Despite saying above that the M20 led doesn't flicker I noticed flicker on the BSA A10 led headlamp last night which got me thinking about Critical Flicker Fusion. We all vary in our sensitivity to percieving flicker (flicker threshold) as individuals and within ourselves for various reasons (fatigue, region of field of view, or health). For a review see...... https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8537539/.