Using SSLD skies with sun

Hi,

I know that not many here use the SSLD skies, but maybe one or another has a comment on it...

I am currently using gensky to find the brightness levels and sun position required to use the SSLD skies. Now I am wondering whether this combination hides some risks.

First, when I add a gensky-sun to the ssld-sky, in my understanding this should not change the horizontal illuminance but be kind of a technical optimization, replacing the bright glow spot where the sun is seen by a source of type light, thus making sure that shadow-test actually hit it. This would require the flux of the sky (glow) part to be covered to be the same as the flux of the sun light source.

Second, gensky manipulates the sun position when coming close to the zenith and sets it to 87.5 altitude. I am in a tropical country, where this 90 degrees can be reached. In such a case, I would have two suns in my sky definition - the one in the ssld glow source (at 90 degree) PLUS the gensky light source at 87.5 degree, right?

I am just trying to debug my sky setups, so any comments are appreciated :smiley:

Cheers Lars.

Hi Lars,

The SSLD sky generator creates a diffuse sky distribution. If you don't put a sun in there, there won't be one. If you're sky model involves a diffuse sky that is brightest in the sun direction, but the sun itself is not directly visible, then you'll just need to ensure you get the diffuse horizontal illuminance you're looking for (and that's the hard part - what should the diffuse horizontal illuminance be?).

So you won't end up with two sun definitions. And you control the diffuse and direct components of global horizontal illuminance. If the sun is not visible, it shouldn't be in the model, the direct illuminance component will be zero and the global will equal the diffuse illuminance.

This is assuming you are using my old SSLD sky generator / cal file. Could be a different story if you're using something else?

Cheers,
Phil.

···

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Lars O. Grobe
Sent: Wednesday, 12 August 2009 6:13 PM
To: Radiance general discussion
Subject: [Radiance-general] Using SSLD skies with sun

Hi,

I know that not many here use the SSLD skies, but maybe one or another
has a comment on it...

I am currently using gensky to find the brightness levels and sun
position required to use the SSLD skies. Now I am wondering whether this
combination hides some risks.

First, when I add a gensky-sun to the ssld-sky, in my understanding this
should not change the horizontal illuminance but be kind of a technical
optimization, replacing the bright glow spot where the sun is seen by a
source of type light, thus making sure that shadow-test actually hit it.
This would require the flux of the sky (glow) part to be covered to be
the same as the flux of the sun light source.

Second, gensky manipulates the sun position when coming close to the
zenith and sets it to 87.5 altitude. I am in a tropical country, where
this 90 degrees can be reached. In such a case, I would have two suns in
my sky definition - the one in the ssld glow source (at 90 degree) PLUS
the gensky light source at 87.5 degree, right?

I am just trying to debug my sky setups, so any comments are appreciated :smiley:

Cheers Lars.

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Hi Phillip!

So you won't end up with two sun definitions. And you control the
diffuse and direct components of global horizontal illuminance. If
the sun is not visible, it shouldn't be in the model, the direct
illuminance component will be zero and the global will equal the
diffuse illuminance.

Hm, my concern was maybe a bit academic :slight_smile: For a sky with a very steep gradient, the sun (light) would cover the brightest spot of the sky (glow) - so light replaces glow. But because of the altitude modification gensky does for high altitudes, the sun (light) is 2.5 degrees off the brightest spot of the sky glow - leading to light plus glow spot. Probably the fact that the solid angle covered by the sun is small, the difference is not that high, but in cases where my illuminance is really only due to a small circumsolar domain of the sky, this may lead to some higher illuminances, right? Would need some test models to see whether this becomes visible with a skylight. I just realized that I have two different sun positions in my model - the 'real' one by the ssld background sky, and the manipulated on by gensky.

Yes, I am using you cal-file, thank you by the way :slight_smile:

Cheers

Lars.

OK - thanks for the explanation. All clearer now!

I'm thinking that this shouldn't really cause you much problems. You may be right that the sun will block that part of the sky nearest to the solar position, but that part of the sky that is blocked will be really small, and much less bright than the sun, so it should (I think?) make very little, to no, difference. And because the diffuse sky is only part of the indirect calculation, a small change in directionality should again be almost unnoticeable.

BTW, you could always set the sun direction input for the SSLD definition (A8, A9, A10) equal to that generated by gensky?

Cheers!
Phil.

···

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Lars O. Grobe
Sent: Wednesday, 12 August 2009 6:53 PM
To: Radiance general discussion
Subject: Re: [Radiance-general] Using SSLD skies with sun

Hi Phillip!

So you won't end up with two sun definitions. And you control the
diffuse and direct components of global horizontal illuminance. If
the sun is not visible, it shouldn't be in the model, the direct
illuminance component will be zero and the global will equal the
diffuse illuminance.

Hm, my concern was maybe a bit academic :slight_smile: For a sky with a very steep
gradient, the sun (light) would cover the brightest spot of the sky
(glow) - so light replaces glow. But because of the altitude
modification gensky does for high altitudes, the sun (light) is 2.5
degrees off the brightest spot of the sky glow - leading to light plus
glow spot. Probably the fact that the solid angle covered by the sun is
small, the difference is not that high, but in cases where my
illuminance is really only due to a small circumsolar domain of the sky,
this may lead to some higher illuminances, right? Would need some test
models to see whether this becomes visible with a skylight. I just
realized that I have two different sun positions in my model - the
'real' one by the ssld background sky, and the manipulated on by gensky.

Yes, I am using you cal-file, thank you by the way :slight_smile:

Cheers

Lars.

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Second, gensky manipulates the sun position when coming close to the
zenith and sets it to 87.5 altitude. I am in a tropical country, where
this 90 degrees can be reached.

The sun hits a zenith of 90 at any position between the Tropic of
Cancer and the Tropic of Capricorn. A few degrees above and below the
tropics hit a zenith of 87.5.

A short while ago I modified and recompiled gensky, changing 87.5 to
89.9 in lines 218-225 (
http://www.radiance-online.org/pipermail/radiance-general/2005-December/003118.html
).

Has anybody else done this and run into problems?

I think if the zenith is at 90.0, the -vu vector may get messy if
you're translating the gensky output into a camera showing the sun's
view, which was my reasoning for 89.9.

--Dave

Hi Dave.

A short while ago I modified and recompiled gensky, changing 87.5 to
89.9 in lines 218-225 (
http://www.radiance-online.org/pipermail/radiance-general/2005-December/003118.html
).

Has anybody else done this and run into problems?

I did that back in the days when I compiled my own stuff. You can't
got to 90 degrees
but everything else should work. I remember that the positions I got
were still credible
and far better than the results form another algorithm I used that
distorted the sun's
path around the zenith. Gensky may be a bit inaccurate, too, but i
don't think it is
noticeable.

I checked the output by plotting a sunpath with the values I got from the gensky
comments in the output. I think Axel Jacobs has written a script to do
that but you
can use any plotting app you want.

Regards,
Thomas

···

On Wed, Aug 12, 2009 at 2:24 PM, David Smith<[email protected]> wrote:

Sort of related to this discussion:

When diffuse and direct input parameters are taken from measurements (e.g. a climate file), the value for direct normal will usually be based on an instrument that has an acceptance angle quite a bit larger than the solar disc, e.g. 6 degrees for the Eppley solar tracker. So, a reasonable portion of the most intense circumsolar region will end up be apportioned to the sun luminance (actually, radiance), and therefore handled by the deterministic part of the calculation. This effect is described in more detail in Chapter 3, Section 3.2.3 (p60-61) here:

http://www.iesd.dmu.ac.uk/~jm/doku.php?id=resources:thesis

-John

···

-----------------------------------------------
Dr. John Mardaljevic
Reader in Daylight Modelling
Institute of Energy and Sustainable Development
De Montfort University
The Gateway
Leicester
LE1 9BH, UK
+44 (0) 116 257 7972
+44 (0) 116 257 7981 (fax)

[email protected]
http://www.iesd.dmu.ac.uk/~jm