Spotflare is a procedural flare generator, that generates a general radial light but also allow for light shimmer and "cone" masking.
At the core of spotflare is a radial light, that is generated from the inverse square of the distance to the center. Unlike a liniear falloff, a inverse square falloff gives a very realistic light look.
If you look at the diffrence between a gaussian and a gamma adjusted inverse square profile you can clearly see the effect.
This is the gaussian profile of the Radial Node. As the highlight rolls off at 1.0 any luminance adjustments down the line can make the light look flat.
This is the profile of Spotflare. The highlights have a steep peak, making it always blow out even if luminance is adjusted. The spread of this profile can be adjusted in the gamma control of the toolset.
The Nuke Expression Node contains a fBm noise function that takes a X,Y and Z offset as input. To generate a 2D cartesian Noise you simply add the 2D X and Y coordinates into the X and Y offset and leave the Z as a static.
To generate the light shimmer i calcualte the polar coordinates from the center of the flare, so that X becomes the rotation and Y becomes the distance from the center. I then apply thoes values as the X and Y offset, this gives the light shimmer streaks we need.
Noise based on polar coordinates
Noise based on cartesian coordinates