[Radiance-general] Low simulated illuminance levels under direct sun

[Krystyna Zelenay]

Hi Greg,

I got a response from the DIVA mailing list - the program does not
allow the user to input calibration factors (-r -R -b or -B) for the
gensky command (I chose to use Rhino in combination w/ the plugin
DIVA-for-Rhino since I do not have access to a Unix machine and
because of its user-friendly interface). Will see if I can workaround
this problem as described below.

Thank you,

Krystyna


On Mon, Feb 14, 2011 at 7:09 PM, J. Alstan Jakubiec <alstan at jakubiec.net> wrote:
Dear Krystyna,

DIVA does not pass any calibration factors to gensky from the weather
file. It is called as in the below example,
!gensky 09 21 09 +s -a -16 -o 48 -m 45

You can't put in the calibration factors using -r -R -b or -B as Greg
suggests via the DIVA GUI. However, by editing the filename_sky.rad in
your DIVA temp folder and by running the simulation using
filenameimg.bat you could short-circuit the GUI and add a calibration
flag to the gensky command. Note that every time you run a
visualization from the GUI, you'll need to re-edit the sky.rad file if
you decide to do this.

The other option, if you have direct and diffuse radiation data, is to
use the Perez sky distribution via gendaylit. DIVA has a gui frontend
to this which you can choose by selecting the Perez Sky option under
sky condition in the metrics dialog.

Best,
Alstan




On Mon, Feb 14, 2011 at 4:02 PM, Greg Ward <gregoryjward at gmail.com> wrote:
>
> Hi Krystyna,
>
> Did you use any of the calibration options to gensky (-r, -R, -b, or -B)?  If not, then it's surprising that your results are as close as this.  If you did use these options, then we should check the conversion factors and what measurements you based them on.  Provided you have a separate measure of sun and sky and the rest of the parameters are correct, you should get good agreement with the open-sky test case.
>
> Some good tips on setting gensky parameters may be found in Axel Jacob's Radiance tutorial, which is currently linked at:
>
>        http://luminance.londonmet.ac.uk/learnix/docs/radiance_tutorial.pdf
>
> Best,
> -Greg
>
> > From: Krystyna Zelenay <kzelenay at berkeley.edu>
> > Date: February 14, 2011 3:16:58 PM PST
> >
> > Hi,
> >
> > I am trying to develop a Radiance model of an actual building.  I have
> > collected hourly global horiz. illum. on roof and vertical illum. on
> > facade (using a LI-COR sensor) and have been making hourly comparisons
> > between measured and simulated data. The building is surrounded by
> > several buildings which block much of the direct sun reaching the
> > facade - I've included the obstructions in the computer model.
> >
> > While the simulated  global vertical illuminance levels on the facade
> > are within +/-10% of those measured on site under diffuse light
> > conditions (much of the light reaching the facade is diffuse due to
> > heavy exterior obstructions), I am getting a high error when for
> > simulated vs actual illuminance under direct sun. Under direct sun,
> > illuminance levels are underpredicted by 30% in the computer model.
> >
> > The site is Berkeley, CA, and I used the Oakland weather file, so the
> > location should be very close in terms of latitude/longitude and I
> > double-checked the facade orientation (8 deg east of South). I
> > excluded all of the exterior obstructions and compared to illuminance
> > levels one might expect  under direct sun for that orientation using
> > daylight availability tables for San Francisco and the levels once
> > again seem low.
> >
> > Here is a bit more info on the model:
> > CIE clear sky
> > -ab 5 -ad 1024 -as 256 -ar 1064 -aa 0.1
> > Max. dimension of the site/model modeled is 4,200 ft
> >
> > Does anyone have any idea why illuminance levels under direct sun
> > might be underpredicted?
> >
> > Thank you,
> >
> > Krystyna
>
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