(en) Bengt Strömgren, « Two-dimensional spectral classification of F stars through photoelectric photometry with interference filters », Vistas in Astronomy, vol. 2, no 1, , p. 1336-1346 (DOI10.1016/0083-6656(56)90060-5, Bibcode1956VA......2.1336S)
(en) Bengt Strömgren, « Two-dimensional spectral classification of F stars through photoelectric photometry with interference filters », Vistas in Astronomy, vol. 2, no 1, , p. 1336-1346 (DOI10.1016/0083-6656(56)90060-5, Bibcode1956VA......2.1336S)