Hello,

I am having some problems in calculating illuminance levels for my model. My
model was built and imported into Ecotect and then exported to Radiance for
lighting analysis. The Radiance-calculated value for luminance (cdm-2) are
similar to what I measured on-site. However, once I exported my model to
Radiance for illuminance calculations (lux), the lux values were ten times
higher than what I measured from the site! Can anyone advise what might go
wrong with my simulation? The simulation was conducted for a night time
scenario and with artificial lighting only. Does Radiance take into account
the luminous efficacy of the lamps? I read other threads too and it seems
that the default luminous efficacy value is set to 179 and that is for
daylit condition. However, a general efficacy value for, say, incandescent
lamp is 14. I do not know where to set the luminous efficacy in
Ecotect/Radiance and I am not sure if this is the reason why the computed
lux values are approximately ten times higher than in the reality?

Many Thanks,

vil

Vil,
your is a very wide and general question.
So I think the best advice is to look for tutorials over the internet and start from the basics.
WIth the command line.

The 179 number is not intuitive, but has nothing to do with practical things.
For historic reasons (fee free to dig in the archive for an answer) we all divide the photometrical measures by 179 and use the converted radiometric values in Radiance.
Then we get the radiometric results from Radiance and multiply them again by 179 to retrieve the photometrical equivalent;
So we all divide the input by 179 and multiply the output by 179 in a simulation that is linear.
We could do without multiplying…. and would be the same
If you know what you are doing, then you can skip the above, but as we all collaborate with others it is not advisable… because your colleagues could be assuming that you are using the 179 conversion anyway…
So we all do it.

If you are confused because there is a luminous efficacy of lamps and then the Radiance 179 value, be reassured:
There is anything to do between each other.

To get the luminance of a surface, knowing the geometry and the flux you should use the command line:
lampcolor
This will spit out the radiometrical quantities that you need to use in the light/glow/illum primitives

To convert from a ies file to a radiance luminaire file you should use:
This will do all for you, all conversion, but beware that unless you force the lamp to be 'white' you could be using a lumen depreciation from the radiance library (as opposed to a lumen depreciation that you would like to use based on a particular design and cleaning/relamping regime).

in all cases you should check what has come out of the commands.
To do this you should familiarise with the rcalc and rtrace commands.
rcalc
rtrace
You should for example measure the illuminance around the luminaire to estimate the luminous intensity and check that the light fixture is pointing in the right way.
Everybody has his own script/method to do this.
When I started I found easy to create a big sphere around the fixture, and render two fisheye views of illuminance. if you divide by the square radius you get the intensity plot...

About your issue, the 10 coefficient is a bit strange. Could it be a setting like lux vs Fc? Maybe this is something in Ecotect?

Good luck,
G

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On 12 Jan 2012, at 12:50, Villian Wing-Lam LO wrote:

I am having some problems in calculating illuminance levels for my model. My model was built and imported into Ecotect and then exported to Radiance for lighting analysis. The Radiance-calculated value for luminance (cdm-2) are similar to what I measured on-site. However, once I exported my model to Radiance for illuminance calculations (lux), the lux values were ten times higher than what I measured from the site! Can anyone advise what might go wrong with my simulation? The simulation was conducted for a night time scenario and with artificial lighting only. Does Radiance take into account the luminous efficacy of the lamps? I read other threads too and it seems that the default luminous efficacy value is set to 179 and that is for daylit condition. However, a general efficacy value for, say, incandescent lamp is 14. I do not know where to set the luminous efficacy in Ecotect/Radiance and I am not sure if this is the reason why the computed lux values are approximately ten times higher than in the reality?

May seem silly, but a 10x difference seems like the difference between lux and footcandles. Maybe your site measurements are in fc, or maybe you are multiplying your Radiance results to convert to lux when they were already in lux?

If you know your illuminance and lux measurement for one point, and can estimate the surface reflectance, you can check the formula L = E * R / Pi to see if it all makes sense. If L is in cd/m2 then E must be in lux for that formula.

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