[Radiance-general] Re: IES2Rad questions

[Greg Ward]

Hi Zack,

Sorry for the late response, but as I mentioned at the workshop, I  
was out of the country.

Have a look at the following change, made back in 4/93:

	http://www.radiance-online.org/cgi-bin/viewcvs.cgi/ray/src/cv/ 
source.cal.diff?r1=2.5&r2=2.6

When you say that Type C photometry is measured in the counter- 
clockwise direction, do you mean as seen from below (looking in the  
positive-Z direction)?  If so, then the comment at the top of  
source.cal is correct, but it looks like I messed up on the actual  
definition of phi, which should be:

	src_phi = mod( atan2(-Dy, Dx) / DEGREE, 360 );  { 0-360 }

rather than what it is, now.  I'm really confused, because this is  
effectively what I had before I made the "correction," since I also  
had a -my transform prior to 4/93.  I don't know where my copy of  
LM-63 is, if I even still have it, so if the above line works for  
you, perhaps I should just change source.cal to agree with its  
comment...

-Greg

> From: Zack Rogers <zrogers at archenergy.com>
> Date: September 21, 2007 8:39:34 PM PDT
>
> Hello,
>
> I am trying to simulate a window optical daylighting system  
> (LightLouver) in Radiance and am seeing some weird behaviour.
>
> The IES file I am modeling is completely asymmetric and it appears  
> as if Radiance treats the horizontal angles in a clockwise fashion  
> whereas the IESNA standard is to treat the horizontal angles in a  
> counter-clockwise fashion.  A photometric representation of the  
> file can be seen here:
>
> http://home.comcast.net/~z.rogers/LiLo_photometric1.jpg
>
> and here:
>
> http://home.comcast.net/~z.rogers/LiLo_photometric2.jpg
>
> This is an IES file that I made for a daylighting system that sits  
> vertically in a window using a forward ray-tracing program  
> (TracePro).  I defined it with Type C photometry; the 0 vertical  
> angle is perpendicular to the window pointing into the room and the  
> 0 horizontal angle is up (+Z).  The window is roughly 5' wide by 2'  
> high and can be seen in these images as the blue rectangle.  It is  
> apparent that the main daylight distribution is entering the room,  
> above the horizontal and angled to the left as it enters the  
> space.  This is as expected as this is for a south-west facade at  
> noon.  This illustrates that the IES file was defined with the  
> horizontal angles proceeding in a counter-clockwise fashion  
> according to a right-hand rule.  And the the 0 horizontal angle is  
> defined as up or in the direction of the smaller window dimension.
>
> I then attempted to place this IES file into the room I am  
> modeling, seen here:
>
> http://home.comcast.net/~z.rogers/Lilo_room.jpg
>
> which gives me this rendering (low quality but the distribution of  
> light is apparent):
>
> http://home.comcast.net/~z.rogers/lilo_e12pm_bad.gif
>
> So my first question is relative to the 0 horizontal angle and  
> which axis IES2RAD assumes for this.  It reads in the source.cal  
> file that +x axis is the 0 horizontal angle and the -y is the 90  
> deg angle.  However, to get the rendering shown above I had to have  
> my replmarks marker with +y up which seems to disagree with the  
> header of the source.cal file.  I also have to define the longer  
> dimension as x and the shorter dimension as y, similarly in  
> disagreement with the source.cal file header.  When I try rendering  
> it with +x as up I get a strange and obviously incorrect  
> rendering.  Is this a bug or am I mis-interpreting this?
>
> My second question is relative the progression of horizontal  
> angles.  The rendering obviously disagrees with the photometric  
> illustration as the light is entering the room to the right rather  
> than to the left as it was in the photometric view and as it should  
> be given my daylight conditions.  Does Radiance treat the  
> horizontal angles in a clockwise (left-hand rule) fashion or am I  
> mis-interpretting this?  IESNA does define the other way.
>
> The fix I figured out for this second issue is in the source.cal  
> file.  Instead of this:
>
> src_phi = mod( Atan2(Dy, -Dx) / DEGREE, 360 );    { 0-360 }
>
> I made the -Dx a positive like this:
>
> src_phi = mod( Atan2(Dy, Dx) / DEGREE, 360 );    { 0-360 }
>
> and this gave me a rendering with the light going in the direction  
> I was expecting.
>
> One final question; anybody have a way to get Type B photometry  
> into Radiance?  It seems like IES2Rad does not like this type.
>
> I hope all that made sense.  Any help is much appreciated!  I hope  
> to see some of you at the upcoming conference!
>
> Regards,
> Zack
>