The stability of ultra-high voltage grid is related to the working condition of the insulators

whose insulation performance degrades during working period with surface discharge.The ultraviolet camera（UV-camera） can be used to detect the working condition of the insulators for fault diagnosis.The main parameters of UV-camera are spot area and spot diameter.In this paper

five kinds of insulation states of a same kind insulator

i.e.normal

good

slight trace

degradation and severe aging

are selected through the water spray classification method to compare the effect of the two parameters on insulator fault diagnosis.A platform is constructed in laboratory to take sufficient ultraviolet pictures of insulator discharge with a CoroCAM504 type UV camera（made in South Africa）at the voltage of 8 kV

10 kV and 15 kV

respectively.The two parameters are extracted by using the digital image processing algorithm

and the variation of the two parameters with insulation condition of the insulator is obtained through digital simulation by MATLAB.The results indicate that:When the insulation condition of the insulator is perfect

the spot area and spot diameter are almost negligible;Smaller values of the two parameters correspond to nice insulation;When the insulation get worse

both parameters show incremental tendency

but only spot area appears a turning point "4"

which suggests spot diameter is more sensitive to insulation degradation as the degradation reaches a certain degree.This study may provide a basis for quantitative assessment of insulator working condition.