The problem with using a plane to do the light like a gel, is that Superfly is a ray trace engine. Firefly is a rasterization engine which works backwards and differently at the same time, as to how ray tracing works, as far as light is concerned.

In a rasterization engine, the light goes out the light, to the object, to the camera. Which makes it fairly easy to control light with gels, etc. Gels literally limit where the light can go in a rasterization engine as soon as it passes thru the gel, or exits a spot light that is a gel in itself. But that is not the case with a ray tracing engine. To a point....

A ray trace engine looks out of the camera, finding a lit surface that uses global illumination and the lighting it finds is entirely controlled by the number of samples you set in the render engine, not by the light itself. I know that sounds odd, but that is how works. Do a superfly render with one bounce and see what you get versus 60 in a fully enclosed scene and see... You never touched the light, and it will be lit very differently from 1 to 60 bounces.... Same lights thou....

As you add render samples to a path tracing engine, it not only adds bounces, it adds to where it bounces based on output of the camera, the shaders, and angles it encounters, caustics, etc. Basically what this all comes down to with a gel light. Is that the more samples you use, the less effective the gel setup is due to bounces and scattering off of objects and shaders. Caustics just adds far more chaos to that bouncing..... Worse yet, we can't control the light exit direction of any light. Say to make the all parallel paths..... Even if you put it in a tube that only one end is open, it will still bounce around and exit the tube in a cone.

What the engine sees thru the camera, is based on the number of samples. But what left the light is not controllable, path wise, very well at all...

The method I showed can project a light source in Superfly, but it has to be very close to what it projects on or it washes out rather quickly, based on render settings, shaders, etc.

You can add to what I did by limiting the number of bounces on materials in your scene, but that gets very complex, very quickly. 

The only options we presently have to control light as far as its path, is what is in the Lightpath node, and what you can do with vector math to change some other things. Blender Cycles has additional ways of controlling the light. It even has sort of a rasterization fall back to mimic rasterization light control. And now the newest version of Cycles has Path Guiding controls as well.

Using my light setup you can project light onto a scene from the plane itself. No other light is required for it to work, and it doesn't depend on other light or gel either either. It is basically just a custom area light when you use the shader on a plane. Use it on a sphere and it is a point light.. But the scene has to be set up properly for them to work, and the distance cutoff for the ray length is crucial as to getting it to look right and not show up directly in the cameras view. You don't want to see the source, only what is was shining on.

Remember, at the end of the day, it is all in the illusion.......

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