# Spectrum of Radiance simulation using Ladybug and Honeybee (Which wavelengths are simulated for?)

Dear all

I’ve been working on a solar altitude dependant conversion factor between irradiance in watt pr. m2 and photosynthetically photon flux density in micromoles pr. m2 pr. s.

In order to find the correct conversion factor I need to know the radiation spectrum used by Radiance.
I’m using both Ladybug and Honeybee[+] to do analysis, but I assume that the Radiance computations use the same spectrum. I assume the radiation spectrum goes beyond the visible spectrum, but how much? My geuss is that it goes from approx. 250-3000. Though I can’t seem to find any information of the topic on Radiance’s page.

I hope that some of you have some insight you would like to share would me. I would really appreciate any link or redirection to a Radiance source of any kind.

/Lasse Korsholm

Hi Lasse,

Radiance provides three channels for calculation (we’ll call them R, G, and B). Radiance doesn’t know or care what spectrum appears in each channel. It is up to you to choose this and to scale your source and reflection parameters appropriately.

Nathaniel

Hi Nathaniel,

Alright, so my question should really be: Which spectrum is the default spectrum when Honeybee[+] uses Radiance to perform annual radiation analysis?

/Lasse

Radiance doesn’t represent spectral data as such. It has three channels, which are nominally red green and blue values with specific chromaticities that allow conversion to other color spaces such as CIE XYZ. However, you can interpret the three channels differently if you like. This topic has been covered quite a bit on the older Radiance mailing list, and I suggest you start by searching some of the archives.

It seems that the input “O1” in gendaylit equals the use of solar radiation. I assume that this means that the power of the combined R, G and B channels equal the summation of the of power for each wavelength of the sun. I understand that Radiance does not use channels for each wavelength, but just three channels ‘loaded’ with a value. Is it safe to assume that when speaking ‘solar radiation’, that covers the full extraterrestrial solar radiation (250-3000)?

/Lasse

My interpretation has always been that if you use the O1 option you do get total solar spectrum and should not apply the 179 conversion factor later. Probably the distinction between R, G, and B is no longer relevant (I’ll rely on someone to confirm if that’s correct). If you use the default O0 it restricts to just the Watts of radiation within the visible part of the spectrum in the RGB channels for application of the human eye response formula across RGB and 179 unit conversion multiplier to get to lux.

I did a small comparison of O0 and O1 results ;
here are the results, with comparison (O0/O1) which can be compared to expected visible/total radiance

visible/total solar is 54 % which is in the ballpark of 48% by textbooks (But it could be my wishfull thinking)
I 'm not sure if read the perezlum function variables correctly.

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