A lamp assembly adapted for use in a substrate thermal processing chamber to heat the substrate to temperatures up to at least about 1100.degree. C. is disclosed. In one embodiment, the lamp assembly comprises a bulb enclosing at least one radiation generating filament attached to a pair of leads, a lamp base configured to receive the pair of leads, a sleeve having a wall thickness of at least about 0.013 inches and a potting compound having a thermal conductivity greater than about 100 W/(K-m).