[Radiance-general] Again transmissivity for color filter / glass

[Gregory J. Ward]

Hi Christian,

I'm afraid you lost me, here.

> Forget my other question, at first i want to make sure that i model  
> a simple filter in the right way.
> And that is why i want to stick to the values LEE gives for a 3200K  
> lamp.
> These are: X= 58.8 , Y=62.4 , Z=37.2

OK, I see these values on the URL you sent earlier <http:// 
www.leefilters.com/LPFD.asp?PageID=193>.

> with xyz_srgb.cal i get: R= 76.088002   G=61.6146944   B=  
> 29.8622268, ( i want to calculate this with the D65 white point)

Since the XYZ values were measured using a 3200 K tungsten  
illuminant, converting to sRGB only gives you back the same orange  
color.  You haven't really changed illuminants, because xyz_srgb.cal  
doesn't do a white-point conversion.

> As the filter won't have a bigger transmission than 1, i divide  
> through 100.
> Now i get values of: R= .761   G= .616   B= .299;
> If i apply these to glass polygon i get a yellow/orange filter with  
> my tungsten 3200 K source in the scene.

Does the scene your simulating use a tungsten source or not?  If it  
does, you should model it as a pure white light, because you have  
incorporated the illuminant color into your filter.  (This isn't a  
good idea for transmission as the angular function will be incorrect.)

> Then is subtract 1 from RGB =  .239 , .384,  .701;

This where you lost me completely.  What made you think to subtract  
each value from 1?  It doesn't work that way, even if it gets you  
what you were expecting.

> These values should now represent my blue filter, and it looks  
> quite good!

If you want blue out of your filter from a tungsten source, you won't  
get it.  You'll get something bluer than the source, but not truly  
blue.  This all gets involved in the white balance issue, which I  
cover in depth in the paper I keep mentioning:

	http://www.anyhere.com/gward/papers/egwr02/

I know it's a difficult read, but until you understand white  
balancing, colors are going to look consistently wrong to your eyes.

-Greg