The emission of light by a substance that has absorbed light or other electromagnetic radiation such as ultraviolet light. Fluorescence is a useful indicator of organic matter type by distinguishing the fluorescent liptinic components from non-fluorescent ones. In addition to indicating organic matter maturation due to intensity of the fluorescence, which decreases with increasing maturity. Immature source rocks show a positive alteration (increase of fluorescence intensity during irradiation), mature source rocks show a negative alteration and at the over mature stage fluorescence is no longer visible.