Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
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, and Perez defines the distribution in several papers which I can't find at the moment. I recommend using gendaylit, which automatically takes diffuse horizontal and direct normal irradiance or illuminance and makes a reasonable distribution based on that.
| | 2 | No.2 Revision |
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, 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. moment, used by gendaylit. I recommend using gendaylit, gendaylit, which automatically takes diffuse horizontal and direct normal irradiance or illuminance and makes a reasonable distribution based on that.
| | 3 | No.3 Revision |
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.
