Most phosphors appear white in ordinary light. When the fluorescent lamp is turned on, mercury in the lamp emits UV radiation that causes the phosphor to emit its characteristic color. In the case of the BL phosphor, the tube emits a wide band of color peaking in the near UV area where alternative emulsions are most sensitive. The BLB lamps have a purple filter to cut out most of the visible light from the BL phosphor, but that light is useful for photography, so the BL lamps are better for our uses.