A device for monitoring radiation flux from a surface. The flux monitor is based on the photoelectric effect that occurs inherently when a reflective metal optic is exposed to a beam of energetic radiation. The incoming beam of energetic radiation is not totally reflected by the optic surface. That portion of the radiation absorbed by the optic generates photoelectrons producing a signal proportional to the incident radiation flux. By measuring this signal, an accurate determination of the radiation reflected by the optic surface can be made.
 This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention.