Visibility limit of naked-eye sunspots

Based on observations of 20 observers, a visibility limit for naked-eye sunspot observations has been calculated. A sunspot with a penumbral diameter of at least 41 arcsec and an umbral diameter of at least 15 arcsec can be detected by an average eye. An eyesight test was designed to solve the question whether the penumbral or the umbral diameter is conclusive for the visibility of a sunspot. The measured limit of 19.3 arcsec indicates that a combination of both is the determining factor.