• Over the life of an incandescent light bulb the filament begins to deteriorate and the particles will settle on the inside of the glass. In return the bulb will take on a grayish appearance and a slight decrease in light output may occur. • For fluorescent bulbs only the ends will blacken as the bulb is burned. Each time the lamp is turned on and off the emissions material settles on the inside of the glass. Extreme darkening generally indicates the bulb is about to burn out.