[Radiance-general] Ximage illuminance readings

[steve michel]

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


----------------------------------------
> From: gward at lmi.net
> Subject: Re: [Radiance-general] Ximage illuminance readings
> Date: Mon, 10 Mar 2008 14:04:06 -0700
> To: radiance-general at radiance-online.org
> 
> Dear Steve,
> 
> The 'l' command in ximage applies the 179 lumens/watt conversion  
> factor, so if the input is a picture created by rpict with the -i  
> option, you should be seeing lux values on your opaque surfaces.  If  
> you are off by a factor of ten, then I would suspect the input values  
> or the calculation are at fault.
> 
> Best,
> -Greg
> 
>> From: steve michel 
>> Date: March 10, 2008 10:48:33 AM PDT
>>
>> I know this is supposed to be fairly basic but needs clarification:  
>> I want to get lux readings from my pic files
>> When generating *.pic files (with -i option for irradiance) ,  
>> viewing them with ximage and
>> selecting a point with the ximage 'l' command gives me values that  
>> are clearly NOT lux values (values appear to be 1/10th of my  
>> expected lux measurement).
>> Correct me if I'm wrong, but isn't the value in W/m2 at an efficacy  
>> of 179lm/watt? For daylight only or all irradiances? If so, what  
>> conversion to get lux values?
>>
>>
>> regards
>> Steve
> 
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