[Radiance-general] Determining average reflectance from RGB values

[Mark Steven Baker]

I'm a new radiance user and I'm try to use the software to 
correlate results from measures taken using a detailed architectural model. 
The goal is to try a number of variations using radiance
that would be prohibitive revising the model.

The reflectance values of the various surfaces in the model were
painted a neutral color (shades of gray) and the average reflectance was 
measured. I don't have access to equipment that would measure the
reflectances at different wavelengths (separate RGB reflectance 
values).

In the radiance models, the goal is to provide some color
other than gray for these surfaces but with the same average
reflectances. Is there an obvious way to correlate RGB values in 
a radiance material description with the average relectance for a
surface?

I am assuming that the following wall material would have an
average reflectance of 0.75:

# adobe texture function
void texfunc ad
6 cdx cdy cdz  adobe.cal -s .1
0
1 .015

# Sample stucco wall of adobe material, reflectance = 75%
ad plastic stucco_75
0
0
5  .75 .75 .75 0 0

Can you average the RGB values for a material to get
the average reflectance (as in the plaster_white) material below 
or do you need to use some more complex calculation? Does the
adobe calculation function significantly affect the average
reflectance value?

--- excerpt from Building.mat file ----

# adobe texture function
void texfunc ad
6 cdx cdy cdz  adobe.cal -s .1
0
1 .015

# Sample wall of adobe material, reflectance = 75%
ad plastic plaster_white
0
0
5  .80 .75 .70 0 0

-------- excerpt ends -----------

Also, how would this apply to the average reflectance values for 
carpet that are generated using the radiance carpet.cal function.
For example, how would the average reflectance value be calculated
for the sample carpet below (one of the book samples):

------ material from "Rendering with Radiance" book sample ----

# carpet pattern function
void colorfunc carp
6 carpr carpg carpb carpet.cal -s .0175
0
4 .7 .825  .9  1

carp plastic carpet_blue
0
0
5 .9 .9 .9 0 0

------  sample ends ---------------

Any help understanding the conversion of RGB values to 
average reflectance values for materials and texture (calculation)
functions would be most appreciated.

Thanks,
-Steve

Steven Baker
msbaker@cs.uoregon.edu