[Radiance-general] getting irradiance from source defined in photometric units

[Guglielmetti, Robert]

Hi Lars,

lamp.tab provides information to adjust the lamp chromaticity and apply a lamp lumen depreciation factor based on the source type. Up until now, I always thought using -t white for all sources in the scene was safer (OK, easier) because I like to roll in my lamp lumen depreciation factor with all the other things that go into a total light loss factor (LLF). I consider lamp lumen depreciation, luminaire dirt depreciation and room surface dirt depreciation, maintenance factor, and the product of all those is the LLF, which I apply all at once with the -m option of ies2rad. This was my main rationale for using -t white for all sources, but also the distinct coloration of the different sources was a problem. I forgot that you could white balance the image after the fact with pfilt, though, which leads me to my next rabbit hole:

Given that one can do a final white balancing after the fact, it does seem that we should be striving for maximum accuracy with our electric light source descriptions and specifying the lamp type/color. Just playing with the lampcolor script, I get some different luminances by changing the lamp type. 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. Since lampcolor uses the same lamp.tab data file as ies2rad does, we can expect similar variation in light sources created using ies2rad and using different lamp types. Seems like a potential for error to me, but then again obtaining accurate CIExy data and lumen maintenance values for all the new lamps coming out seems a bit daunting too. Is this splitting hairs? I'm curious what others in the community are doing in this regard.

--
Robert Guglielmetti  IES, LEED AP
Building Energy Efficiency Engineer
National Renewable Energy Laboratory (NREL)
1617 Cole Blvd, MS-5202
Golden, CO 80401
robert.guglielmetti at nrel.gov
303.275.4319

________________________________
From: "Lars O. Grobe" <grobe at gmx.net>
Reply-To: Radiance general discussion <radiance-general at radiance-online.org>
Date: Thu, 8 Oct 2009 05:52:00 -0600
To: Radiance general discussion <radiance-general at radiance-online.org>
Subject: Re: [Radiance-general] getting irradiance from source defined in photometric units

Jack de Valpine wrote:
> No problem! I just wanted to make sure that I had not missed some
> critical change somewhere along the way....
I hope not ;-) Still, I am not really sure whether I am doing right...
Does anyone know whether the lamp,tab-lookup done by ies2rad also
accounts for the luminous efficacy of a source? This would be required
to get correct radiance values for rgb from the candela-distribution of
a source. 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...

Cheers, Lars.

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