Creating/configuring an Iray mesh light?

PotatoHead

I tried the built-in Light Plane, then tried setting the shader for that to the Iray Emission one. Neither of those produced any kind of light. The only light I could get in the scene was via a distant light.

I haven't found much about mesh lights for Iray via Google or searching the forums, is this concept possible in Iray ? I rather not use point lights. I have mesh lights working fine in Reality/LuxRender but no tweaking or intensity, luminance or strength on either shader works.

IceCrMn

Iray doesn't handle mesh lights in the same way that luxredner does, so the results will not be the same.

In iray, each pixel of the mesh that was used for the light is considered a light source.This makes for very noisey scenes and long render times if these are used as the primary or only light source.

The better solution would be to use the buit-in spotlights (top toolbar Create > New Spotlight ).If you have the iray engine selected as your render engine, then all ths spot/point lights you make have the photometric setting turned on by default.

Change the shape of the light fom point to rectangle and that should give you a much better light.These clean up faster, and you can also switch the viewport view to look thought the light itself to point it.

That being said,,,,

To make a mesh light.Make a new primitive plane , 1M should do.

Apply the iray emissive shader to it.

On the Surfaces pane, scroll down to "Luminance Units" and switch it to "w" for watts.

Set the "luminous Efficiency" from 15 watts to something more useable( for this you will need to experiment ) .

The yellow arrow should be the direction the light points.

Hope this helps out :)

Tobor

icecrmn said:

In iray, each pixel of the mesh that was used for the light is considered a light source.

 Each facet. A simple mesh doesn't tax Iray too much, but one with smooth compound curves is going to add a lot of processing. 

A simple plane with 2 divisions creates a mesh with two triangles, which isn't bad. That would be the most efficient mesh light.

Suggestion about the luminance units: If you set the units of the emitter to Watts or Lumens, then the value is for the entire surface. The apparent light output will change if the size of the mesh changes. If you choose one of the per-square values, the light output remains consistent for the specified area, even if the size of the mesh is altered. I prefer cd/cm^2, which is candles per centimeter square. The rest of D|S uses centimeters as the basic scene unit, so this helps keep things consistent. The per-square values also don't use the efficacy unit, making setting the values a little more direct.

Because the cd/cm^2 is a fairly small area, the luminosity value doesn't have to be very high. A value of 100 will start to produce some strong scene lighting. That equates to 100 candles per square centimeter, which is quite bright. Though there is no direct conversion between lumens and candles, for the purposes of a flat emissive plane, you can consider about 10-12 lumens per candle. 

I concur about using spotlights instead, and changing the emitter size. I almost never use mesh lights these days, unless I need in-scene lighting, like a desklamp or glowing eyes, or something.

 

mjc1016

Tobor said:

 

A simple plane with 2 divisions creates a mesh with two triangles, which isn't bad. That would be the most efficient mesh light.

No...a plane created by using a Studio primitive will, with 2 divisions, actually give you 8 tris/ 4 quads.  With only 1 division it will give you 2 tris/1 quad.

Tobor

Urp. That's what I meant to write, actually. Since you can make a square plane with 1 division, it stands it would be the one with 2 tris.

PotatoHead

So I tried both the emission parameter for the Iray shader and the spot light and I didn't really see any difference in terms of render, but using an array of spot lights was giving me a hard time, while the array of meshes worked fine. I put my spot lights to rectangle, increased the beam angle to 120-180 degres and setup the luminace to match.

All my meshes are actual scene objects but I could get away by using a spot light for them, but as it stands the only reason to do so would be to improve the render speed, which doesn't seem to vary much. My light meshes are simple though, just 4 triangles actually. There is a 5th ceiling light which is a rectangle with 9 faces or 18 triangles, but unless I can make the spot light match the mesh light, I'll just stick with the mesh.

Tobor

If they're scene objects just keep them as meshes. You're meshes are simple, and that's the main point.

Expect these to put out diffuse omnidirectional light, though, which is the natgure of emissive mesh objects. That's not very "spotlighty." You can "shape" the light if you use IES profiles.