Dear everyone:
in material definition, trans is a special type of glass. i used trans to
define a glass like the following:
void trans window
7 0.325 0.325 0.325 0 0 1 0.3
in glass definition we can do like the following
void glass window
3 0.325 0.325 0.325
i used the different definition to simulae daylight illuminance and i found
the result is different. Would you tell me what the two difference is?
Best Regards!
Hi Victor,
The setting for trans are confusing to say the least (might be worth searching the archives for threads on the topic). The mapping between trans and something like glass is not 1:1. Note also that the Radiance glass material uses "transmissivity" not transmittance, this term can be calculated from transmittance. How did you figure out the parameters that you have for your trans material?
-Jack
--
# Jack de Valpine
# president
#
# visarc incorporated
# http://www.visarc.com
#
# channeling technology for superior design and construction
Victor Li wrote:
Dear everyone:
in material definition, trans is a special type of glass. i used trans to define a glass like the following:
void trans window
7 0.325 0.325 0.325 0 0 1 0.3
in glass definition we can do like the following
void glass window
3 0.325 0.325 0.325
i used the different definition to simulae daylight illuminance and i found the result is different. Would you tell me what the two difference is?
Best Regards!
------------------------------------------------------------------------
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Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general
Hi!
in material definition, trans is a special type of glass. i used
trans to define a glass like the following:
Hm, I never heard anyone calling trans a special case of glass.
glass: special case of dielectric, for thin objects (a 1d-approach)
plastic: special case of trans with no transmission
The definition of a trans material usually requires some calculation, as the range of the parameters is usually not all that intuitive. I would also not expect exactly the same results, as both materials are optimized for different applications. The transmissive specularity of 0.3 you use in your trans material also certainly does not match the perfectly clear glass material.
Especially I do not understand why using trans to be able to get daylight illuminance. It is not really made for this. Is there a special reason that you cannot use the glass modifier?
Cheers,
Lars.
void trans window
7 0.325 0.325 0.325 0 0 1 0.3
void glass window
3 0.325 0.325 0.325
Page 325 of Rendering with Radiance has almost everything you could want to
know about how the parameters work together. The last piece of info you need
is the flowchart here:
....er, here:
http://www.schorsch.com/rayfront/manual/transdef.html
So, that's:
Page 325 of Rendering with Radiance has almost everything you could want to
know about how the parameters work together. The last piece of info you need
is the flowchart here:
http://www.schorsch.com/rayfront/manual/transdef.html
Sorry about that first email sans link.
--
Robert Guglielmetti IES, LEED AP
Building Energy Efficiency Engineer
National Renewable Energy Laboratory (NREL)
1617 Cole Blvd, MS-5202
Golden, CO 80401
[email protected]
303.275.4319
What do you mean by this, Lars? Trans is merely a material that can model
diffuse transmittance, glass cannot. Both materials are used for daylight
apertures. Maybe I don't understand your statement (always a possibility).
On 12/10/09 11:23 AM, "Lars Grobe" <[email protected]> wrote:
Especially I do not understand why using trans to be able to get daylight
illuminance. It is not really made for this. Is there a special reason that
you cannot use the glass modifier?
--
Robert Guglielmetti IES, LEED AP
Building Energy Efficiency Engineer
National Renewable Energy Laboratory (NREL)
1617 Cole Blvd, MS-5202
Golden, CO 80401
[email protected]
303.275.4319
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).
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.
Actually i had a look the webpage and i define the trans followed the
instruction in the page.
Note also that the Radiance glass material uses "transmissivity" not
transmittance,
so does trans use "transmittance" in the first three paremeters? What is the
difference between transmittance and transmissivity?
Hi Victor,
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.
You can find the equation to get transmisivity given transmittance at the
bottom of this page:
http://www.schorsch.com/rayfront/manual/dielectricdef.html
A glazing with reported VLT of 0.325 has a transmisivity of 0.354 so the
material definition would be as follows:
void glass glazing.325
0
0
3 0.354 0.354 0.354
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.
Also on this page there is a nice diagram describing how trans works:
http://www.schorsch.com/rayfront/manual/transdef.html
Andy
On 12/10/09 7:22 PM, "Victor Li" <[email protected]> wrote:
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).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.Actually i had a look the webpage and i define the trans followed the
instruction in the page.Note also that the Radiance glass material uses "transmissivity" not
transmittance,
so does trans use "transmittance" in the first three paremeters? What is the
difference between transmittance and transmissivity?_______________________________________________
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http://www.radiance-online.org/mailman/listinfo/radiance-general
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Hi...
What do you mean by this, Lars? Trans is merely a material that can model
diffuse transmittance, glass cannot. Both materials are used for daylight
apertures. Maybe I don't understand your statement (always a possibility).
The cause of confusion was the typo in the original email. The original aim of the experiment was to compare glass and trans for the same set-up (so the question could arise why results were not the same). This comparision is only possible for clear glass. I tried to express that, as far as I understood, trans is not ment for this case, and I would not expect 100% identical results.
However, it may be interesting to have a comparision of the three modifiers
- glass
- dielectric
- trans
to really quantify the variance for a given scene. Maybe for only direct calculation, and then for some ambient bounces. I will take a look at my copy of RwR to see where to expect differences. However, we do not even know how far off the two results were, so the discussion is a bit lacking of the numbers.
Cheers,
Lars.
Thank you all very much!
Andrew mentioned "Transmisivity is the amount of light NOT absorbed in one
pass through the glazing. "
Does it mean that the Transmisivity is the amount of light transmitted and
reflected by a meterial?
From the web "http://www.schorsch.com/rayfront/manual/dielectricdef.html" , "
*Transmittance* is the total light transmitted through the pane including
multiple reflections"
So the difference between Transmisivity and Transmittance is the light
reflected?
According to Andrew's email, for glass with VLT of 0.325 has a transmisivity
of 0.354. The glass is defined as
void glass glazing.325
0
0
3 0.354 0.354 0.354
For trans with VLT 0.325 and no diffse transmission. Can we difine it as the
following?
void trans window.0.325
7 0.325 0.325 0.325 0 0 1 1
For glass, the light transmitted would be specular transmitted, is it
right? If right, what is the differrence between above meteirals definition
- glass glazing.325 and trans window.0.325?
From the web "http://www.schorsch.com/rayfront/manual/dielectricdef.html" ,
" Transmittance is the total light transmitted through the pane including
multiple reflections"So the difference between Transmisivity and Transmittance is the light
reflected?
I would go with this definition. But the reflections here are those that
happen between two parallel interfaces of a refracting material as the
front and back of a sheet of float glass. Some light is reflected at
the back of the sheet when the light exits the glass. This light will
travel back through the glass to the front and again a small amount
will be reflected towards the back and so on.
The formula to calculate transmissivity from the transmittance of glass
(VLT) takes these inter-reflections into account and so you can model
the two interfaces of a single pane of float glass with one single polygon
in Radiance and you don't have to calculate the reflections during the
ray tracing process.
According to Andrew's email, for glass with VLT of 0.325
has a transmisivity of 0.354. The glass is defined asvoid glass glazing.325
0
0
3 0.354 0.354 0.354For trans with VLT 0.325 and no diffse transmission.
Can we difine it as the following?void trans window.0.325
7 0.325 0.325 0.325 0 0 1 1
The correct definition would be
void trans window.0.325
0
0
7 0.325 0.325 0.325 0 0 1 1
This produces a dark clear material without any specularity that has
a VTL of 0.325 (or nearly as far as I can tell from my tests). But it
is not the same as glass.
For glass, the light transmitted would be specular transmitted,
is it right? If right, what is the differrence between above meteirals
definition - glass glazing.325 and trans window.0.325?
The most obvious one is that "glass" is a specialised "dielectric"
material type which modulates the reflection according to the
angle and refraction index of the material. So if you want to
reflections/refractions of glass with another material you should
start with "dielectric" not "trans"
The "window.0.325" material defined above has not specularity
and no roughness. The glass material has a visible specularity.
Just create a small test scene with a bubble of glass/trans and
a light source (like the sun) and you will see the difference.
If you don't care about the particular refraction of glass and
specular refraction and only focus on VTL you could say the
materials are similar or equal depending on your use.
Regards,
Thomas
On Sun, Dec 13, 2009 at 3:36 AM, Victor Li <[email protected]> wrote:
I would go with this definition. But the reflections here are those that
happen between two parallel interfaces of a refracting material as the
front and back of a sheet of float glass. Some light is reflected at
the back of the sheet when the light exits the glass. This light will
travel back through the glass to the front and again a small amount
will be reflected towards the back and so on.I know what you mean but i am sitill confused the difference between
Transmisivity and Transmittance?
What is the difference between the first three parameters and trans (the
second to last parameter) in trans definition?
The formula to calculate transmissivity from the transmittance of glass
(VLT) takes these inter-reflections into account and so you can model
the two interfaces of a single pane of float glass with one single polygon
in Radiance and you don't have to calculate the reflections during the
ray tracing process.> According to Andrew's email, for glass with VLT of 0.325
> has a transmisivity of 0.354. The glass is defined as
>
> void glass glazing.325
> 0
> 0
> 3 0.354 0.354 0.354
>
> For trans with VLT 0.325 and no diffse transmission.
> Can we difine it as the following?
>
> void trans window.0.325
>
> 7 0.325 0.325 0.325 0 0 1 1The correct definition would be
void trans window.0.325
0
0
7 0.325 0.325 0.325 0 0 1 1This produces a dark clear material without any specularity that has
a VTL of 0.325 (or nearly as far as I can tell from my tests). But it
is not the same as glass.> For glass, the light transmitted would be specular transmitted,
> is it right? If right, what is the differrence between above meteirals
> definition - glass glazing.325 and trans window.0.325?The most obvious one is that "glass" is a specialised "dielectric"
material type which modulates the reflection according to the
angle and refraction index of the material. So if you want to
reflections/refractions of glass with another material you should
start with "dielectric" not "trans"The "window.0.325" material defined above has not specularity
and no roughness. The glass material has a visible specularity.
Just create a small test scene with a bubble of glass/trans and
a light source (like the sun) and you will see the difference.If you don't care about the particular refraction of glass and
specular refraction and only focus on VTL you could say the
materials are similar or equal depending on your use.Actually i run the simulations by two different materials i found the
illuminance in the room is a little different. Why is it different?
For "dielectric" definition, is anyone can tell me where i can find the
detailed definition and the instruction fo corresponding parameters?
Best Regards!
I know what you mean but i am sitill confused the difference between
Transmisivity and Transmittance?
It's just how it is. If you want to define the glass material
correctly you have to convert VTL to transittance via formula.
End of existencial uncertainty.
What is the difference between the first three parameters and trans (the
second to last parameter) in trans definition?
Obviously the material could have any color. You define the
color of the transmitted and reflected light via the first three
parameters. The actual transmittance is the sixth parameter
(second to last). If you'd set this to 0 you would effectively
get an opaque material of the specified color.
In your example the color is identical to the transmittance
because you set the transmittance parameter to 1 which
eliminates the complexity of finding the right values for all
7 parameters. If your transmittance in less than 1 you have
to calculate the red, green and blue components from their
average value and the transmittance. The formula for this is
given in the Rendering with Radiance book.
[... on difference between glass and trans]
Actually i run the simulations by two different materials i found the
illuminance in the room is a little different. Why is it different?
We need to know how much different it is to comment on that.
Can you please quote the numbers you get from the simulations.
One obviouse difference is that the glass material will appear
less translucent at an angle than orthogonal to the surface.
The trans material will keep it's 0.325 transmittance at all
angles.
If you try to compare the two materials you have to use
a method that is not susceptible to these differences.
For "dielectric" definition, is anyone can tell me where i can find the
detailed definition and the instruction fo corresponding parameters?
You can find documentation for Radiance here:
http://radsite.lbl.gov/radiance/framer.html
In particular this guide might be helpful. I think it lists all
material types and parameters.
http://radsite.lbl.gov/radiance/refer/usman1.pdf
Regards,
Thomas
On Wed, Dec 16, 2009 at 10:14 AM, Victor Li <[email protected]> wrote:
Hi Andy,
I followed your link and studied the description of the material GLASS in
the rayfront manual (see below).
I found, that GLASS is a special form of dielectric for modelling a thin
glas pane without internal reflections within the glas pane itself.
GLASS produces one ray, which is transmitted, and a second ray, which is
reflected - so far, so good.
Now my question: I am looking for a diagram or an equation of GLASS, which
shows the angle-dependancies.
It is well known, that with more normal incidence transmission of rays is
increasing and reflection is decreasing;
with more sloped incidence angle transmission of rays decreases and
refelctions increases ....
Where can I find a documentation about this angle dependancies of the
material GLASS ?
-Gunter
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Andrew
McNeil
Sent: Friday, December 11, 2009 5:35 PM
To: Radiance general discussion
Subject: Re: [Radiance-general] material trans and glass?
Hi Victor,
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.
You can find the equation to get transmisivity given transmittance at the
bottom of this page:
http://www.schorsch.com/rayfront/manual/dielectricdef.html
<http://www.schorsch.com/rayfront/manual/dielectricdef.html>
A glazing with reported VLT of 0.325 has a transmisivity of 0.354 so the
material definition would be as follows:
void glass glazing.325
0
0
3 0.354 0.354 0.354
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.
Also on this page there is a nice diagram describing how trans works:
http://www.schorsch.com/rayfront/manual/transdef.html
<http://www.schorsch.com/rayfront/manual/transdef.html>
Andy
On 12/10/09 7:22 PM, "Victor Li" <[email protected]> wrote:
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).
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.
Actually i had a look the webpage and i define the trans followed the
instruction in the page.
Note also that the Radiance glass material uses "transmissivity" not
transmittance,
so does trans use "transmittance" in the first three paremeters? What is the
difference between transmittance and transmissivity?
_____
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Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general
<http://www.radiance-online.org/mailman/listinfo/radiance-general>
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Gunter.
Some heavy reading on the materials can be found hear:
http://radsite.lbl.gov/radiance/refer/materials.pdf
Hope you kept your math skills ...
Regards,
Thomas
On Thu, Dec 17, 2009 at 10:12 AM, Pueltz, Gunter <[email protected]> wrote:
Hi Andy,
I followed your link and studied the description of the material GLASS in
the rayfront manual (see below).
I found, that GLASS is a special form of dielectric for modelling a thin
glas pane without internal reflections within the glas pane itself.
GLASS produces one ray, which is transmitted, and a second ray, which is
reflected - so far, so good.Now my question: I am looking for a diagram or an equation of GLASS, which
shows the angle-dependancies.
It is well known, that with more normal incidence transmission of rays is
increasing and reflection is decreasing;
with more sloped incidence angle transmission of rays decreases and
refelctions increases ....Where can I find a documentation about this angle dependancies of the
material GLASS ?-Gunter
Does anyone tell me the definition and difference between Transmisivity and
Transmittance?
What is the definition of the frist 3 numbers and the second to last number
of trans definitions?
The formula I have to go from total transmission to transmissivity is
=(SQRT(0.8402528435+0.0072522239*Transmission^2)-0.9166530661)/0.0036261
19/Transmission
For trans definition from page 325-326 of Rendering with Radiance:
Where:
Cr,Cg,Cb are the diffuse reflectance in R,G,B channels, and Rd is the
photopic average of those.
Rs is reflected specularity (fraction reflected off first surface in
mirror fashion)
Sr is surface roughness similar to plastic
Td is diffuse transmissivity (fraction transmitted diffusely in a
scattering fashion)
Ts is fraction transmitted as a beam (like through clear glass, not
scattered)
In reverse order, the parameters for trans can be calculated as:
A7=Ts / (Td + Ts)
A6=(Td + Ts) / ( Rd + Td + Ts)
A5=Sr
A4=Rs
A3=Cb / ( (1-Rs)*(1-A6) )
A2=Cg / ( (1-Rs)*(1-A6) )
A1=Cr / ( (1-Rs)*(1-A6) )
Then the -st setting for rpict or rtrace should be:
St=A6*A7*( 1 - photopic average(A1,A2,A3) * A4 )
From: [email protected]
[mailto:[email protected]] On Behalf Of
Victor Li
Sent: Sunday, December 20, 2009 3:24 AM
To: Radiance general discussion
Subject: Re: [Radiance-general] material trans and glass?
Does anyone tell me the definition and difference between Transmisivity
and Transmittance?
What is the definition of the frist 3 numbers and the second to last
number of trans definitions?
____________________________________________________________
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Hi,
Christopher gave you the formulae. To understand the difference, the definitions in RwR are quite useful. To make it easy to understand, the transmittance is what you can measure from a real, physical sample. It is light out / light in. Simple. But as we know, there are more effects then absorption affecting the transmittance, namely it is not only the body but also the surfaces of it (the interface between the solid and, e.g., air) which influence the amount of light traveling through a dielectric object. These surface effects, depending on the index of refraction of the solid and surrounding media, tell you about the light which is reflected. This happens both at the outside of a glass pane (which is what you see when looking at your mirror image in the subway while in a dark tunnel) and at the inside, which is used e.g. for transmitting light through optical fibre. I am sure that you can get a better explanaition from any physics book. So what this means is that while the!
transmissivity of a cube of glass would be constant wherever it is, its transmittance would change when surrounded by air, water or oil, due to the different refractive indices of the surrounding media.
Now why is it that complicated, why do trans and glass expect different characteristics?
Remember that glass is just a handy way to model thin dielectric surfaces for windows. This means that a lot is known that would have to be modeled for a generic material as trans. A window glass has a known refractive index as the surrounding media (air) has, the approximate light transport by in the material can be estimated. So you just give the materials property as what light is NOT absorbed when traveling through it once, the surfaces' influence can be calculated by the software.
Trans is generic, it could describe a thick block of material, or a material with any refractive index. So the software needs the information on what happens on the surfaces - so you need to know how much light is reflected. In other words, you are the one measuring or calculating the reflectance, radiance expects the result and thus does not have to care about the refractive properties any more.
However, I would still refuse to take trans as the counterpart for glass - better look at dielectric. This is actually the more generic type for glass, and it again allows you to define the refractive indices of the object and the surrounding media. Other than trans, it will use these to calculate the surface effects. But other then glass, it HAS to calculate them according to the thickness, and refractive indices as given, and cannot assume a simplified model with a thin object and standard refractive properties. So again, you have to give the transmissivity. Other then for glass, it needs to be defined per unit length - while the surface properties only depend on the refractive indices, the absorption depends on the thickness, or betther on the length that light travels through the material.
So it is all about using the appropriate modifier for the given task. Glass for thin glass surfaces in air, dielectric for thicker dielectric surfaces or objects which are not surrounded by air (then you need the interface modifier and dielectric), and trans when you know from measurements or calcultation the reflective properties and need the additional parameters and do not need the optimizations of the others. The rest can be found in literature on physics and optics. As I surely made some mistakes while typing the above late at night, I would recommend some good book instead as this is not specific to Radiance.
Cheers,
Lars.
Does anyone tell me the definition and difference between Transmisivity
and Transmittance?