Hello everyone I’m new to the forum, but have been a reading member for a long time,
I’m currently trying to derive irradiance from illuminance values for a scene, lit by an IES light source.
I do have the SPD and the IES data of the LED spot lights.
I do also have several measurement points of a real scene, where I took measurements with a Pyranometer and a Lux-meter. This scene was later modelled in Radiance accurately. I’m trying to get close to those measured values through Radiance rtrace simulations with the -I option on. The command I used for the simulations is:
rtrace -I -faa -aa 0.1 -ab 2 -ad 1024 -dc 0.75 -st 0.15 -lw 0.005 -as 1024 -ar "128
" -lr 8 -dt 0.15 -dr 4 -ds 0.05 -dp 2000
For comparing I ran rtrace simulations for a single point. At the real scene, I have measurement values for the exact same point poisition in lux and W/m2.
Results from RTRACE:
1588.8 1588.8 1588.8 (W/m2)
and
1588.8 * 179 = 284395.2 lux
Compared to measured results (pyranometer/lux-meter):
78 W/m2
and
26490 lux
(1)
As it can be seen the correlation of lm to W in this setup is 339.6 (far from 179).
So for scaling irradiance correctly in my setup, I would simply take the illuminance and devide it by the 339.6, for obtaining irradiance, as I suppose that’s the correct correlation for this specific LED light source. Does this make sense?
(2)
As it is can be seen from the results given above, there is a bias, which corresponds almost exactly to the ratio of m2 to sqft. It’s too exact to be coincidence (10.73 vs 10.76). But I already double and triple-checked everything, to be sure not to use feet anywhere in my workflow. Since the photometric data in the IES is given in candela, it’s not even sensitive to m/ft. Also, the IES is set to “2”, corresponding to m.
extract from the ies file:
TILT=NONE
1 700 1 91 37 1 2 -0.048 0.0 0.00
1.0 1 1
For IES2RAD the following command was used:
ies2rad -dm -o cNAME -p IES_PATH -m 1.0
Did anyone already experience such a (conversion-)issue? Can anyone give me feedback if it could really be about sqft/m2?
Sorry for the long message, thank you already in advance for your input!
Best, Rafael