[Radiance-general] Re: sky definition part 2

[Greg Ward]

Hi Zack,

> From: Zack Rogers <zrogers@archenergy.com>
> Date: Thu Oct 16, 2003  10:25:29  PM US/Pacific
>>> I should clarify what I mean when I say the CIE sky function tells 
>>> me....I am refering to both the sky and sun calculations laid out in 
>>> the latest IESNA lighting handbook which uses Kittlers CIE adopted 
>>> sky function (which I understand is from his 1967 studies) to 
>>> determine the sky component.  So I probably should have said that 
>>> IESNA sky/sun functions tell me.  I am not sure if this is the 
>>> accepted method used by others.  The direct component is calculated 
>>> as this;
>>>
>>> Edn = Ext * e ^ (-cm)
>>>
>>> where;
>>> Edn = direct normal solar illuminance
>>> Ext = extraterrestrial solar illuminance = Esc * (1 + 0.034 * cos ( 
>>> (2pi/365)*(J-2)))
>>>                where;
>>>                Esc = solar illumination constant = 127.5 klx (11,850 
>>> fc)
>>>                J = julian date
>>> c = atmospheric extinction coefficient (given as c = 0.21 for clear 
>>> ; c = 0.8 for partly cloudy)
>>> m = optical air mass = 1 / sin (solar altitude)
>>>
>>> and Direct Horizontal (Edh) would then be;
>>> Edh = Edn * sin (solar altitude)
>>>
>>> This gives the Direct Horiz to be 8,821 fc on a clear summer 
>>> solstice at 40 lat.  This is what I would like Radiance to give me 
>>> without having to specify with the -r parameter.  Currently it is 
>>> giving me 6,836fc.
>>
>> The formula above for Ext must be specific to the northern 
>> hemisphere.  I don't see how the same seasonal correction factor 
>> could apply everywhere on the globe.  I don't think this is the 
>> declination angle, as the solar altitude includes that.  I'm not sure 
>> what this is about.
>
> I'm confused.  What seasonal correction factor, c?  The declination 
> angle does go into the solar altitude angle calc, but it is not used 
> elsewhere in this calc.

I'm referring to the expression for Ext = Esc * (1 + 0.034 * cos ( 
(2pi/365)*(J-2))), which looks like a seasonal correction much like 
declination angle to me.  I really don't know what it's doing in there.

>> The variation of atmospheric extinction is quite large.  I wouldn't 
>> trust this, either.  In other words, this gives you a value, but I'm 
>> not sure it's any better than the guess provided by gensky.  It's 
>> still just a guess.
>
> I'm sure this atmospheric extinction factor was derived from weather 
> data.  I don't think IES would publish unless there was atleast some 
> verification.  I think this variable is trying to take into account 
> the different turbidities for clear and partly cloudy skies.

Your trust is greater than mine if you believe you can take weather 
data and come up with one value for clear and another for cloudy.

> I'm not sure what the ZL table is based on, again I am guessing it is 
> derived from weather data.  Yes, from what I understand the CIE clear 
> sky is Kittler's function from the 1967 studies.  Yeah, I'm not sure 
> where the A, B, and C variables are derived from either.  I just do 
> what IES tells me to.  Actually, I tend to trust these calcs for 
> several reasons; 1) they have never seemed to be off more than 20% 
> from weather data and measurements I've looked at (but I've done no 
> extensive study by any means) and 2) while I'm not one to question 
> master Greg Ward, I am also not one to question David DiLaura or the 
> IES.  One of the other sky calculation programs I'm using for these 
> comparisons is one he wrote based on these equations, it was discussed 
> as one of the best methodologies to estimate daylight levels, given 
> that exact predictions are always impossible.

It's good to get your experience, but I'd like to hear others.  You 
should *always* question these calculations, no matter what the source. 
  Especially if it's from me!

>> However, if you like this calculation, you could certainly add it as 
>> a wrapper to gensky using a simple shell script that computes the -r 
>> and -b options based on these factors and I'd be happy to add it to 
>> the distribution.  To my sensibilities, it seems like a lot of effort 
>> for what must be a pretty crude estimate.
>
> I do like this calculation.  If it is not too much trouble, I for one, 
> would like to see it added to gensky.  Otherwise, I can always apply a 
> multiplier outside of gensky to adjust.  BTW - I think what this has 
> shown is that it is dangerous to apply the -r option without applying 
> the -b options or adjusting the turbidity and vice versa.  Since that 
> would change the sky to sun ratio and give you a false sky/sun 
> condition?  When I was only specifying a -R the change in the sky 
> illuminance was less than a 1 fc while the change in the solar 
> illuminance was roughly 2000fc.

The ratios between skylight and sunlight are no more reliable than the 
absolutes of the two in gensky.  It is indeed a very crude estimate I'm 
using.  If you have a look at the source, you'll see what I mean.  I do 
recommend specifying -r and -b options together, though -- if you're 
using one, you should use the other.  I would do this rather than 
applying a multiplier outside of gensky.

>> It seems that the correct input to these equations is every bit as 
>> difficult as turbidity to obtain, if not moreso, and just using the 
>> values in the book is equivalent to using some rule-of-thumb value 
>> for turbidity in most respects.  It's just as likely to give you a 
>> reasonable value, not!
>
> Well, I guess since the difficult input has be done for us with the 
> constants, and if you trust the constants its not as difficult.

As I indicated, I am agnostic towards these absolute sky level 
estimates, leaning towards athiest.  I am happy to implement the IES 
calcs in gensky, as I see them as being no worse than what's in there, 
but I would like to hear a general "huzzah!" from the group before 
doing so.

-Greg