pointlight / light emitter

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Ron_Bruins
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pointlight / light emitter

Post by Ron_Bruins » Thu Dec 09, 2010 5:39 am

Hi,

I have read many topics about light emitting surfaces and that they are favoured somehow over the point/spotlights.

So my question is how to do this?

eg. the twilight can light has a tiny spotlight inside to lit the can and a pointlight to light the scene. How do i use a light emitter for this purpose.

do I need to:
- make a very small cube paint it with LEM material (but wouldn't this be like having 6 lightbulbs all pointing different directions, especially when loaded an IES file?)
- make a small square surface (but how to get decent power as the watts go per m2
- make a small rectangular surface, but again if loading an IES what will be the center of the 'bulb' ?
- make a small triangle surface, ......

The basic thing I don't get is if you have a normal bulb (real life) the light comes from a single source and with the IES characteristics it spreads the light according to it.
So how will this be setup with a light emitting surface and if you have a plane to light a scene, where is this bulb then located and where does the IES originate from?

Not sure if I can make it clear what I escatly mean, not being native in english and trying to explain a technical subject I also don't have much knowledge in....

Ron
Regards, Ron

Fletch
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Re: pointlight / light emitter

Post by Fletch » Thu Dec 09, 2010 10:57 am

Hi Ron, this is a very good question. I hope the attached image and scene can help you. All the best,
Please note the color assigned to the light emitting materials in the 2 examples. The color you assign the light can make a big difference for realism. See the Tips and Tricks Section Master List and find/read what you can there about lights, speed of render for different light sources, and about the color of light.

In the case of "Can" Downlights as shown below, I would not suggest the light emitter approach. I would find an IES file from a lighting manufacturer that specifically looks like the downlight you want to represent, and load that into the spotlight. Position it correctly inside the can so that it will light the inside of the can as well, with only one light source, it will look the most realistic, and render relatively quickly in any setting. You can download some free IES lights and see what they look like in the "Freebies" section here.

:>: Download This Scene In SketchUp7 Format Here (0.08Mb)

Image

Fletch
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Re: pointlight / light emitter

Post by Fletch » Tue Dec 14, 2010 9:19 pm

Ron, was this answer helpful?

Ron_Bruins
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Re: pointlight / light emitter

Post by Ron_Bruins » Wed Dec 15, 2010 5:35 am

Hi Fletch,

Sorry for the delay, caught a bit up in other stuff so 24 hrs a day were barely enough :)
Though I did check your example out and it was definitely helpful, thanks for that!

I (of course) still have a few questions:

- So best approach is to use spot/point lights to light a scene and use (invisble) light emitters where additional lighting is needed (eg you pointed out in a previous post to lit the inside of a pool to get caustics out of it)?

- If I use a light emitter and load an IES for it how will the light be affected by the size of the surface? the IES originates from a ver small point and the light will be spread acoording to the characteristics of the bulb with these IES specs. Now if you apply these specs to a large LEM painted surface, how will these specs be applicable?

Thanks,
Ron
Regards, Ron

Fletch
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Re: pointlight / light emitter

Post by Fletch » Wed Dec 15, 2010 10:31 am

Regarding Photometric Data on surface vs. inside spot light:
As you may know, IES photometric data like these represent the light distribution of a specific light fixture. So if you have the IES for a fluorescent 2x4 ceiling light, then the IES inside a rectangular light emitter material would be appropriate. If you have the IES data file for a light such as a spot light, or can light, then this would not be as quick to render nor as accurate. So it is advisable to load the IES into the appropriate object.

It is much easier to visualize the distribution of the light from the IES file if you load it into a spot light, as the spot light will change immediately to roughly mimic the IES data. There is no way to pre-visualize this same manner for a light emitting material surface... so it becomes a "wild guess". An IES file loaded into a spotlight will lengthen render times a little in comparison to a standard boring spot light. An IES file loaded into a rectangular face light emitting surface will take more than 2x longer to render.

Regarding Invisible Light Emitters:
You can have it invisible or visible. It depends simply on whether or not you want to actually see the light source. If you don't mind seeing the light source, leaving it as a visible emitter is fine.

Regarding Light Emitters and "Cast Shadow" checkbox:
The "Cast Shadow" check box for Light Emitter Materials is NOT to determine whether or not the light will cast shadows for objects inside the room such as chairs, etc. It is telling the render engine whether or not the light emitter material ITSELF would cast a shadow if OTHER lights shine on it.

The perfect example is light from the sun coming through a window. A common way, with unbiased render engines, to get a nice bright light shining in through the window is to put a rectangle just outside the window that covers the whole window and place it facing IN to the space. Making this a LIGHT EMITTING MATERIAL would then give a nice glow through the window. But now it would block the SUN from shining in. So setting that LIGHT EMITTING MATERIAL to NOT cast shadows, AND be invisible would allow the sun to still shine through the window and cast sunny shadows, and also allow the camera to see OUT of the window to the sky/objects outside.

Regarding Position of Spot Light within Can Light Fixture, and lighting the inside of the can:
The IES data file, combined with positioning the spot light just right inside of the can, causes it to light the inside of the can as well as providing a nice realistic distribution of light to the scene. It is centered and positioned about 5 cm up inside of the can. This light does NOT have any other light such as a point light inside of the can, and it gives a great "value" for light visual quality vs. render time.
Attachments
IN THIS EXAMPLE you see the identical Photometric IES data file for a strong pendant light from an actual light manufacturer's website loaded into the Light Emitting Surface on the right, and into the down light (can light) on the left.
IN THIS EXAMPLE you see the identical Photometric IES data file for a strong pendant light from an actual light manufacturer's website loaded into the Light Emitting Surface on the right, and into the down light (can light) on the left.
IES-LightTypeComparison.jpg (81.74 KiB) Viewed 19696 times

Ron_Bruins
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Re: pointlight / light emitter

Post by Ron_Bruins » Wed Dec 15, 2010 10:46 am

Aha, now it starts to make a bit more sense....IES data is not just for a bulb you fit into a fixture, it is related to the fixture itself.... pieces coming together :)

So last question standing:

general rule of thumb?
- light a scene with point/spot lights as you would in real life and load IES where needed.
- If additional lighting is needed (eg the caustics in the pool as previous mentioned topic) then use some (invisble) emitters

Kind of funny that TR is very easy to use but also if you want complexity you can get as complex as any wishes for :)

Tx Fletch!
Regards, Ron

Fletch
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Re: pointlight / light emitter

Post by Fletch » Wed Dec 15, 2010 11:42 am

you are correct.
I have added some additional information to the above post, please review.

Yes Twilight Render is basically as easy or as complex/advanced as you want it to be.

Ron_Bruins
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Re: pointlight / light emitter

Post by Ron_Bruins » Wed Dec 15, 2010 12:03 pm

Case closed, at least for now ;)

Tx Fletch, you've been of great help again :hug:
Regards, Ron

Fletch
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Re: pointlight / light emitter

Post by Fletch » Wed Dec 15, 2010 12:18 pm

:hat:
I should have mentioned, there is a very nice comparison of these lights you are asking about done by Frederik (and me) here.

light render speed comparison results:
Image

Ron_Bruins
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Re: pointlight / light emitter

Post by Ron_Bruins » Wed Dec 15, 2010 12:41 pm

Yeah, saw this one before and used the info as well (actually been through the whole forum almost post by post as many subjects are very useful, not only for lighting but for anything!)
Though it did not answer my remaining questions, which are now.

Will be around, so I hope you are as well in case I am stuck again ;)
Regards, Ron

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