[Radiance-general] Problem with Metal Roughness Parameter

[Lars O. Grobe]

Hi Kevin!

> I'm having a problem with the metal material when it is both highly
> specular and rough. In my room model, I have a vertical array of
> specular louvers at the top of a glazed facade which bounce light onto
> the ceiling.
(...)
> The problem I'm seeing is that as I increase the roughness of the metal
> ceiling from 0 (I've tried 0, .125, .2, and .9) the total amount of
> light in the room increases significantly.

I am just following your textual description as such:

1) direct sunlight gets specularly reflected from louvers to ceiling

2) light gets reflected from ceiling to sensor (and you have a very high 
specular reflectance of .95)

As long as the sun is not perfectly aligned perpendicular with your 
facade, you will probably also have some diffuse reflection over the 
wall, but that should not cause these effects.

Is it that surprising?

In case of zero roughness, all light reflected by the ceiling probably 
hits the back wall. Your sensor should probably only get hit by light 
reflected of the back wall (if there is any), light from the sky and 
maybe scattered light from the louvers.

In case of higher roughnesses, the distribution after the reflection on 
the ceiling gets wider - less light reaches the back wall, more light 
reaches the sensor. Seen from below, the ceiling would appear brighter 
(unless you are at the exact sun position where the sun would get 
reflected at by a mirror-like ceiling).

All this can be visualized by holding a mirror, a scattering mirror and 
a piece of paper in front of a (small) lamp in a dark room. The mirror 
will appear black - unless you find the exact one position where light 
gets reflected straight into your eyes. The scatter mirror will appear 
brighter when seen close to the mirror angle, paper will appear bright 
at any angle.

My guess that this is all the reason for your observation. Am I right?

Cheers, Lars.