[Radiance-general] Illuminance and coloured lights

[Rob Guglielmetti]

loscotec at libero.it wrote:
> Mmm.. I see.. Thank you very much.. but...
>
> what actually is depreciation? 
>   

We're talking about lamp lumen depreciation (LLD) here, i.e. the gradual 
decrease in lamp output over the life of the lamp--which can be 
significant in certain lamps.  This is the reason why you will see two 
columns for lumen output in any respectable lamp manufacturer's catalog, 
for sources where this is an issue. Often called "initial" and "design" 
lumens, the design lumens are adjusted for a particular lamp's lumen 
depreciation over its rated life.  Design lumens will always be lower 
than initial, unless if you're looking at a first edition Philips lamp 
catalog from 2005 (or 2004, can't remember) which, very embarrassingly, 
inverted the labels on the two values!  (I still have my copy somewhere, 
but I doubt it will become a collector's item (except among a bunch of 
lighting geeks.))

Anyway, back to LLDs... The LLD is but one of the elements that comprise 
the overall light loss factor (LLF) that is commonly used in lighting 
calculations to account for these and other losses inherent to a 
lighting design (room surface dirt, ballast factors, etc).  In Radiance, 
this is accounted for with the multiplier (-m) in ies2rad (though you 
can also use it to simulate higher or lower wattage lamps, or dimming, etc).

***Which brings up a good point: if you are using colored lights in 
Radiance by referencing lamp.tab it's not a bad idea to check what 
lamp.tab thinks the LLD is and make sure it jives with your actual lamp, 
AND make sure that factor is REMOVED from the LLF that you are 
(hopefully) using in your multiplier.***

> Should I run the command "ies2rad -m 1.1764706 -t fluorescent lamp.ies" to have right illumination ? If this is right, is there a reason why ies2rad does not do it automatically?
>   
Hopefully the above paragraphs shed light on the answers to these 
questions, which are "no", and "yes", respectively.  =8-)

> If it isn't right, are Calculux and Dialux using badly lights ? 15 % illuminance less is really something.. !!
>   

See above...

> And, there is some documentation about all this stuff ? It would be an interesting chapter for my thesys I guess..
>
>   
The ies2rad man page does explain the lamp color and the lamp.tab 
lookups.  There is more info in lamp.tab itself.  General info about 
light loss factors can be found in any decent lighting design 
calculation text. 

Hope this helps!

Rob Guglielmetti
www.rumblestrip.org