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This parameter lets you emit particles from lights. The Emitter Type Light(s) needs to be selected for this parameter to be active.
The advantages to using Light Emitters rather than the built-in Emitter is that Lights in After Effects are true 3D objects. This means their 3D motion path can be conveniently edited, and many Lights can be used to emit multiple Emitters at once. See our tutorial for 'Emitting Particles from Lights' at the bottom of this page.
When Light(s) is active in the Emitter Type popup, Particles/sec Modifier can tell the Light Emitter to emit different amounts of particles at various times. The parameter does this by modifying certain layer properties of the composition Light that is being used. Essentially, particles/sec Modifier is altering the value set in Particles/sec in the Emitter group.
Particles/sec Modifier has four choices. Each choice affects a different Light property, which is located in the Light's layer properties in the Timeline. For example, if you set the popup to Light Intensity and the emitter's Light Intensity value is set to 90%, then the Particle/sec value will be 90% of the set value in the plug-in. In other words, a value of 100 particles/sec would become 90 particles/sec.
The Light's layer properties that can be modified.
The Light Emitter is recognized with the Light Emitter feature in the Options panel. When a comp Light has the same name as the Options Light Emitter name, that Light will be used to control the particle emission.
Enter a Light Name in the Name field, and use that same name for the Light layer in your Timeline. This will make the Light recognize the Light Emitter.
By default, this Light Name is named 'Emitter' and that is often an easy standard to use. But the Light Name can be changed to anything as long as the Light's layer name matches. More info on the Options Panel page.
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), choose Make Comp Size, then click 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:
Particles are emanating from the Light! Now bring up the Rotation properties for the Light by selecting the layer in the Timeline and hitting 'R'. Try scrubbing the X Rotation back and forth a bit and notice how the particle beam corresponds to the Light's 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
Go to Time 0:00 (zero) and create a new Spot Light. Name it 'Emitter2' and make it blue. Twirl down its layer properties. 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.