[Radiance-general] Ximage illuminance readings

[Greg Ward]

Hi Steve,

A direct/indirect fixture needs an interreflection calculation for  
any level of accuracy.  I recommend setting -ab 1 on the rtrace  
command line, maybe even -ab 2.  This will take considerably longer  
to produce a result, but should be closer to what you expect if there  
is nothing amiss with your model.  The command you are using  
otherwise looks OK.

I'm a little surprised by the factor of 10, unless the fixture throws  
over 90% of its light towards the ceiling.

-Greg

> From: steve michel <smichel_designer at hotmail.com>
> Date: March 10, 2008 6:56:26 PM PDT
>
> Greg,
>
> Thanks for confirming the conversion factor..that was a the gap in  
> my knowledge of radiance.  Now,  without getting into into sordid  
> details, the scene pic in question has a skylight and some T5  
> direct/indirect lighting. From the rendered night and day pics all  
> seems well with ximage's human exposure setting corroborating with  
> what  I would expect. The daytime pic shows strong illumination on  
> the  work plane but yields a 'lux' value lower than I expected (250  
> to 300). I did not use a secondary lightsource for the skylight.   
> With a night, with only the T5 luminaires (ies2rad in meters),  
> which, from manufacturer's data and experience, should give a 300  
> to 500lux illuminance, gives me a reading approx. 40lux(?).
>
>
> I tried rtrace (courtesy of man pages) with ximage -t command on  
> the daytime pic
> $ ximage office-i.pic | rtrace  -h  -x  1  -i office2xx.oct | rcalc  
> -e '$1=47.4*$1+120*$2+11.6*$3'
> 18.3891712
> 21.1014687
> 20.6663913
> ???
>
> I like ximage's interactive features, but something may be missing  
> in that pipe  to rcalc.
>
> Im using radiance to better (re)present my interior designs but
> feel out place here using scientific instruments for my art supplies
>
>
> thnks always
> Steve