DF verification study

radiance-general
1

Best all,
I am new to Radiance but I am using it as an executable from C++ in my thesis to evaluate rooms using the Daylight Factor (DF). I am trying to verify the DF however I haven’t succeeded.

In most cases, my ADF is +/- 1,3x too high.
I am not sure why my values vary so much.
If anybody knows what parameters or settings I should check, all the help is welcome.
Below I will present my results and process.
PS: I already tried more “accurate” rtrace settings, such as: -I -h -ab 5 -ad 2048 -as 512 -aa 0.1 -ar 256 -oov. However, this resulted in my computer running for almost 5 hours, instead of 20 seconds. The results remain comparable (0.05 difference of ADF).

Model setup:
First, I am creating materials with the corresponding reflectance values.
I am using polygons to create the 3D scene: scene.rad
my Gensky settings are:
. 3 21 12:00CET -a 52.2 -o -4.5 -c -B 55.865 -g 0.2 > sky.rad
. (55.865 = 10000/179)
with oconv I compile the scene:
. oconv sky.rad scene.rad > room.oct
my Rtrace settings are:
. -I -h -ab 4 -ad 256 -as 32 -aa 0.9 -ar 256 -oov < gridfile > results.txt
I calculate the illuminance values then with rcalc:
. -e ‘$1=179*(.265*$4+.670*$5+.065*$6)’ results.txt > lux.txt
. (resulting in Epoint, per point specified in the gridfile)
Then I calculate the DF: (Epoint/Eout)*100% = (Epoint/10000)*100%
Then I calculate the average of all points to derive the ADF.

I modelled the following rooms (vD) of: Compare effect of window placement
Room x: room name: ADF study : ADF my result : percentage difference
Room1: A4 : 7,2 : 11,69 : 162%
Room 2: A3 : 2,2 : 2,62 : 119%
Room 3: C2 : 2,9 : 4,4 : 152%
Room 4: B3 : 0,7 ; 0,54 : 77%

Furthermore, I am also comparing rooms from this paper (VK):
Salah Vaisi, Farzam Kharvari,
Evaluation of Daylight regulations in buildings using daylight factor analysis method by radiance
Room a = block 2, horizontal window
WFR: ADF study : ADF my result : percentage difference
WFR 12: 2,47 : 3,75 : 152%
WFR 15: 3,43 : 4,62 : 135%
WFR 18: 3,85 : 5,47 : 142%
WFR 24: 5,03 : 7,12 : 142%

And I also modelled a simple room in Velux Daylight Visualiser:
This allowed me to control more settings. Still with no correct result.
5x5x3 room, with window of 3x2 centrally placed in the wall.
ADF study : ADF my result : percentage difference
3,86 : 5,22 : 135%

2

Hi @quimara,

Welcome to the Radiance discourse forum. I’m quite rusty with daylight factor (we don’t use it anymore where I’m from), but I understand that it is defined based on an idealized unform sky (which doesn’t exist in reality). I don’t think your gensky settings using -c will achieve a uniform sky. Instead, you should use gensky with the -u setting or model your sky as a source without using gensky at all.

3

Hi all,

for DF calculations, the -c option is correct. It gives you an idealized overcast sky according to CIE, which is the used sky definition for daylight factor simulations.

Best,
David

4

My mistake, @David_Geisler-Morod1, thanks for the correction.

@quimara, I would also recommend that you check the number of ambient bounces in your simulations compared to the references you chose. The ab setting can have a significant effect on measured indoor illuminance as you move from 1 bounce to whatever limit is established by your -lr parameter. My personal recommendation is to use -ab 6 with rtrace and -ab 5 with rpict, and you’ll get lower illuminance readings with smaller -ab values. It looks like you tried increasing -ad already, which is also a good idea, but the effect of that won’t be so noticeable because you are taking an average of many points.

5

If your grid points are facing upwards on the work plane, and you have side lighting in your space, then results accuracy will depend heavily on the -ad and -as settings as a significant portion of the light comes from low angles through the window. As you are averaging together the results, you might get better accuracy with a strict MC path tracing, so try the following settings in rtrace:

-aa 0 -ab 6 -ad 10240 -as 2560 -lw 1e-6 -n 4

You can adjust the -n option to the number of processor cores available and your calculation should be reasonably fast. If there is still a large systematic error, like you are seeing in your VK paper comparisons, then I would check the surface reflectances, exterior description, and geometry very closely to see that it matches the publication. (Also, the -as option sometimes causes bias, so you can try -as 0 and see if that helps.)

By the way, which version of Radiance are you using. I.e., what does “rtrace -version” say?

Best,
-Greg

P.S. The only gensky settings that matter in DF calculations are -c and -B. The rest you can leave off, since they only set the zenith brightness in the absence of a -B or -b option when you have a sunless sky.