The CoroCAM® cameras are used to image the UV light signature of external discharge activity (Corona, Sparking and Arcing). The UV light is mainly (~95%) in the UVa & UVb bands, but it’s nearly impossible to image the signatures due to the intensity of sunlight in those bands.
The UV light signature of discharge activity (purple) after it was stretched vertically 1000x and overlaid onto the spectrum of light from the sun (300 to the Near Infra-Red)
No light from the sun in the UVc / Solar Blind (SB) / <300nm band passes through the O-zone barrier, making the discharge signature nearly unique. There are a few natural or man-made sources of UVc light, but they have other characteristics which makes it possible to identify them. 

The SB band is therefore ideal for uniquely imaging the signature of discharge activity. However, the resulting image is just a blob in an otherwise black image.

Binarised UVc image
This is clearly not useful in identifying the location of the discharge activity.
The blob has to be co-located with a visible structure in order to identify the location of the discharge activity.


A visible video camera is used to record the visible image while a bore-sighted and electronically fine aligned UV camera records the UV image. A high throughput prism and mirror is used to direct only the UV light from the incoming beam path at the UV camera. A solar blind filter is used to reject / block all the light outside the UVc band.

Image Intensifier

The light intensity from a corona discharge below 280nm is very low, therefore an image intensifier is used to amplify the light to levels where the camera pixels can sense it.
The resulting image looks like this.
The spurious noise is removed using temporal and morphological filters and finally the image is binarized, false coloured and the coloured blob overlaid onto the visible image, yielding the final image.
The camera user-interface is overlaid onto the image and displayed to the user.

UV Intensity Expression

The initial users of corona cameras wanted to know if the discharge being observed was “severe”, thereby justifying the maintenance expense to correct the situation. Based on their thermography knowledge it stood to reason that the more intense the discharge was the more severe it was.

Logically counting the photons received from a discharge activity could be used as an indicator of the severity.

At low intensities in the laboratory the blobs were separately visible and could be counted individually. The count increased/decreased with changes in intensity.

With increases in sensitivity/ higher intensity discharges observed the number of photons detected increased, resulting in the blobs overlapping or touching, which would then be counted as one blob.

Drawing a graph of the count to intensity results in the below:
Bell curve with right ending at 1 not zero as on the left.
The count starts at zero for zero intensity, as the intensity increases the count increased. As the blobs start merging the count increase per intensity increase reduces until the count starts reducing as the intensity increases, ultimately at maximum intensity a single blob is observed – resulting in a count of 1.

The blob counting regime also newer specified the conditions, distance or camera setting under which expressions of low to high severity was assessed.
This is clearly not useful.

A new technique was introduced with the CoroCAM® 6D, which matches the total observed intensity in an image with the intensity of a source and returns a “count” equivalent.
The resulting intensity to “count” graph grows nearly linearly from low intensity to saturation – this is a far better way of expressing the intensity of a discharge.
Graph of CoroCAM® Intensity to count
However, since the intensity measurement is only valid at that time and it changes with changes in the conditions and observers it is clearly not ideal as a severity indicator or for trending.

US EPRI has developed a better regime to assess the maintenance priority of a discharge location. 


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