[Radiance-general] Infrared scene simulation

[Greg Ward]

Hi Alex,

Unless you are concerned about how the surfaces look to a human  
observer, there's little sense in modeling a 300K blackbody as a  
color.  You are better off treating total radiance, using identical  
RGB intensities and reflectances throughout, and just considering the  
energy transfer.

The main problem with Radiance as an IR engine is that it doesn't  
consider the ambient temperature and radiation of all surfaces --  
only light sources and reflections thereof.  Some people have fudged  
this by considering a surfaces absorption and re-radiation as a kind  
of diffuse reflection, but this is a poor approximation for 300K,  
which is just above room temperature.  It only works when there is  
one high-temperature surface acting as a heat source.

Bottom line:  I don't think Radiance is well-suited to your problem.

Best,
-Greg

P.S.  The cct_x(t) and cct_y(t) functions in blackbody.cal are also  
not reliable for such low temperatures -- it's meant for visible  
blackbodies.  300K is in the far infrared.

> From: Alexander Utz <AlexanderUtz at web.de>
> Date: May 28, 2009 3:52:14 AM PDT
>
> Hi All,
>
> I'm currently trying to find a way to simulate the radiant power  
> distribution for a given scene in the far infrared (about 8-14um  
> wavelength). Using the search option I saw that there already was a  
> discussion on this subject, but I can only find fragments of it -  
> so please forgive me if I ask questions already answered ;).
>
> So my question is: How can I model a blackbody source at about 300° 
> K as radiation source?
>
> I tried using blackbody.cal and and xyz_rgb.cal as I saw in another  
> posting:
>
> echo 300 100 | rcalc -f blackbody.cal -f xyz_rgb.cal -e 'xi=cct_x 
> ($1);yi=cct_y($1)' -Yi=$2;Xi=Yi*yi/xi;Zi=Yi*(1-xi-yi)/yi' -e '$1=R  
> ( Xi,Yi,Zi ) ; $2=G ( Xi,Yi,Zi ) ; $3=B ( Xi,Yi,Zi ) '
>
> and it results in
>
> 18.3632205      143.92786       0
>
> So I modeled the blackbody as polygon light source with this RGB  
> values and it's real physical extent.
>
> First of all I must admit I don't understand the YL lightning  
> parameter (100 in this case). Shouldn't this already be clear by  
> giving the temperature of the blackbody?
>
> And are these values correct for the infrared radiation the  
> blackbody emits at such low temperatures? Or do I still have to  
> define the spectral range I'm interested in somehow?
>
> It would be great if someone has already experience with thie and  
> could give me some tips.
>
> Thenks in advance and best regards
>
> Alex