Hey all, this is a super brief tutorial, complete with scene files on how to "split" white light into red, green, and blue rainbow patterns in Carrara. I want to stress right off the bat that this isn't physically accurate. My example uses a prism shape and since objects really only have just a skin with no thickness and no internal volume, light will not act the way it does in the real world. Well, maybe if you had a refracting balloon it would look right.
What you need to do is set up a spot light. Adjust it's parameters such as half angle and edge falloff. You will want to position the light and orient the light as well. If you want the final brightness to be around 100%, then set the lights brightness to 33%.
Duplicate your light two time so that you have three spotlights. They should all be in the exact same spot and in the exact same orientation.
Adjust the colors of each light. One light should be pure red, the next, pure blue and the final light pure green. If you do a test render without caustics, the light will look white.
Insert an object and apply a shader with transparency and refraction. You will notice the light splits and forms rainbow patterns.
I've included sample images of using the above method with and without cuastics and also some images with and without caustics that use a standard white spotlight.
The .zip file with the scene file is below the images.
Thats a great tip Evil!
I have not done anything with caustics yet, but I will keep this in mind.
So this is kind of a cheat because Carrara's redering engine is not physically accurate?
The real reason has to do with the lights. In the real world, white light is made up of all the colors. The prism breaks the light apart through refraction. Carrara's white light is just white. I give credit to the renderer for making the combined red, blue and green lights appear white.
It is important to note that there are differences in rendering engines. Carrara uses a biased engine, but perhaps more physically accurate lighting would be achieved by an un-biased renderer. I don't know enough to break it down specifically. Artistically it's more important to know how to light the scene than what engine you use. Whichn leads to my next point. Carrara is basically a 3D art and animation suite, and not a real world simulator. It has the ability to do some of those things, but it's not the main point of the program.
The way the light moves through the prism is also effected by the fact that it is essentially an empty space surrounded by a skin. A real prism is solid glass which will effect how the light is refracted. If I recall my high school science correctly, a real, solid, prism would have broken the light apart into a rainbow pattern where it shines on the wall. Maybe if I built a triangular shape out of panes of glass so that it would be hollow in the real world, and shone a light through it would look as it does in my renders. It's hard to say without doing it.
Actually someone posted a prism breaking the light into the spectrum using LuxRender and the new Carrara plugin.
It is actually better that Carrara is NOT physically accurate, since the render times would go way up. It is good to know alternative ways to do things, so that if you know how it should look, you can "fake" it if you need to.
That is what inspired me to post this thread! That and Hollywetcircuit saying, Tutorial! Tutorial! Tutorial! 
I bet that method could create a wicked disco ball effect!
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