[Radiance-general] Comparison of values measured by sensors and calculated with Radiance

[Greg Ward]

Hi Sung,

Try adding the following parameters to your rtrace command:

	-dj .7 -dt 0

How sure are you of your surface reflectances?  Did you actually  
measure the lamp output, or are you assuming the manufacturer's lumen  
specification is correct?  How far off are your values?

Since you notice the most errors near to the lamp and at low angles,  
it's possible that the Fresnel effects are important.  The emission  
of the fluorescent phosphor may be considered diffuse, but the actual  
output of the lamp will be a function of angle due to Fresnel  
reflection of the glass bulb.  You can account for this  
(approximately) by applying the "winxmit.cal" function included with  
Radiance, like so:

void brightfunc Fresnel_effect
2 winxmit winxmit.cal
0
0

Fresnel_effect light lamp_light
0
0
3 X X X

lamp_light cylinder fluorescent_tube
(etc...)

I think you get the idea.

-Greg

> From: Minki Sung <mkii at iis.u-tokyo.ac.jp>
> Date: April 1, 2009 11:27:29 AM PDT
>
> Dear all,
>
> A bared cylinder lamp (with 60cm length and 3.2cm diameter) is  
> installed on the wall and illuminance values were measured at some  
> points and to some directions as shown in the linked image.  
> Unfortunately, the illuminance values calculated with Radiance are  
> different from measured values, especially when the sensor is near  
> the lamp or  the incident angles are high (K~O).
>
> Link image
>
> What can be supposed as the cause of the discrepancy?
>
> rtrace command was as follow.
> cat pts.pts | rtrace -I -oov -ar 128 -ad 512 -as 256 -ab 2 -ds 0.02  
> test.oct > test.dat
>
> and all the walls, floor and ceiling was covered with non-glossy  
> black paper. (reflectance 0.05)
>
> Sung.