[Radiance-general] Re Re: trans or glass material, rectangular, luminous opening problem

[Gregory J. Ward]

Hi Christian,

> Concerning the glass with the values of a Lee color filter i just  
> want to make sure that i got all right:
> I calculated the RGB values from the spectrum, taking in  
> consideration that Y is not =1 but 0.642.
> Then i got the transmissivity which is 0.92.
> I multiplied my RGB values with 0.92 and thats it, or not?

I really don't know where you got the 0.92 value from a normal  
transmittance of 0.642, assuming that's what Y is.  The proper  
procedure is to figure out the transmission from the transmittance  
using ray/src/cal/cal/trans.cal, which is also discussed on p. 240 of  
"Rendering with Radiance."  First, you need to normalize your RGB  
values to match your Y normal transmittance.  For this, you can use  
ray/src/cal/xyz_rgb.cal.  Run the following:

	cd ray/src/cal/cal
	icalc xyz_rgb.cal trans.cal

Let's say you have a measured RGB of 82 53 19, you would define a  
normalization factor like so:

	nf = 0.642 / Y(82,53,19)

Then, compute the actual RGB normal transmittances using:

	82 * nf
	$1=0.900124166

	53 * nf
	$2=0.581787571

	19 * nf
	$3=0.208565356

(The lines with '$' in front are the outputs of icalc.)  Then, set  
these values one by one to the Tn (normal transmittance) variable  
used in trans.cal, and query the tn (normal transmission) value that  
corresponds:

	Tn = $1
	tn
	$4=0.980257446

	Tn = $2
	tn
	$5=0.6342239

	Tn = $3
	tn
	$6=0.227507866

These are the values you actually use in glass:

	void glass LeeGlass
	0
	0
	3 0.980 0.634 0.228

This assumes your glass has an index of refraction of 1.52.  If this  
is wrong, then you should assign a different value of n before you  
compute the tn values, and add the appropriate value to the glass  
primitive.

> Is there some kind of "roughness" for glass too? The glass with the  
> filter on top is too "glossy", reflective, shiny or whatever you  
> would call it. I would want it more "dull". ( I dont know if this  
> is the right term in this context)

I have no clue what a Lee filter looks like.  Is it smokey?  Is it  
rough?  There is no roughness value for glass -- Radiance assumes  
this is a polished surface.  In case of a rough front surface and a  
smooth back surface, you will have to do something special.  In the  
case of scattering in the transmission distribution, you can use the  
trans type and use the winxmit.cal file with a brightfunc to  
approximate the angular transmission.

-Greg