In this paper, we review the LED flicker metrics as defined by the IEEE P2020 working group. The goal of these metrics is to quantify the flicker behaviour of a camera system, to enable engineers to quantify flicker mitigation, and to identify and explore challenging flicker use cases and system limitations. In brief, Flicker Modulation Index quantifies the modulation of a flickering light source, and is particularly useful for quantifying banding effects in rolling shutter cameras. Flicker Detection Index quantifies the ability of a camera system to distinguish a flickering light source from the background signal level. Modulation Mitigation Probably quantifies the ability of a camera system to mitigate modulation of a flickering light source. This paper explores various use cases of flicker, how the IEEE P2020 metrics can be used to quantify camera system performance in these use cases, and discusses measurement and reporting considerations for lab based flicker assessment.