During a partial discharge the air around HV equipment is caused to fluoresce. The air is mainly composed of Nitrogen, which emits UV light (UVa, UVb & UVc) during fluorescence. Fluorescence does not generate any heat therefore the phenomenon is invisible to thermal cameras.
All modern CoroCAM’s feature at least 2 internal cameras: one UV camera and one visible camera. The UV image is overlaid onto the visible image to locate the source of the discharge or the UV image is viewed alone to see the shape of the discharge.
CoroCAM models can be non solar blind (NSB) or solar blind (SB).
Non-Solar Blind cameras feature a UV camera which is sensitive to UVa & UVb light. UVa & UVb is abundant in sunlight, therefore these CoroCAM’s can only be used indoors or at night, else it may be blinded by the ambient sunlight.
Solar blind cameras feature a special filter which blocks all light frequencies from the sun observed at ground level, rendering it blind to solar (sun) light. UVc light (100-280nm) is not blocked. Some UVc light is emitted by the fluorescence of air caused by discharges, this allows for inspections to be performed in full daylight.
Some solar blind models also have thermal imaging modules allowing for multi-spectral imaging, correlating the thermal image with the UV image of the corona allows operators to determine the type of discharge activity.