The Electromagnetic particle Detectors (ED) within the 1 km² array (KM2A)  Used for measuring the secondary electro-magnetic particles in the EAS. The detecting medium is plastic scintillator. The scintillation light generated by charged particles in the scintillator is collected through wavelength-shift optical fibers, conducted to photomultiplier tubes (PMTs) and converted to electrical signals for measurement. The effective area of each ED is 1 m².