A fused quartz bubble window approximately 0.115 mm thick and several centimeters in diameter has been constructed. It has high far infrared transmittance (∼70–90% for 180 &mgr;<&lgr;<3 mm), can pass a beam of large cross section, and can support a vacuum. The window is a bubble blown inward (convex toward low pressure) to be under tension. Its use avoids the temperature dependent transmission characteristics of crystal quartz, a material often used for vacuum envelope windows because of its high far infrared transmittance. Unlike crystal quartz windows, the fused quartz window can be fused to the envelope. Transmission measurements, lens effects, and reflection losses are discussed.