[Radiance-general] Re: query about accurately modelling glazing
alex Summerfield
[email protected]
Fri, 16 Jan 2004 12:49:37 +0000
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Hi All,
Thanks everyone for this interesting thread. Since there is a fair bit=20=
of discussion on precise measurement of transmission and reflection=20
properties, this might be a good opportunity to check about Radiance=20
handling of glazing with the greater wisdom out there.
Ok so as my limited understanding has it:
refractive index: in Radiance this is only used for tracing 'view rays'=20=
- (so that we see a view refracted through the glazing) but that the=20
light is transmitted through and cast into the space is unaffected.=20
When tracing a ray back towards a direct source, such as the sun - if=20
it were refracted it would be deflected and might miss a small target=20
source.
(hmmm, might create some surreal situations - suppose we are standing=20
in a space and could just see a refracted sun through the edge of the=20
window but the line on the floor indicates we are standing in shadow.)
Probably the most common example where refraction of direct sources=20
might be visually important would be glass bricks.
I think refractive index also does not apply when tracing rays as part=20=
of the indirect calculation - even though missing a large area source=20
(sky) is not likely to be a problem.
Reflectance - in atria there are likely to be complex and multiple=20
glazing surfaces - eg the roof and then in large areas along the=20
offices lining the edge. The intricate reflections may be visible in=20
the scene, but specular reflections from the glazing (or anything else)=20=
will not contribute to illuminance levels in the atrium - unless=20
specified as secondary sources (ie generate 'virtual' sources). This is=20=
not going to handle anything other than smooth flat surfaces (eg not=20
water).
(Daniel - this may have relevance for your work) it does not include=20
the contribution of indirect sources eg bright overcast skies are not=20
going to be specularly reflected around the atrium. The Radiance=20
indirect calculation essentially deals with diffuse-diffuse pathways=20
only.
Ok so does that mean specs and functions for reflectance changing with=20=
angle of incidence are going to effect the appearance of the scene=20
visible in the glazing, the light transmitted, but not the light=20
reflected?
And what about direct sunlight and diffusing glass? Is the light=20
transmitted - or does the random scattering cause the same problem as=20
refraction about missing the target when tracing the ray backwards?
Anyway synthetic images of what we see in these types of scenes - with=20=
all those glossy hard surfaces and intricate multiple reflection of sky=20=
sources etc. - may be substantially different to what the walls, floor=20=
etc 'see'.
well hope this is right - i await heavy editing/ comments.
cheers
alex
*******************************************************
A. J. Summerfield [email protected]
Faculty of Architecture, University of Sydney
Outside of a dog, a book is man's best friend.
Inside a dog, it's too dark to read. Groucho Marx
*******************************************************
On 15 Jan 2004, at 7:08 pm, Jack de Valpine wrote:
> Hi All,
>
> Well here are some further things to confuse/clarify the =
possibilities.
>
> The BRTDfunc is very powerful and has multiple uses. Several things=20
> that I actually find quite nice about it are:
>
> =95 values represent transmittance and reflectance, as one =
might=20
> determine from manufacturers data or from Optics 5 (a side note, I=20
> believe that Optics 5 is actually used to calculate the values that=20
> manufacturers then report for the measurements, this information is=20
> incorporated into tools like the Window 4 and 5 software, some one=20
> from the LBL optics group could probably give a better summary of the=20=
> capabilities).
> =95 allocation of values is clear, that is values are =
allocated to=20
> specular and diffuse components of the material definition (the=20
> problem here if you are trying to simulate a glazing material that has=20=
> diffuse transmittance/reflectance performance is that manufacturers=20
> are only reporting total reflectance and transmittance and not diffuse=20=
> and specular measures, so then you have to measure by hand. I think=20
> that Rob G. could give everyone a really superb summary on how to=20
> setup a measuring environment for glazing materials).
> =95 enables simulation of more complex glazing materials =
such as those=20
> that have higher or differing front/back side reflectances than=20
> accounted for in the typical "glass" material type ( I believe that=20
> the reflectance for the glass material type is around 7%).
> Note that it is possible to use one BRTDfunc definition to define=20
> front and backside reflectances. Note also that transmittance as I=20
> understand it must be the same front and back (this is supported by=20
> the fact that there is only one transmittance that is supplied in=20
> manufacturers data and software such as Optics 5).
>
> Here is a simple definition for a "simple" glazing material (I am=20
> pretty confident that I am going to get some corrective feedback on=20
> this ;->):
>
> void BRTDfunc gl.test
> 10
> =A0=A0=A0 if(Rdot, .12, .11)=A0=A0=A0 if(Rdot, .12, .11)=A0=A0=A0 =A0=A0=
=A0 if(Rdot, .12, .11)
> =A0=A0=A0 0.70=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0=
0.70=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 0.70
> =A0=A0=A0 0=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =
=A0=A0=A0 =A00=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =
=A0=A0=A0 =A00
> =A0=A0=A0 .
> 0
> 9
> =A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=
=A0 0=A0=A0=A0 0=A0=A0=A0 0
>
> This would define a simple glazing material with a total front side=20
> reflectance of 12%, a total backside reflectance of 11% and a total=20
> transmittance of 70%. This material could be applied to a single=20
> surface and does not need separate front and backside polygons.=20
> However what this definition does not account for is the impact of the=20=
> incident angle of the incoming ray on the resulting transmittance and=20=
> reflectance. It is not clear to me what kind of error this might=20
> actually introduce, however, I believe that a definition such as this=20=
> could easily take the place of the two (front and back) BRTDfunc=20
> definitions that get exported from Optics 5.
>
> I believe that there is some trickiness with normals and BRTDfunc, so=20=
> I hope that I am using Rdot to calculate orientation correctly?
>
> Note that the section in the definition with the 10 arguments relates=20=
> to the specular behavior of the material and the section with 9(+)=20
> arguments relates to the diffuse behavior of the material. Summing=20
> between these gives us total reflectance and transmittance. In this=20
> case the there is no diffuse behavior to the material.
>
> Depending on the application is might be worth checking out glaze.csh=20=
> in the latest release. This calculates models for both single and=20
> double surface glazing materials. I am not sure how it compares to=20
> glazing.cal in terms of the accuracy of the underlying model. Note=20
> however that glaze.csh is built around a handful (5 or 6) sets of=20
> reflectance and transmittance values as it "database". Some of these=20=
> were actually measure by me and looking back on it, I would prefer to=20=
> measure the "pvb" materials under a more precise protocol than I did=20=
> originally (again Rob G. would be the real resource here). I would=20
> think that the tool could be extended to account for many things that=20=
> can be exported from Optics 5. Perhaps the Optics 5 group might be=20
> interested in having Greg develop a more robust solution for what gets=20=
> exported from Optics 5 to account for a more sophisticated model of=20
> angular dependance and other properties?
>
> Well, all in all probably more confusion that clarity.
>
> Best,
>
> -Jack
>
>
> Zack Rogers wrote:
>
> Hello,
>
>
> secondly, what does this do to the transmittance?=A0 physically, the=20=
> transmittance going one way should be the same as the transmittance=20
> going the other way.=A0 should the (rtrns, gtrns, btrns) variables =
then=20
> be the same for both front and back panes?=A0 or does it not matter, =
as=20
> long as the product of their transmittances is equal to that of the=20
> combination.
>
>
> This is true for a symetric glass composition (ie. single pane no=20
> low-e, double pane, clear, no low-e) but not true for non-symetric=20
> glass which is often the case for glass with low-e and glass with one=20=
> of the panes tinted.=A0 I believe the front and back transmittance is=20=
> always really close, (ie. it can't be 10% in one direction and 90% in=20=
> the other) but they can vary slightly.=A0 This is what desktop =
radiance=20
> and optics 5 definition method allows you to do.
>
>
> finally, there are no functions built in.=A0 that means that there is =
no=20
> dependence on angle of incidence for either transmittance or=20
> reflectance.=A0 this is unrealistic, as transmittance generally =
reduces=20
> and reflectance generally increases with increasing angle of=20
> incidence.
>
>
> I do think Desktop Radiance and Optics 5 both make use of the angular=20=
> transmittance function.=A0 This is part of the glass primitive.=A0 And =
in=20
> my example, the BRTDFunc calls glazing.cal which provides the angular=20=
> transmittance function.=A0 Also, from my comparisions a while ago now,=20=
> the two methods gave me identical results.=A0 That is:
>
> void glass=A0=A0=A0=A0=A0 clear3_glass
> 0
> 0
> 3=A0=A0=A0 0.92189=A0=A0=A0 0.98612=A0=A0=A0 0.972
> =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 void =
BRTDfunc=A0=A0=A0 clear3_front
> 10=A0=A0 =A0=A0 0.84636=A0=A0=A0 0.90553=A0=A0=A0 0.89251=A0=A0 =A0=A0 =
0.07428=A0=A0=A0 0.08322=A0=A0=A0=20
> 0.08556=A0=A0 =A0=A0 0 0 0
> =A0=A0 .=A0=A0 0
> 9 0 0 0 0 0 0 0 0 0
> =A0 void BRTDfunc=A0=A0=A0 clear3_back=A0=A0 10
> =A0=A0 0.84636=A0=A0=A0 0.90553=A0=A0=A0 0.89251
> =A0=A0 0.07567=A0=A0=A0 0.08418=A0=A0=A0 0.08538
> =A0=A0 0 0 0
> =A0=A0 .=A0=A0 0
> 9 0 0 0 0 0 0 0 0 0
>
> is equivalent to:
>
> void BRTDfunc clear3_glass
> =A0=A0 10=A0=A0=A0 rrho=A0=A0=A0 grho=A0=A0=A0 brho
> =A0=A0=A0=A0=A0=A0=A0=A0 rtau=A0=A0=A0 gtau=A0=A0=A0 btau
> =A0=A0=A0=A0=A0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0
> =A0=A0=A0=A0=A0=A0=A0=A0 glazing.cal
> =A0=A0 0
> =A0=A0 18=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0
> =A0=A0=A0=A0=A0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0
> =A0=A0=A0=A0=A0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0
> =A0=A0=A0=A0=A0=A0=A0=A0 0.07428=A0=A0=A0 0.08322=A0=A0=A0 0.08556
> =A0=A0=A0=A0=A0=A0=A0=A0 0.07567=A0=A0=A0 0.08418=A0=A0=A0 0.08538
> =A0=A0=A0=A0=A0=A0=A0=A0 0.84636=A0=A0=A0 0.90553=A0=A0=A0 0.89251
>
> Also, you might want to check out glass1.cal and glass2.cal which is=20=
> part of the newest HEAD radiance release.=A0 It just distinguishes=20
> between single pane and double pane which have different angular=20
> dependance functions.=A0 Also, check out glaze.csh in the latest=20
> release, I was just informed of this and have not checked it out=20
> personally, but I understand it helps create these definitions.
>
> So, I think the BRTDFunc methods can provide greater accuracy than the=20=
> glass primitive.
>
> I hope I have not mistated any of this, anyone please correct me if =
so.
>
> Regards,
> Zack
>
>
>
> _______________________________________________
> Radiance-general mailing list
> [email protected]
> http://www.radiance-online.org/mailman/listinfo/radiance-general
>
>
>
>
>
> --=20
> # John E. de Valpine
> # president
> #
> # visarc incorporated
> # http://www.visarc.com
> #
> # channeling technology for superior design and construction
>
>
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Hi All,
Thanks everyone for this interesting thread. Since there is a fair bit
of discussion on precise measurement of transmission and reflection
properties, this might be a good opportunity to check about Radiance
handling of glazing with the greater wisdom out there.
Ok so as my limited understanding has it:
refractive index: in Radiance this is only used for tracing 'view
rays' - (so that we see a view refracted through the glazing) but that
the light is transmitted through and cast into the space is
unaffected. When tracing a ray back towards a direct source, such as
the sun - if it were refracted it would be deflected and might miss a
small target source.=20
(hmmm, might create some surreal situations - suppose we are standing
in a space and could just see a refracted sun through the edge of the
window but the line on the floor indicates we are standing in shadow.)
Probably the most common example where refraction of direct sources
might be visually important would be glass bricks.
I think refractive index also does not apply when tracing rays as part
of the indirect calculation - even though missing a large area source
(sky) is not likely to be a problem.
Reflectance - in atria there are likely to be complex and multiple
glazing surfaces - eg the roof and then in large areas along the
offices lining the edge. The intricate reflections may be visible in
the scene, but specular reflections from the glazing (or anything
else) will not contribute to illuminance levels in the atrium - unless
specified as secondary sources (ie generate 'virtual' sources). This
is not going to handle anything other than smooth flat surfaces (eg
not water).=20
(Daniel - this may have relevance for your work) it does not include
the contribution of indirect sources eg bright overcast skies are not
going to be specularly reflected around the atrium. The Radiance
indirect calculation essentially deals with diffuse-diffuse pathways
only.
Ok so does that mean specs and functions for reflectance changing with
angle of incidence are going to effect the appearance of the scene
visible in the glazing, the light transmitted, but not the light
reflected?
And what about direct sunlight and diffusing glass? Is the light
transmitted - or does the random scattering cause the same problem as
refraction about missing the target when tracing the ray backwards?
Anyway synthetic images of what we see in these types of scenes - with
all those glossy hard surfaces and intricate multiple reflection of
sky sources etc. - may be substantially different to what the walls,
floor etc 'see'.
well hope this is right - i await heavy editing/ comments.
cheers
alex
*******************************************************
A. J. Summerfield [email protected]
Faculty of Architecture, University of Sydney
Outside of a dog, a book is man's best friend.
Inside a dog, it's too dark to read. Groucho Marx
*******************************************************
On 15 Jan 2004, at 7:08 pm, Jack de Valpine wrote:
<excerpt>Hi All,
Well here are some further things to confuse/clarify the possibilities.
The BRTDfunc is very powerful and has multiple uses. Several things
that I actually find quite nice about it are:
=95 values represent transmittance and reflectance, as one =
might
determine from manufacturers data or from Optics 5 (a side note, I
believe that Optics 5 is actually used to calculate the values that
manufacturers then report for the measurements, this information is
incorporated into tools like the Window 4 and 5 software, some one
from the LBL optics group could probably give a better summary of the
capabilities).
=95 allocation of values is clear, that is values are =
allocated to
specular and diffuse components of the material definition (the
problem here if you are trying to simulate a glazing material that has
diffuse transmittance/reflectance performance is that manufacturers
are only reporting total reflectance and transmittance and not diffuse
and specular measures, so then you have to measure by hand. I think
that Rob G. could give everyone a really superb summary on how to
setup a measuring environment for glazing materials).
=95 enables simulation of more complex glazing materials =
such as those
that have higher or differing front/back side reflectances than
accounted for in the typical "glass" material type ( I believe that
the reflectance for the glass material type is around 7%).
Note that it is possible to use one BRTDfunc definition to define
front and backside reflectances. Note also that transmittance as I
understand it must be the same front and back (this is supported by
the fact that there is only one transmittance that is supplied in
manufacturers data and software such as Optics 5).=20
Here is a simple definition for a "simple" glazing material (I am
pretty confident that I am going to get some corrective feedback on
this ;->):
void BRTDfunc gl.test
10
=A0=A0=A0 if(Rdot, .12, .11)=A0=A0=A0 if(Rdot, .12, .11)=A0=A0=A0 =A0=A0=A0=
if(Rdot, .12, .11)
=A0=A0=A0 0.70=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =
0.70=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 0.70
=A0=A0=A0 0=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =
=A0=A0=A0 =A00=A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =A0=A0=A0 =
=A0=A0=A0 =A00
=A0=A0=A0 .
0
9
=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0=
0=A0=A0=A0 0=A0=A0=A0 0
This would define a simple glazing material with a total front side
reflectance of 12%, a total backside reflectance of 11% and a total
transmittance of 70%. This material could be applied to a single
surface and does not need separate front and backside polygons.
However what this definition does not account for is the impact of the
incident angle of the incoming ray on the resulting transmittance and
reflectance. It is not clear to me what kind of error this might
actually introduce, however, I believe that a definition such as this
could easily take the place of the two (front and back) BRTDfunc
definitions that get exported from Optics 5.=20
I believe that there is some trickiness with normals and BRTDfunc, so
I hope that I am using Rdot to calculate orientation correctly?
Note that the section in the definition with the 10 arguments relates
to the specular behavior of the material and the section with 9(+)
arguments relates to the diffuse behavior of the material. Summing
between these gives us total reflectance and transmittance. In this
case the there is no diffuse behavior to the material.
Depending on the application is might be worth checking out glaze.csh
in the latest release. This calculates models for both single and
double surface glazing materials. I am not sure how it compares to
glazing.cal in terms of the accuracy of the underlying model. Note
however that glaze.csh is built around a handful (5 or 6) sets of
reflectance and transmittance values as it "database". Some of these
were actually measure by me and looking back on it, I would prefer to
measure the "pvb" materials under a more precise protocol than I did
originally (again Rob G. would be the real resource here). I would
think that the tool could be extended to account for many things that
can be exported from Optics 5. Perhaps the Optics 5 group might be
interested in having Greg develop a more robust solution for what gets
exported from Optics 5 to account for a more sophisticated model of
angular dependance and other properties?
Well, all in all probably more confusion that clarity.
Best,
-Jack
Zack Rogers wrote:
Hello,
secondly, what does this do to the transmittance?=A0 physically, the
transmittance going one way should be the same as the transmittance
going the other way.=A0 should the (rtrns, gtrns, btrns) variables then
be the same for both front and back panes?=A0 or does it not matter, as
long as the product of their transmittances is equal to that of the
combination.
This is true for a symetric glass composition (ie. single pane no
low-e, double pane, clear, no low-e) but not true for non-symetric
glass which is often the case for glass with low-e and glass with one
of the panes tinted.=A0 I believe the front and back transmittance is
always really close, (ie. it can't be 10% in one direction and 90% in
the other) but they can vary slightly.=A0 This is what desktop radiance
and optics 5 definition method allows you to do.
finally, there are no functions built in.=A0 that means that there is no
dependence on angle of incidence for either transmittance or
reflectance.=A0 this is unrealistic, as transmittance generally reduces
and reflectance generally increases with increasing angle of incidence.
I do think Desktop Radiance and Optics 5 both make use of the angular
transmittance function.=A0 This is part of the glass primitive.=A0 And =
in
my example, the BRTDFunc calls glazing.cal which provides the angular
transmittance function.=A0 Also, from my comparisions a while ago now,
the two methods gave me identical results.=A0 That is:
void glass=A0=A0=A0=A0=A0 clear3_glass
0
0
3=A0=A0=A0 0.92189=A0=A0=A0 0.98612=A0=A0=A0 0.972
=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 void =
BRTDfunc=A0=A0=A0 clear3_front
10=A0=A0 =A0=A0 0.84636=A0=A0=A0 0.90553=A0=A0=A0 0.89251=A0=A0 =A0=A0 =
0.07428=A0=A0=A0 0.08322=A0=A0=A0
0.08556=A0=A0 =A0=A0 0 0 0
=A0=A0 .=A0=A0 0
9 0 0 0 0 0 0 0 0 0
=A0 void BRTDfunc=A0=A0=A0 clear3_back=A0=A0 10
=A0=A0 0.84636=A0=A0=A0 0.90553=A0=A0=A0 0.89251
=A0=A0 0.07567=A0=A0=A0 0.08418=A0=A0=A0 0.08538
=A0=A0 0 0 0
=A0=A0 .=A0=A0 0
9 0 0 0 0 0 0 0 0 0
is equivalent to:
void BRTDfunc clear3_glass
=A0=A0 10=A0=A0=A0 rrho=A0=A0=A0 grho=A0=A0=A0 brho
=A0=A0=A0=A0=A0=A0=A0=A0 rtau=A0=A0=A0 gtau=A0=A0=A0 btau
=A0=A0=A0=A0=A0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0
=A0=A0=A0=A0=A0=A0=A0=A0 glazing.cal
=A0=A0 0
=A0=A0 18=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0
=A0=A0=A0=A0=A0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0
=A0=A0=A0=A0=A0=A0=A0=A0 0=A0=A0=A0 0=A0=A0=A0 0
=A0=A0=A0=A0=A0=A0=A0=A0 0.07428=A0=A0=A0 0.08322=A0=A0=A0 0.08556
=A0=A0=A0=A0=A0=A0=A0=A0 0.07567=A0=A0=A0 0.08418=A0=A0=A0 0.08538
=A0=A0=A0=A0=A0=A0=A0=A0 0.84636=A0=A0=A0 0.90553=A0=A0=A0 0.89251
Also, you might want to check out glass1.cal and glass2.cal which is
part of the newest HEAD radiance release.=A0 It just distinguishes
between single pane and double pane which have different angular
dependance functions.=A0 Also, check out glaze.csh in the latest
release, I was just informed of this and have not checked it out
personally, but I understand it helps create these definitions.
So, I think the BRTDFunc methods can provide greater accuracy than the
glass primitive.
I hope I have not mistated any of this, anyone please correct me if so.
Regards,
Zack
_______________________________________________
Radiance-general mailing list
=
<color><param>0000,0000,EEEE</param>[email protected]</=
color>=20
=
<color><param>0000,0000,EEEE</param>http://www.radiance-online.org/mailman=
/listinfo/radiance-general</color>=20
<fixed>--=20
# John E. de Valpine
# president
#
# visarc incorporated
# =
<color><param>0000,0000,EEEE</param>http://www.visarc.com</color>
#
# channeling technology for superior design and =
construction</fixed>
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