An intense, monochromatic, extreme ultraviolet (121.6 nm) light source has been constructed for testing extreme ultraviolet light rejecting filters. The source and detection system operates at atmospheric pressure, is physically compact, and is relatively inexpensive to build. A calibrated neutral density filter with a transmission of1×10−6is used as a reference. The light source has been used to measure transmissions as small as9×10−6quickly (in less than 1 min) with negligible background levels. ©1998 American Institute of Physics.