# Re Re: trans or glass material, rectangular, luminous opening problem

Hi Greg, hi Rob,

thanks so far with the -ds and -dj options. Yes Rob i meant the appearance of the illuminance patch.
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?

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)

Christian

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
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