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The interstellar radiation field (ISRF) is the electromagnetic radiation (EMR) that occupies the "empty" (i.e., transparent) space of a galaxy. The radiation may be considered part of the interstellar medium (ISM). Clearly its nature depends upon its location, particularly on details of the nearby EMR sources (e.g., nearby stars, and their spectral class and luminosity). The radiation has effects on the ISM as well as all objects within the galaxy. A clear example of this is the ionization of hydrogen clouds.