Simply use a separate layer with a "Circle" effect and play around with colors, belnding modes and possibly also matte options.
I'm still having a bit of trouble getting the desired result following your suggestion.. Could you elaborate a bit, or possibly post an example? Much thanks!
if this is a 2D light effect, you don't need a 3D Light. 2d options include these options :
1. can create a vignette effect use CC Vignette and set it to minus values to make it white.
2. if you want a specific shape you can use a solid layer with a mask and invert the mask + feather it.