Absolute intensity measurements in the extreme ultraviolet spectrum of solar radiation

Experimental results and problems of absolute intensity measurements of solar electromagnetic radiation in the extreme ultraviolet (EUV) and soft X-ray region of the spectrum (designated cumulatively as XUV for brevity) are reviewed. The numerous practical problems are divided in two major areas, (a) general problems of heterochromatic absolute XUV spectrophotometry in the laboratory and (b) specific problems characteristic of requirements of solar physics, the physics of planetary atmospheres, and existing restrictions of space technology. Within the first area (a) emphasis is placed on recent progress toward justified reliance on ionization detectors without necessary connection to source standards. For the second area (b), emphasis is placed on the immediate need to have existing exploratory observations followed by a new phase of more systematic experiments of increased accuracy.