Headlamp/ lazer!

Bought a new 100w bulb from ebay. Very bright with a hint of blue to the light, nice i thought... only thing is after a short time the headlamp glass gets so hot you cannot touch it. And the lens is not glass, its plastic. Now i'm guessing its a heat resistant unit, but to what point? Dont want to find the headlamp melted onto the mudguard after a longer ride! I haven't noticed how hot the original bulb got to, but i'm sure i've never burnt myself before! Anyone had a similar experience?