Transmittance and reflectance dependent on incidence angle

Thanks for your reply Chris, I have tried to alter my specular treshold
(-st) to 0.05, 0.01 or even 0.
However I still get that with an incidence angle higher than 45 degress no
reflectance occur, and my transmittance increases! This was not what I would
have expected. I would have expected an increase in reflectance and a
decrease in transmittance with higher incidence angle.
My question is therefore, am I using a wrong approach to test the incidence
angle-dependency on transmittance and reflectance of my "sample"?

My approach has been to vary the rotation of the sample from 'seq 10 10 80'
plus 87 around the x-axis, and do the rtrace from "above" to achieve the
reflectance and from "below" to achieve the transmittance:
oconv Materials.mat Glow.sky lamellaGlow.rad > glow.oct

echo "reflectance:"
echo '1.75 0.1 4 0 0 -1' | rtrace -h -w -dt 0.05 -st 0.01 glow.oct
echo "transmittance:"
echo '1.75 0.1 -4 0 0 1' | rtrace -h -w -dt 0.05 -st 0.01 glow.oct

The output is attached to this mail. My trans description is given below--
(you have seen it here on the list before ;o))
# a1, a2 og a3 = 1, due to the color of the glass is included in the
transmittance end reflectance given by the manufacturer
# a4 = 0.31
# a5 = 0
# a6 = trans, determined from t_s=a6*a7(1-a4), where t_s is 0.65 and a7=1
# a7 = 1

void trans lamel_trans
0 0 7 1 1 1 0.31 0 0.942028986 1

And the glow for the sky is:

skyfunc glow sky_mat
0 0 4 1 1 1 0
sky_mat source sky
0 0 4 0 0 1 180

Thanks!
Anne

Message: 5

GlowOutput.txt (1.36 KB)

···

Date: Thu, 21 May 2009 09:56:56 -0400
From: "Christopher Rush" <[email protected]>
mirror vs.BRTDfunc
<[email protected]>
Message-ID:
Content-Type: text/plain; charset="us-ascii"

You may need to decrease the 'specular threshhold' setting. The default
is 0.15, which crudely means that any specular reflection less than 15%
is not accounted for. Depending on how precise you want to go you could
go down to -dt .05 or less, or even to 0, but that may bog down the
calculation.

________________________________

Sent: Thursday, May 21, 2009 5:01 AM

... I have tried to test my "sample" with a glow sky where I rotate
the sample around the x axis. However when I am above a tilt of 45
degrees I get a reflectance of 0... ( My sample have the dimensions 3.5
x 3.5 in x and y and my rtrace point is placed close to the x axis
(0.2))
...
Anne

Dumb question, but are you sure your rays are passing through the window at all angles? Is it possible that when the angle is greater than 45, the test ray misses the sample?

-Greg

···

From: Anne Iversen <[email protected]>
Date: May 23, 2009 5:26:11 AM PDT

Thanks for your reply Chris, I have tried to alter my specular treshold (-st) to 0.05, 0.01 or even 0.
However I still get that with an incidence angle higher than 45 degress no reflectance occur, and my transmittance increases! This was not what I would have expected. I would have expected an increase in reflectance and a decrease in transmittance with higher incidence angle.
My question is therefore, am I using a wrong approach to test the incidence angle-dependency on transmittance and reflectance of my "sample"?

My approach has been to vary the rotation of the sample from 'seq 10 10 80' plus 87 around the x-axis, and do the rtrace from "above" to achieve the reflectance and from "below" to achieve the transmittance:
oconv Materials.mat Glow.sky lamellaGlow.rad > glow.oct

echo "reflectance:"
echo '1.75 0.1 4 0 0 -1' | rtrace -h -w -dt 0.05 -st 0.01 glow.oct
echo "transmittance:"
echo '1.75 0.1 -4 0 0 1' | rtrace -h -w -dt 0.05 -st 0.01 glow.oct
...