Warning message and glass definition

radiance-general
1

Hello Everyone,
It was really nice meeting some of you at the Radiance conference. As a few of you already know I am really new with Radiance and am now shifting from Desktop Radiance to Radiance. (Thanks a lot Francesco.. Cygwin is a real hit, Thanks Rob for getting me started )
I have been trying to do some runs in Radiance wherein I am calculating illuminance levels at a workplane grid for 4 daylighting configurations. I will be comparing these results with results from Superlite and eventually with DOE 2 so as to determine the error in the calcs in DOE 2
The configurations are 36wide x 30 deep x 10' high room with one window on the whole south face.
The same room with 2). a light shelf 3). 4 skylights 4) roof monitor.
Presently I am still doing the calcs with simple room with one window.

Now for the newbie questions.
1. When I am running rtrace I am getting a message warning no light source found. I am using a sky without sun and specifying -B. Is this because of the sky type or some other reason since I am getting the values which seem reasonable.
2. Since I am comparing results from superlite it is important that the geometry and material properties are exactly the same.The glass type in Superlite is a clear glass with 85% transmittance and 5% reflectance. How do I specify a similar glass type in radiance. The lib shows one with same properties as

void glass E178-6_glass
0
0
3 0.61271 0.84589 0.68805

void BRTDfunc E178-6_front
10
      0.56202 0.77639 0.63125
      0.24487 0.09924 0.22543
      0 0 0
      .
0
9 0 0 0 0 0 0 0 0 0
void BRTDfunc E178-6_back
10
      0.56202 0.77639 0.63125
      0.22373 0.13212 0.24437
      0 0 0
      .
0
9 0 0 0 0 0 0 0 0 0

Is this what I should be using or simply put it as (for 85%transmittance)

void glass class_lglass
0
0
3
0.927 0.927 0.927

I know the questions may seem really simple for the advanced users but I need your guidance to proceed.

Thanks
Arvinder

···

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2

Hello Everyone,
It was really nice�meeting��some of �you at the Radiance conference. As a few of you already know I am really new with Radiance and am now shifting from Desktop Radiance to Radiance. (Thanks a lot�Francesco.. Cygwin is a real hit, Thanks Rob for getting me started�)

Hi Arvinder,

It was a pleasure to meet you as well, and to watch your face light up as you pressed return following "ximage test1.pic" for the first time! =8-)

Now for the newbie questions.
1. When I am running rtrace I am getting a message warning no light source found. I am using a sky without sun and specifying -B. Is this because of the sky type or some other reason since I am getting the values which seem reasonable.

Yep, the warning is because you are using a sky without sun, nothing to worry about. Happens anytime you spec an overcast sky too...

2.�Since I am comparing results from superlite it is important that the geometry and material properties are exactly the same.The glass type in Superlite is a clear glass with� 85% transmittance and 5% reflectance. How do I specify a similar glass type in radiance.

I know nothing about Superlite, and little more about Radiance, but a clear 85% transmittance glass in Radiance looks like this:

void glass Avrinder's_glass
0
3
.85 .85 .85

Now, the glass material in Radiance has a fixed reflectance value, but I don't recall what it is (but it's not 5%). If you wanna explicitly spec the reflectance you will have to use the fancier BRTDfunc, which is the domain of the gurus on the list. Sorry I can't be of additional help on that one.

Rob Guglielmetti
[email protected]

···

On Friday, October 3, 2003, at 04:39 PM, Arvinder Dang wrote:

3

Thanks, Rob, for responding to Arvinder's questions. I have nothing to add to your comments on the warning, but let me add a bit to your response on glazings.

First, I agree that Arvinder should go with the glass type. The library glass type is probably not appropriate, and you are better off defining your own. The specification you came up with:

void glass class_lglass
0
3
0.927 0.927 0.927

is very close to correct. I assume you used the formula recommended in the Radiance reference manual, or else the identical one from Chapter 4 (page 240) of "Rendering with Radiance":

{ ---------------- trans1.52.cal --------------- }
sq(x) : x*x;
n = 1.52; { index of refraction }
rn = sq((1-n)/(1+n)); { normal reflectivity }
          { normal transmissivity }
tn = (sqrt(sq(sq(1-rn))+4*sq(rn*Tn))-sq(1-rn))
      / (2*rn*rn*Tn);

I actually got a slightly different value when I used 85% as the transmittance: tn=0.926 for Tn=0.85. However, the part that isn't documented for you is the formula for normal reflectance, which is:

Rn=rn+rn*(1-rn)*sq(tn)*(1-sq(tn)*sq(rn))/(2-sq(tn)*sq(rn));

If you add this to the above file and run calc on, you'll find that by plugging in an index of refraction of 1.52 (the default for glass), your normal reflectance is 6% rather than your desired 5%. To get a value of 5%, I had to play around with the index of refraction in calc until I got something close, as the formula is rather nasty to solve exactly. What I came up with was n=1.465, which gives a transmissivity of 0.913. Your glass specification then becomes:

void glass class_lglass
0
4
0.913 0.913 0.913 1.465

This is the specification you should use. Notice the extra argument to specify the index of refraction.

It would be much simpler if Radiance took the normal reflectance and transmittance of the glazing and figured out the transmissivity and index of refraction for you. The only downside is that you wouldn't really know whether the values you'd be giving it were physically realizable. You might want to give a transmittance of 90% and a reflectance of 8%, thinking that were perfectly reasonable as it doesn't sum to 100%. Oddly enough, no such pane of glass exists or could be made in the real world. This is why the specification is for the normal transmissivity (the light not absorbed in one pass through the glass interior), which is legal anywhere between 0 and 1, and the index of refraction, which is well-studied and available in tables and can be inferred by measurements.

The exact behavior of glass as a function of angle is a bit complicated, even though it is physically one of the simplest materials to model. Section 3.3 of <http://floyd.lbl.gov/radiance/refer/materials.pdf> gives the full formula if you feel like diving into the deep end, as it seems you do.

-Greg

4

On the off-chance that someone asks a question I know the answer to, time permitting, I'm thrilled to be able to answer them! =8-) I also believe that everyone benefits from these exchanges, from the gurus on down. Your glazing reply is a perfect example, because I just learned a whole lot about what you can do with the glass primitive. In a way, it's good to ask lots of questions on here, because Greg's noggin has so much Radiance info locked up in it that it's good for the regular users of the software to ask questions, to open the tap every once in a while!

Rob Guglielmetti
[email protected]

···

On Sunday, October 5, 2003, at 01:40 AM, Greg Ward wrote:

Thanks, Rob, for responding to Arvinder's questions. I have nothing to add to your comments on the warning, but let me add a bit to your response on glazings.