The best for fire starting Ive found is a big and powerful magnifying glass, what you say about surface area of the lens seems right. I don’t think I could start a fire with a rifle scope, however well it concentrates its focus, just as I can’t with a small loupe.
Perhaps not the locals headlights despite the plush Landrovers that abound, it’s all chainsaws, tree surgeons and sawmills round here, most of them mates
. I might try the local breakers yard though. It’s one of those things that’ll be the first to break even in a minor shunt and involve replacing the whole headlamp unit. There must be hundreds of them about being chucked away. I’ve no idea what that lens bit is even called to be honest, they just intrigue me.
Edit: They are called projection modules says google.