At the present time the most efficient insulating crystal laser is &agr;‐YAG (Ho3+in sensitized YAG), but the 2.1‐&mgr; cw output from many &agr;‐YAG crystals undergoes a degradation under continuous tungsten lamp illumination. With the aid of a series of liquid optical filters, it is shown that degradation is caused by high‐energy pump photons. At least three different types of degradation are produced depending on the wavelength of pump radiation, but all may be eliminated with a liquid filter that absorbs radiation below 5000 Å.