a wavelength corresponding to an absorption band of a color center accelerates the destruction of the color centers and the reduction of Yb 2+ ions to the trivalent state in proportion to the density of the radiation. A model is constructed for the bleaching mechanism that can be used to estimate the ionization cross section of the color centers.Introduction. Because of their increased availability, the demand for UV lasers is increasing in modern science, medicine, and industry [1]. The conventional UV sources employing lasers with nonlinear conversion from longer wavelengths to shorter harmonics or parametric lasers require complicated adjustments and are costly. At present the most promising sources are lasers with tuneable active media capable of generating UV radiation directly without conversion of any kind. These active media are based on interconfi gurational transitions of rare earth ions in wide-band dielectric crystals, such as Ce-activated fl uoride crystals [2]. However, there are a number of diffi culties with the use of solid materials such as these for active media of UV lasers. Here the major diffi culty is the photodynamic processes induced by intense UV pump radiation which form color centers in the active medium. These color centers cause additional loss of the pump radiation and losses in the luminescence range of the active medium, which reduces the quality factor of the cavities of lasers based on them and, thereby, lead to a drop in the laser output effi ciency; in a number of cases, this makes it impossible to obtain lasing with this active medium. Thus, studies of the photodynamic processes induced in the active medium by UV radiation and the search for effective methods of combating them are important tasks for the creation of new and effi cient crystalline active media for UV lasers. Knowledge of the properties and parameters of these processes are needed to solve this problem successfully, so the development of ways to measure them is also an important task.Various methods are currently in use to combat the color centers produced in Ce-activated crystals by UV radiation. One such method is to choose a basic matrix with as wide a band as possible that is specially extended into the UV range. But few such crystals are currently known. Another method is to improve the technology for growing activated crystals, since reducing the number of crystal lattice defects (which are potential traps for free charge carriers in the crystals) leads to a lower rate of color center formation. However, growing high quality Ce-activated crystals is a separate, and complicated, technological problem.One effective way of battling color centers in UV active media is coactivation of Ce-activated crystals with ions that create recombination channels for the free charge carriers produced in the active medium by the UV pump light. Yb 3+ ions are suitable for this purpose and their coactivation effi ciency for Ce-activated crystals has been confi rmed experimentally [3]. This coactivation technique is used in th...