After earning his Doctor of Science degree from Liege in 1937 and supported by a grant from the Belgian government, he engaged in a year of research in molecular physics under Peter Debye's mentorship, attending seminars led by Victor Henri in which he established connections with significant fellow students.
A pivotal phase of his career was the next three years, 1938–40, spent at Columbia University in New York as a Fellow of the Belgian American Educational Foundation where he collaborated with Selig Hecht to explore the biophysics of vision.
[2] After experiments they reported to the American Association for the Advancement of Science that received attention oil the media,[3][4][5] in 1942, a joint paper authored by Hecht, Shlaer, and Pirenne marked a turning point in the understanding of visual perception near the absolute threshold level by measuring the minimum number of photons the human eye can detect 60% of the time.
[6][7][8] This paper highlighted that the perceived variability, previously attributed to biological causes, predominantly stemmed from physical fluctuations in the small quantity of light quanta absorbed by the visual photo-pigment.
[9] On his return to England, Pirenne's intricate neurophysiological studies of 'on' and 'off' neuronal units and their interactions found practical application in screening military personnel for night blindness which he carried out there until 1945.