imbert|first2=Michel|title=Vision|url=https://archive.org/details/vision0000buse|url-access=registration|access-date=11 October 2013|date=1992|publisher=MIT Press|isbn=978-0-262-02336-8|page=[https://archive.org/details/vision0000buse/page/50 50]|quote=Light is a special class of radiant energy embracing wavelengths between 400 and 700 nm (or mμ), or 4000 to 7000 Å.}}</ref> and given optimal conditions the absorption and scattering by Earth's atmosphere produces illumination that approximates an [[Standard illuminant|equal-energy illuminant]] for most of this range.<ref name="MacEvoy">{{cite book|last1=MacEvoy|first1=Bruce|title=color vision|date=2008|url=http://www.handprint.com/HP/WCL/color1.html|access-date=27 August 2015|quote=Noon sunlight (D55) has a nearly flat distribution...|url-status=live|archive-url=https://web.archive.org/web/20150924024814/http://www.handprint.com/HP/WCL/color1.html|archive-date=24 September 2015}}</ref> The useful range for color vision in humans, for example, is approximately 450–650&nbsp;nm. Aside from effects that arise at sunset and sunrise, the spectral composition changes primarily in respect to how directly sunlight is able to illuminate. When illumination is indirect, [[Rayleigh scattering]] in the upper atmosphere will lead blue wavelengths to dominate. Water vapour in the lower atmosphere produces further scattering and ozone, dust and water particles will also absorb particular wavelengths.<ref name="Wyszecki_and_Stiles">{{cite book|last1=Wyszecki|first1=Günter|last2=Stiles|first2=W. S.|title=Color Science: Concepts and Methods, Quantitative Data and Formulas|date=1967|publisher=John Wiley & Sons|page=8}}</ref><ref name="MacAdam">{{cite book|last1=MacAdam|first1=David L.|title=Color Measurement: Theme and Variations|url=https://archive.org/details/colormeasurement00ddav|url-access=limited|edition=Second Revised|date=1985|isbn=0-387-15573-2|publisher=Springer|pages=[https://archive.org/details/colormeasurement00ddav/page/n46 33]–35}}</ref>

[[File:Spectrum of Sunlight en.svg|thumb|center|upright=3.5|Spectrum of the visible wavelengths at approximately sea level; illumination by direct sunlight compared with direct sunlight scattered by cloud cover and with indirect sunlight by varying degrees of cloud cover. The yellow line shows the power spectrum of direct sunlight under optimal conditions. To aid comparison, the other illumination conditions are scaled by the factor shown in the key so they match at about 470&nbsp;nm (blue light).]]