[Radiance-general] Randance and different sky descriptions

Thu Feb 28 12:58:22 PST 2013

Hi Allison,

I have been doing some comparisons of some of these new daylight simulation
approaches recently as well.  What we are seeing here is really why I
created an IES_gensky.py module that comes with SPOT and creates a clear,
pt cldy, and cldy skies according to IESNA recommendations (pre handbook
10, not sure if there were any changes).  Software 2 (SPOT) uses
IES_gensky.py (and IES_skybright.cal) for it's "design day" calculations
and we are seeing the output from that.  It then uses weather data to
adjust for its annual or specific day calculations.  In creating SPOT we
felt these "design day" skies were still good to review as it can give a
better idea of the extremes a space might see.  Relying solely on Perez
skies and weather data skies might not yield a perfectly sunny winter
solstice and while this may certainly be representative of a given climate
(Seattle ;-) one cannot claim a sunny winter equinox will never exist.  Now
the town might be outside celebrating if this ever happens, making it all a
moot point, but I digress...

Here are the numbers I get for a Boulder, CO equinox clear and sunny sky at
noon from TMY2 weather data, IES_gensky, and gensky
Weather data = 7,893fc
IES_gensky (5,000elev) = 9,416fc
IES_gensky (0) = 8212fc
gensky = 5823fc

Your current SPOT sky is almost 2x that of gensky which should explain much
of your interior discrepancies.  This sky did have an elevation adjustment
for 5,000ft.  I would recommend taking that out and just using 0 elevation
which results in exact IES recommendations.  The elevation factor was
derived from a weather study for higher elevation locations, and while I
think it works ok for really high elevations and high sun angles (it
matches our high mountain Colorado skies fairly well during the summer
months) it does not seem to help the Boulder sky.  I guess there is too
much smoke in the air ;-)

I also looked at the difference in distribution and found none - which is
good as they both should be using the CIE recommended clear sky
distributions, just not the same magnitude.  I put the sky definitions into
files and ran:
oconv 3_21_12.01_1.sky >  SPOTsky.oct
rpict @calc.opt -vta -vp 0 0 0 -vd 0 0 1 -vu 0 1 0 -vv 180 -vh 180 -x 1250
-y 1250  SPOTsky.oct >  SPOTsky.hdr
oconv e12pm_gensky.sky >  Gensky.oct
rpict @calc.opt -vta -vp 0 0 0 -vd 0 0 1 -vu 0 1 0 -vv 180 -vh 180 -x 1250
-y 1250  Gensky.oct >  Gensky.hdr
pcomb -e "ro=ri(1)/ri(2);go=gi(1)/gi(2);bo=bi(1)/bi(2)" SPOTsky.hdr
Gensky.hdr > SPOTgen_ratio.hdr (single back quotes on linux)

you can compare resulting illuminance (in fc) from these skies for global
and all orientations like so:
echo 0 0 0 0 0 1 | rtrace @calc.opt -h -I -faa -ov  SPOTsky.oct | rcalc -e
"$1=$1*4.40805+$2*11.1436+$3*1.07777" >  SPOTsky.dat
echo 0 0 0 1 0 0 | rtrace @calc.opt -h -I -faa -ov  SPOTsky.oct | rcalc -e
"$1=$1*4.40805+$2*11.1436+$3*1.07777" >  SPOTskyE.dat
echo 0 0 0 0 -1 0 | rtrace @calc.opt -h -I -faa -ov  SPOTsky.oct | rcalc -e
"$1=$1*4.40805+$2*11.1436+$3*1.07777" >  SPOTskyS.dat
echo 0 0 0 -1 0 0 | rtrace @calc.opt -h -I -faa -ov  SPOTsky.oct | rcalc -e
"$1=$1*4.40805+$2*11.1436+$3*1.07777" >  SPOTskyW.dat
echo 0 0 0 0 1 0 | rtrace @calc.opt -h -I -faa -ov  SPOTsky.oct | rcalc -e
"$1=$1*4.40805+$2*11.1436+$3*1.07777" >  SPOTskyN.dat

I hope this helps.

Regards,
Zack

On Tue, Feb 26, 2013 at 5:31 PM, Allison Bygott
<Allison.Bygott at megroup.com>wrote:

>  Hello.
>
>
>
> I have been evaluating a couple of different software programs that
> interface with Radiance. Unfortunately, my "standard" test models aren't
> producing the same results in each software; the results are different by
> at least a factor of 2. I'm trying to understand how each software is
> separately using Radiance to come up with daylight results. I believe this
> requires an understanding of how the sky files are generated. I spent some
> time today poking around the Radiance-online mailing lists, looking for sky
> file content clues, but I suspect that some of these concepts are above me.
> I did review the gensky document dated 4/24/98 and I found it informative.
> I'm hoping that the community might provide some insight on the content of
> two "comparable" sky files; please see below:
>
>
>
> Software 1:
>
>
>
> !gensky 3 21 12.01 +s -a 40.03 -o 105.28 -m 105.00
>
>
>
> skyfunc glow skyglow
>
> 0
>
> 0
>
> 4  0.7  0.8  1.0 0
>
>
>
> skyglow source sky
>
> 0
>
> 0
>
> 4 0 0 1 180
>
>
>
> skyfunc glow grndglow
>
> 0
>
> 0
>
> 4 0.05 0.1 0.07 0
>
>
>
> grndglow source ground
>
> 0
>
> 0
>
> 4 0 0 -1 180
>
>
>
>
>
>
>
> Software 2:
>
>
>
> void light solar
>
> 0
>
> 0
>
> 3 11009526.873677 11009526.873677 11009526.873677
>
>
>
> solar source sun
>
> 0
>
> 0
>
> 4 0.039983 -0.647611 0.760921 0.5
>
>
>
> void brightfunc skyfunc
>
> 2 skybr IES_skybright.cal
>
> 0
>
> 7 1 15.092265 18.041592 0.649859 0.039983 -0.647611 0.760921
>
>
>
> skyfunc glow skyglow
>
> 0
>
> 0
>
> 4 0.989 0.989 1.159 0
>
>
>
> skyglow source sky
>
> 0
>
> 0
>
> 4 0.0 0.0 1.0 180
>
>
>
> skyfunc glow groundglow
>
> 0
>
> 0
>
> 4 1.0 1.0 1.0 0
>
>
>
> groundglow source ground
>
> 0
>
> 0
>
> 4 0.0 0.0 -1.0 180
>
>
>
>
>
> Thank you. I appreciate any thoughts on the matter.
>
>
>
> [image: Element Logo 4-27-12.png] <http://www.megroup.com/>
>
> ALLISON BYGOTT, PE, LC, LEED-AP
>
> ENGINEER
>
> t: 303.382.1920 x4132
>
> www.megroup.com
>
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>
>
>
>
>
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-- 
Zack Rogers, P.E., LEED AP BD+C
Daylighting Innovations, LLC
211 North Public Road, Suite 220
Lafayette, CO 80026
(303)946-2310
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