Many studies have reported dosimetric characteristics of amorphous silicon electronic portal imaging devices (EPIDs). Some studies ascribed a non‐linear signal to gain ghosting and image lag. Other reports, however, state the effect is negligible. This study compares the signal‐to‐monitor unit (MU) ratio for three different brands of EPID systems. The signal was measured for a wide range of monitor units (5–1000), dose‐rates, and beam energies. All EPIDs exhibited a relative under‐response for beams of few MUs; giving 4 to 10% lower signal‐to‐MU ratios relative to that of . This under‐response is consistent with ghosting effects due to charge trapping.