Saw a documentary on Johann Reichhart, who modified the guillotine allowing executions to be “less stressful” for the prisoners taking 3-4 seconds. From 1924 on, during the Weimar Republic and the period of National Socialism, he executed 2,951 people (250 of them women) via guillotine, and 59 by hanging. He also executed Hans Scholl and Sophie Scholl (d. 22 February 1943), the most famous members of the resistance group White Rose. Reichhart later said that he had never seen anyone die as bravely as Sophie Scholl.
it also depends on the medium which the light pass through.