Potential or light induces degration reduce the performance continually, without being noticed. And without constant voltage measurement in the module, PID recognition in the field is simply impossible.
 

Common detection:            Electroluminescence / open-circuit voltage measurements
Necessary requirements:   Plant inspecton / good access to the modules

Detection by SunSniffer:   Measurement and analysis of module voltage

One defective diode reduces modules voltage to 66%, two defective diodes down to 33%. This is not noticeable without measurement of modules voltages.

Common detection:            Junction box opening / open-circuit voltage measurements
Necessary requirements:   Plant inspecton / good access to the modules

Detection by SunSniffer:   Measurement and analysis of module voltage 

Badly-soldered connections or structural defects in the cell cause selective heat productions at those spots. Any hot spot is a power leakage. 

Common detection:            IR-thermography / visual inspection
Necessary requirements:   Plant inspecton / good access to the modules

Detection by SunSniffer:   Measurement and analysis of module voltage

Hail can cause serious defects of cells. Cracks in a cell may be small, but can cause or accelerate degradation.

Common detection:            Electroluminescence / visual inspection
Necessary requirements:   Plant inspecton / good access to the modules

Detection by SunSniffer:   Measurement and analysis of module voltage