So I'm modeling a space ship, and I want to make a pattern on it glow with an iridescent sheen only when in direct light. When strong light hits that specific area, I want a bunch of lines of iridescent patters to appear, but not where there isn't strong light. I hope this makes sense. I've been trying a lot of different things, but none of them work. Any suggestions would help.
As Nathan mentioned in his comment, you can make use of Shader to RGB. Used a Diffuse into Shader to RGB into a ColorRamp (your choice whether to use linear or constant) like a basic Toon Shader setup, but only using 2 shades - black and white. Use this as a mix factor between your base material and your Iridescent material.
Here's the setup in its entirety with a slightly better Iridescent texture:
EDIT - Sorry, I just saw you wanted a "glow". If you want this, try mixing your glossy shader with an emission shader of the same color. If you just want the "glow sheen" to overlay the current material (without replacing it entirely), you can try something like the setup below: