In this paper, the operating principle of daylight UV imagers was introduced first, emphasizing the SBUV(solar blind UV) technology, which utilizes the fact that ultra violet between 240nm~280nm is being absorbed by the ozone so that signals detected on earth in this spectral range originate on earth. And then several influencing factors were explained, including observation distance, observing angle, imager gain settings and environmental conditions. Experimental data measured in the UHV(ultra high-voltage) DC converter station in Changzhi, Shanxi, China were analyzed using SRA(single regression analysis) method, and mathematical equations with acceptable deviation were calculated, with simulating curves plotted. The results show that environmental conditions including humidity and temperature, observation distance and imager gain settings all contribute to the measuring result, exhibiting as exponential function and convex function respectively. Concluded from the above analysis and calculation, observing conditions of a clear observing angle at the same observation distance with mediate gain settings on days of low humidity were recommended. This conclusion may guide further ultra-violet imaging appliance in high-voltage electrical devices corona discharge sensing.