Editing Spectral power distribution (section)

==Source SPD and matter==
[[File:Rayleigh sunlight scattering.svg|thumb|right|250px|Figure showing the greater proportion of blue light scattered by the atmosphere relative to red light.]]The spectral power distribution over the [[visible spectrum]] from a source can have varying concentrations of relative SPDs. The interactions between light and matter affect the absorption and reflectance properties of materials and subsequently produces a color that varies with source illumination.<ref name=mcol>{{cite book | title = The Measurement of Colour | author = William David Wright | date = 1969 | publisher = New York: Van Nostrand Reinhold Co. }}</ref>

For example, the relative spectral power distribution of the sun produces a white appearance if observed directly, but when the sunlight illuminates the Earth's atmosphere the sky appears blue under normal daylight conditions. This stems from the optical phenomenon called [[Rayleigh scattering]] which produces a concentration of shorter wavelengths and hence the blue color appearance.<ref name=intro/>