5-phase method tutorial - section 6.1

Dear list,

Following example 1 in 5-phase method tutorial, I am trying to model
venetian blinds with frosted glass as blind material. The properties of the
glass are as follows:

Visible Transmittance: 67%
Reflectance: 18%
Clarity (small-angle scattering): 4%
Thickness: 8mm

To generate accurate BSDF files, I thought I should model the blind slats
as trans material, but I am struggling with translating above properties
into trans material modifiers.
I'd appreciate it if you helped me with writing the blind material script.
Also, I am not sure where to include the glass thickness. genblinds does
not seem to have an option for blind thickness.
Does the glass thickness in this case affect the BSDF results?

Thanks,
Leyla

Hi Leyla,

I was thinking someone else might answer this one, as a lot of folks have worked on similar things, but here goes...

Figuring out Radiance "trans" parameters is a classic challenge, and there are a few spreadsheets out there to make this a bit easier. The basic reference is section 5.2.6 from "Rendering with Radiance," attached below for your convenience. (Responders please delete attachment in your replies.)

Plugging in your parameters of:

Ts = .04
Td = .67 - .04 = .63
Rd = .18
Rs = 0 (guessing no clear reflections)

These yield:

A7 = Ts/(Td+Ts) = .04/(.63+.04) = .0597

A6 = (Td+Ts)/(Rd+Td+Ts) = (.63+.04)/(.18+.63+.04) = .7882

A5 = 0 (adjust if you want to scatter your transmitted rays a bit)

A4 = Rs = 0

A1 = A2 = A3 = .18/((1-0)*(1-A6) = .18/(1-.7882) = .85 (assumes uncolored material)

So, there's your material:

  void trans frosted_glass
  0
  0
  7 0.85 0.85 0.85 0 0 0.7882 0.0597

Regarding thickness, so long as the distance between slats is 16 cm or greater, you needn't worry about it. If they are closer together, you may want to use the xform -a option to create your own array of slats. It's actually pretty simple -- genblinds isn't doing that much for you with flat slats, anyway.

Cheers,
-Greg

.

TransParams.pdf (80 KB)

···

From: Leyla Sanati <leylasanati@gmail.com>
Date: August 29, 2013 10:00:45 PM PDT

Dear list,

Following example 1 in 5-phase method tutorial, I am trying to model venetian blinds with frosted glass as blind material. The properties of the glass are as follows:

Visible Transmittance: 67%
Reflectance: 18%
Clarity (small-angle scattering): 4%
Thickness: 8mm

To generate accurate BSDF files, I thought I should model the blind slats as trans material, but I am struggling with translating above properties into trans material modifiers.
I'd appreciate it if you helped me with writing the blind material script. Also, I am not sure where to include the glass thickness. genblinds does not seem to have an option for blind thickness.
Does the glass thickness in this case affect the BSDF results?

Thanks,
Leyla

1 Like

Thank you Greg. I really appreciate your help.

Leyla

···

On Fri, Aug 30, 2013 at 5:43 PM, Greg Ward <gregoryjward@gmail.com> wrote:

Hi Leyla,

I was thinking someone else might answer this one, as a lot of folks have
worked on similar things, but here goes...

Figuring out Radiance "trans" parameters is a classic challenge, and there
are a few spreadsheets out there to make this a bit easier. The basic
reference is section 5.2.6 from "Rendering with Radiance," attached below
for your convenience. (Responders please delete attachment in your
replies.)

Plugging in your parameters of:

Ts = .04
Td = .67 - .04 = .63
Rd = .18
Rs = 0 (guessing no clear reflections)

These yield:

A7 = Ts/(Td+Ts) = .04/(.63+.04) = .0597

A6 = (Td+Ts)/(Rd+Td+Ts) = (.63+.04)/(.18+.63+.04) = .7882

A5 = 0 (adjust if you want to scatter your transmitted rays a bit)

A4 = Rs = 0

A1 = A2 = A3 = .18/((1-0)*(1-A6) = .18/(1-.7882) = .85 (assumes uncolored
material)

So, there's your material:

void trans frosted_glass
0
0
7 0.85 0.85 0.85 0 0 0.7882 0.0597

Regarding thickness, so long as the distance between slats is 16 cm or
greater, you needn't worry about it. If they are closer together, you may
want to use the xform -a option to create your own array of slats. It's
actually pretty simple -- genblinds isn't doing that much for you with flat
slats, anyway.

Cheers,
-Greg

.

*From: *Leyla Sanati <leylasanati@gmail.com>

*Date: *August 29, 2013 10:00:45 PM PDT

*
*

Dear list,

Following example 1 in 5-phase method tutorial, I am trying to model
venetian blinds with frosted glass as blind material. The properties of the
glass are as follows:

Visible Transmittance: 67%
Reflectance: 18%
Clarity (small-angle scattering): 4%
Thickness: 8mm

To generate accurate BSDF files, I thought I should model the blind slats
as trans material, but I am struggling with translating above properties
into trans material modifiers.
I'd appreciate it if you helped me with writing the blind material script.
Also, I am not sure where to include the glass thickness. genblinds does
not seem to have an option for blind thickness.
Does the glass thickness in this case affect the BSDF results?

Thanks,
Leyla

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Hi Greg,
I used this frosted glass to simulate in a room and compared its DGP. I found that the DGP of frosted glass is significantly higher than that of ordinary glass in the same viewpoint. And the luminance of the whole windows with frosted glass was about 15000 cd/m2, while the maximum luminance of windows using ordinary glass was 12000 cd/m2, and the lower half of the window was 2000-4000 cd/m2. I feel very strange about this phenomenon. So, I want to ask if there are some problems with my simulation results, as shown in Figure 1 and Figure 2.

Results depend on the angle of the sun and the transmission parameters of your glazing surface. From your renderings, it looks like the transmission scattering of the frosted glass is making the window brighter from this viewpoint, which does not have the sun in view. This makes sense, as the intense beam radiation is bent in the direction of the virtual camera with frosted glass, but passes straight through to the floor with clear glass.

In other words, it looks right to me.

Not sure if this will help, but here’s a measured benchmark for a clerestory diffusing panel – luminance levels were surprisingly high: see https://eta-publications.lbl.gov/sites/default/files/lbnl-4583e.pdf, Table 5, clear sky conditions, E diffuse-VB, upper window, maximum luminance = 10,462 cd/m2 (description of upper translucent diffusing panel (Tvis=0.47) on p.33). See also Kyle Konis’ study for more in-depth info: https://eta-publications.lbl.gov/sites/default/files/lbnl-4417e.pdf

Thank you Greg. I really appreciate your help,too.

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