US 1.888.950.5557 | CN 86.1580.215.4221


US 1.888.950.5557 | CN 86.1580.215.4221


UV & Acoustic Cameras
  1.  UV camera
  2.  US camera
  3.  US predicted corona
  4.  UV photo of corona

Electric Corona PD on high/medium voltage installations leads to loss of energy and to an accelerated aging of insulators. 

Corona produces 

  • Ozone, acids
  • Electromagnetic emissions
  • Ultraviolet light
  • High-frequency sound
  • Audio noise

Thermodynamically, corona creates a non-thermal plasma where not enough energy is released to heat the gas in the corona region. Thermal cameras, therefore, miss to detect corona (not arcing nor sparks).  

High-frequency sound, ranging 40-100kHz, is also created during the process due to collisions between space charges and air molecules. The human ear is sensitive to audible range of 20 to 20,000 hertz and therefore ultrasonic sensors are required to detect PD sound and transform them into audible noise. 

Electric corona PD emits electromagnetic radiation in the UV region of the spectrum (240 – 400nm), and hence the term “corona”.  Human eyes respond to wavelength from 380 to about 750nm and as a result need a DayCor® UV camera to see corona in daytime.  

Sniffers that can detect chemical reactions outcome such as the formation of Ozone and nitrous oxide gases will indicate exiting ionization processes of partial discharge, but these are not feasible for monitoring powerlines. 

As soon as the electric Corona PD is detected maintenance teams or manufacturers perform corrective actions in order to reduce the discharge and avoid further power loses. 

"Optical detection techniques provide many advantages in the consideration of accuracy and suitability for the applications when compared to other techniques"

UV & US testing for Corona PD
UV & US testing methods


US discharge prediction
Corona representation the outcome of a Sonic imager
Acoustic Discharge Recorded
The acoustic source for the corona image (up)
Corona UV Photo
Photo of UV Corona PD by a UV camera
corona imager - false PD
Audio noise - as displayed by a sonic corona imager

What do Acoustic/Ultrasound Detectors Look for?

Acoustic detectors aim at finding distinctive sound signatures of partial discharge as well as gas and vacuum leaks. These detectors are effective in places that have no external noise , such as underground or indoor networks as ambient noise is a problem at lower frequencies near 20 kHz, where the sensitivity is greater Acoustic detectors are effective for locating within a distance range of 0.3-m (1 ft) to 9 m (30 ft). When used outdoors, this method is disturbed by environmental noise. To limit the amount of ambient noise that gets into the receiver there is a need to reduce their sensing angle. 

What Are the Main Advantages of Acoustic Detectors? 

  • Acoustic detectors can direct inspectors to the approximate location of corona discharges, suggesting a rather wide area as the suspicious target. 
  • Acoustic detectors are most effective diagnostic tools for the detection of enclosed defects in GIS (Gas Insulated Systems) and within solid insulation.  


What Are the Drawbacks of Using Acoustic Detectors?

  • Bias – Portable acoustic detectors provide information via headphones or loudspeakers and rely heavily on operators’ ability to distinguish between nuances of noise.  The results were considered as unreliably biased for interpretation and analysis.
  • Graphs – Acoustic detectors display metric oscillating graphs to indicate the captured sound and need further analysis and proficiency. 
  • Attenuation -Sound intensity decays logarithmically with the distance between the measurement point. Sonic detectors were tested and the decay exceeded that of the  [1] and the atual decay rate was even higher. These detector are less fit for remote inspection of OHT lines.
  • Reflections – Sound intensity is affected strongly by reflections from objects such as walls, ground expected results
    should differ somewhat from this law

What are Acoustic Imagers?

Acknowledging the advantages of displaying corona on its emitting source, acoustic detectors use a matrix of microphones and a standard digital camera. The microphone sonic recordings are processed and displayed as a schematic representation of corona  superimposed on the visible image generated by the digital camera. 

What are the drawback of Acoustic Imagers?

  • Unable to pinpoint the discharge. The image of corona covers a large area.
  • Reflections – a major issue with acoustic imagers, in particular in the lower audio frequencies where the sensitivity of the acoustic imager is high.
  • Logarithmic attenuation of sonic signals, more than calculated or planned, and therefore not effective for remote inspection.
  • The microphones’ matrix is exposed to dust and humidity.
  • Too large probe, unfit for UAS 
  • The process of transforming audio signals into an image is time consuming and slows down inspection 
Discharge decay
Logarithmic decay of the sound intensity of the discharges that were detected by the acoustic NL camera. Experimental points and logarithmic fit (R 2 = 0.9891) that was measured with the sphere-plane gap with 50 Hz ac supply
[1] MM Yaacob1, MA Alsaedi, JR Rashed, AM Dakhil, and SF Atyah, “Review on Partial Discharge Detection Techniques Related to High Voltage Power Equipment Using Different Sensors,” Photonic Sensors, DOI: 10.1007/s13320-014-0146-7.
Needle-plane-gap, Left: Corona imager, Right: SLR image of corona
Sphere-pane-gap, Left: Corona imager, Right: SLR image of corona

The SLR photos demonstrate the meaning of pinpointing. SLR pictures were taken in dark rooms and involved long exposure. Acoustic imagers, as shown do not pinpoint the discharge, though they were taken under light. 

Arcing on Railways insulator
Arcing on Railways insulator - pinpointed discharge
UV camera - Pd on end windings
UV inspection, PD on end windings

UV imaging technology is the only technique able to provide to technician a direct true photo of the discharge. 

SENSITIVITY – The DayCor® solar blind UV detection cameras are highly sensitive to UV radiation and detect electric corona PD from distances of 150m. Therefore DayCor® cameras are fit for remote inspection of OHTL. 

REAL TIME NO DELAY – Ofil’s cameras implement the DayCor® bi-spectral technology. DayCor® technology uses a smart combination of optical components that enable seeing the UV radiation of corona in real time without any  processing. As a result, Ofil’s DayCor® cameras can be used while driving, flying, walking without losing any discharge. 

RELIABLE – Professional users know that  DayCor® cameras are reliable and therefore DayCor® are being used for research and investigation in HV labs and by rotating machine workshop that fix motors and generators.  

PINPOINTED INFORMATION – The outstanding optics and the HD digital camera ensure taking clear bright video and photos of pinpointed corona overplayed on the visible digital camera output.  Thereby, investigation and inspection duration is shorter with results that help prioritize corrective maintenance.

COMPARATIVE DATA – Ambient conditions highly affect corona intensity, and must be added to the recording. DayCor® cameras have a plug-in TRH sensor whose reading are printed on the captured video/phots. Thereby it is possible to follow up, trend and compare results. 


UVolle-and-defected end fitting fault
UV Corona Camera pinpoints end fitting suspected problem


  • Acoustic imagers are sensitive corona PD detectors but for short distances. The detection capability of those equipment deteriorates drastically as the distance from the corona source increases. This technology is safe and satisfying for very intense coronas. The DayCor®  corona camera, on the other hand, detects electric corona UV PD easily even from distances of 150m and more.

  • The exact location of corona discharge indicates severity of damages and leads to the actions needed to be taken by maintenance teams. acoustic imagers  do not pinpoint corona sources but direct experienced operators towards suspicious defective insulators of high voltage pylons. While corona UV cameras pinpoint corona on any installation and component as long as there is a line of sight.

  • Whether a discharge is on an arcing horn, on a corona ring, on an end fitting of a polymer insulator or on the cap of a porcelain insulator etc… can easily be revealed by an optical device like a corona camera.

  • Acoustic detectors can detect enclosed corona PD sources. 

  • Noisy environments, as in urban locations are not ignored by acoustic imagers, despite their published specifications.  Corona UV cameras, on the other hand are disturbed by sporadic sparks and welding but are easily filtered out. 

  • Due to noise sonic imagers detectors cannot be used from helicopters. Corona cameras, on the other hand, are fit for airborne inspections both by helicopters and UAS.

  • Sonic imagers are seemingly less expensive to buy but the trade-off is in saving labor cost. Inspection team using a corona camera spend less time in investigating, finding and reports generations. 

Acoustic detectors
Corona UV Cameras
Measurement method
Targets for corona PD detection
Sound waves and electromagnetic waves
UV radiation, photons
Data processing
Processing the amplitude attenuation or phase delay of the acoustic waves propagating from the PD
No processing: Based on UV photons produced as the result of ionization during the discharge.
Incapable of locating EXACT location of corona PD. Even when corona is seemingly displayed as a target, it covers too many objects
Getting pinpointed image of the discharge shape, location & direction
Noise elimination is imperative, affects sensitivity.
No need but there is an option to filter noise without impairing sensitivity
Noise sources
Discrete spectral interference, Periodic pulse shaped interferences and random interference. These noise may overlap and superpose the actual PD.
Welding sparks. This kind of noise can easily be eliminated because sparks appear randomly vs corona PD is repetitive, appears in the same location
Detecting gas leaks, Inspecting GIS
Locating sources for audio noise, Assessing pollution before and during washing, Periodic inspection for condition based maintenance; Solving customers’ complaints Investigating electrical faults; Research in laboratories; Aerial scanning of overhead lines and more

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