Radiance-general Digest, Vol 38, Issue 14

radiance-general
1

Steve,

Rob is pointing you in the right direction. But be careful when you wrap the fixture in an illum box as the light output may be quite right. It's been a while since I've had to do this, but I recall having issues with "flatcorr" and "lboxcorr" in my brightdata definitions (created by IES2RAD). I'm not sure where these are documented, but I'm sure other functions are used for spheres and rings. I believe the sphere option is the easiest to work out, but only works if you can wrap your fixture in the illum sphere and the sphere does not bisect geometry. In my case I've had to work out linear fluorescent pendants which have required a box.

Mark

···

------------------------------

Message: 3
Date: Wed, 25 Apr 2007 13:03:30 -0600
From: Rob Guglielmetti <[email protected]>
Subject: Re: [Radiance-general]ies photometry+geometry?
To: Radiance general discussion <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

steve michel wrote:

I am trying to include in a scene file, lamp fixture photometry with a
object geometry (dxf model supplied by the manufacturer for that same
fixture. Is there a straighforward way of combining both such that
the fixture model doesn't compromise the photometry??

You need to use a procedure where you wrap your geometry with an illum
sphere or box and you apply the luminous distribution to that illum
enclosure. Then you also have to generally create some glow polygons
within the fixture geometry to make the geometry appear correct (the
glow will not be evaluated beyond the illum boundary so you won't affect
the distribution of the fixture on the scene). I would say it's not
straightforward, but the procedure is outlined in Charles Ehrlich's
chapter of Rendering with Radiance. The process is very much streamlined
when you use Georg Mischler's Rayfront interface. Personally, I am
usually interested in the calculation result from electric light
sources, so I tend to just use the simple geometry that ies2rad creates
for me. But the visual result of going the distance and actually
setting up the whole illum enclosure can be fantastic. I have very
limited experience with this, but I hope others will chime in here with
some tips.

- Rob Guglielmetti

2

Mark de la Fuente wrote:

Steve,
Rob is pointing you in the right direction. But be careful when you wrap the fixture in an illum box as the light output may be quite right. It's been a while since I've had to do this, but I recall having issues with "flatcorr" and "lboxcorr" in my brightdata definitions (created by IES2RAD). I'm not sure where these are documented, but I'm sure other functions are used for spheres and rings. I believe the sphere option is the easiest to work out, but only works if you can wrap your fixture in the illum sphere and the sphere does not bisect geometry. In my case I've had to work out linear fluorescent pendants which have required a box.

Yeah, like I said, it's far from straightforward. =8-) Mark is right, the sphere is easiest, but since the illum enclosure must completely contain the luminaire geometry, it's not the best choice for a long skinny thing like a fluorescent strip. The sphere ends up so big you introduce inaccuracies. So those cal files lboxcorr and flatcorr take the ies2rad .dat output -- which is essentially a point distribution -- and wraps it around more apporpriate geometry: a box in the case of lboxcorr, (for a linear pendant fixture) and a flat rectangular polygon in the case of flatcorr (for say a recessed fixture where the luminous aperture is a single plane).

- Rob Guglielmetti

3

A linear fixture is exactly what I had in mind. For a while I even searched for 'bare bulb' photometry files from big lamp manufacturers (GE or Philips) but none exist for fluor tubes (only par and mr16 spots). But is the illum distribution method another one or a subset of using flatcorr and lboxcorr techniques you describe??

···

From: Rob Guglielmetti <[email protected]>
Reply-To: [email protected],Radiance general discussion <[email protected]>
To: Radiance general discussion <[email protected]>
Subject: Re: [Radiance-general] Re: Radiance-general Digest, Vol 38, Issue 14
Date: Thu, 26 Apr 2007 09:31:02 -0600

Mark de la Fuente wrote:

Steve,
Rob is pointing you in the right direction. But be careful when you wrap the fixture in an illum box as the light output may be quite right. It's been a while since I've had to do this, but I recall having issues with "flatcorr" and "lboxcorr" in my brightdata definitions (created by IES2RAD). I'm not sure where these are documented, but I'm sure other functions are used for spheres and rings. I believe the sphere option is the easiest to work out, but only works if you can wrap your fixture in the illum sphere and the sphere does not bisect geometry. In my case I've had to work out linear fluorescent pendants which have required a box.

Yeah, like I said, it's far from straightforward. =8-) Mark is right, the sphere is easiest, but since the illum enclosure must completely contain the luminaire geometry, it's not the best choice for a long skinny thing like a fluorescent strip. The sphere ends up so big you introduce inaccuracies. So those cal files lboxcorr and flatcorr take the ies2rad .dat output -- which is essentially a point distribution -- and wraps it around more apporpriate geometry: a box in the case of lboxcorr, (for a linear pendant fixture) and a flat rectangular polygon in the case of flatcorr (for say a recessed fixture where the luminous aperture is a single plane).

- Rob Guglielmetti

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4

steve michel wrote:

A linear fixture is exactly what I had in mind. For a while I even searched for 'bare bulb' photometry files from big lamp manufacturers (GE or Philips) but none exist for fluor tubes (only par and mr16 spots). But is the illum distribution method another one or a subset of using flatcorr and lboxcorr techniques you describe??

No photometric files exist for bare fluorescent tubes probably because you can't really test that condition, nor would anyone want to spend the money to do so. A fluorescent tube will always be in a fixture, even if it's a simple striplight -- a simple ballast housing and two lampholders. And that striplight housing will block part of the lamp, thereby altering the distribution, and THAT is what you would want to photometer. There are photometric files for simple fluorescent striplights, I believe people like H.E. Williams have them. The reason there are photometric files for PARs -- a seemingly plain bare lamp similar to the fluorescent lamp example -- is that those lamps have a reflector and that reflector's effect IS worth evaluating. PARs are in essence a "fixture", since they contain a light emitting filament as well as an optical device to take that light and do something with it. Photometric files are all about describing what a fixture does with those lumens the lamp starts out with.

Anyway, the flatcorr and lboxcorr techniques are really enhancements to the "illum distribution method" we've been discussing, and all of this is a more complex but elegant way of describing the luminous output of a given photometric file. Think of a photometric file as a description of the fixture performance at a single point, because that's what it is. This works great for a 5" round aperture recessed downlight; stick that description on a disc-shaped piece of geometry, slap it on your ceiling and render away. But when your photometric file is describing the luminous performance of a linear fixture, you need to spread this out along the lamp length. This is what the -ds option to rpict does, as we discussed earlier. Yes, it's a hack, yes, it's less than perfect, and yes, many smart people have offered alternate ways to photometer fixtures to improve on this, but implementation and adoption of a standard for that is not on the horizon, as far as I know.

So, we take our point "distribution description" and array it along a polygon, at a frequency determined by the -ds option. This works, until you have a three dimensional fixture model, or even a large flat fixture that is close to other geometry. Then you must stretch and pull that point description into a box shape, and apply it to an illum material. Illum is a magical material (I know Greg is cringing hearing anything in Radiance described as 'magic'; trust me, it's still physically-based!) that can emit light but becomes invisible when you look directly at it. So the idea is to create this "imposter geometry" that acts as the light-emitting boundary and origin for the fixture. Within the box you have 3D geometry that represents the physical appearance of the fixture, but the impostor box is doing all the lighting. So flatcorr and lboxcorr are just little helper files that take that point "distribution description" and do all the stretching and pulling. If the ies file was for a table lamp, you might be able to use a simple sphere to do the same thing (you'd describe a sphere whose radius is larger than the lampshade, which in this case is the thing doing the emitting, in the physical world).

Are we having fun yet? Well, that's the general idea of what these do. This is not an alternate description of the distribution! It's a way of taking the distribution (again, described as a point) and mapping it to a 3D form. For raw quantitative analysis, it's unnecessary. But if you want 3D fixtures in your renderings, it's a process you'll need to understand. The juicy bits of the technique start on page 320 in Rendering with Radiance.

- Rob Guglielmetti