Emitting Particles from Lights
The advantages to using Light Emitters rather than the built-in emitter is that Lights in AE are true 3D objects so the 3D motion path can be conveniently edited, and the fact that many lights can be used to emit from multiple emitters at once.

Trapcode Particular does not use all lights in the comp, only the lights that has a name that starts with a certain word are use. By default this is "Emitter" - this name can be changed in Options.

When emitting particles from Lights, the particle color by default comes from the Light's color. It can be changed to use the Color over Life gradient with the Particle>Set Color pop-up.

When the Light Emitters are keyframed and the keyframes are edited a lot, the cache sometimes works inappropriately. This will manifest as jerkiness in the animation (the emitter jumps from one frame to the next). To get rid of the erroneous frames from the cache, use Edit>Purge>Image Caches and re-render.

Step-by-step instructions on using Light emitters
Start with a new comp (Ctrl/Cmd-N), make it 640*480 at 30 fps. 5 seconds long.
Create a Spot Light (Layer>New>Light). Name it "Emitter1" and make it red.
Create a comp-sized solid (Ctrl/Cmd-Y), make sure to click "Make Comp Size" before OK.
Apply Effect>Trapcode>Particular to the solid.
In Particular, set Emitter>Emitter Type to Light.
Set Emitter>Direction to Directional.
Now go forward in time to 1:00 (one sec).
The comp window looks like this:

X Rotation = 0	X Rotation = -120

Particles are coming from the Light! Bring up the rotation properties for the light by selecting it in the TLW and hitting 'r'. Try scrubbing the X Rotation back and forth a bit and notice how the particle beam corresponds to the Lights orientation.

Twirl open Options for the light and edit Cone Angle. Notice that the width of the particle beam depends on Cone Angle for the Light.

You will notice that particles are born in a volume around the light rather than at the exact point of the light. This is because the Emitter Size is non-zero by default. Try changing the Emitter>Emitter Size values to 0. Now hit RAM preview:



Adding a second emitter to the scene
Goto to time 0:00 (zero) and create a new spot light, name it "Emitter2" and make it blue. Set position to (170,270,-200). Set X Rotation to -40 and Y Rotation to 120. Hit RAM preview:



Have fun with the light emitters! And remember:

When the Light Emitters are keyframed and the keyframes are edited a lot, the cache sometimes works inappropriately. This will manifest as jerkiness in the animation (the emitter jumps from one frame to the next). To get rid of the erroneous frames from the cache, use Edit>Purge>Image Caches and re-render.