###### Abstract  
The detection of corona-related defects in transmission lines has been included in the work of transmission line defect detection. Ultraviolet detection technology has gradually been widely applied in the field of corona discharge detection. However, the current research on the application of ultraviolet detection technology in transmission lines is relatively simple and mainly limited to using changes in the photon count to determine whether corona discharge has occurred. To address this, this study used a mini corona cage to simulate transmission lines and measured the changes in the photon count, spot area, and corona current in the corona inception process of different types of smooth, stainless-steel conductors. This study also investigated the variations in the photon count and spot area depending on conductor corona intensity, ultraviolet imager gain, and observation distance. The results show that the photon count and spot area can, to some extent, reflect the intensity of corona discharge. Both the photon count and spot area exhibited quadratic relationships with the voltage. As the observation distance increased, both the photon count and spot area showed exponential decay. The photon count exhibited a trend of initially increasing, then decreasing, and finally increasing again with the increase of gain, while the spot area showed exponential growth with increasing gain. The photon count and spot area can complement each other to identify and characterize the intensity of corona discharge.  
corona discharge; UV imaging; photon number; gain; observation distance +
Footnote †: journal: Medicalmodels [17]. Therefore, it is necessary to study the variations in the photon count and light spot area depending on conductor voltage, as well as the gain and observation distance settings of the ultraviolet imaging system, to provide references for the detection of corona discharge on transmission lines using ultraviolet imaging instruments. Studying the corona discharge process of conductors using a corona cage is closer to the actual conditions of power transmission lines and offers advantages, such as low cost, controllable experimental conditions, convenient structural adjustments, ease of measurement, and shorter experimental cycles [18; 19; 20]. Therefore, utilizing a corona cage in the laboratory to investigate the variations in the photon count and light spot area with conductor voltage, as well as changes in the gain and observation distance of the ultraviolet imaging system, represents an effective experimental approach. This study utilized a mini corona cage to measure the variations in photon count, spot area, and corona current during the corona inception process of different types of smooth, stainless-steel conductors. The functions relating the ultraviolet photon count and spot area to conductor voltage for different conductors were obtained through fitting using the least squares method. The fitting results indicate that both the ultraviolet photon count and spot area exhibited a quadratic relationship with the conductor voltage. Additionally, this study investigated the variations in the photon count and spot area with respect to the gain and observation distance of the ultraviolet imaging system and performed relevant function fittings to capture these relationships. The spot area data obtained at different gains and observation distances were integrated to reference gains and spot areas. By combining the fitting functions of the photon count and spot area with respect to the conductor voltage at the reference gain and reference observation distance, the measured ultraviolet photon count or measured spot area can be used to estimate the corresponding conductor voltage, facilitating the assessment of the discharge severity. The above-mentioned research provides valuable application references for utilizing ultraviolet imaging to assess corona discharges in power transmission lines.