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Exist some relationship between irradiance units and wavelength of the incident sunlight? What about irradiance? I want to establish a relationship between wavelength and irradiance, because I would try to model photosynthesis on Vensim.

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Have a look at http://en.wikipedia.org/wiki/Sunlight as this gives the irradience of sunlight as a function of wavelength. The spectrum is very similar to a black body temperature of 5,600K.

I wasn't sure from your question if you were asking about the units used for irradience. The units used in the Wikipedia article (and in the ASTM page it references) are power per square metre per nanometre of bandwidth. For example a value of 1.5 at 400nm means that the total power of all the light between 400nm and 401nm is 1.5W/m$^2$. To model photosynthesis you need to multiply the spectrum by the absorption spectrum of chlorophyll then integrate across all wavelengths to get the total power per square metre.

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