[esp-r] Re: question for ESP-r+Radiance users

[Lars O. Grobe]

Hi Gian Luca!

> I am trying to measure the radiance  at a point in a scene with Radiance

Are you sure that you want to measure the radiance at a point, not the 
irradiance?

> with the command "rtrace", but I always get values "0" if the given
> point is inside a zone. I get  values>  0 only if the point is outside
> the building.

(...)

> Then I issue in bash the following command to test the radiance of a
> sensor positioned at coordinates 8, 5, 1.5 (thus, inside the room)
> looking upwards (0 0 1):
>
>        echo 8 5 1.5 0 0 1 | rtrace  reception_Intern.oct
>
> and I obtain the following result:
>
>        0.000000e+00    0.000000e+00    0.000000e+00
>
> But if I now place the point outside the room, say at position 10, 5,
> 1.5, I obtain:
>
>        9.416492e+00    9.416492e+00    1.147933e+01
>
> Does anybody have an idea of why is this?

A rough idea, yes ;-) Type "rtrace -defaults", and you get the default 
settings of the rtrace command, which will be appiled unless you specify 
others on the command line.

One of these settings is "-ab 0". This means, that only direct light 
from light sources will be considered. No light reflected from other 
surfaces, and also no light from glow-objects. So, if you have a sunny 
sky, and your sensor is located outside, it will receive light from the 
sun with -ab 0, but not from the remaining hemisphere which is typically 
defined as glow. And if it inside, it will receive any light unless it 
can directly see the sun.

I would recommend Axel Jacob's Radiance tutorial as a start:

http://www.jaloxa.eu/resources/radiance/documentation/index.shtml

There are also some strategies described how to optimize calculations 
avoiding too high -ab values, which slow down the process (such as 
mkillum). And there has been a lot of development during the last year 
especially in fenestration, a look at the last workshop's slides at 
radiance-online.org may be useful.

Cheers, Lars.

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