Corona Effect Cameras

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In the case of N2 (the main component of the air around us), the frequencies of light emitted are mainly in the UV range, with only 5% above 400 nm, which is considered the lower end of visible blue.

The light spectrum of N2 fluorescence, after having been stretched vertically 1000 times larger.

Corona discharge detection

Corona downloads have several signatures that can be detected:

UV cameras are the best for detecting corona discharges.

The Sun emits ultraviolet light in all bands.

The ozone layer only lets through some frequencies at various volumes.

Type of UV radiationWavelengthEffectTransmission through Zone O
UVA, long wave400-315 nmSkin tanning95%
UVB, mid-wave range315-280 nmDNA damage5%
UVС, Shortwave range280-100 nmCell death0%

Below 280 nm the light from the N2 fluorescence is only visible

Below 280 nm very little light remains, so an image intensifier is used to amplify the light to levels where the camera can perceive it.

Image intensifiers

Due to the spacing and misalignment of the microchannel plates, some electron scattering occurs during the transition from one plate to the next, resulting in satellite spots around the projection of the discharge site.

CCD operation

When the phosphor screen is impacted by electrons, it glows green.

The green light is detected by the CCD pixels, where it is converted back into electrons.

The CCD has a green light.

The CCD has a maximum charge that it can accept between readings before overflowing.

When a pixel "spills over" it is called blooming."

The blooming of a pixel is called "blooming."

The overflowing electrons make their way to neighboring pixels, increasing their readout value, resulting in a blurry looking image.

When a pixel saturates and "overflows", the additional intensity values that overflow are lost.

Ideally, the ideal is to reduce the gain until the pixel does not saturate, but then lower intensity signals will be lost, which will reduce the camera's sensitivity.

Image Processing - Display

The raw image of the corona is not useful to the inspector, as it contains no structural information that would allow him to identify the physical location of the discharge.

Corona discharge detection

In order to display UVC and visible images in a useful way you have to use 2 types of cameras together with suitable optics.

The detected UV image is superimposed on a visible image to show where the discharge occurs. The UV spot is false-colored to make it observable.


Once a discharge has been detected, the camera should zoom in to show the operator the physical cause of the discharge.

The camera is zoomed in to show the operator the physical cause of the discharge.

Zooming in is the common term for changing the field of view.

The camera is zoomed in.

Field of view (FOV)

The field of view is the solid angle through which a detector is sensitive to light.

Field of view is the solid angle through which a detector is sensitive to light.

FOV = Field of view

DFOV = Diagonal field of view

HFOV = Horizontal field of view


When a camera is zoomed in, the FOV is reduced.

The black frame is the initial FOV

The green frame shows the zoomed-in FOV.

The green frame shows the zoomed-in FOV.

The green frame shows the zoomed-out FOV.

Optical zoom

Optical zoom is the result of changing the field of view (FOV) of the camera, which is achieved by mechanically changing the relative position of the lens elements.

This is the best type of zoom, since it makes the smallest details visible.

The strength of the zoom is the ability to zoom in and out.

The zoom strength can be expressed as "##x" zoom, but this is not useful since you don't know the initial or final FOV.

So, it is better to indicate the initial and final FOV.

It is better to indicate the initial and final FOV.


Digital zoom

Digital zoom magnifies every pixel.

Digital zoom.

Enlargement can be interpolative or non-interpolative. Interpolative gives gradient edges as opposed to non-interpolative, which gives sharp block edges.

RIV - In theory, actually calculated from pC

IEC 60270:2000 - refers to how pC was measured, not the configuration to generate the discharge. The standard was written before sun shading corona chambers existed and does not address any of the variables that affect the use of corona chambers.

NEMA 107-1987 - addresses how RIV was measured, again it was written long before corona chambers were available.

Given the number of uncontrolled variables (and their effects), it is not appropriate to use a discharge spot as a source of ultraviolet light for camera sensitivity testing.

The use of a device called a blackbody is more appropriate, since few of the variables that affect the discharges affect the blackbody.

The blackbody is used to test the sensitivity of the cameras.

The blackbody is used in physics as a stable source of light over a range of frequencies. The intensity of the light per frequency is easily and precisely controlled by adjusting the temperature of the device.

The only variable is the distance the camera is held from the blackbody aperture.

The only thing to get right is the calculation of the ultraviolet light emitted by the BB through its aperture.

What does the detection interpret?

Most practical users consider a spot in the same location at 50% of the frames per second to be an indication that there is a UV source at that location.


An expression for the intensity of the discharge was needed to calculate the decay rate of the affected components.

Initially, spot counting was applied.

The only effective result was the decay rate of the affected components.

It was only effective in low-intensity discharges when the camera is out of focus, as overlapping or contacting spots skewed the results.

Initially, spot counting was applied.

Summing intensities proved to be a better approach, as its results were only skewed when the image reached saturation.

The grayscale values of all pixels are summed, and a "blank image" value is subtracted.

This value is then adjusted to the UVc irradiance needed to generate the same intensity.

By changing the sampling area, the intensity of the light received from individual discharges can be measured.

Discharge intensity

To obtain the apparent discharge intensity, a number of factors must be taken into account: