Light decay generally refers to its light flux. When charging the surface of the light drum, with the accumulation of the charge on the surface of the sensor, the potential continues to rise. Finally, the "saturation" potential is the highest potential. The surface potential will decrease over time. Generally, the potential at the time of work is lower than this potential. The process of naturally decreased by this potential is called the "dark decay" process. During the exposure of the light drum scanning, the potential of the dark region (referring to the surface of the optical conductor that did not be illuminated by the light) is still in the dark decay process; The conductivity rate rose rapidly, forming optical guide voltage, the charge disappeared rapidly, and the surface potential of the optical conductor also dropped rapidly. It is called "light decay" and finally slows down.