Using IES lights

[Alejandro Testa]

Hi guys, I start using IES lighting (also Jorge  Javier Lara Dominguez is doing some search and tests)  and I wish to know which are the benefits. I read some post about IES on the net and it looks that this is the way to go.  Any  know problem using IES?

Is there any difference in rendering time between photometric lighting and not photometric lighting?  What about  image quality?

What happen if you change the wattiage, it works like  any other non photometric lighting?

Any suggestion will be trully apprecciated.

Regards,

Alejandro

[George Ioannidis]

From my experience with IES lights it is really easy to use them. Nice free utility to manage IES files and see how they emit light in instant review is the "IES Viewer" program you can find here . Most of known light manufacturers give ies light data files free (I could find a load of various lights for indoor and outdoor use from: CooperLighting, Erco, GE, Lithonia and Visa) and also accurender has several generic IES light data files. Never caused any problem to me when used in scenes.

I think that they are way more accurate than the general light fixtures in terms of light distribution in space. The truth is that if you model exactly the light fixture and place light source in it, it should work similarly to IES light, but I think would render slower than using IES. Never tested it though.

IES files contain the real and exact wattage of the particular fixture. Of course you can change it manually. :)

Roy for sure can provide you with more informations "from the other side". ;)

Regards

[Alejandro Testa]

Thanks George, for your information.  Do you usually use IES lighting ?

[George Ioannidis]

When doing internal renderings (which is really rear :D ) I like to use IES lights for spots and building surrounding light fixtures almost exclusively. For other fixtures like LED ropes I use simple light emitting material and play with its intensity through light channels. I never use simple point lights and always use sun for exteriors, not HDR images. If you want I could send you zipped my IES lights collection :) Using IES viewer you can then easily chose one suitable.

[Roy Hirshkowitz]

Like everything else, IES data has both good and bad aspects.  It can be more accurate than our standard distributions under many circumstances.  However, since it is (usually) measured data acquired by placing a luminaire into a special rig in a lab, it already includes effects caused by the geometry of the luminaire.  When you use complex geometry for your luminaire you need to be careful that things like shadow-casting and reflection by the luminaire itself are not counted twice, once in the IES data and once by nXt during the simulation.  We have some switches for these-- they sometimes work.  Best, in terms of simulation, is to use very simple, planar, geometry when using IES data.  Another potential issue is that IES data is an example of "far-field" photometry.  It works better when the objects to be illuminated are at least 5 X the light sources dimension away from the source.  Illuminating objects which are close is less accurate.  People often like the patterns that IES data can cause on a wall, for example-- but these are often not accurate.

[George Ioannidis]

>>People often like the patterns that IES data can cause on a wall, for example-- but these are often not accurate.>>

Maybe not accurate, but to my eye something I am used to see in real life :) Without wondering if what I see is as accurate as it should be actually, in terms of accurate light distribution model. Rendering which is not done as lighting analysis of particular space for me is more "artistic" rather than "technocratic" matter of work. First of all it must please my eye and aesthetic criteria and creativity (mostly learned in photography courses) of things I want to show to customer (and others I don't want to show :) ) and secondly the really engineering approach of accurate light design of living spaces. On the other hand I really love Roy's accuracy loyalty, which guarantees us the correctness  of light solution nXt is delivering. As result of my previous "meditations"  IES lights work quite nice for my needs. :D

[Roy Hirshkowitz]

Yep.  My job is to be accurate and let you know when the tool might not be accurate.  Your job is to use the tool however you want.

[Alejandro Testa]

Thanks Roy, more clear for me now. Is there any difference in rendering time and definition?

[Roy Hirshkowitz]

I doubt if it's noticeable.  As far as setup goes, asymmetric distributions can be a little harder to work with since you'll need to deal with rotation.  Symmetric distributions tend to be very easy.

[Aleksander Nowacki]

I have found the solution for the IES 'double information' problem (IES information plus luminaire's geometry properties used by nXt):

I use to build the luminaire model as precise as needed, with different materials and "light sources" (only glowing material attached to the object) and then I add the last object that surrounds the lamp. This object is tagged with the IES file and set as INVISIBLE (in light properties window) - see the files attached.

This trick makes the lamp looks good and the light it produces is correct.

But well, maybe someone knows better way to manage IES files?

[George Ioannidis]

Clever workaround. The only drawback of the method is the fact, that the "dummy" light fixture looks turned off while it obviously emits light. Maybe some light emitting material could solve this?

[akinlolu olugboji]

I now use IES lights for interior since George Ioannidis intervention in my last interior forum thread. Here also I believe is a much simpler IES viewer and editor. 

I have a bunch of IES files for sharing should anyone be interested.  

[Alejandro Testa]

Thanks Akin