[Radiance-general] Guidelines for trans Material

[Rob Guglielmetti]

On Jun 3, 2006, at 8:30 AM, Marcus Jacobs wrote:
> So I think they are actually more complicated that the example that  
> I showed to you before. Out of curiosity, just as many modeling  
> software estimate a complex surface as a mesh of planes or  
> triangles, could the same principle be used for the lighting  
> fixtures themselves. In other words, instead of seeing each fixture  
> as a single complex surface, can it be considered it as just a  
> group of flat planes? One item that I need to have addressed is  
> whether I should model the diffuse shades a single surface or if it  
> acceptable to have inner and outer surfaces (you can think of it as  
> having a thickness to it). See here:
>
> http://i35.photobucket.com/albums/d178/marcdevon/luminaireview.jpg

Hi Marcus,

Without actual photometric data (such as an ies file) to apply to  
your group of flat planes, you have to model the actual comple  
geometry.  But I'm afraid I may have misunderstood your question.  As  
for the single/dual surface question, I guess it depends on how close  
your views will bring you to the shade.  The trans material is  
infinitely thin, and it'd be best to model the thng as a single  
surface.  For closeup work, you may need to introduce thickness but  
that's gonna get a lot more complicated from a valid material  
definition standpoint.


>
>
> The information in Rayfront's user manual and pages 325-326 from  
> RwR were VERY useful to me in gaining insight as to how Radiance  
> treats light. There was a word that was mentioned that has me  
> worried. Measure. Aside from not having any experience with  
> performing a empirical test of light distribution, measuring light  
> sounds very expensive. May I ask, would purchasing the needed  
> equipment require a second mortgage to be taken out or giving up  
> one's first born? Is it required to use transfunc or transdata in  
> lieu of trans to obtain acceptable accuracy?

Mmmm, measurements, yeah.  The gear can be expensive.  Let's see, at  
my old job, where I did these measurements, I used an illuminance  
meter, a luminance meter and a theatrical spotlight (ETC Source Four  
HID, aka "leko", or ERS).  The light fixture was a free sample, but  
you could use other sources too.  The illuminance meter (Minolta  
Somethingorother-nowdiscontinued) was about $800 and the luminance  
meter (Minolta LS-100) was more expensive, I believe almost three  
thousand USD.  Actually, here are a couple of links I just found for  
the meters I used (the illuminance meter I link to here is a newer  
model than I used, but it's basically the same thing).

http://www.tequipment.net/MinoltaTL1.asp
http://www.tequipment.net/MinoltaLS100.asp

It's expensive not to mention time-consuming, but it's also fun and  
is an excellent example of the power of Radiance, in that Radiance is  
happy to accept material data as complex as you are willing to gather.

Maybe Peter can chime in with his thoughts on this stuff, and maybe  
an update on his goniophotometer project?

- Rob