Irradiance from illuminance for scene with IES - Compared to real-world measurements

radiance-general
1

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

2

Hi Rafael,

I don’t see anything obviously wrong in what you’ve done. Is it possible to post your model and the IES file you are using, somewhere? You can also e-mail them to me directly if that’s easier.

Cheers,
-Greg

3

Hi Rafael,

Any follow-up to this? Last we spoke, it seemed like there was something amiss with the IES file describing the individual LED sources. Was this the problem, or was it something else?

Cheers,
-Greg

4

Hello Greg,
happy new year and sorry for the late reply!

Yes, the IES file was the actual problem, paired with the inaccuracy of our Pyranometer in use. So it resulted that the IES described the power of 7 LED spots. When I took measured photometry values as “correct”, rather than irradiance, results became very close between Radiance and the values measured from a quite accurate lux-meter.

Thank you very much for following up, best,
Rafael