user6057: I had this idea for an effect and I wanted to know if something like this has been done and if so how to do it:
Goal:
I want a lighting effect for an action scene where I will have an area start lighting up, while I would like to simultaneously have another area start darkening in a similar manner, all in a (relatively) lit environment. I want an effect that looks basically in all ways like a light, but its something like a black-colored light that paints its surrounding black in a light-like way.
Idea:
Basically I wanted to know if there is any way I can do some kind of post processing where I can use an LED as a green screen of sorts for the darkening effect. So wherever the LED is shining more or less maps to some kind of darkening effect/transformation.

user5546: I've got another idea, but it's a little kooky.  You light the WHOLE SCENE with spotlights - the fewer the better.  On the key spotlight, you add a gel that has an opaque black circle on it.  Then as you move that around, that section of darkness will pass over parts of the set/props/actors.
For a more focused spot, you could use a bright digital projector to project the image of a black circle on a white field.  Focus it based on the parts of the scene you're moving it across.

user6207: What you're talking about is essentially a negative light. (There's a scene in the Simpson's where Moe doesn't want his fancy customers seeing Homer and the guys, so he unscrews a lightbulb and it casts negative light over them, leaving them in total blackness.) Unfortunately, they don't exist in real life. So your options are:

Light things in such a way that the area you want to be black is not lit (either directly or by reflections)
Use CGI for the various elements and use negative lights to light the "black" ones. I believe most 3D packages will allow this, even though there's no real-world analog
Film the two parts separately and use post production compositing techniques to mix them together with the appropriate lighting


user3592: Fake it.  Use your preferred compositing application to turn down the exposure value over a specified region.  For example, if you're talking about a round spotlight affecting a flat region, create an appropriately shaped ellipse, feather it to taste, and turn down the exposure.  If instead you're talking about volumetric lighting (like shining a light through fog or smoke), you'd use a cone shape, again feathered appropriately.  If your light/darkness cone intersects objects at various distances, you'll need to model those shapes in a 3d app, matchmove the camera, and use the volumetric light source from the 3d app as your exposure mask in your compositing app.
If you'd rather not fake it, then you're limited to the real world behavior of light.  In real life, "negative light" is shadow.  So the trick is casting shadows where you want your negative light to appear.  If it's important that your negative light appears as a beam, you'll need to introduce particles such as fog or smoke into the atmosphere, and as brettfromLA pointed out, use a "black circle" to mask out your beam.  In lighting parlance, these optical elements are usually called gobos.  I would guess that a single diffuse light source with a gobo would work better than brettfromLA's suggestion, though.  Maybe a single china ball with a gobo, but this would definitely require some experimentation.
Remember, darkness is just a relative lack of brightness.

6057: <p>I had this idea for an effect and I wanted to know if something like this has been done and if so how to do it:</p>

<p>Goal:</p>

<p>I want a lighting effect for an action scene where I will have an area start lighting up, while I would like to simultaneously have another area start darkening in a similar manner, all in a (relatively) lit environment. I want an effect that looks basically in all ways like a light, but its something like a black-colored light that paints its surrounding black in a light-like way.</p>

<p>Idea:</p>

<p>Basically I wanted to know if there is any way I can do some kind of post processing where I can use an LED as a green screen of sorts for the darkening effect. So wherever the LED is shining more or less maps to some kind of darkening effect/transformation.</p>
 5546: <p>I've got another idea, but it's a little kooky.  You light the WHOLE SCENE with spotlights - the fewer the better.  On the key spotlight, you add a gel that has an opaque black circle on it.  Then as you move that around, that section of darkness will pass over parts of the set/props/actors.</p>

<p>For a more focused spot, you could use a bright digital projector to project the image of a black circle on a white field.  Focus it based on the parts of the scene you're moving it across.</p>
 6207: <p>What you're talking about is essentially a negative light. (There's a scene in the Simpson's where Moe doesn't want his fancy customers seeing Homer and the guys, so he unscrews a lightbulb and it casts negative light over them, leaving them in total blackness.) Unfortunately, they don't exist in real life. So your options are:</p>

<ol>
<li>Light things in such a way that the area you want to be black is not lit (either directly or by reflections)</li>
<li>Use CGI for the various elements and use negative lights to light the "black" ones. I believe most 3D packages will allow this, even though there's no real-world analog</li>
<li>Film the two parts separately and use post production compositing techniques to mix them together with the appropriate lighting</li>
</ol>
 3592: <p>Fake it.  Use your preferred compositing application to turn down the exposure value over a specified region.  For example, if you're talking about a round spotlight affecting a flat region, create an appropriately shaped ellipse, feather it to taste, and turn down the exposure.  If instead you're talking about volumetric lighting (like shining a light through fog or smoke), you'd use a cone shape, again feathered appropriately.  If your light/darkness cone intersects objects at various distances, you'll need to model those shapes in a 3d app, matchmove the camera, and use the volumetric light source from the 3d app as your exposure mask in your compositing app.</p>

<p>If you'd rather not fake it, then you're limited to the real world behavior of light.  In real life, "negative light" is shadow.  So the trick is casting shadows where you want your negative light to appear.  If it's important that your negative light appears as a beam, you'll need to introduce particles such as fog or smoke into the atmosphere, and as brettfromLA pointed out, use a "black circle" to mask out your beam.  In lighting parlance, these optical elements are usually called <a href="http://en.wikipedia.org/wiki/Gobo_(lighting)" rel="nofollow">gobos</a>.  I would guess that a single diffuse light source with a gobo would work better than brettfromLA's suggestion, though.  Maybe a single china ball with a gobo, but this would definitely require some experimentation.</p>

<p>Remember, darkness is just a relative lack of brightness.</p>
