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

[Greg Ward]

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