Hey guys, I’ve been trying to make realistic headlamps for a vehicle build, but it just wasn’t seeming good. I finally narrowed it down to the fact the car headlamps have diffusers and reflectors, which I need to add.
Modelling - or a combination of hand modelling and bump/normal maps would probably work best. The material itself should be easy, essentially based on a glossy shader.
Both are quite difficult to achieve… Modelling will give you good results but the number of bounces needed for a correct result will raise the rendering time.
Bump mapping will only work for the diffusers… not really for the reflectors. On the other hand, reflectors, if seen from the distance, will look as a glossy shader with it’s normal equal to the incoming vector. But for close ups, there’s no way to invert the reflection (as reflectors do) using simple shaders. Maybe rendering the reflection enviroment into a texture, and inverting the coordinates for a fake reflection… 
In terms of reflectors - the actual retroreflective effect is easy to achieve. Check these links out:
These may not help you - but the tape one would be good to use if you were modelling a police car or ambulance which have retroreflective stickers.
