[Radiance-general] Re: sky definition part 2

[Greg Ward]

Hi Zack,

It was great to meet you at the workshop, and I really enjoyed your 
presentation.

I think you're making trouble for yourself by trying to adjust the 
turbidity factor in gensky.  If you have the horizontal and diffuse 
irradiance, or facsimilies thereof, you should be giving those to 
gensky directly using the -B or -b and -R or -r options as we have been 
discussing.

Just to clarify, gensky ALWAYS produces an accurate CIE sky 
distribution.  Adjusting the turbidity or these other factors only 
affects the absolute levels and the ratio between solar and sky 
components.  It does not change the distribution of skylight.

This is not to say that the CIE sky model is the best.  It is just an 
agreed upon standard, and therefore serves as a reasonable point for 
comparison.  The Perez sky is probably a better approximation to actual 
skies, particularly for the intermediate case.

As I said in the bit of e-mail you so aptly quoted, the absolute values 
produced by gensky with the default settings are unreliable because 
they are not based on weather data for your area.  If you have such 
measurements, by all means use them, but don't adjust the turbidity -- 
go straight for the horizontal and diffuse specifications to gensky.  
This overrides the turbidity approximation and makes it irrelevant.

I hope this helps.
-Greg

P.S.  I got an e-mail from Martin Moeck on the related topic of solar 
efficacy, and include it here for discussion.  I don't know the right 
answer, but I'm sure someone out there does...

Begin forwarded message:

> From: Greg Ward <gward@lmi.net>
> Date: Wed Oct 15, 2003  2:33:06  PM US/Pacific
>
> 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 
> apologies to radiance-general.
>
> Thanks!
> -Greg
>
>> From: "Martin Moeck" <MMoeck@engr.psu.edu>
>> Date: Wed Oct 15, 2003  2:26:55  PM US/Pacific
>>
>> 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
>