Glow Writer is a phosphorescent marking pen for annotating or marking chemiluminescent and isotope-labeled substrates for X-ray film autography. The unique marker delivers a phosphor-containing marking ink through a fine point porous nib. Before exposure to X-ray film, the phosphorescent writing is excited by bright room light. A phosphor ink having an optimum concentration of phosphor and a relatively short afterglow half-life (approx. 2-3 minutes) has been selected so that upon exposure to X-ray film, autographic exposures of almost any duration will be easily readable on the film. The fine point nib is easily replaceable if it dries out.