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# Understanding Radiance: The relationship between sky conditions and irradiance

Hi,

My question relates to the relationship in Radiance between sky conditions and irradiance. I understand that the packages Gendaymtx,Gendaylit and GenCumulativeSky generate a Radiance scene with sky conditions and they also use both direct and diffuse irradiance data from epw files.

What I don't understand is the relationship between the sky conditions (the R,G,B) values of the sky scene the irradiance data and how irradiance in W/Sr/m2 is calculated for each patch of the sky.

For example the default sky color of Gendaymtx according to the documentation - http://www.radiance-online.org/learni... is:

0.960 1.004 1.118

So what is the relationship between the sky color above and the epw irradiance values how are they combined to calculate the irradiance or luminance falling on a surface?

Perhaps I am just missing something but the radiance documentation is very sparse on this particularly GenCumulativeSky. Any insights would be greatly appreciated I am trying my best to understand the code behind Radiance. Thank you!

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The best places to start would be the man pages for gensky and gendaylit. GenCumulativeSky is not part of the Radiance distribution and references to documentation and downloads are spotty, but you can find links in the radiance-general archives and elsewhere.

It is a little crazy that you can edit the RGB values for the skyfunc, but that's part of the joy of using Radiance. You can make the sky any color you want but it's up to you to make sure the values integrate photometrically to 1. If you're mostly interested in accurate values (and not pretty pictures), you can just use greyscale values.

As for how the radiance is assigned across the hemisphere, that's based on the sky type and algorithm selected. The CIE specifies 15 types in CIE S 011/E:2003 (a few of these are what gensky uses), and Perez defines the distribution in several papers which I can't find at the moment, used by gendaylit. I recommend using gendaylit, which automatically takes diffuse horizontal and direct normal irradiance or illuminance and makes a reasonable distribution based on that.

UPDATED 14-Jun-2016

Since this discussion went on (and on) in the comments, here's a bit of detail about this whole sky color business. It's actually discussed pretty well in this ancient thread on the Radiance-general list, to wit:

The color is totally arbitrary, however this is very important, the weighted average RGB must add up to 1! If you pick values that give you a pleasing sky color but add up to greater or less than 1, your sky luminance will be incorrect. It will be factored by the deviation from 1 in your average.

Be sure to read Greg's response in that thread, regarding the solar efficacy value. There's also good stuff on ground reflectance values.

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@rpg777 thank you for your answer, I have read through gensky and gendaylit thoroughly I understand that the Perez model is used to set the distribution of radiance in most of the models however I still don't understand the relationship between sky color and the irradiance. Would you be able to elaborate on this further?

Also when you say integrate photometrically do you mean the following equation? .265 * Red + .67 * Green + .065 * Blue

( 2016-05-26 18:05:13 -0600 )edit

Again, the sky color is something you can change (if you wish) in the Radiance sky/scene description. If you leave it alone Radiance will set a default color which will not affect the total radiation. I recommend using greyscale values for the model materials as well as the skies for doing numeric work (i.e. not pretty pictures). And yes I mean the RGB values should be weighted by V lambda as you have above, times the luminous efficacy value of 179 (this is a Radiance thing). Does that make sense?

( 2016-06-01 09:45:07 -0600 )edit

@rpg777 again thanks for your reply, I think I now understand in summary: - Sky color doesn't affect total radiation that is drawn from location data say a epw file however - the RGB sky color does affect the lluminance through the photo metric equation is this correct?

( 2016-06-13 14:38:40 -0600 )edit
1

In the context of defining skies for Radiance, yes. The RGB values will affect the appearance (color) of the sky in a Radiance rendering, but there is no "sanity testing" in Radiance for the validity of the input, so you could end up defining a sky model that adds or subtracts energy from the input. This is because the total energy (flux) is mapped to a "glow" source, which accepts a final set of RGB modifiers. By default they are "1 1 1" for uncolored skies, but you can go hog wild.

( 2016-06-13 15:11:56 -0600 )edit

Thanks I see clearly now that its imperative to set the correct sky colors in Radiance otherwise the results will be crazy. Perhaps I should open another question but I also want to ask, if all the RGB values are below 1 does this mean that there is no glow and therefore no light will be emitted?

( 2016-06-13 16:59:44 -0600 )edit

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