[Radiance-general] getting irradiance from source defined inphotometric units

[Guglielmetti, Robert]

Thanks Chris,

You are right-on, the reductions in luminance I was seeing were directly attributable to the lumen depreciation factors used for each light source. The color shifts are not having a net impact on the photometric values. I looked at five different light sources with lampcolor, converted the rgb radiance to luminance values, and the differences all correlated to the lamp lumen depreciation being applied. Phew.

I will continue using white as a default light source color (and remind myself once again that that lumen depreciation is being applied if I do stray from white!).

- Rob

________________________________
From: Christopher Rush <Christopher.Rush at arup.com>
Reply-To: Radiance general discussion <radiance-general at radiance-online.org>
Date: Thu, 8 Oct 2009 12:33:32 -0600
To: Radiance general discussion <radiance-general at radiance-online.org>
Subject: RE: [Radiance-general] getting irradiance from source defined inphotometric units

Someone please speak up if my understanding is flawed, but ...


> I would e.g. assume an efficacy of 100lm/W for my source, and if
ies2rad
> would use 179 instead, the radiance values in my simulation would be
by
> far too low...

If you're using IES files via ies2rad, the IES files are measured in
candela, and the physically measured values already account for
efficiency of the source from electrical power in - through to the
lighting power out. The factor of 179 is just a conversion from W/m2 to
lumens/m2, as a unit conversion, not an efficiency factor. The source
efficiency is already incorporated into the physical measurements
documented in the IES file.


> I used white, incandescent, and fluorescent, and spread 200 lumens
over
> a 1 meter polygon, and ended up with 63.7, 60.5, and 54.1 cd/m^2
> respectively.

I had understood the factors lamp.tab are mainly intended to account for
lumen depreciation, and just happen to have some color offset included
as part of this. In Rob's tests the decreasing values roughly correspond
to sources with more lumens lost over their life. I think if you test
again with metal halide or mercury you'll find those a little lower
still.

If accurate results are the goal, I would typically use the approach of
specifying everything as white, with my own -m multiplier calculated
from lamp manufacturer maintained lumen data.

> obtaining accurate CIExy data and lumen maintenance values for all the
new
> lamps coming out seems a bit daunting too.

Regarding accuracy of factors in lamp.tab, the values for metal halide
and high pressure sodium seem higher than real world lumen depreciation,
and halogen is defined with a factor of 1, but should still have some
depreciation over time. Considering that these factors vary so much,
particularly between different types of metal halide, this could be
difficult to keep accurate as part of the distribution.

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