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Resize luminaire when IES file is absolute photometry?

I am somewhat new to AGI32 but I'm learning fast, can you help?

I am looking for help/info to resize luminare's in AGI32 when the IES file is absolute photometry and still get accurate calcs.

For example: A manufacturer supplies an IES file for a 4ft long fixture but they do not supply an IES file for the same fixture at 2ft long. Using the IES file for the 4ft fixture I can define the luminaire in AGI32 and I can change the luminous box and symbol size from 4ft to 2ft. However because the file is in absolute photometry, there is no way adjust the lumen output and so I wonder what is happening to my clacs when I adjust the fixture size whithout being able to modify the lumens accordingly?

Is there a workaround?

Thanks in advance for any advice or links,


e l i

Hi, Eli.

First off, some words of caution connected with your desire to still get accurate calcs.

Making assumptions about pro-rating lumen data can be fine in some circumstances but not others. Sometimes you'll know when those circumstances are in play, and other times you won't.

My first question would be: Has the supplier provided you with the 2ft lumens that the fixture would produce? If yes, even if they're wrong, you've got a fallback if the installation doesn't perform according to the design. If no, you're taking on the risk of doing a design without accurate data. It is very possible for a 2ft fixture to perform at less than half of the fixture output of its 4ft counterpart. I can expand upon that concept if you need me to.

My second question would be: Is the 2ft fixture a significant contributor to the design, or is it merely an auxiliary fixture required due to physical constraints? In other words, will this 2ft fixture be a primary means of illumination throughout a space (perhaps the typical fixture in a room), or will the main fixture be the 4ft version and you just need a couple of 2ft fixtures near some walls or columns where you can't fit the full 4ft fixture? If the 2ft fixture doesn't play a major role in the design, then at least if you find that the 2ft fixture is only 40% of the output of the 4ft fixture (and you assumed it was 50%), in practical applications I'd defy anyone to be able to visually pick up that the differences. However, if you design an entire office floor with the 2ft fixture only, assuming 50% output of the 4ft version, if it only supplies 40% of the output of the 4ft fixture then the installation will only produce 80% of the light that you calculated it will.

A third question is Has the manufacturer confirmed that they can actually produce the 2ft fixture? This might seem strange, but almost half of the linear extrusion based fixtures that my company supplies cannot supply a 'stand alone' 2ft fixture due to a few technical restraints in terms of component size and placement. Ensure that you're not assuming something can be made that is physically impossible (or at least commercially unviable - perhaps something could technically be made, but the difficulties render it far to costly to be a serious undertaking). If you find that the product cannot actually be made as a 2ft fixture, but you've got the issue of the 4ft not being able to fit with columns or walls, there are other ways to approach this. Using downlights, wall lights, etc. can be a good solution both in terms of aesthetics and performance.

That's the cautionary part. Like I said, if you need any of that clarified, I can do so. As for how to deal with your issue, I'll assume you don't have Photometric Toolbox, and that you haven't got any program assigned to open *.ies files

The steps required (assuming the file is called 4ft.ies) are:

  1. Open the 4ft.ies file in AGi32's define lumininaire dialog box
  2. Write down (or better yet, Ctrl+C copy) the figure you see in the field "Luminaire Lumens"
  3. Go to Windows Explorer, find and 'Right-Click' on the 4ft.ies file
  4. Choose "Open with..." from the pop-up menu
  5. If the list of programs shows 'Notepad', select it and skip ahead to Step 11
  6. If not, select 'Choose another app"
  7. From the "How do you want to open this file?" dialog box, either select "Notepad" if it's there, or "More apps".
  8. If you then see Notepad, select it and skip ahead to Step 11
  9. If not select "Look for another app on this PC"
  10. Navigate to C:\Windows folder and select Notepad.exe
  11. You should now see the 4ft.ies file in all its glory in your Notepad app (if not, something has gone wrong in the above steps
  12. In the line under the one that reads "TILT=..." (probably TILT=NONE), you'll see a line where the second number reads '-1'. Select that '-1' and overwrite it with the Luminaire Lumens you either wrote down or copied in Step 2
  13. Save the file (perhaps as a new file)
  14. Open that new file in AGi32's Define Luminaire process.
  15. You can now change the "Lumens Per Lamp" figure by whatever percentage you deem to correctly represent the 2ft fixture, and then of course change your luminous box and symbol dimensions accordingly, plus all the other things you'll need to do to make it a correctly defined luminaire (e.g. description, code, wattage, etc.)

Hope that helps!

Hello Lou,

Thanks for your in-depth reply. All of your cautionary advice makes sense to me. In our case the 2ft fixture is indeed manufactured but is seems common that given the multitude of fixtures produced it is rare to get .ies files for every single one.

I am going to try an experiment with opening and editing the .ies file as you suggest in Notepad and report back.

In the meantime I'd like to ask about Photometric toolbox. Our office was considering purchasing it and I assume it would give more functionality and ease than editing in Notepad?

But whether Photometric Toolbox or Notepad, assuming you are correct, you have answered my original question which is that you can indeed edit the Lumen output for .ies files that are originally supplied as absolute photometric.


In amongst its many capabilities, PTB32 has the ability to do what you’re going to do in Notepad (although I think it only allows changing absolute photometry to relative photometry via ‘raw editing, which is basically the same as using Notepad), but with the added benefit that when the file is a relative photometric one you can then create the 2ft version file yourself.

As an alternate means to get the reduced output from an absolute IES file, you can also take advantage of the "User Defined Factor" within the LLF portion of the Define Luminaire window.  Since the Total LLF is a direct multiplier to the computed values, you can use some quick math to "scale" your output to the final absolute value you need. 

For example, if a 4-foot luminaire has an absolute value of 2000 lumens, and you've confirmed with the manufacturer that a 2-foot version of the fixture would be half of that, once you've made the size adjustments to the fixture, putting a 0.5 factor in the User Defined Factor will simulate the reduced lumen output of 1000 lumens (2000 * 0.5 = 1000).  You won't see the Luminaire Lumens update when you do this, though (I think), so you may want to make a note somewhere in the Label, Tag, or Description that reminds you that it's a 2 foot fixture.

Correct, adding the necessary multiplier as a 'User Defined Factor' in the LLF calculator could be used as a quick fix for some people.

You are also correct that the lumens do not change, so it all depends on what information is shown on any reports/schedules that are output.

For my work, I always show the luminaire lumens and the Light Loss Factor used for transparency of the design factors, so the method described would technically work but in practise would create confusing results in the output I provide to clients.

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