Flicker is a difficult problem to quantify and to solve. The untimely combination of the following factors is required for flicker to be a problem: 1) some deviation in voltage supplying lighting circuits and 2) a person being present to view the possible change in light intensity due to the voltage deviation. The human factor significantly complicates the issue and for this reason flicker has historically been deemed "a problem of perception." (grouper.ieee.org)

Voltage flicker tracking is essential for electric power quality analysis. The instantaneous voltage flicker magnitude, frequency and phase are estimated using Kalman filtering technique. (Acta Press)

This flicker is objectionable only when the magnitude and frequency of occurrence of the voltage drop exceed certain thresholds. (cipco.apogee.net)