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<TITLE>Re: [Radiance-general] material trans and glass?</TITLE>
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<FONT SIZE="4"><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>Hi Victor,<BR>
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Transmisivity is the amount of light NOT absorbed in one pass through the glazing. Glass is a special case of dielectric optimized for glass that accounts for angular dependence of transmission and inter-reflection between front and back surfaces (but is only modeled as a single surface in radiance). The reason for using transmissivity I believe is related to the desire to maintain a valid input range of 0-1 for the parameters instead of something like 0 - 0.92.<BR>
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You can find the equation to get transmisivity given transmittance at the bottom of this page:<BR>
<a href="http://www.schorsch.com/rayfront/manual/dielectricdef.html">http://www.schorsch.com/rayfront/manual/dielectricdef.html</a> <BR>
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A glazing with reported VLT of 0.325 has a transmisivity of 0.354 so the material definition would be as follows:<BR>
void glass glazing.325<BR>
0<BR>
0<BR>
3 0.354 0.354 0.354<BR>
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And your other question - The first three parameters of trans is color which affect transmission and reflection. Trans does not include inter-reflection or angular transmission properties exhibited by glass so the input values are transmission.<BR>
Also on this page there is a nice diagram describing how trans works:<BR>
<a href="http://www.schorsch.com/rayfront/manual/transdef.html">http://www.schorsch.com/rayfront/manual/transdef.html</a><BR>
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Andy<BR>
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On 12/10/09 7:22 PM, "Victor Li" <victorpermanent@gmail.com> wrote:<BR>
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</SPAN></FONT></FONT><BLOCKQUOTE><FONT SIZE="4"><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>Sorry i made a mistake. I just want to compare the glass 3 0.325 0.325 0.325 and trans 7 0.325 0.325 0.325 0 0 1 1 (not 0.3). <BR>
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In trans, the transmissive specularity is 1 which means the total daylight transmitted through the material will be specularly transmitted, like glass. So i defined the transmissive specularity is 1. <BR>
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Actually i had a look the webpage and i define the trans followed the instruction in the page. <BR>
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Note also that the Radiance glass material uses "transmissivity" not transmittance, <BR>
so does trans use "transmittance" in the first three paremeters? What is the difference between transmittance and transmissivity?<BR>
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