# FW: sky definition

Greg suggested yesterday:
"so you should probably intervene with your own correction if you are working from physical
data. Therefore, I recommend multiplying values to the -r and -R options by:

208/179 the ratio between the sun's efficacy and the standard

Likewise, values to the -b and -B options should be multiplied by:

110/179 the ratio for sky radiation efficacy over the standard

I wrote back:

Greg,

are you sure? The sun's luminous efficacy above 20 deg. altitude is
somewhere around 95-100 lm/W, overcast skies are around 120 lmW, clear
skies around 150 lm/W. Therefore, your multipliers for the -R/r
options should be smaller than the multipliers for the -B/b options?

Martin

Hi Martin,

Now that you mention it, I'm not sure at all. I got my number from the
efficacy of Standard Illuminant B, which is simulated (rather than
real) sunlight, and it could be way off. Where do you get your
numbers? If you are sure of them, please forward this message with my

Thanks!
-Greg

The numbers for the solar/beam lumens/watt can be obtained from the Illuminating Engineering Society Handbook , and they are listed as 95 lm/W. If you need more precise data, you can go to

as one example. I do not think that taking the CIE daylight illuminants and their luminous efficacy is a good way to do it.

The following book

"Daylighting in Architecture" edited by Baker, Fanchiotti, Steemers and published by James & James (ISBN 1-873936-21-4) lists luminous efficacy functions for various sky/cloud conditions and sun altitudes on pages 10.4 and 10.5. Input needed is the global irradiance for calculating the overcast efficacy. They also give a formula for calculating luminous efficacies for skies with both clear and diffuse components by taking their ratio.

From those values, they offer a formula to obtain internal illuminance subdivided into the internal direct sunlight factor, the clear sky factor and the overcast sky factor.

Nevertheless, I had good experiences with actual weather data from www.meteotest.com (software Meteonorm) and "gendaylit".

Martin, Penn State