DETECTING CORONA
Corona generates very little heat and therefore is not detectable with thermal cameras.
The corona discharge emits radiation in the 280nm-405nm spectral range, mostly in the
ultraviolet (UV) and therefore is invisible to the human eye, though relatively weak
emission at about 400 nm might be observed at night under conditions of absolute
darkness. The corona emission in the 280nm-405nm spectral range cannot be detected
during daytime due to the large amount of natural solar radiation. The CoronaFinder can
only be effectively used outdoors at night time or can be used indoors, with or without
the room lights on.
USING CORONA FILTERS
Both filters can be used indoors or outdoors, under all conditions. Both filters are equally
useful, but usually under different conditions. An experienced operator will quickly learn
which filter works best under a variety of lighting and searching conditions.
Corona Filter #1 is a very broad spectrum bandpass UV filter. It passes the entire corona
spectrum from deep UV to visible. It has a peak transmission of about 80% in the middle
of its passband. It also passes a small amount in the near IR.
Corona Filter #4 is a very narrow bandpass UV filter. It only passes a narrow section in
the middle of the corona spectrum. It has a peak transmission of about 35-40%. It does
not pass any in the visible or the near IR.
Corona Filter #1 passes light in the UV and the near IR part of the spectrum. This allows
the operator to see the corona and some of the equipment surrounding the corona. This
makes it easier for the operator to pinpoint the exact cause and location of the corona.
Since Corona Filter #1 also passes some of the light in the near IR, a few bright shiny
objects may falsely appear to be emitting corona. Some of these can be eliminated by
dimming the lights or by viewing from a different angle. Light reflections will move or
change intensity depending on viewing angle, but actual corona emission will not move.
Corona filter #4 only passes light in the UV part of the spectrum. This allows the
operator to have maximum contrast and minimum false detections of corona. This has
the disadvantage in that it is more difficult to pinpoint the exact location of the corona.
One may be able to see the corona but still cannot tell exactly where it is. With practice
the operator will be able to view the corona, and then lift the CoronaFinder to see the
location.
Since Corona Filter #1 has a higher peak transmission and a wider passband than Corona
Filter #4; it will always be able to detect lower levels of corona than Corona Filter #4. To
achieve maximum sensitivity and minimum false detections the room lights may need to
be dimmed when using Corona Filter #1.
Corona Filter #4 is fairly insensitive to room lighting.
The amount of room lighting and the quantity of corona will influence which filter will
work better
