A : The WitchFire has a single-element, large-area shortwave ultraviolet detector that senses the very faint emissions from corona discharge.  The WitchFire is a camera with only one wide field of view pixel.  The lens has a large aperture that greatly enhance the system sensitivity, as well as limiting the field of view of the detector (the detection cone) so that the source of the UV emissions can be localized. The WitchFire has a cone-shaped field of view that enables the operator to locate the position of corona discharge within a 12 inch diameter spot at a range of 20 feet.  At 40 feet, the spot will be 24 inches across.

A: The special detector inside does not respond to either reflected or direct sunlight which could otherwise look like corona.  The sensors can be used outside on the brightest of days since it is “solar-blind.”

A:  The Shortwave Ultraviolet, also known as the UVc or solar-blind band, is a part of the electromagnetic spectrum with a wavelength range of 200nm to 280nm.  It is invisible to the human eye.  The ozone layer absorbs sunlight with wavelengths below 280nm, and thus there is negligible solar radiation in this band at ground level, hence the name solar-blind.  The WitchFire sensor operates exclusively in this band and is immune to sunlight, even when pointed directly at the sun.

* If you decide to point the sensors at the sun, do it for a short time only, since the lenses in the systems will focus the sunlight on the detector and could cause heat damage.

A : Yes. The electromagnetic emission from corona discharge in air is primarily in the longwave ultraviolet band peaking around 350nm, and there is a small component in the visible band, such that the dark adapted human eye will see a very faint purple glow.  The longwave UV band is also known as UVa, with spectral limits from 320 to 400nm and is invisible to the unaided human eye. It is straightforward to capture images and video of corona in low light conditions in the longwave UV band, but there is abundant sunlight in this region of the spectrum, making it impossible to see corona discharge during the day, unless the discharge current is very high.  The corona emissions in the shortwave UV band are approximately 100 times fainter than in the longwave UV band.  

A: Because the shortwave UV emissions from corona are 100 times fainter than in the longwave UV, a very high gain imaging sensor is required. This sensor is typically an image intensifier tube with a special photocathode and faceplate that operates in the shortwave UV band and outputs a visible-light image stream that is a replica of the shortwave UV scene.  A complex bandpass filter that rejects EM radiation above 280nm must be inserted into the optical path between the image intensifier and a shortwave UV lens, since otherwise the image intensifier will also detect the much brighter ambient illumination outside the solar-blind band.  This filter must have a tremendous out-of-band rejection, on the order of 1 part in a trillion, in order to make the corona camera immune to sunlight. That is quite difficult to achieve.

The image intensifier has a phosphor screen analog output of visible video which is coupled to a CMOS imaging sensor. The optical system has a dichroic beamsplitter in it so that a color camera video stream can be combined with the UV image stream, with the UV stream overlaid on the color video stream from the CMOS imager to a high degree of precision.  All these engineering requirements make for a very expensive product.

A:  The sensitivity is comparable to a typical corona camera, in that a corona discharge with a fixed intensity can be located out to about the same maximum range with either device. The corona camera has the advantage of locating the position of corona much more precisely than the prüfentronik sensors, but in many use cases, the operator need only to determine that corona is present to warrant further investigation. 

The strength of the corona emission is indicated by the pulse rate of the green indicator light and the beeper, combined with the range to the emission point.  Once a source of corona is located and it is determined to be intense enough to warrant further investigation (a judgement call for corona camera operators as well), the location of the source can be examined visually (and audibly) from the ground with binoculars, from a cherry picker, or via an inspection drone.  The closer the sensors get to the source of the corona, the smaller the detection spot size, and the easier it is to precisely locate the corona source.

Corona discharge can occur on high voltage insulators (bushings) that have surface contamination in the form of dust, dirt or salt encrustation due to proximity to the ocean.  The surface contamination can get damp due to fog or moisture condensation, and moisture combined with contamination leads to a conductive path that causes partial discharge, with its attendant UV emission.  A buzzing sound is often heard as well.  If the partial discharge current gets high enough, the surface of the insulators can get damaged, leading to higher corona current and a runaway process.  Bird droppings on insulators can lead to equipment failure, especially if the birds repeatedly soil the same insulator.

Another common cause of corona is broken strands on high voltage transmission line cables.  These strands can be broken by lightning strikes or by malicious people with rifles shooting at the lines.  If this is not repaired, the nitric acid produced by corona discharge and moisture and the ozone can eventually corrode through the remaining strands and drop an energized line into dry brush, causing wildfires.