Light Emitting Objects

[Bullet]

I've been searching all over the place and i can't seem to find an answer. My problem is that I want to create an object that emits light, not just glow. I've tried the glow feature in both the A&D material and also the Architectural material and I can't get it work right. There doesn't seem to be a way to control how far the light travels if you use glow, meaning that at a certain distance the light disappears and there's no value to change to increase that distance. In Vray you can use a Vray light material and you can adjust how much light an object gives off and it would also affect the travel distance of the light. So my question is, is there a way to do this in mental ray?

 

Any help would be great.

[Justin Hunt]

With A&D look at the self-illumination properties, there is an option to "emit Final gather samples" and have control over the intensity either calculated with inverce square (real world) or unitless.

 

It works really well with large planes but not so well with really thin objects. Mainly because thin objects cant emit enough fg samples.

 

Of cause these dont emit photons.

 

jhv

[amer abidi]

Thats the one thing that mental ray is really lagging behind vray in.

If your Object is a sphere, rectangle, cylinder, etc... then you can replace it with a photometric light and tick the "show light shape in render" box to show the shape emitting the; but if your object is something else, then self illuminated Material with "Illuminate the scene" is your only option at the moment. Make sure your Exposure settings are low enough to allow it, otherwise, you'll have to Jolt up the Luminance (Unitless) Multipler Quite a lot to get the FG effect.

[Justin Hunt]

I use the SI all the time with no hassles at all.

 

I do a lot of shopping malls were you need 100's of lights to light it up. With SI I can use a few planes, a sun and sky (if nessesary) and I am done.

 

Of cause if you need really detailed shadows then you will still have to use real lights.

 

If you set up your physical scale correctly then it is not necessary to crank up the multipliers.

 

jhv

[Bullet]

I guess A&D materials with SI is the only option. Also i think i found where my problem was in using the SI and that was I didn't have enough diffuse bounces set under the Final Gather render options. For an interior scene using SI what values everyone usually set the diffuse bounce to? I usually just leave the diffuse bounce set to the default which is 0 but i guess that's not going to work if I use SI right?

 

Thanks everyone for the help.

 

Amer - yes i'm trying to make a hanging light fixture that's not a normal shape emit light.

 

Justin - on a different note. If you use planes to light up an interior of a mall do you apply the material with SI and then make the object not visible to the camera? I'm just curious because I work on a lot buildings with large open areas that i used to use vray lights for but not i think i'll be using area lights. What would be the advantage be of using an SI plane over a large area light?

[Bullet]

This might be worth mentioning that instead of using the self illumination portion of the A&D material you can use just the Architectural material and change the luminance setting. Mental ray treats this kind of illumination just like an omni light. That is it creates shadows as well. Also it's pretty easy to control, just set the colour and the luminance value.

[Justin Hunt]

For light spilling out of the shops I apply a shop interior texture to the plane and keep it visible but for fill light I make the plane invisible to both camera and reflections.

 

Even with 0 bounce SI will work, of cause no light will be bounced so you will only see the direct light.

 

Nice point on the architectural material I try that. Only thing to look out for is that architectural materials use raytrace reflections which dont work nicley with mentalray.

 

jhv