Defects of high-voltage transmission lines are often accompanied with corona discharge; and corona discharge would produce ultraviolet. Based on this principle

the UV imager can be used for detecting the defect. In this paper

photon number and the number of corona current pulse per unit time is investigated with a rod-plane gap. And the relationship among the photon number

the number of corona current pulse per unit time

and the applied voltage are acquired. The results indicate that three stages exist in positive DC corona discharge. In stage 1

while the distance from UV imager to cathode and the UV gain are the same

the number of corona current pulse per unit time is approximately proportional to the applied voltage for different distance of rod-plane

and the UV photon number has approximate quadratic relationship with the applied voltage and the number of corona current pulse per unit time. In stage 2

the UV photon number decreases rapidly with increasing voltage without obvious rule. In stage 3

the UV photon number shows an exponential relationship with the positive applied voltage（N=a×ebx）

where parameter a decreases with the increase in the gap distance

depending on electrode structure and corona inception voltage

and parameter b is in the range of 0.2 and 0.3.