This study estimated thresholds and locus for human phosphene perception during non-invasive transcranial alternating current stimulation at 20, 50, 60, and 100 Hz. Perception depended significantly on stimulation intensity, with the lowest threshold at 20 Hz and no reported phosphenes at 100 Hz. The authors report dosimetry consistent with a retinal origin and frame the findings as relevant for informing cautious ELF exposure limits in safety guidelines.