A double-enveloped lamp assembly includes a light-source capsule subject to burst on rare occasions, a light-transmissive shield substantially surrounding the light-source capsule for absorbing and dissipating a portion of the energy when the light-source capsule bursts, a mesh of substantially nonconducting fiber for reinforcing the shield, and a light-transmissive outer envelope. The mesh is fabricated of ceramic fibers having sufficient strength to reinforce the shield. Since the fibers are nonconducting, sodium migration is minimized. The ceramic fiber mesh is particularly useful for high wattage lamps where thick-walled outer envelopes are not practical.